ArticlePDF Available

Magnesium-Based Sacrificial Anode Cathodic Protection Coatings (Mg-Rich Primers) for Aluminum Alloys

Authors:

Abstract and Figures

Magnesium is electrochemically the most active metal employed in common structural alloys of iron and aluminum. Mg is widely used as a sacrificial anode to provide cathodic protection of underground and undersea metallic structures, ships, submarines, bridges, decks, aircraft and ground transportation systems. Following the same principle of utilizing Mg characteristics in engineering advantages in a decade-long successful R&D effort, Mg powder is now employed in organic coatings (termed as Mg-rich primers) as a sacrificial anode pigment to protect aerospace grade aluminum alloys against corrosion. Mg-rich primers have performed very well on aluminum alloys when compared against the current chromate standard, but the carcinogenic chromate-based coatings/pretreatments are being widely used by the Department of Defense (DoD) to protect its infrastructure and fleets against corrosion damage. Factors such as reactivity of Mg particles in the coating matrix during exposure to aggressive corrosion environments, interaction of atmospheric gases with Mg particles and the impact of Mg dissolution, increases in pH and hydrogen gas liberation at coating-metal interface, and primer adhesion need to be considered for further development of Mg-rich primer technology.
Content may be subject to copyright.
Metals 2012, 2, 353-376; doi:10.3390/met2030353
metals
ISSN 2075-4701
www.mdpi.com/journal/metals/
Review
Magnesium-Based Sacrificial Anode Cathodic Protection
Coatings (Mg-Rich Primers) for Aluminum Alloys
Shashi S. Pathak, Sharathkumar K. Mendon, Michael D. Blanton and James W. Rawlins *
School of Polymers and High Performance Materials, The University of Southern Mississippi,
Hattiesburg, MS 39406, USA; E-Mails: shashi.pathak@usm.edu (S.S.P.);
sharathkumar.mendon@usm.edu (S.K.M.); michael.blanton@usm.edu (M.D.B.)
* Author to whom correspondence should be addressed; E-Mail: james.rawlins@usm.edu;
Tel.: +1-601-266-4781; Fax: +1-601-266-5880.
Received: 31 May 2012; in revised form: 6 August 2012 / Accepted: 21 August 2012 /
Published: 14 September 2012
Abstract: Magnesium is electrochemically the most active metal employed in common
structural alloys of iron and aluminum. Mg is widely used as a sacrificial anode to provide
cathodic protection of underground and undersea metallic structures, ships, submarines,
bridges, decks, aircraft and ground transportation systems. Following the same principle of
utilizing Mg characteristics in engineering advantages in a decade-long successful R&D
effort, Mg powder is now employed in organic coatings (termed as Mg-rich primers) as a
sacrificial anode pigment to protect aerospace grade aluminum alloys against corrosion.
Mg-rich primers have performed very well on aluminum alloys when compared against the
current chromate standard, but the carcinogenic chromate-based coatings/pretreatments are
being widely used by the Department of Defense (DoD) to protect its infrastructure and
fleets against corrosion damage. Factors such as reactivity of Mg particles in the coating
matrix during exposure to aggressive corrosion environments, interaction of atmospheric
gases with Mg particles and the impact of Mg dissolution, increases in pH and hydrogen
gas liberation at coating-metal interface, and primer adhesion need to be considered for
further development of Mg-rich primer technology.
Keywords: magnesium; sacrificial anode; cathodic protection; Mg-rich primers;
anticorrosive coatings; aluminum alloys; corrosion protection
OPEN ACCESS
Metals 2012, 2
354
1. Introduction
Magnesium is the sixth most abundant element found in the earth’s crust, occurring in over 80 minerals
that contain more than 20% Mg by weight. Carbonates are the most common form of Mg in nature.
Magnesite [MgCO3], dolomite [CaCO3·MgCO3], brucite [Mg(OH)2], bishovite [MgCl2·6H2O],
carnallite (KCl·MgCl2·6H2O) and olivine [(MgFe)2SiO4] have been considered as raw materials for
Mg metal production [1]. The other major source of Mg is MgCl2 from seawater.
Mg is the most electrochemically active metal used in engineering applications (Table 1), and
corrodes so readily in some environments that Mg and Mg alloys are purposely utilized as sacrificial
anodes on steel structures, such as ship hulls and steel pipes. Mg and Mg alloys stored at room
temperature (STP) or in humid atmospheric conditions develop a compositionally varied surface film,
consisting of Mg oxide, hydroxide, and carbonates. These films are less stable than the passive films
formed on metals such as aluminum and stainless steels. The corrosion protection ability of the surface
film is highly dependent on environmental conditions, such as humidity, chloride ion concentration,
and interaction with atmospheric gases. Corrosion behavior and corrosion protection methods of Mg
and its alloys have been reviewed by several researchers [2–5]. Corrosion resistance of Mg alloys
decreases with increasing relative humidity (RH) and decreasing pH (below 11.5). Consistently, Mg
resists corrosion in alkaline solutions when the pH value is above 11.5.
Table 1. Property comparison.
Properties of Magnesium Aluminum Iron
Crystal structure Hcp Fcc Bcc
Ease of fabrication and joining of Mg
Density at 20 °C (g/cc) 1.74 2.70 7.86
Al is ~2.9 × and Mg is ~4.5 × lighter than Fe
Coefficient of thermal expansion 20–100 (×106/°C) 25.2 23.6 11.7
Elastic modulus (106 psi) 6.4 10 30
Melting point (°C) 650 660 1536
Standard reduction potential (V vs. Standard
Hydrogen Electrode)
2.37 1.66 0.447
Higher reactivity of Mg and Al with atmosphere than Fe
Mg can act as sacrificial anode
Cost Mg > Al > Fe
Mg’s high strength to weight ratio, low density (~66% of aluminum and 25% of iron), high specific
stiffness, high thermal conductivity and electromagnetic shielding properties make it a popular choice
in various lightweight applications in aircrafts, automobiles, electronics, and medical implant components.
Although Mg is available commercially with purity above 99.5%, it is rarely used in engineering
applications without being alloyed, due to inherent limitations such as low elastic modulus, limited
high strength and creep resistance at elevated temperatures, high degree of shrinkage on solidification,
high chemical reactivity and limited corrosion resistance. The most commonly used Mg alloys contain
Al, Zn, and Mn; Al and Zn are added in ingot form, dissolving readily at normal melt temperature
(~700 °C). Al, Zn and Mn are the major alloying elements in Mg alloys. Keeping corrosion protection
in mind, alloying elements (Al, Zn, Mn) and heavy-metal impurities (Ni, Fe, Cu) of Mg alloys have to
Metals 2012, 2
355
be controlled according to the endurance limit. Excess addition of Mn enhances the formation of
nobler Al Mn (Fe) secondary phase with Al or Fe which is detrimental to the corrosion resistance of
Mg alloys. Mg alloys such as AZ91D, AZ91E, and AM60B contain very low levels of the heavy metal
impurities and offer far better corrosion resistance than ordinary Mg alloys. The number of
applications for Mg alloys is increasing every year, primarily in the automotive and aerospace sectors.
The established corrosion rate of the secondary phases in Mg alloys is very low when the pH is
between 4 and 14. Corrosion of Mg alloys in neutral or alkaline salt solutions is usually initiated as
pitting at secondary phase particles. The thickness of the oxide film on secondary phases formed in
solution (pH 12) was shown to be several times the thickness formed in air and increased with
decreasing pH [6]. The type of processing (ingot, die-cast and extruded Mg or its alloys) also
influenced the nature and severity of the corrosion process due to subtle but important morphology
differences. For instance, the corrosion rate of AZ91 ingot and die-cast was higher in acidic solutions
(pH 1–2) than in neutral and highly alkaline solutions (pH 4.5–12) [7]. In comparison, extruded
Mg alloy AM60 does not exhibit the same phenomenon in 3.5% NaCl solution at different pH values,
however, it undergoes severe pitting corrosion at pH 7 in the absence of pitting (except on the edges) at
pH 12. With the shift towards higher chloride ion concentration, the open circuit corrosion potentials
shift to more negative values and the corrosion rates increase at all pH levels.
Mg possesses a strong thermodynamic driving force for corrosion and its surface film does not
present a very protective kinetic barrier to corrosion [8]. Consequently, Mg is unsuitable for use
singularly in applications involving humid and aqueous environments with pH < 12. On the contrary,
structural metals such as steel and Al are cathodic to (more noble in galvanic series than) Mg. In these
cases, the undesirably high electrochemical activity of Mg has been capitalized upon to provide
cathodic protection of nobler structural metals/alloys in pipelines, tanks, and marine structures. The
premise of cathodic protection is for an electrical circuit to be established as a means to control the
corrosion of a structural metal surface by rendering it as the cathode of a galvanic cell. The use of Al,
Zn, Mg, and Sn/In to create activated aluminum alloys, as (1) sacrificial anodes and (2) impressed
current cathodic protection systems, are established methods to protect steel structures against
corrosion [9,10]. Mg and its alloys have been used as sacrificial anodes for several decades in cathodic
protection of oil and gas pipelines, oil drilling platforms cables, heat exchangers, aircraft, ships, and
bridges. In these applications, Mg acts sacrificially and transfer corrosion activity away from the
structural materials to be protected (cathode). The process results in Mg (anode) dissolution over a
given period of time. Figure 1 provides the schematic of sacrificial anode and impressed current
protection methods of a pipeline.
Metals 2012, 2
356
Figure 1. Schematic showing cathodic protection methods using sacrificial anode and
impressed current.
2. Interaction of Mg and Its Alloys with the Atmosphere
Corrosion behavior of Mg and its alloys in the atmosphere differs considerably from their behavior
in solution. For example, in the presence of sodium chloride in humid air, the surface of Mg is rapidly
converted and covered by the white, flaky corrosion products of magnesium hydroxide [11]. Chloride
ions are known to promote the corrosion of Mg in aqueous solutions [12]. The anodic reaction
(Mg dissolution) under a thin electrolyte layer is diminished compared to a bulk electrolyte [13].
Additionally, the main cathodic process in solution is water reduction, while oxygen reduction is the
main cathodic reaction during atmospheric corrosion in thin electrolyte layers [14,15]. The overall
rate of corrosion and nature of corrosion products are strongly influenced by the RH and chloride
ion concentration.
The natural affinity of Mg and magnesium hydroxide for carbon dioxide has been exploited for CO2
sequestration. Studies investigating the effects of CO2 pressure, temperature, and aqueous solution pH
on rates and mechanisms of magnesium oxide and magnesium hydroxide conversion to magnesium
carbonate have established that the combination of high CO2 pressure and high temperature increased
the rate of carbonate formation [16,17]. Dissolution of CO2 in the surface electrolyte neutralizes the
alkali formed in the cathodic reaction, which initially reduces pH in the surface electrolyte and
increases the dissolution rate of the surface film. The hydroxide ions (formed during the cathodic
reaction or dissolved from the film) react with carbonic acid, forming carbonates, which enhance the
corrosion performance of Mg alloys in humid air by forming a physical barrier [18]. The presence of
CO2, even at the atmospheric level of CO2, i.e., 350 ppm, reduces the corrosion rate by a factor of 3–4
compared that in a CO2-free atmosphere in the presence of NaCl (0–70 mg/cm2) [19,20]. Lin et al.
studied the role of CO2 in the initial stage of atmospheric corrosion of AZ91 magnesium alloy in the
presence of NaCl and concluded that CO2 inhibited NaCl-induced corrosion by generating the slightly
soluble hydroxy carbonates that provided a partly protective layer on the surface of the Mg alloy [21].
In general, magnesium carbonates, such as hydromagnesite [Mg5(CO3)4(OH)2·4H2O) and nesquehonite
Metals 2012, 2
357
(MgCO3·3H2O), have been determined to be the dominant corrosion products on the surface of Mg
and its alloys during NaCl-induced atmospheric corrosion [22]. The protective property of Mg
carbonates on atmospheric corrosion resistance of Mg alloys has been reported in a number of
investigations [23]. In the absence of CO2, Mg(OH)2 is the dominant corrosion product formed on Mg
surfaces in aqueous solutions and high humidity environments in the presence of NaCl [24].
Through the proper selection of Mg alloy components and selective use of coatings and insulation
materials, the risk and rates of corrosion can be significantly reduced. Some metallurgical processes,
such as rapid solidification and heat treatment, improve mechanical properties and also improve the
corrosion resistance of Mg alloys by refining grain size and distributing the β phase along grain
boundaries. Carbon inoculation (for Mg alloys containing Al and Mg-Zr hardeners (for Mg alloys
lacking Al) are commonly used for grain refining/modification.
3. Magnesium-Rich Primer
In the early 2000s, following, by analogy the formulation of Zn-rich primer coatings for the
protection of steel, researchers at North Dakota State University (NDSU) developed and refined the
concept of a Mg-rich primer for cathodic corrosion protection of Al alloys without the use of
chromate-based pretreatments or chromate pigments. The research was facilitated by the timely
availability of particulate Mg appropriate for use as a pigment in coatings. While particulate Mg can
pose a fire hazard, the thin layer of Mg oxide (4% by weight) on the Mg particles has been reported to
stabilize the bulk Mg against further oxidation [25]. Moreover, although the natural Mg oxidation
products are basic, they do not yield a pH high enough to directly corrode and dissolve Al. Since Mg is
more electronegative (2.37 V vs. SHE) than Al (1.67 V vs. SHE), the more noble Al substrate in this
galvanic couple is cathodically polarized, while the less noble Mg particles in the coating matrix are
anodically dissolved (Figure 2). Sacrificial Mg particles serve as a source of electrical energy. The
protective cathodic current generated by contact between Mg particles in the coating matrix is used in
the polarizing cathodic reaction on the Al substrate.
Figure 2. Schematic showing the open circuit potential (OCP) of Mg-rich primer coated
AA2024 substrate.
Mg-rich coatings, termed as such because they are formulated to ensure that the Mg loading
exceeds the critical pigment volume concentration (CPVC), contain Mg particles in physical and
Metals 2012, 2
358
electrical contact with each other as well as with the substrate. The formulation variables facilitate the
flow of cathodic protection current from Mg particles to the Al substrate with minimal resistance and
protect the underlying substrate from corrosion. Pigment volume concentration (PVC) is defined as the
ratio of pigment(s) by volume to the sum volume of pigment(s) and non-volatile binder. The CPVC is
the point at which just enough binder exists to cover all the pigment(s) at the densest possible packing
and fill the voids between the pigment particles. Beyond the CPVC, therefore, there is insufficient
polymer binder to coat the pigment surfaces completely, and the pigment particles are in physical
contact with each other. The CPVC is also the point where the dry coating film transitions from a
two-phase system of pigment and binder to a three-phase system through the introduction of trapped
air voids in the matrix. Mathematically, CPVC can be calculated for a solvent-based coating using
Equation 1 [26].
CPVC = 1/(1 + OAv) (1)
where OAv is the volumetric oil absorption, expressed as milliliter oil/milliliter pigment.
Mg-rich primers have proven in certain applications to be viable or potentially viable alternatives
for replacing chromate-based pretreatment and coatings for high strength and light weight aerospace
grade Al alloys such as AA2024 T-3 and AA7075 T-6. These phase-separated Al alloys are susceptible
to galvanic corrosion due to their highly complex metal-in-metal composite form. Chromates in the
form of pigments in primers and as pretreatments for the substrates perform exceptionally well in
protecting these alloys from corrosion as they function uniquely as anodic and cathodic inhibitors
at very low concentrations in electrolyte solutions, especially with chloride ions that affect Al
substrates [27,28]. However, hexavalent Cr has been recognized as a human respiratory carcinogen,
based on epidemiological and medical evidence accumulated for more than a century [29]. The
elimination of toxic Cr(VI) species in current painting and de-painting operations will have
tremendous environmental impact as the waste stream generated through these materials incurs
significant disposal costs to the Air Force. It is estimated that the Air Force spends over a billion
dollars annually in stripping and repainting aircraft [30]. Furthermore, recent reductions in the Cr(VI)
personnel exposure limit by the Occupational Safety and Health Administration will result
in increased compliance costs unless a viable alternative is identified and implemented. A
non-chrome/chrome replacement coating system would need to extend the life of existing Al-based
assets and facilitate expanded use of economical aluminum alloys in both DoD and commercial
applications [31]. Consequently, extensive research has been conducted in search of alternative
technologies, such as anodization [32], sol–gel treatment [33–35], pigmented coatings [36], plasma
polymer layers [37], conductive pigments/polymers [38,39], and pigment-based cathodic protection [40].
NDSU sought a patent on their research [41] and subsequently licensed the Mg-rich primer technology
to Akzo Nobel Aerospace Coatings who further improved upon the original formulation and resolved
the rougher than desirable appearance and usability issues by using smaller Mg particles. Akzo Nobel
further optimized the PVC, lowered the volatile organic compound (VOC) levels, and modified the
resin system to improve coating flexibility. In 2007, Akzo Nobel produced Aerodur® 2100 MgRP that
contained green pigment to increase the opacity and facilitate a better contrast ratio for painters to
judge wet film thickness [42]. Since that time, Mg-rich coatings have been the topic of research by
several researchers and evaluated in many potential applications. Mg-rich primers have proven to be
Metals 2012, 2
359
quite effective as part of a completely chromate-free coating system comprising a non-film forming
surface treatment and an Advanced Performance Coating (APC) grade topcoat, which exhibits
excellent corrosion protection of scribed AA2024-T3 panels in both ASTM B 117 and outdoor
exposure tests at Daytona Beach, FL [43]. Before discussing the detailed characteristics, stability and
performance of corrosion performance of Mg-rich primers, the principle of sacrificial cathodic
protection and metallurgical and electrochemical properties of Mg will be briefly discussed.
4. Principle of Sacrificial Anode Cathodic Protection
Cathodic protection is the most widely adopted electrochemical corrosion control technique and is
accomplished by applying a direct cathodic protection current (Figure 3) to a structure, effecting a
change in potential from the natural corrosion potential (Ecorr) to a protective potential in the immunity
region. Cathodic polarization of the structure controls the kinetics of the electrode processes occurring
on the metal-electrolyte interface. The required cathodic current is supplied by means of an impressed
current or attachment to a sacrificial anode. The metal structure in contact with an aqueous environment
having a near neutral pH is thereby cathodically protected.
Figure 3. Evans diagram explaining the principle of cathodic protection.
Corrosion involves the active dissolution of metal at anodic sites and reduction of oxygen and/or
water at cathodic sites. The severity of corrosion is directly proportional to the magnitude of the
difference in potential between the anode and the cathode. Electrons liberated in anodic reactions are
consumed in the cathodic reaction. Upon cathodic polarization, the potential of cathodic sites shifts the
Metals 2012, 2
360
potential of the anodic area to the point at which there is no potential difference between the anode and
cathode, thereby minimizing or even eliminating corrosion at the protected substrate. Complete
cathodic protection is achieved when the metallic structure becomes the cathode, i.e., more electronegative.
The principle of cathodic protection is well explained by the Wagner-Traud mixed potential theory.
According to this theory, any corrosion process can be divided into two or more oxidation and
reduction partial reactions with no net accumulation of electric charge during the process. The
corrosion reactions occurring in aluminum in an aqueous medium are shown in Equations 2–4:
Anodic reaction:
Al Al3+ + 3e (Aluminum dissolution) (2)
Cathodic reactions: O2 + 2H2O + 4e 4OH (Oxygen reduction on Al in neutral or basic solution) (3)
O2 + 4H+ + 4e H2O (Oxygen reduction on Al in acid solutions) (4)
Corrosion is initiated only when both the anodic and cathodic reactions occur simultaneously. The
total rate of oxidation must equal the total rate of reduction in any system. In Figure 3, the relationship
between the anodic and cathodic partial corrosion currents for Al has been shown, using mixed
potential theory and kinetic equations. As shown in Figure 1, polarization of the cathode in a negative
direction from the corrosion potential decreases the corrosion rate. By polarizing the system from Ecorr
to E’corr with a known applied current through sacrificial anode or direct current source, the corrosion
current density decreases from Icorr to I'corr. For complete inhibition of the corrosion processes, it is
necessary to polarize the metal to its reversible potential EAl/Al3+. The applied current at EAl/Al3+
potential is termed as the protection current [10,44]. We will limit our discussion here on sacrificial
anodes considering only their relevance to Mg-rich primers.
5. Open Circuit Potential and Potentiodynamic Dynamic Polarization Measurement of
Mg-Rich Primer
Driven by the electrical connection between the Mg-rich primer and the Al substrate, the
substrate/primer interface is polarized to the mixed potential of the Mg particles/Al substrate. The
mixed potential of Al substrate coated with Mg-rich primer is a cathodic potential relative to the open
circuit potential (OCP) of the Al substrate itself. OCP and potentiodynamic polarization plots provide
an idea of the extent of the cathodic protection versus time during service. An OCP below ~0.9 V
(SCE) for Al 2024 T provides an indication of the cathodic protection provided by Mg-rich primers [45].
Figure 4 provides a visual summary of the cathodic protection offered by Mg-rich primer to Al
alloy AA2024 T3.
Mg shifts the potential (cathodically polarizes) of the Al substrate towards more negative
potential than the AA2024-T3 substrate. The variation in OCP of AA 2024-T3 (0.5 VAg/AgCl) and
Mg (1.3 to 1.5 VAg/AgCl) in 3.5 wt % NaCl solution are shown in Figure 4. The Mg-rich primer
coated AA2024-T3 aluminum alloy achieved a mix potential (about 0.9 VAg/AgCl) between those of
the bare AA2024 T3 substrate and the Mg particles.
Metals 2012, 2
361
Figure 4. Open circuit potential of Mg-rich primer coated AA2024T3, bare AA2024 T3,
and bare Mg in 3.5 wt % NaCl solution.
The DC potentiodynamic plot for bare Al alloy AA2024-T3, bare Mg, and Mg-rich primer coated
AA2024-T3 (Figure 5) support the concept of mixed potential theory describing the galvanic coupling
behavior between the primer and alloy substrate. Potentiodynamic scans show a bare Mg-rich primer
coated AA2024-T3 having an OCP in 3.5 wt % NaCl solution of about 1.26 V vs. Ag/AgCl
(sat. AgCl) while bare AA2024-T351 and bare Mg exhibit OCP values of about 0.56 V and 1.66 vs.
Ag/AgCl (sat. AgCl), respectively. The potentiodynamic plots of coated AA2024-T3 substrate are
shifted to lower currents when compared to the bare AA2024-T3.
6. Performance and Mechanism of Corrosion Protection by Mg-rich Primer
Mg-rich primers on Al alloys have been shown to perform very well on outdoor exposure at various
sites across the US in a variety of applications. The performance of Mg-rich primer depends on various
factors such as polymer properties, Mg PVC, type of Mg particle (pure Mg, Mg alloys or the presence
of oxide/hydroxide/carbonate layer on Mg particles) and the environment. Several electrochemical
studies, e.g., electrochemical impedance spectroscopy (EIS), potentiodynamic polarization, scanning
vibrating electrode technique (SVET) and scanning electrochemical microscopy (SECM), and OCP
studies have been conducted to understand the corrosion protection mechanism offered by Mg-rich
primers on aerospace grade Al alloys. These studies most often suggest that cathodic protection was
due to uniform corrosion/dissolution of the Mg particles in coatings matrix.
Metals 2012, 2
362
Figure 5. DC potentiodynamic polarization measurement of Mg-rich primer coated
AA2024T3, bare AA2024 T3 and bare Mg in 3.5 wt% NaCl solution. (scan rate of
0.166 mV/s, polarization range: OCP 0.25 mV and OCP +0.25 mV).
Battocchi et al. employed EIS, OCP and potentiodynamic polarization to study the electrochemical
behavior of Mg-rich primer on alloys AA2024 and AA7074, and showed that the Mg-rich primer
provides sacrificial protection to the Al substrate by a two-stage mechanism [46]. In the first stage, Mg
polarizes Al cathodically, shifting its potential below the pitting corrosion potential. The consequence
of this polarization can be either the prevention of pit nucleation at the exposed Al areas, or the
inhibition of pit growth for the nucleated pits. During this stage, any defects on the surface will
become cathodic, whereas the Mg particles will be anodic. At the cathodic areas, reduction of
hydrogen and possibly dissolved oxygen increases the pH above the threshold for the precipitation of
magnesium oxide. This precipitation leads to the formation of a porous layer that further inhibits
corrosion by a barrier mechanism. The typically high dissolution rate of Mg is significantly decreased
by its incorporation in the polymer (a polymeric membrane controlling water, oxygen and electrolytes
to varying degrees). In a subsequent paper, the same authors reported the corrosion behavior of the
same alloys coated with a magnesium-rich coating, of pure magnesium and of the bare aluminum
substrates in 0.1% NaCl solution and dilute Harrison’s solution (DHS), assessed using electrochemical
techniques [47]. The change from 0.1% NaCl to dilute Harrison solution (DHS) affected the OCP, the
corrosion rates and the equivalent circuits of the systems studied. All along, the Mg in the coating
maintained its protective properties by cathodically polarizing the Al substrates away from their pitting
potential. DHS caused pure magnesium to corrode faster due to cathodic de-polarization, which the
authors mentioned as being possibly due to the formation of a sulfate ion pair (or complex) of Mg, and
also resulted in increased electromotive force for cathodic protection of Al alloys by the
Mg-rich primer.
Allahar et al. modeled EIS data of a Mg-rich primer on a gold substrate under immersion in DHS
and analyzed it for consistency with Kramers-Kronig relations and applicability for use with a
transmission-line model [48]. The data in the frequency range of 1 mHz to 100 kHz were
Kramers-Kronig consistent, while the transmission-line model was shown to be applicable for data in
Metals 2012, 2
363
the 1 mHz to 10 kHz range. In a subsequent paper, Allahar et al. monitored the performance of a
Mg-rich primer with a standard US Air Force topcoat on an AA2024 T3 substrate via embedded
electrodes placed between the primer and the topcoat, where the coatings were subject to ASTM B117
exposure [49]. EIS and electrochemical noise method experiments indicated that cathodic protection
was due to a more uniform corrosion of the Mg particles, and the loss of cathodic protection resulted in
a shift toward a more localized corrosion.
Simões et al. investigated the mechanism of corrosion protection of AA2024 T3 by a Mg-rich
coating using SVET and SECM [40,50]. SVET measured the evolution of pit activity with time under
sacrificial protection, while SECM allowed indirect sensing of the cathodic activity above the
electrodes. The study was complemented by EIS and OCP measurements. The results showed that in
the first stage, Mg acted both by preventing pit nucleation as well as by inhibiting the growth of the
already existing ones, whereas at a later stage, the precipitation of a porous layer of magnesium oxide
at defective areas was seen to lead to some degree of barrier protection. Cathodic protection provided
by the Mg-rich coating was capable of inhibiting pit nucleation by shifting the potential of the system
towards the cathodic direction and decreasing the anodic activity at pre-existing pits. Changes in
oxygen reduction current indicated that the high corrosion activity of Mg led to some maintenance of
the cathodic reaction on Mg surface even when it behaved as a sacrificial anode for Al.
Li et al. investigated the effects of compositional variables associated with formulating a
two-component epoxy-amine based Mg-rich primer for protecting alloy AA20224 T3 [51]. An
optimized coating composition based on high molecular weight epoxy resin, amide-functional curing
agent, epoxy:amine ratio of 1, and Mg volume content of 50% passed over 3,000 hours of ASTM
B117 salt spray exposure. Corrosion protection was shown to occur through galvanic coupling
between Mg in the primer and the aluminum substrate. SEM-EDX mapping and electrochemical
measurements indicated that Mg oxidation products may also be playing a role in corrosion protection
by increasing the barrier properties over the coating lifetime.
King and Scully attempted to investigate the primary sacrificial and secondary barrier mechanisms
of protection afforded to the alloy AA2024-T351 substrate by a Mg-rich primer to estimate the total
residual stored Mg anode capacity and electrically “well-connected” Mg in the primer as sensed
electrochemically, after various environmental exposures [52]. Two possible modes of protection:
long-range protection of remote defects and local or short-range Mg pigment-based protection of local
and buried defects were suggested. Both modes of protection were believed to be mediated by the high
ionic and electrical resistance of the coating system as a function of PVC and primer/topcoat properties.
While most Mg-rich primers have been formulated with either thermoplastic epoxy resins or
thermosetting epoxy-amine systems, Ravindran et al. reported employing a silane-modified glycidyl
carbamate binder crosslinked with a polyamide or a polyamine as the continuous phase of their
Mg-rich primer [53]. Trimethoxy aminosilane was reacted with hexamethylene diisocyanate-based
biuret (10%, 15% and 20% silane modification) and the product was reacted with glycidol to
synthesize the silane-modified binder. While no corrosion data was presented, the authors reported that
the coatings possessed excellent thermal stability as determined via thermogravimetric analysis. As the
PVC was increased from 20% to 40%, the char content increased from ~40% to 80% (under nitrogen).
The weight gain was attributed to the formation of Mg3N2, which decomposes rapidly upon exposure
to air to form MgO/Mg(OH)2.
Metals 2012, 2
364
Hayes et al. reported that unlike Mg-rich primers, topcoating had negative impact on the corrosion
performance of commercial chromate primers. The authors opined that the topcoat may be acting as a
vapor/water barrier in limiting the amount of water that penetrated through to the chromate primer, a
necessary step which depends on inhibitor solubilization to provide corrosion protection. While the
Mg-rich primer also requires contact with water to function, the galvanic corrosion protection provided
by the Mg-rich primer does not require the transport of an active corrosion inhibiting species. The
authors also suggested that the galvanic protection mechanism could be robust enough where the
topcoat does not affect it significantly [54].
Lu et al. evaluated an epoxy primer with and without Mg particles on AZ91D alloy using EIS,
scanning electron microscopy (SEM) and X-ray diffraction (XRD), and concluded that the Mg-rich
primer provided better protection for the alloy than the coating without Mg particles. Upon immersion
in 3 wt % NaCl solution for 100 days, Mg(OH)2 was observed to have been formed that precipitates
and blocks micropores in the coating, which is beneficial for the coating structure and resistance
properties [55].
7. Unnoticed Factors that Influence the Behavior of Mg-Rich Primers
Bierwagen concluded that the total system performance of the Mg-rich primer + topcoat was a
synergistic blend of the cathodic/sacrificial protection of the primer, the inhibition/thin barrier layer
effects of the MgO/Mg(OH)2 formed as oxidation products of the Mg, the barrier properties of the
polymer in the Mg-rich primer, plus the barrier properties of the topcoat [56]. Indeed, panels coated
with Mg-rich primers have performed very well on outdoor exposure at various sites across the US.
However, it has also been observed that these same Mg-rich primers fail rapidly and exhibit heavy
blistering very early on in salt spray tests (ASTM B117), which is still a key MIL-SPEC test in
certifying coatings for corrosion protection. This duality in performance could not be explained by the
Mg products mentioned above. The performance contradiction is possibly unique to Mg-rich primers
and magnifies the importance of matching mechanisms to outdoor performance in judging and
specifying accelerated weathering tests. Failure in salt spray testing may result in a complete dismissal
of otherwise viable new technologies for corrosion control materials. Pathak et al. investigated the
behavioral dichotomy by exposing Mg-rich primers to salt spray testing and natural weathering and
characterizing them at periodic intervals [57]. The coatings were formulated at a PVC of 45% in a high
molecular weight high performance thermoplastic epoxy resin, Eponol®, that is supplied as a 35%
solution (by weight) in a blend of methyl ethyl ketone and propylene glycol methyl ether (75:25 by
weight), and is reported to have a specific gravity of 0.934 at 25 °C and 26.6% volume solids. Eponol
was employed as a model polymer in this study as it closely mimics the thermosetting epoxy-amine
systems commonly employed in anticorrosive coatings, while affording broader characterization
studies because of its thermoplastic nature. The authors reported the presence of a thin and porous
magnesium hydroxide layer in Mg-rich primers exposed to salt spray, while a thicker, protective
magnesium carbonate layer was detected in the samples when subject to natural weathering. The
carbonate film was shown to inhibit both the anodic and the cathodic corrosion processes and does not
result in blister formation. Consequently, Mg-rich primers exposed to natural weathering exhibit
Metals 2012, 2
365
excellent corrosion resistance. However, salt spray conditions are not conducive to facilitate
magnesium carbonate formation at a rate versus the rate of dissolution and corrosion.
Strekalov inferred that the amount of adsorbed water present on a magnesium surface at 95% RH
and 22 °C corresponds to more than 16 monolayers [58]. At very low concentrations of CO2, this
adsorbed water will react with the surface film to form magnesium hydroxide, i.e., brucite, (Equation 5).
Mg + 2H2O Mg(OH)2 + H2 (5)
In the presence of CO2, protolysis of carbonic acid decreases the surface pH (Equations 6 and 7) [59].
CO2 (aq) + H2O HCO3 + H+ (6)
HCO3 CO32 + H+ (7)
Magnesium hydroxide is thermodynamically stable only at low CO2 partial pressure and is
converted into magnesite (MgCO3) in the presence of atmospheric levels of CO2 (Equation 8) [60–62].
CO2 + Mg(OH)2 MgCO3 (s) + H2O (8)
At high RH, magnesite forms a stable hydrated magnesium carbonate, i.e., nesquehonite (Equation 9).
MgCO3 (s) + 3H2O MgCO3·3H2O (s) (9)
Equations 5–9 help to explain why Mg(OH)2 was not transformed into magnesium carbonate in
Mg-rich primers at the relatively low CO2 content in the salt-spray chamber environment as well as
primer film exposed to humid environments in glass jars. The relative proportions of magnesium
hydroxide and magnesium carbonate are influenced by CO2 concentration (the salt-spray chamber has
less CO2 than field exposure), CO2 solubility (in water/salt water) [63], and chloride concentration [64]
all relative to the rates of dissolution and corrosion. Moreover, the solubility of CO2 in water decreases
in the presence of sodium chloride (NaCl concentration is 5 wt % in salt-spray chamber and 3.5 wt %
in sea water) [65]. The limited availability of CO2 at the surface of the Mg-rich primer and the reduced
solubility of CO2 in 5 wt % salt solution favors rapid dissolution of Mg in water to form magnesium
hydroxide (brucite) with liberation of hydrogen that results in blister formation. Brucite was shown to
be only semi-protective due to its plate-like structure. Moreover, the film undergoes compressive
rupture due to the higher molar volume of magnesium hydroxide compared to metallic magnesium,
resulting in the constant exposure of fresh metal and allowing direct and facile electrolyte ingress [8].
The absence of magnesium carbonate passivation also contributes to poor corrosion protection in
Mg-rich primers exposed to salt spray test. Lindström et al. studied the influence of ambient
concentrations of CO2 on the atmospheric corrosion of Mg and reported that in the absence of CO2, a
passivating magnesium hydroxide film forms on the Mg surface that is unable to act as a cathode [20].
In the presence of CO2, the Mg surface was rendered more passive due to the formation of a thick
magnesium hydroxy carbonate film that inhibited both the anodic and the cathodic processes. The
authors also reported that immersing Mg in aqueous NaCl solution with a limited supply of CO2
resulted in rapid corrosion, which is consistent with the performance of Mg-rich primers in salt spray test.
Pathak et al. reported that SEM micrographs of Mg-rich primer films exposed to salt spray for
30 days exhibited the characteristic “sand rose” [66] or “sunflower” [67] morphology of brucite that
was also noted by Bierwagen et al. during electrochemical dissolution of magnesium pigment in water
Metals 2012, 2
366
and chloride environment (Figure 6). XRD and FTIR data also validated the formation of magnesium
hydroxide on Mg-rich primers in the salt-spray chamber. On the other hand, SEM micrographs of free
primer films exposed to natural weathering showed the presence of needle-like crystals reported for
magnesium carbonate by several authors [22,68,69].
Figure 6. Characterization of Mg particles used in the formulation of Mg-rich primers.
Metals 2012, 2
367
Pathak et al. formulated Mg-rich primers with Eponol, applied them on AA 2024 panels and
exposed the coated panels at Daytona, FL and Hawaii (Rain forest) for 6 months [70]. The panels were
evaluated via FT-IR (ATR) spectroscopy and compared to similar spectra obtained for Mg particles
treated for 3 h in carbonic acid (Figure 7a). The peaks around 845 cm1 and in between 1380 and
1530 cm1 suggest the formation of magnesium carbonates (nesquehonite and/or hydroxy magnesium
carbonates). The peak around 3695 cm1 is due to magnesium hydroxide which appears along with
hydroxy magnesium carbonate (3650 cm1 from O-H stretching of water molecule, 845 cm1 C-O
anti-symmetric stretching of carbonate, 1380–1530 cm1 C-O symmetric stretching of carbonate) on
Daytona and Hawaii exposed Mg-rich primer. The formation of magnesium hydroxide and carbonates
in coatings exposed to natural weathering support the explanation behind the behavioral dichotomy of
Mg-rich primers.
Figure 7b,c summarize the Raman spectra corroborating the FTIR spectra data. Raman spectra
(Figure 7b) of Mg particles treated with carbonic acid solution showing the presence of peak around
1101 cm1 (Raman shift) corresponding to magnesium carbonate. The Raman spectra of Mg particles
and Mg-rich primer (Figure 7c) under various exposure/treatment condition showing the formation of
magnesium carbonate naturally with interaction of atmospheric CO2 with Mg particles in primer and
on treated Mg particles in simulated carbonic acid solution. The peak around 1099 cm1 (Raman shift)
in spectra of treated Mg particles and Mg-rich primers validate the interaction of Mg with CO2. An
overlapping peak around 1108 cm1 (Raman shift) also comes from the resin Eponol.
Figure 7. (a) FT-IR (ATR) spectra of Mg-rich primer coated on AA2024 and Mg (untreated
and treated) particles; (b) Raman spectra of Mg powder (untreated and treated for 30 min and
120 min); and (c) Raman spectra of Mg particles treated with carbonic acid, Mg-rich primer
exposed outdoors, and Eponol coating.
(a)
Metals 2012, 2
368
Figure 7. Cont.
(b)
(c)
8. Further Technology Development
8.1. Improving Electrochemical Stability by Surface Treatment of Magnesium Particles
Building upon their earlier work, Pathak et al. treated Mg powder to develop a layer of protective
magnesium carbonate on or within Mg particles at ambient conditions and evaluated the pretreated Mg
in Mg-rich primers [71]. Specifically, Mg powder was treated with aqueous carbonic acid (CO2–H2O
solution) for varying lengths of time at ambient conditions and the resulting products were formulated
into systematically varying but controllably treated Mg-rich primers with Eponol. The same
formulation with the untreated Mg powder was employed as the control. While nesquehonite was
Metals 2012, 2
369
identified as a reaction product (Figure 8), magnesium hydroxide formation was not detected in any of
the XRD patterns, possibly due to being below the lower detection limit of standard XRD (2%) and the
fact that the hydroxide under these conditions was converted quickly to the carbonate Mg counterpart [72].
FTIR and FT-Raman analysis indicated that MgCO
3
·3H
2
O appeared after 20 min of treating Mg while
XRD analysis indicated the presence of MgCO
3
·3H
2
O after 30 min of treatment. Extended treatment
resulted in a flower-like morphology, possibly due to conversion of nesquehonite to hydroxy
magnesium carbonate.
Figure 8. Treatment of Mg particles prior to coating formulation.
The primer formulated with untreated Mg powder exhibited severe blistering within 4 h of being
placed in a salt spray chamber (following ASTM B117). However, the primer formulated with Mg
powder treated for 30 min did not exhibit any detectable or visible changes like blisters even after
being in the salt spray chamber for 2160 h. This confirms that coatings formulated with the aqueous
carbonic acid-treated Mg pigment performed similarly to Mg-rich primers exposed to natural
weathering where few, if any, failures have been reported in a variety of applications and
environmental conditions. The carbonic acid treatment was thus proven to be effective in stabilizing
the Mg particles by reducing its reactivity and rate of dissolution with water and is a more facile
approach than other techniques, such as the use of Mg-Al alloy instead of pure Mg [26] or surface
treatment of Mg by organic coatings [73]. Turel et al. established that adding Mg to an aqueous
solution of carbonic acid was the optimal method of generating significant amounts of nesquehonite
from Mg and that a combination of nesquehonite and free Mg is necessary to protect Al substrates
from corrosion [74].
Maier and Frankel studied the behavior of Mg-rich primers on AA2024 T3 panels and observed that
basic or cathodic corrosion of AA2024-T3 is possible for samples in contact with Mg-rich primers [75].
Thin electrolyte layer experiments and cathodic polarization curves in solutions equilibrated with
different gases showed that CO
2
in high concentration shifts the corrosion potential of Mg towards
cathodic direction and buffers the pH on the AA2024-T3 surface such that no basic corrosion occurred.
However, the amount of atmospheric CO
2
was not enough to prevent corrosion in an air-exposed
Metals 2012, 2
370
AA2024-T3 sample polarized cathodically. The authors also proposed that dissolved Mg ions could
play a role in the protection provided by Mg-rich primers and this influence could be different for
exposures in outdoor environments and accelerated weathering environments. Insoluble Mg
compounds formed in the coating pores could function as a protective barrier and such precipitates
could form more readily in the moist and cyclic exposure of real environments than in the constant
wetness of a salt spray chamber.
8.2. Use of Mg Alloys instead of Pure Mg Particles
Xu et al. investigated three different Mg alloys (AM60, AZ91B, and LNR91 with Al content of 5%,
8.5% and 50%, respectively) as pigments in an epoxy-polyamide system at various PVC values [76]. The
alloy pigments were characterized by large particle sizes (>60 microns) and varying shapes,
e.g., AM60 has a plate-like shape with a smooth edge, AZ91B has a chip-like shape, and LNR91 has a
cubic-like shape with a sharp edge. EIS and SEM studies indicated that the Mg alloy pigments
provided sacrificial protection to the Al alloy substrates and that precipitates formed from oxidation of
the Mg alloy particles were similar to the ones found in pure Mg-rich primers. In a subsequent paper,
Bierwagen et al. discussed the surface compositions of two Mg alloy pigments, AM60 and AZ91B,
and how coatings formulated with them changed in Prohesion chamber (where DHS is employed as
the spray solution) studies [77]. For both these alloy pigments, XPS depth profile revealed a three layer
structure (from outside to inside) as (a) MgCO3; (b) MgCO3; MgO and metallic Mg, Al mixture; and
(c) metallic core of Mg and Al. In Prohesion chamber studies with AM60, the nature of the corrosion
products changed as a function of the PVC. Below the critical PVC (CPVC), the major corrosion
products were identified as MgAl2O4, Al2O3, and AlOOH. Above CPVC, the major corrosion product
changed to MgCO3. Below CPVC, the DHS acts primarily at the coating surface where it oxidizes the
Mg first to Mg(OH)2 and then the Al to Al(OH)3 at higher pH. In the drying cycle, these products form
MgAl2O4, Al2O3, and AlOOH. Above the CPVC, the DHS is able to penetrate into the coating where it
neutralizes the area around the Mg particles as it is oxidized and maintains the pH low enough to
prevent the oxidation of Al. The coating porosity also facilitates the penetration of CO2 into the coating
matrix, which converts the Mg(OH)2 to MgCO3.
8.3. Additional Corrosion Inhibitive Components to Improve Mg-rich Performance
Addition of small amounts of cerium oxide (0.5% by weight) to a Mg-rich primer was shown to
significantly improve the protection performance of a Mg-rich primer on AZ91D magnesium alloy [78].
While the ceria particles did not change the protection mechanisms of the Mg-rich primer on AZ91D
magnesium alloy, the authors claimed that the electrochemical activity of the Mg particles increased
the service life of the Mg-rich primer. Apart from providing a barrier effect, ceria particles increased
the corrosion potential and decreased the current density of the AZ91D alloy, which is beneficial for
cathodic protection of the Mg particles. Lu et al. reported improved adhesion and better corrosion
protection when the surface of AZ91D magnesium alloy substrates were coated with γ-glycidoxy
propyl trimethoxy silane, due to the formation of Si–O–Mg covalent bonds between the silane film
and the substrate and Si-O-Si bonds within the silane film, each shifting the water and oxygen
permeability drastically [79].
Metals 2012, 2
371
9. Conclusion
Mg-rich primers represent the first commercially viable, high performance (in corrosion control
terms) and non-toxic alternative to the use of carcinogenic Cr(VI) pretreatments and pigments for
preventing corrosion on metal substrates, especially on aluminum alloys used in the aircraft industry.
Mg-rich primer technology has advanced drastically since its conception with many of the limitations
having been overcome via thorough understanding of the mechanisms by which Mg particles afford
their corrosion protection abilities. Yet, incremental and important improvements are still occurring in
Mg-rich primer technology, both in the academic and industrial laboratories and applications, and it is
possible that it will not be too long before these primers become the standard against which all other
anti-corrosive primer alternatives will be evaluated.
Acknowledgments
The authors gratefully acknowledge the financial support of Mandaree Enterprise Corporation
(FA8501-06-D-0001), Engineer Research and Development Center (ERDC W9132T-09-2-0019) and
The United States Air Force (FA7000-10-2-0014) through funding by the Department of Defense and
collaborative efforts for Corrosion Prevention and Understanding via the Technical Corrosion
Collaboration working group comprising The University of Virginia, The University of Hawaii, The
Ohio State University, the Air Force Academy, The University of Akron, The University of Southern
Mississippi, the Air Force Institute of Technology, the Naval Postgraduate School, the US Naval
Academy, and Nippon Paper Chemicals Co., Ltd.
Conflict of Interest
The authors declare no conflict of interest.
References
1. Simandl, G.J.; Schultes, H.; Simandl, J.; Paradis, S. Magnesium-raw materials, metal extraction
and economics—Global picture. In Digging Deeper, Proceedings of the Ninth Biennial SGA
Meeting; Irish Association for Economic Geology: Dublin, UK, 2007; pp. 827–831.
2. Guo, K.W. A review of magnesium/magnesium alloys corrosion. Recent Pat. Corros. Sci. 2011,
1, 72–90.
3. Wu, C.-Y.; Zhang, J. State-of-art on corrosion and protection of magnesium alloys based on
patent literatures. Trans. Nonferrous Met. Soc. China 2011, 21, 892–902.
4. Gray, J.E.; Luan, B. Protective coatings on magnesium and its alloys—A critical review. J. Alloy
Compd. 2002, 336, 88–113.
5. Zeng, R.-C.; Zhang, J.; Huang, W.-J.; Dietzel, W.; Kainer, K.U.; Blawert, C.; Ke, W. Review of
studies on corrosion of magnesium alloys. Trans. Nonferrous Met. Soc. China 2006, 16, 763–771.
6. Song, G.; Atrens, A. Corrosion mechanisms of magnesium alloys. Adv. Eng. Mater. 1999, 1,
11–33.
7. Ambat, R.; Aung, N.N.; Zhou, W. Evaluation of microstructural effects on corrosion behavior of
AZ91D magnesium alloy. Corros. Sci. 2000, 42, 1433–1455.
Metals 2012, 2
372
8. Shaw, B.A.; Wolfe, R.C. Corrosion of Magnesium and Magnesium-Base Alloys. In ASM
Handbook, Corrosion: Materials; Cramer, S.D., Covino, B.S., Jr., Eds.; ASM International:
Russell Township, OH, USA, 2005; Volume 13B, pp. 205–227.
9. Gurrappa, I. Cathodic protection of cooling water systems and selection of appropriate materials.
J. Mater. Process. Technol. 2005, 166, 256–267.
10. Popov, B.N.; Kumaraguru, S.P. Cathodic Protection of Pipelines. In Handbook of Environmental
Degradation of Materials; Myer, K., Ed.; William Andrew Publishing: Norwich, NY, USA, 2005;
Chapter 24, pp. 503–521.
11. Lindström, R. Atmospheric Corrosion of Magnesium alloys Influence of Microstructure and
Environment. Ph.D. Thesis, Göteborg University, Göteborg, Sweden, 2007.
12. Loose, W.S. Corrosion and Protection of Magnesium. In Metals Handbook; Pidgeon, L.M.,
Mathes, J.C., Woldmen. N.E., Eds.; ASM International: Russell Township, OH, USA, 1946;
pp. 173–260.
13. Rozenfeld, I.L. Atmospheric Corrosion of Metals; National Association of Corrosion Engineers:
Houston, TX, USA, 1972.
14. Tomashov, N.D. Theory and Protection of Metals: The Science of Corrosion; The Macmillan
Company: London, UK, 1966; pp. 367–398.
15. Jönsson, M.; Persson, D.; Leygraf, C. Atmospheric corrosion of field-exposed magnesium alloy
AZ91D. Corros. Sci. 2008, 50, 1406–1413.
16. Prigiobbe, V.; Hänchen, M.; Werner, M.; Baciocchi, R.; Mazzotti, M. Mineral carbonation
process for CO2 sequestration. Energy Proced. 2009, 1, 4885–4890.
17. Bruant, R.G., Jr.; Giammar, D.E.; Myneni, S.C.B.; Peters, C.A. Effect of pressure, temperature,
and aqueous carbon dioxide concentration on mineral weathering as applied to geologic storage of
carbon dioxide. In Proceedings of the 6th International Conference on Greenhouse Gas Control
Technologies, Kyoto, Japan, 1–4 October 2002; pp. 1609–1612.
18. Feliu, S., Jr.; Pardo, A.; Merino, M.C.; Coy, A.E.; Viejo, F.; Arrabal, R. Correlation between the
surface chemistry and the atmospheric corrosion of AZ31, AZ80 and AZ91D magnesium alloys.
Appl. Surf. Sci. 2009, 255, 4102–4108.
19. Lindström, R.; Johansson, L.G.; Svensson, J.E. The influence of NaCl and CO2 on the
atmospheric corrosion of magnesium alloy AZ91. Mater. Corros. 2003, 54, 587–594.
20. Lindström, R.; Johansson, L.G.; Thompson, G.E.; Skeldon, P.; Svensson, J.E. Corrosion of
magnesium in humid air. Corros. Sci. 2004, 46, 1141–1158.
21. Lin, C.; Li, X. Role of CO2 in the initial stage of atmospheric corrosion of AZ91 magnesium alloy
in the presence of NaCl. Rare Met. 2006, 25, 190–196.
22. Hao, Z.; Du, F. Synthesis of basic magnesium carbonate microrods with a “house of cards”
surface structure using rod-like particle template. J. Phys. Chem. Solids 2009, 70, 401–404.
23. Blücher, D.B.; Svensson, J.-E.; Johansson, L.-G.; Rohwerder, M.; Stratmann, M. Scanning Kelvin
probe force microscopy: A useful tool for studying atmospheric corrosion of MgAl alloys in situ.
J. Electrochem. Soc. 2004, 151, B621–B626.
24. Lindström, R.; Svensson, J.-E.; Johansson, L.-G. The influence of carbon dioxide on the
atmospheric corrosion of some magnesium alloys in the presence of NaCl. J. Electrochem. Soc.
2002, 149, B103–B107.
Metals 2012, 2
373
25. Nanna, M.E.; Bierwagen, G.P. Mg-rich coatings: A new paradigm for Cr-free corrosion protection
of Al aerospace alloys. J. Coat. Technol. Res. 2004, 1, 69–80.
26. Glancy, C.W. Oil Absorption of Pigments. In Paint and Coating Testing Manual: 15th Edition of
the Gardner-Sward Handbook; Joseph, K., Ed.; ASTM International: West Conshohocken, PA,
USA, 2012; Chapter 29, pp. 304–305.
27. Bierwagen, G.P.; Brown, R.; Battocchi, D.; Hayes, S. Active metal-based corrosion protective
coating systems for aircraft requiring no-chromate pretreatment. Prog. Org. Coat. 2010, 68, 48–61.
28. Osborne, J.H.; Blohowiak, K.Y.; Taylor, S.R.; Hunter, C.; Bierwagen, G.P.; Carslon, B.;
Bernard, D.; Donley, M.S. Testing and evaluation of non-chromated coating systems for
aerospace applications. Prog. Org. Coat. 2001, 41, 217–225.
29. Covino, J.J.; Sugden, K.D. Genotoxicity of chromate. Adv. Mol. Toxic. 2008, 2, 1–24.
30. Morris, E.; Ray, C.; Albers, R.; McLaughlin, J.; Bean, S.; DeAntoni, A.; Patel, R. Using chrome-free
primer technology to develop a chrome-free pretreatment. In Proceedings of the 2007
Tri-Service Corrosion Conference, Denver, CO, USA, 3–6 December 2007; Available online:
https://www.corrdefense.org/technical%20papers/using%20chrome-free%20primer%20technol
ogy%20to%20develop%20a%20chrome-free%20pretreatment.pdf (accessed on 30 May 2012).
31. Joint DoD Demonstration and Validation of Magnesium Rich Primer Coating Technology.
Available online: http://www.serdp.org/Program-Areas/Weapons-Systems-and-Platforms/Surface-
Engineering-and-Structural-Materials/Coatings/WP-200731 (accessed on 30 April 2012).
32. Twite, R.L.; Bierwagen, G.P. Review of alternatives to chromate for corrosion protection of
aluminum aerospace alloys. Prog. Org. Coat. 1998, 33, 91–100.
33. Pathak, S.S.; Khanna, A.S. Synthesis and performance evaluation of environmentally compliant
epoxysilane coatings for aluminum alloy. Prog. Org. Coat. 2008, 62, 409–416.
34. Pathak, S.S.; Khanna, A.S. Investigation of anti-corrosion behavior of waterborne
organosilane–polyester coatings for AA6011 aluminum alloy. Prog. Org. Coat. 2009, 65,
288–294.
35. Pathak, S.S.; Sharma, A.; Khanna, A.S. Value addition to waterborne polyurethane resin by
silicone modification for developing high performance coating on aluminum alloy. Prog. Org.
Coat. 2009, 65, 206–216.
36. Hamdy, A.S. Enhancing corrosion resistance of aluminum composites in 3.5% NaCl using
pigmented epoxy fluoropolymer. Prog. Org. Coat. 2006, 55, 218–224.
37. Yasuda, H.K.; Reddy, C.M.; Yu, Q.S.; Deffeyes, J.; Bierwagen, G.P.; He, L. Effect of scribing on
corrosion test results. Corrosion 2001, 57, 29–34.
38. He, J.; Gelling, V.J.; Tallman, D.E.; Bierwagen, G.P.; Wallace, G.G. Conducting polymers and
corrosion. III. A scanning vibrating electrode study of poly(3-octyl pyrrole) on steel and
aluminum. J. Electrochem. Soc. 2000, 147, 3667–3672.
39. Tallman, D.E.; Pae, Y.; Bierwagen, G.P. Conducting polymers and corrosion. 2. Polyaniline on
aluminum alloys. Corrosion 2000, 56, 401–410.
40. Simões, A.; Battocchi, D.; Tallman, D.; Bierwagen, G.P. Assessment of the corrosion protection
of aluminium substrates by a Mg-rich primer: EIS, SVET and SECM study. Prog. Org. Coat.
2008, 63, 260–266.
Metals 2012, 2
374
41. Bierwagen, G.P.; Nanna, M.E.; Battocchi, D. Magnesium Rich Coatings and Coating Systems.
U.S. Patent 20,070,128,351, 7 October 2004.
42. Price, C.J.; Johnson, J. Performance Evaluation of a Magnesium-Rich Primer for Chrome-Free
Aerospace Coating Systems. Available online: http://symposiumarchive.serdp-estcp.org/
symposium2008/posters/upload/w189-joseph.pdf (accessed on 30 May 2012).
43. Johnson, J.A. Magnesium rich primer for chrome free protection of aluminum alloys.
In Proceedings of the Tri-Service Corrosion Conference 2007, Denver, CO, USA,
3–7 December 2007.
44. Ahmad, Z. Cathodic Protection. In Principles of Corrosion Engineering and Corrosion Control;
Butterworth-Heinemann: Oxford, UK, 2006; pp. 271–351.
45. Bierwagen, G.; Battocchi, D.; Simoes, A.; Stamness, A.; Tallman, D. The use of multiple
electrochemical techniques to characterize Mg-rich primers for Al alloys. Prog. Org. Coat. 2007,
59, 172–178.
46. Battocchi, D.; Simões, A.M.; Tallman, D.E.; Bierwagen, G.P. Electrochemical behaviour of a
Mg-rich primer in the protection of Al alloys. Corros. Sci. 2006, 48, 1292–1306.
47. Battocchi, D.; Simões, A.M.; Tallman, D.E.; Bierwagen, G.P. Comparison of testing solutions on
the protection of Al-alloys using a Mg-rich primer. Corros. Sci. 2006, 48, 2226–2240.
48. Allahar, K.N.; Battocchi, D.; Orazem, M.E.; Bierwagen, G.P.; Tallman, D.E. Modeling of
electrochemical impedance data of a magnesium-rich primer. J. Electrochem. Soc. 2008, 155,
E143–E149.
49. Allahar, K.N.; Wang, D.; Battocchi, D.; Bierwagen, G.P.; Balbyshev, S. Real-time monitoring of
a United States air force topcoat/Mg-rich primer system in ASTM B117 exposure by embedded
electrodes. Corrosion 2010, 66, 075003:1–075003:11.
50. Simões, A.M.; Battocchi, D.; Tallman, D.E.; Bierwagen, G.P. SVET and SECM imaging of
cathodic protection of aluminium by a Mg-rich coating. Corros. Sci. 2007, 49, 3838–3849.
51. Li, J.; He, J.; Chisholm, B.J.; Stafslien, M.; Battocchi, D.; Bierwagen, G.P. An investigation of the
effects of polymer binder compositional variables on the corrosion control of aluminum alloys
using magnesium-rich primers. J. Coat. Technol. Res. 2010, 7, 757–764.
52. King, A.D.; Scully, J.R. Sacrificial anode-based galvanic and barrier corrosion protection of
2024-T351 by a Mg-rich primer and development of test methods for remaining life assessment.
Corrosion 2011, 67, 055004:1–055004:22.
53. Ravindran, N.; Chattopadhyay, D. K.; Zakula, A.; Battocchi, D.; Webster, D.C.; Bierwagen, G.P.
Thermal stability of magnesium-rich primers based on glycidyl carbamate resins. Polym Degrad.
Stab. 2010, 95, 1160–1166.
54. Hayes, S.; Brown, R.; Visser, P.; Adams, P.; Chapman, M. Magnesium rich primers and related
developments for the replacement of chromium containing aerospace primers. In Proceedings of
the 2011 Corrosion Conference, Houston, TX, USA, 13–17 March 2011; Available online:
https://www.corrdefense.org/Spotlight/2011%20Corrosion%20Conference%20Presentations/Mag
nesium%20rich%20primers%20and%20related%20developments%20for%20the%20replacement
%20of%20chromium%20contraining%20aerospace%20primers.pdf (accessed on 30 April 2012).
55. Lu, X.; Zuo, Y.; Zhao, X.; Tang, Y.; Feng, X. The study of a Mg-rich epoxy primer for protection
of AZ91D magnesium alloy. Corros. Sci. 2011, 53, 153–160.
Metals 2012, 2
375
56. Bierwagen, G. The physical chemistry of organic coatings revisited—Viewing coatings as a
materials scientist. J. Coat. Technol. Res. 2008, 5, 133–155.
57. Pathak, S.S.; Blanton, M.D.; Mendon, S.K.; Rawlins, J.W. Investigation on dual corrosion
performance of magnesium-rich primer for aluminum alloys under salt spray test (ASTM B117)
and natural exposure. Corros. Sci. 2010, 52, 1453–1463.
58. Strekalov, P.V. The atmospheric corrosion of metals by adsorbed polymolecular moisture layers.
Prot. Met. 1998, 34, 501–519.
59. Smith, R.M.; Martell, A.E. Critical Stability Constants; Plenum Press: New York, NY,
USA, 1973.
60. Jönsson, M.; Persson, D.; Thierry, D. Corrosion product formation during NaCl induced
atmospheric corrosion of magnesium alloy AZ91D. Corros. Sci. 2007, 49, 1540–1558.
61. White, W.B. Thermodynamic equilibrium kinetics, activation barriers, and reaction mechanisms
for chemical reactions in Karst Terrains. Environ. Geol. 1997, 30, 46–58.
62. Hosking, N.C.; Ström, M.A.; Shipway, P.H.; Rudd, C.D. Corrosion resistance of zinc–magnesium
coated steel. Corros. Sci. 2007, 49, 3669–3695.
63. Duan, Z.; Sun, R. An improved model calculating CO2 solubility in pure water and aqueous NaCl
solutions from 273 to 533 K and from 0 to 2000 bar. Chem. Geol. 2002, 193, 257–271.
64. Yao, H.B.; Li, Y.; Wee, A.T.S. An XPS investigation of the oxidation/corrosion of melt-spun Mg.
Appl. Surf. Sci. 2000, 158, 112–119.
65. Portier, S.; Rochelle, C. Modelling CO2 solubility in pure water and NaCl-type waters from
0 to 300 °C and from 1 to 300 bar: Application to the Utsira Formation at Sleipner. Chem. Geol.
2005, 217, 187–199.
66. Henrist, C.; Mathieu, J.-P.; Vogels, C.; Rulmont, A.; Cloots, R. Morphological study of
magnesium hydroxide nanoparticles precipitated in dilute aqueous solution. J. Cryst. Growth
2003, 249, 321–330.
67. Gao, Y.; Wang, H.; Su, Y.; Shen, Q.; Wang, D. Influence of magnesium source on the
crystallization behaviors of magnesium hydroxide. J. Cryst. Growth 2008, 310, 3771–3778.
68. Wang, Y.; Li, Z.; Demopoulos, G.P. Controlled precipitation of nesquehonite (MgCO3·3H2O) by
the reaction of MgCl2 with (NH4)2CO3. J. Cryst. Growth 2008, 310, 1220–1227.
69. Mitsuhashi, K.; Tagami, N.; Tanabe, K.; Ohkubo, T.; Sakai, H.; Koishi, M.; Abe, M. Synthesis of
microtubes with a surface of “house of cards” structure via needlelike particles and control of
their pore size. Langmuir 2005, 21, 3659–3663.
70. Pathak, S.S.; Blanton, M.D.; Mendon, S.K. School of Polymes and High Performance Materials,
The University of Southern Mississippi Hattiesburg, MS, USA. Unpublished work, 2011.
71. Pathak, S.S.; Blanton, M.D.; Mendon, S.K.; Rawlins, J.W. Carbonation of Mg powder to enhance
the corrosion resistance of Mg-rich primers. Corros. Sci. 2010, 52, 3782–3792.
72. Moore, D.M.; Reynolds, R.C. X-ray Diffraction and the Identification and Analysis of Clay
Minerals; Oxford University Press: New York, NY, USA, 1989.
73. Khramov, A.N.; Balbyshev, V.N.; Kasten, L.S.; Mantz, R.A. Sol–gel coatings with phosphonate
functionalities for surface modification of magnesium alloys. Thin Solid Films 2006, 514, 174–181.
Metals 2012, 2
376
74. Turel, T.; Pathak, S.S.; Blanton, M.D.; Mendon, S.K.; Rawlins, J.W. Optimizing the
Transformation of Magnesium Powder to Enhance its Corrosion Protection. In Proceedings of the
38th Annual International Waterborne, High-Solids, and Powder Coatings Symposium, New
Orleans, LA, USA, 28 February–4 March 2011; pp. 430–437.
75. Maier, B.; Frankel, G.S. Behavior of magnesium-rich primers on AA2024-T3. Corrosion 2011,
67, 055001:1–055001:15.
76. Xu, H.; Battocchi, D.; Tallman, D.E.; Bierwagen, G.P. Use of magnesium alloys as pigments in
magnesium-rich primers for protecting aluminum alloys. Corrosion 2009, 65, 318–325.
77. Bierwagen, G.; Brown, R.; Battocchi, D.; Hayes, S. Active metal-based corrosion protective
coating systems for aircraft requiring no-chromate pretreatment. Prog. Org. Coat. 2010, 67,
195–208.
78. Wang, Y.-P.; Zhao, X.-H.; Lu, X.-Y.; Zuo, Y. Corrosion protection of ceria particle in Mg-rich
primer on AZ91D magnesium alloy. Acta Phys. Chim. Sin. 2012, 28, 407–413.
79. Lu, X.; Zuo, Y.; Zhao, X.; Tang, Y. The improved performance of a Mg-rich epoxy coating on
AZ91D magnesium alloy by silane pretreatment. Corros. Sci. 2012, 60, 165–172.
© 2012 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article
distributed under the terms and conditions of the Creative Commons Attribution license
(http://creativecommons.org/licenses/by/3.0/).
... Broadly categorized, the electrochemical protection techniques for steel rebars encompass two primary methods: the impressed current approach and the sacrificial anode technique. The sacrificial anode protection method depicted in Figure 5 involves the utilization of active metals, such as Magnesium alloy, as sacrificial anodes [109]. These anodes sacrificially undergo corrosion, effectively substituting for and mitigating the corrosion of cathodic metals, thereby precluding electrochemical degradation of the matrix. ...
... Electrochemical protection: Sacrificial anode method; Impressed current method. Reprinted from Ref.[109]. ...
Article
Full-text available
The corrosion of steel rebars is a prevalent factor leading to the diminished durability of reinforced concrete structures, posing a significant challenge to the safety of structural engineering. To tackle this issue, extensive research has been conducted, yielding a variety of theoretical insights and remedial measures. This review paper offers an exhaustive analysis of the passivation processes and corrosion mechanisms affecting steel rebars in reinforced concrete. It identifies key factors such as chloride ion penetration and concrete carbonization that primarily influence rebar corrosion. Furthermore, this paper discusses a suite of strategies designed to enhance the longevity of reinforced concrete structures. These include improving the concrete protective layer’s quality and bolstering the rebars’ corrosion resistance. As corrosion testing is essential for evaluating steel rebars’ resistance, this paper also details natural and accelerated corrosion testing methods applicable to rebars in concrete environments. Additionally, this paper deeply presents an exploration of the use of X-ray computed tomography (X-CT) technology for analyzing the corrosion byproducts and the interface characteristics of steel bars. Recognizing the close relationship between steel bar corrosion research and microstructural properties, this paper highlights the pivotal role of X-CT in advancing this field of study. In conclusion, this paper synthesizes the current state of knowledge and provides a prospective outlook on future research directions on the corrosion of steel rebars within reinforced concrete structures.
... The material to be protected is connected to an external direct source of electricity and insoluble materials, such as graphite, stainless steel and scrap iron, as illustrated in Figure 2.8 [37]. [38] b) Sacrificial anodes ...
... Illustration of sacrificial cathodic protection[38] 2.5.2 Conditioning of the corrosive environment ...
Thesis
Full-text available
For several decades, mild steel has been an ideal mandated material in many industrial applications. However, unlike stainless steel and other special steel alloys, mild steel is prone to corrosion attack, leading to devastating failures of equipment and metallic structures over time, causing losses worth billions. This situation has spurred researchers to find solutions for the corrosion phenomenon. The present work investigates the inhibition efficiency as a corrosion inhibitor of an ecofriendly and cost-effective composite developed from organic polymers. Synthesis of an environmentally friendly copolymer composite of Polyethylene glycol L-proline (PEGLP), from polyethylene glycol and L-proline, was carried out. A variety of analytical equipment was used to evaluate the characteristics of PEGLP. Results of Fouriertransform infrared spectroscopy, nuclear magnetic resonance spectroscopy, X-ray diffraction, scanning electron spectroscopy-energy dispersive spectroscopy, thermogravimetric and organic elemental analyser analysis show that a composite was produced. Fourier-transform infrared spectroscopy and nuclear magnetic resonance spectroscopy confirmed the presence of –COOH, -C-H, N-C and –OH in the PEGLP molecules, making the PEGLP ideal for corrosion inhibition. Spectroscopy-energy dispersive spectroscopy and organic elementary analysis showed that the mass percentages of nitrogen, carbon and oxygen were higher on the PEGLP than in the starting materials; which confirms that the PEGLP has bulk molecular weight that will improve inhibition efficiency. Thermogravimetric analysis revealed that PEGLP was thermally stable up to 200oC and could be used for a wide range of applications below 200oC without it degrading. Subsequently, the newly synthesised PEGLP was investigated as a corrosion inhibitor for mild steel in 1 M HCl, and its corrosion inhibition properties were studied by means of gravimetric and electrochemical techniques, from which corrosion rate, inhibition efficiency, thermodynamic parameters and adsorption were determined. The gravimetric technique focused on the effects on the mild steel of concentrations of the inhibitor between 200 ppm and 800 ppm, immersion time from 1 h to 9 h, and temperatures from 298 K to 338 K. It was found that an increase in inhibitor concentration led to a decrease in corrosion rate. The optimal concentration, with the V highest inhibition efficiency of 94.48% at 298 K, was observed while working with 800 ppm after an exposure time of 6 h; after 6 h, the inhibition effect decreased gradually. The effects of temperature were used to assess the adsorption on mild steel and dissolution of the mild steel phenomenon. Results reveal that an increase in temperature led to an increase in corrosion rate, due to the PEGLP decomposing at high temperature. Moreover, it was found that PEGLP adsorption adheres to the Frumkin isotherm model at best fit, with regression coefficient values R2 relatively close to unity and ranging from 0982 to 09953. Thermodynamic parameters indicate that PEGLP adsorption on mild steel had taken place mainly by exothermic physisorption reaction. Scanning electron microscope and atomic force microscopy surface analysis showed a decrease in surface roughness in the presence of the inhibitor. This result was due to the formation of an adsorptive layer that was justified by the presence of nitrogen, carbon and oxygen on the mild steel surface, found by energy dispersive X-ray analysis. Electrochemical measurements were focused on open circuit potential (OCP), potentiodynamic polarisation (PDP) and electrochemical impedance spectroscopy (EIS). The results show that mild steel is less prone to corrosion attack in 1 M HCl in the presence of PEGLP, based on open circuit potential evaluation. The susceptibility and the underlying mechanistic information on corrosion was investigated further by potentiodynamic polarisation (PDP) and EIS. Potentiodynamic polarisation measurement through tafel plots reveals that PEGLP was a mixed-type inhibitor; as its concentration was increased, a decrease in corrosion current density Icorr, from 2556.361 to 162.363 μA, and an increase in resistance polarisation Rp, from 11.86 to 148.503 , were observed. The maximum inhibition efficiency was found to be 93.65% for 800 ppm of PEGLP. EIS data represented in terms of Bode and Nyquist plots also showed an increase in charge transfer resistance, from 9.612 to 464.9 , based on EIS cell simulation to an equivalent electric circuit. In every case, electrochemical studies reaffirm the evaluation based on the effect of inhibitor concentration on the corrosion rate and inhibition due to the formation of an adsorptive layer on mild steel in 1 M HCl, as mentioned above, during gravimetric studies.
... In the contrary sense, adding more electrons from a different source to the metal will decrease reaction 1 which eventually results to decrease in corrosion. Reaction 2 decrease as a result (Shashi, 2012). www.iarjournals.com ...
Article
Full-text available
This publication centres on building a software that has the potency of being used for sizing anodes (sacrificial anodes) to get the most reliable choice of the number of anodes used for Galvanic Cathodic protection of subsea structures.
... The two biggest CPC cost drivers are screening/qualifying of materials used in the design and building of new platforms, and then maintaining those platforms throughout their service lifetimes. For example, the U.S. Department of Defense (DoD) spends around $20 billion per year on corrosion-related maintenance [40,46], while the U.S. Air Force estimates that annual expenditures for stripping and repainting aircraft exceed $1 billion [47]. ...
Preprint
Full-text available
Recent estimates indicate that the U.S. Department of Defense spends over \20billionUSDannuallyoncorrosionrelatedmaintenance.Thisexpenditureisaccompaniedbyasubstantiallossinassetreadiness,rangingfrom1020 billion USD annually on corrosion-related maintenance. This expenditure is accompanied by a substantial loss in asset readiness, ranging from 10% to 30%. Moreover, the global costs associated with corrosion damage have been estimated at an astonishing \2.5 trillion USD per year, or approximately 3.4% of global GDP in 2016. This project aims to describe how quantum computers might be leveraged to fundamentally change the way material-environment interactions are modeled for material discovery, selection, and design. This project also seeks to understand the plausibility and utility of replacing portions of classical computing workflows with algorithms optimized for quantum computing hardware. The utility of quantum computers is explored through the lens of two industrially relevant problems: (1) characterizing magnesium alloy corrosion properties in aqueous environments and (2) identifying stable niobium-rich alloys with corrosion resistance at temperatures above 1500K. This paper presents an end-to-end analysis of the complexity of both classical and quantum algorithms used in application workflows. Resource estimates are produced using a custom software package, pyLIQTR, based on the qubitized Quantum Phase Estimation (QPE) algorithm. Estimates for the two aforementioned applications show that industrially-relevant computational models that have the potential to deliver commercial utility require quantum computers with thousands to hundreds of thousands of logical qubits and the ability to execute 101310^{13} to 101910^{19} T-gates. These estimates represent an upper bound and motivate continued research into improved quantum algorithms and resource reduction techniques.
... The coatings protect the substrate from the corrosive environment, which acts as a barrier (Otsuki et al. 2015). Organic polymers like epoxies (Chen et al. 2023a), Mg-rich primers (Pathak et al. 2012), carbon-based epoxy (Atta et al. 2017), silver-embedded epoxy (El-Faham et al. 2018), graphitic carbon nitride-ZnO nanocomposite embedded pure epoxy (Kumar et al. 2020), polyurethanes, polymeric blend with TiO 2 particles (Benea et al. 2018), polyaniline blends (Sathiyanarayanan et al. 2009), acrylics, alkyd, bio-based polybenzoxazines (Chen et al. 2023b), and fluoropolymers are typically utilized as organic coating materials (Kittel et al. 2001). Also, Pourhashem et al. (2020) exclusively reviewed the application of carbon nanostructures as nanofillers for corrosion-resistant organic coatings. ...
Article
Marine structures are constantly exposed to the corrosive effects of seawater, making effective corrosion protection crucial for their longevity and performance. Sacrificial anodes, commonly made of zinc, aluminum, or magnesium alloys, are widely employed to mitigate corrosion by sacrificing themselves to protect the steel structures. However, the selection and implementation of sacrificial anode materials present various challenges that need to be addressed. This paper explores the challenges associated with sacrificial anode materials for steel structures and provides potential solutions. To overcome these challenges, the paper proposes solutions such as using advanced alloy compositions, protective coatings, hybrid anode systems, and improved design considerations. Furthermore, the importance of monitoring techniques to assess the performance and remaining lifespan of sacrificial anodes is emphasized. Several case studies and experimental findings are discussed to illustrate the effectiveness and limitations of sacrificial anode materials based on zinc alloys, aluminum alloys, and magnesium alloys. The paper highlights the need for ongoing research and development efforts to address the evolving demands of corrosion protection in marine environments.
Article
Full-text available
This review critically examines advancements in anode materials for cathodic protection systems, focusing on overcoming the limitations of traditional materials like magnesium, zinc, aluminum, graphite, lead-silver alloys, and high-silicon cast iron (HSCI). Conventional anode materials, though widely used, face issues such as rapid degradation, high maintenance costs, and environmental harm. Novel materials, including mixed metal oxides (MMO), advanced aluminum-based alloys, nanostructured materials, and conductive polymers, offer superior electrochemical properties, enhanced durability, and improved performance in aggressive environments like seawater. This review also highlights the role of surface modifications and coatings, such as platinum on titanium and ceramic coatings, in boosting corrosion resistance. Moreover, smart monitoring systems, integrated with IoT and SCADA technologies, are explored for their potential to improve the longevity and efficiency of cathodic protection systems. The paper emphasizes the urgent need for sustainable solutions due to the substantial economic and environmental costs of corrosion, particularly in high-risk industries like oil and gas, maritime, and infrastructure. Future research directions, including the development of hybrid systems combining coatings with CP technologies and the application of advanced alloys and nanostructured materials, are proposed to address the long-term performance and ecological impacts of CP systems.
Article
Full-text available
The scanning vibrating electrode technique (SVET) was utilized to monitor localized corrosion and substrate protection of three metal-rich primers (MRP). The ability to suppress localized corrosion and provide widespread cathodic polarization to enable sacrificial anode-based cathodic protection of a AA 7075-T651 substrate with either an aluminum-rich primer (AlRP), magnesium-rich primer (MgRP), or a composite magnesium + aluminum-rich primer (MgAlRP) in a polyamide-based epoxy primer coatings fully immersed in 1 mM NaCl was investigated. Pigments did not activate uniformly in each MRP. The notion of throwing power polarizing the bare substrate and uniform current and potential distributions at scratch sites does not describe the behavior observed. In cases where activation occurred, protection was noticed in the form of suppression of local anodes on bare AA 7075-T651. Local corrosion was suppressed on heterogeneously corroding AA 7075-T651 with strong local anodes and cathodes. Widespread cathodic polarization was absent. The MgRP and MgAlRP were shown to provide superior local corrosion suppression associated with pitting on AA 7075-T651 compared to the AlRP.
Article
Full-text available
The energy sector is the main source of greenhouse gases, so it has the highest potential for improvement. The improvements can be achieved by generating energy from renewable sources. It is necessary to combine production from renewable sources with storage systems. Thermal energy storage using concentrated solar power systems is a promising technology for dispatchable renewable energy that can guarantee a stable energy supply even in remote areas without contributing to greenhouse gas emissions during operation. However, it must be emphasised that greenhouse gases and other impacts can occur during the production process of concentrating solar system components. This paper analyses the receiver design to produce thermal energy for the existing CSP dish plant at the Energy Center of the Politecnico di Torino. The plant is designed to produce electrical energy in the spring and summer periods. In addition to this energy production, the CSP can be adopted to produce thermal energy, through hot water, during the less favourable periods of the year in terms of global solar radiation. The surface heat flux is calculated in the first part of the analysis to obtain the maximum internal temperature in the receiver, which is 873.7 °C. This value is a constraint for the choice of material for the solar receiver. A life cycle assessment is performed to compare the emissions generated during the production of the main components of the CSP system with the emissions generated by the methane-fuelled water heater to produce the same amount of thermal energy. It can be concluded that the production of the main components of the CSP system results in lower greenhouse gas emissions than the operational phase of a conventional system. Given the assumptions made, the utilization of methane leads to the emission of approximately 12,240 kg of CO2, whereas the production of the CSP system results in emissions totalling 5332.8 kg of CO2 equivalent
Chapter
Description The most complete manual of its type ever published. Eleven parts cover optical, physical, mechanical, and chemical properties; weather, film and whole paint testing, raw materials, specific products, instrumentation and specifications.
Chapter
CO2 mediated dissolution of silicate minerals and subsequent precipitation of carbonates in deep saline aquifers may allow permanent trapping of carbon dioxide. However, the time-scales and extents of the reactions are poorly understood for CO2 receptor formation conditions. To address these shortcomings, experiments were conducted to investigate the effects of pressure, temperature, and aqueous solution composition on rates and mechanisms of silicate mineral dissolution and carbonate precipitation. A high pressure/high temperature flow-through reactor system was used to derive steady-state dissolution rates of crushed forsteritic olivine. The system allowed continuous monitoring of temperature, pressure, and pH, and periodic sampling of effluent fluids for dissolved ion concentration analysis. Preliminary measurements of dissolution rates indicate good agreement with previously published measurements at ambient conditions. Increasing the pressure from 1 to 100 bar under constant CO2 conditions increased the dissolution rate by 80%. The same reactions were studied in batch systems using an array of analytical techniques to investigate dissolution mechanisms and secondary precipitate formation. The extent of olivine dissolution in the batch reactors increased with temperature, PCO2 and surface area. Precipitation of magnesium-rich carbonates on reacted olivine was observed at initial magnesite saturation indices greater than 1.6.
Article
A worldwide empirical data file accumulated for the last 30 years and concerning the corrosion of metals by the adsorbed moisture films is systematized. The following problems are considered: the formation of adsorption layers and the dependence of their thickness on the humidity of atmosphere; the effect of the metal surface morphology on the physical adsorption of water; the chemisorption of water on metals; the change in the composition of the adsorbed films with time; the adsorption of gaseous pollutants; the mechanism of the initial corrosion stages in a pure atmosphere (on zinc); and the corrosion of iron, zinc, copper, silver, tin, nickel, and cobalt in the atmosphere containing small amounts (10-4 to 10-7%) of individual or mixed corrosive stimulants H2S, SO2, O3, NO2, and Cl2.
Article
Corrosion is a huge issue for materials, mechanical, civil and petrochemical engineers. With comprehensive coverage of the principles of corrosion engineering, this book is a one-stop text and reference for students and practicing corrosion engineers. Highly illustrated, with worked examples and definitions, it covers basic corrosion principles, and more advanced information for postgraduate students and professionals. Basic principles of electrochemistry and chemical thermodynamics are incorporated to make the book accessible for students and engineers who do not have prior knowledge of this area. Each form of corrosion covered in the book has a definition, description, mechanism, examples and preventative methods. Case histories of failure are cited for each form. End of chapter questions are accompanied by an online solutions manual. * Comprehensively covers the principles of corrosion engineering, methods of corrosion protection and corrosion processes and control in selected engineering environments * Structured for corrosion science and engineering classes at senior undergraduate and graduate level, and is an ideal reference that readers will want to use in their professional work * Worked examples, extensive end of chapter exercises and accompanying online solutions and written by an expert from a key pretochemical university.