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Crevice Corrosion of Stainless Steel
Abstract
Crevice corrosion is a serious problem for stainless steel exposed in corrosive environment. This article describes the corrosion mechanisms, its similarities with other types of localized corrosion, various test methods, factors influencing the corrosion process and different protection methods. Particular emphasis has been placed on the crevice corrosion of austenitic stainless steels, since these are used as major structural materials for nuclear power plants.
... A lot of work has been done on the problem of impact energy degradation of Super304H during aging. Pervious studies have found that Super304H heat-resistant steel of 18-8 type exhibits a common phenomenon of intergranular corrosion during hightemperature aging [29,30]. The shape of stainless steel hardly changes when intergranular corrosion occurs in the metal, but the mechanical properties, especially ductility and toughness, are significantly reduced. ...
In this paper, the evolution of microstructure and mechanical properties of Super304H during aging at 650 °C is investigated, and the effect between microstructure evolution and properties change are discussed. The results shows that the precipitated phases in the material during aging mainly include Cu-rich and MX particles with nano-size dispersed in matrix, M23C6 and σ particles with micron-size located at the grain boundary. These precipitates change significantly in quantity and size with the aging time, which affect the mechanical properties of the material. It helps to improve the tensile performance and hardness of the material, but it has an adverse effect on the impact energy. In addition, it is found that the degradation of impact energy is divided into two stages. Stage I witnesses the rapid drop in impact energy caused by Cr-depletion, which occurs between 0 to 1000 h during aging at 650°C. The content of Cr near the grain boundary rapidly drops from 21% to about 16% due to the precipitation of M23C6 particles on the grain boundary. Stage II is the stage of impact energy drop relatively smoothly, which occurs after aging for 1000 h. In this stage, although the Cr content increases from 16% to about 20% and remains stable due to intracrystalline diffusion, the impact energy decreased slowly, besides the M23C6 particles located at grain boundaries, the micron-sized brittle σ precipitated is another important reason.
... Obviously, ionic and electrical connectivity between anode and cathode must be maintained as well. [1][2][3] Historically, crevice corrosion modeling has assumed a constant potential at the crevice mouth [4][5][6] . In some cases, the external cathode and the crevice have been modeled simultaneously in one code [7][8][9] . ...
For corrosion resistant materials exposed to low-temperature atmospheric environments, the corrosion mode of highest risk is expected to be localized corrosion (pitting, crevice, stress-corrosion cracking) due to accumulation of aggressive species within thin solution layers and/or formation of occluded local geometries. The stability of such a localized corrosion site requires that the corroding site (anode) must dissolve at a sufficient high rate to maintain the critical chemistry, and a robust cathodic area (cathode) must exist that can provide sufficient cathodic current. The characteristics of both the anode and the cathode depend on a large number of physiochemical variables (e.g., temperature, ionic concentration, water layer thickness, etc) and electrochemical parameters (i.e., cathodic and anodic polarization behavior). The effects of all these parameters add significantly to the dimensionality of the problem and a systematic study of these parameters is thus more tractable computationally than experimentally. The objective of this study was to computationally characterize the stability of such a local corrosion site and explore the effects of physiochemical and electrochemical parameters on that stability. The overall goal is to contribute to the establishment of a scientific basis for the prediction of the stabilization of localized attack on wetted, corrosion resistant material surface. A localized corrosion site, illustrated in Figure 1, consists of two parts: (a) the external wetted surface (cathode) and (b) the crevice (anode). This study computationally separated the two and modeled them individually, linking them through the imposition of a common fixed potential at the junction point (i.e., the mouth of the crevice). An objected-oriented computational code, CREVICER, developed at UVa, was extended to study separately both the wet surface (cathode) and the crevice (anode). SS316L was chosen as the material of interest.
... Obviously, ionic and electrical connectivity between anode and cathode must be maintained as well. [1][2][3] Historically, crevice corrosion modeling has assumed a constant potential at the crevice mouth [4][5][6] . In some cases, the external cathode and the crevice have been modeled simultaneously in one code [7][8][9] . ...
The stability of localized corrosion sites on SS 316L exposed to atmospheric conditions was studied computationally. The localized corrosion system was decoupled computationally by considering the wetted cathode and the crevice anode separately and linking them via a constant potential boundary condition at the mouth of the crevice. The potential of interest for stability was the repassivation potential. The limitations on the ability of the cathode that are inherent due to the restricted geometry were assessed in terms of the dependence on physical and electrochemical parameters. Physical parameters studied include temperature, electrolyte layer thickness, solution conductivity, and the size of the cathode, as well as the crevice gap for the anode. The current demand of the crevice was determined considering a constant crevice solution composition that simulates the critical crevice solution as described in the literature. An analysis of variance showed that the solution conductivity and the length of the cathode were the most important parameters in determining the total cathodic current capacity of the external surface. A semi-analytical equation was derived for the total current from a restricted geometry held at a constant potential at one end. The equation was able to reproduce all the model computation results both for the wetted external cathode and the crevice and give good explanation on the effects of physicochemical and kinetic parameters.
... Obviously, ionic and electrical connectivity between anode and cathode must be maintained as well. [1][2][3] Historically, crevice corrosion modeling has assumed a constant potential at the crevice mouth [4][5][6] . In some cases, the external cathode and the crevice have been modeled simultaneously in one code [7][8][9] . ...
No abstract prepared.
The main application in sheet metal forming is austenitic stainless steel (ASS), which is utilised in a various applications, including plate heat exchangers. The martensitic transition can occur during plastic deformation. Temperature, stress and strain are all factors that influence martensite development. The main focus of this research is on the impact of the martensite transformation that occurs during cold working on the corrosion resistance of AISI 316. Corrosion investigation revealed that the martensitic transformation that happens during the cold forming process has a significant impact on the corrosion resistance of AISI 316 ASS plate heat exchangers. The capacity of molybdenum to generate insoluble chloride complexes at the base of pits has been attributed to its role in the creation of passive films. However, due to the cyclic working circumstances of plate heat exchangers, the protective film generated as a result of passivation or repassivation was continuously damaged due to continuous martensitic transformation. The passive layer was continually broken by martensite volume expansion, exposing fresh unprotected surfaces to chlorine-treated water (used in heat exchanger plate as a thermal agent). The martensite transition, which occurs as a result of cold plastic deformation, alters the steel structure as well as its physical and chemical properties. Stainless steel’s magnetic characteristics are highly reliant on the components added to the alloy. Because they have differing corrosion potentials, it is simple for them to become the anode and cathode of a corrosion battery.KeywordsCorrosion resistanceMartensitic transformationCold work
Localized corrosion of metallic biomaterials is a concerning problem for implant applications. In the present work, the interaction between localized corrosion of Ti6Al4V implant material and adsorption of bovine serum albumin (BSA) was investigated by electrochemical tests, surface morphological and compositional analysis. The artificial crevice was set up on the Ti6Al4V surface to simulate the crevice corrosion of metallic implants. The results show that the BSA adsorption is regulated by the heterogeneous metallic ion release resulted from the crevice corrosion. The negatively charged BSA prefers to aggregate at the interior of artificial crevice due to electrostatic interaction with the concentrated metallic cations in the crevice. In return, the non-uniform BSA adsorption can decrease the crevice corrosion susceptibility of the Ti6Al4V implant material, even though the presence of BSA accelerates the dissolution of passive films. For the Ti6Al4V alloys with artificial crevice, stable localized corrosion occurs after 96 h of immersion in the PBS free of BSA while no stable localized corrosion can be determined in the presence of 4% BSA. It is deduced that the presence of BSA can reduce the surface potential discrepancy between the crevice interior and the crevice exterior. The disclosed mechanism of albumin-corrosion interaction is vital to the localized corrosion control of metallic implants in clinical use.
Crevice corrosion is a type of local corrosion which occurs when a metal surface is confined in a narrow gap on the order of 10 μm filled with a solution. Because of the inaccessible geometry, experimental methods to analyze the inner space of the crevice have been limited. In this study, a light-addressable potentiometric sensor (LAPS) was employed to estimate the potential distribution inside the crevice owing to the IR drop by the anodic current flowing out of the structure. Before crevice corrosion, the I–V curve of the LAPS showed a potential shift, depending on the distance from the perimeter. The shift reflected the potential distribution due to the IR drop by the anodic current flowing out of the crevice. After crevice corrosion, the corrosion current increased exponentially, and a local pH change was detected where the corrosion was initiated. A simple model of the IR drop was used to calculate the crevice gap, which was 12 μm—a value close to the previously reported values. Thus, the simultaneous measurement of the I–V curves obtained using a LAPS during potentiostatic electrolysis could be applied as a new method for estimating the potential distribution in the crevice.
Intergranular stress corrosion cracking (IGSCC) is a fracture mechanism in sensitised austenitic stainless steels exposed to critical environments where the intergranular cracks extends along the network of connected susceptible grain boundaries. A constitutive model is presented to estimate the maximum intergranular crack growth by taking into consideration the materials mechanical properties and microstructure characters distribution. This constitutive model is constructed based on the assumption that each grain is a two phase material comprising of grain interior and grain boundary zone. The inherent micro-mechanisms active in the grain interior during IGSCC is based on crystal plasticity theory, while the grain boundary zone has been modelled by proposing a phenomenological constitutive model motivated from cohesive zone modelling approach. Overall, response of the representative volume is calculated by volume averaging of individual grain behaviour. Model is assessed by performing rigorous parametric studies, followed by validation and verification of the proposed constitutive model using representative volume element based FE simulations reported in the literature. In the last section, model application is demonstrated using intergranular stress corrosion cracking experiments which shows a good agreement.
In the first part of this review a number of theoretical aspects of the mechanism of crevice corrosion are dealt with, mainly related to the crevice attack of stainless steels in chloride-containing solutions. Next the test methods are discussed, paying attention to the geometrical conditions and the electrochemical test procedure. A number of practical applications are given, both as regards mechanistic testing and also for ranking purposes. Finally a selected review is given of the results obtained during the past few years.
Electrochemical dissolution of in deaerated was found to produce thiosulfate ions. In addition, the presence of both chloride and thiosulfate ions above critical levels of concentration was found to cause rapid depassivation of 304 stainless steel in well‐stirred deaerated solutions. Therefore, the hypothesis was explored that initiation of crevice corrosion in this system is caused by entrapped thiosulfate ions produced by electrochemical dissolution of inclusions. The hypothesis differs from the view that breakdown is caused by acidification owing to Cr dissolution and hydrolysis. An artificial crevice cell, constructed from two optically flat surfaces, was used to measure the initiation time under controlled variation of crevice gap, applied potential, and sulfide inclusion density. It was found that crevice corrosion did not initiate for sufficiently large crevice gaps, negative (cathodic) applied potentials, and low inclusion densities. In Part II of this series of papers, data are compared to predictions based on a mathematical model of the proposed “thiosulfate entrapment” mechanism.
Pitting and crevice corrosion are sustained by accumulation of highly corrosive solutions in restricted regions (crevices and pits). Thus, electrochemical properties change appreciably when localized corrosion is established on an otherwise passive metal surface. Changes in corrosion potential and polarization resistance have been correlated with visual observations during crevice corrosion of Type 304 stainless steel. Thus, the initiation of localized corrosion can be detected remotely by electrochemical methods.
It is proposed that the various localized forms of corrosion (pitting, stress corrosion cracking, intergranular corrosion, crevice corrosion, filiform corrosion, tuberculation, and exfoliation) have the common feature of local acidification by hydrolysis. The acidity is viewed as having a responsible role in the self-perpetuating character of these corrosion forms. Three possible benefits of this rationale are offered.
Cathodic polarization studies of crevices in Type 304 stainless steel and copper in 0.6M NaCl solution show that crevices with a distance to crevice opening ratio of 12,000:1 can be polarized and that the pH shifted in the alkaline direction to the domain on the potential-pH diagram (Pourbaix) where water is unstable and hydrogen discharge would be expected. The potential was shifted to values where experience has shown that cathodic protection has been achieved. These potential and pH shifts occurred with cathodic polarization even when the solution initially was acid (pH 1.9) simulating the condition in an actively corroding stainless steel crevice. One can, therefore, deduce from the data presented that cathodic protection can both prevent and stop crevice corrosion to great distances within deep, narrow crevices.
A series of investigations were carried out to determine the critical potentials for the growth of localized corrosion on Type 316 and Type 316L stainless steel in chloride media using artificial specimens having the macroscopic local anode separated from the cathode. The critical potential value for the pitting in 0.5N NaCl solution at 70 C (158 F) was shown to be approximately −0.25 V to a saturated calomel electrode and that for the crevice corrosion was about −0.36 V SCE. The latter was nearly equal to the critical potential for the crack propagation of stress corrosion cracking (SCC) in boiling 35% MgCl2 solution. These results indicate that the critical potential can vary, depending on the degree of occlusion on the anode. The critical potential seems to be determined mainly by the hydrogen ion concentrations inside the occluded anodes. The critical potential for pit growth was not affected by the environmental factors as the pitting potential.
In occluded cells, such as crevices, the chemical composition of the electrolyte tends to change dramatically with time from the original bulk composition. The consequences of such changes are reviewed using the binary Cu-Ni and binary Fe-Cr systems as examples. In the case of the copper base alloys, conditions leading to certain dealloying reactions are reported. The concept of a crevice protection potential is confirmed for chromium modified 70-30 Cu-Ni. The possible influence of the presence of complex ionic species in determining the extent of acidification within crevices in Fe-Cr alloys in saline solutions is reported using thermodynamic models in combinations with experimental observations.
A crevice assembly and electrochemical method have been developed to study crevice corrosion resistance as a function of metallurgical variables. The existence of a critical crevice potential (E//c//c) analogous to the critical pitting potential is independent of scan rate in the range 5-100 mV/min. Three different types of austenitic stainless steel (304, 310 and 316) were investigated in 0. 5 M NaCl solution and their resistance was in the order 316 greater than 304 greater than 310.
Crevice corrosion greatly restricts the use of stainless steels in chloride media. To aid in overcoming this difficulty a laboratory study was made of the stages of development of naturally occurring crevice corrosion. Also studied were the response of crevice and bulk specimen to cathodic polarization. The development of naturally occurring crevice corrosion was followed by measuring the potential within the crevice. Typically, there were three relatively rapid changes in potential separated by plateaus in the potential-time curve. These changes are postulated to correspond to (1) oxygen depletion in the crevice, (2) passivity breakdown in the crevice, and (3) acidification in the crevice. It was shown that crevice corrosion can be stopped by extended polarization somewhat more noble than the protection potential, and that on specimens with multiple active crevices some will be stifled at lower current densities than others. The findings indicate that intermittent application of cathodic protect...
On the basis of different surface treatments of a Type 316 stainless steel, results of protective film composition as obtained by Auger electron spectroscopy (AES) are related to resistance to initiation of crevice corrosion. The nickel content of the protective film was similar after all treatments, whereas the chromium surface content was found to differ between different surface treatments. The results suggest that initiation of crevice corrosion and initiation of pitting corrosion are different in nature.
Tests on commercial alloys and a number of new Fe-Cr-Mo compositions show that pitting in 10% FeCl//3 multiplied by (times) 6H//2O solution at 50 C on specimens containing crevices is more severe than on specimens immersed in a solution of 2% KMnO//4-2% NaCl at 90 C. Among nonferrous-base metals, only Hastelloy C (or C-276), titanium, tantalum and columbium (Nb) resist pitting corrosion in the above tests. An alloy with 28% Cr and 4% Mo additives is representative of a formulation that is resistant to pitting in both chloride tests.
The purpose of this study was to establish a simple way of making pH measurements using common commerical electrodes to evaluate the relative crevice corrosion susceptibility of various materials. For titanium alloys, no acidification occurred during a 40-hour test. With type 316 stainless, a pH of 4. 75 was recorded, much higher than is reported for pits. A pH 3 value resulted with 6061 aluminum alloy.
Using temperature as a crevice corrosion criterion, a number of commercial and experimental Mo-bearing 18% Cr austenitic stainless steels have been evaluated for crevice corrosion resistance. These data have been correlated with the effect of Mo content on the rate of active corrosion in the very aggressive acid solution expected in the crevice itself. Alloyed Mo has been found to play the dominant role in reducing the relative rate of crevice corrosion and in defining the go/no-go crevice corrosion behavior in terms of the crevice corrosion temperature (CCT) in oxidizing acid chloride solutions.
Using Russian steels of 13 Cr and 18–9 analyses in general experiments are conducted varying pH and oxygen concentrations in sea water to determine the relative effect of these variables on corrosion rates in crevices. Effect of oxygen concentrations in the range 1–9 mg/1 and pH 8.3 to 2.3 are reported.
Changes in pH were found to strongly and changes in oxygen concentration weakly influence the corrosion rate. Authors postulate that anodic passivity is effected after application of tenths of a milliampere per square centimeter whereas when the pH in the anodic space is lowered, critical current density required for passivation increases 40–50 fold.
Heat treatment of the steel does not influence electrode potentials when solution pH is lowered.
Failure of structures and of many types of automatic equipment is often due to crevice corrosion. Differential aeration was advanced some time ago as a major cause of such attack. The present investigation was made to demonstrate other factors in the mechanism of crevice corrosion.
Electrochemical methods were used to determine effects and relationships of polarization potentials and corrosion current densities in the main volume of electrolyte (which has free access to the open surface of the metal tested) and in the metal's crevices and clearances. Special laboratory apparatus was used with which crevice corrosion conditions could be simulated and in which “crevice” width was adjustable.
Iron, aluminum and stainless steels were tested in 0.5N NaCl. Iron was also exposed to a mixture of Na2SO4, NaCl and NaNO2, to 8.6N HNO3 and to 0.2N H2SO4. Observations were also made of linear selective type crevice corrosion at the metal-dielectric interface perimeter in acid and of comparative corrosion rates above, at, and below the water line of partly-immersed iron. (In the latter case, a “crevice” is considered to exist near the liquid's meniscus.)
It was concluded that much destruction of metal in clearances is due to peculiar electrochemical behavior which results in acceleration of the anodic metal ionization reaction and deceleration of the cathodic reaction. Insignificant differences in potential between metal on the open surface and that in the crevices initiate operation of corrosion cells.
Linear-selective dissolution of iron in acids, on the phase boundaries of the metal-dielectric-acid system, proceeds according to the crevice corrosion mechanism.
In water-line zone corrosion, the crevice corrosion mechanism is also in evidence, rate depending on whether the electrolyte is neutral, acid or alkaline. In inhibited media, metal potential near the water line becomes negative, due to difficulty of access of inhibitor, so that macrocells are activated.
Unacceptably high leakage rates from a large number of type 304 stainless steel tubes forced the scrapping of two carbonate reboiler heat exchangers from the carbon dioxide removal system of a fertilizer plant. Extensive failure investigation carried out on the failed heat exchangers established that the main factor responsible for premature failure of the heat heat exchangers was partial expansion of the stainless steel tubes into the carbon steel tube sheet as against the design specification for the expansion of almost the full length of the tubes inside the tube sheet. Partial expansion of the tubes into the tube sheet left undesirable tube-to-tube sheet gaps, which subsequently acted as crevices and led to localized corrosion of the tube sheet as well as the tubes.
The changes in concentration of hydrogen and chloride ions during corrosion in an artificial crevice on steel were observed from potential changes of Pd-H and Ag/AgCl micro-electrodes inserted in the crevice. Increasing the bulk chloride ion concentration accelerated the rate of pH change and resulted in more acidic conditions in the crevice. Such pH changes were delayed by the addition of inhibitor such as dialkylamines. Though the free chloride ion concentration did not change appreciably, the total concentration of free and complex ions in the crevice increased with continuous dissolution of the steel. Increasing the hydrogen and total chloride ion concentration in the crevice accelerated the anodic dissolution of steels within the crevice.
Nucleation of crevice corrosion of five stainless steels in NaCl solution has been studied using potentiokinetic and galvanostatic methods. It is inferred that a well reproducible critical potential for crevice corrosion nucleation exists. This potential depends on the type of steel and is more negative than the critical potential for pit nucleation. The difference between the potential for crevice corrosion and that for pitting is higher for more resistant steels than for less resistant ones. A mechanism explaining the crevice corrosion in chloride solutions is proposed.
The crevice corrosion resistance of AISI type 316 stainless steel was investigated as a function of carbide precipitation and grain size in 0.5 M NaCl solution using a potentiodynamic technique and a specially designed crevice assembly. It has been found that if there is chromium depletion along the grain boundaries (for sensitized material) or if there is a large grain boundary area per unit volume (for fine grained material), the intergranular attack takes place in the crevice due to the presence of more active areas along the grain boundaries.
Internal cathodic reactions can have an important role in determining the local chemistry in cracks and crevices, depending on the details of the application.1-4. However, for stainless steels in chloride environments at ambient temperatures,4 the range of potentials for which the internal cathodic reactions would be important was predicted to be below the values of practical relevance in seawater, which approach +400 mV (SCE) at ambient temperatures due to biofilm activity. If the kinetics of the internal cathodic reactions could be stimulated, the consequent increase in pH could, in principal, act to prevent the onset of crevice corrosion. In order to achieve this, the cathodic kinetics of hydrogen ion reduction on the stainless steel would have to be enhanced or a gasket material introduced within the crevice which acted as a net cathode for the potentials of relevance or at least increased the consumption of hydrogen ions sufficiently to elevate the pH. The use of a gasket material offers the more viable approach. It is hoped that this note will stimulte wider interest in this unusual approach to prevention of crevice corrosion in order to substantiate the method or to expose its limitations.
Repassivation potentials, ER , for crevice corrosion of metal/metal-crevices were measured with more than 50 kinds of stainless steels and nickel base alloys in solutions containing 3×10−1, 3 and 20% NaCl at 80 °C. Critical temperature and critical Nacl concentration for repassivation of the crevice corrosion were determined after successive decreasing of temperature and/or NaCl solution under an electrode potential kept at −0.20 V,SCE. The obtained boundary conditions for repassivation depend on both the temperature and NaCl concentration (Type A) for high alloys containing Ni and Mo as expressed by Mo (§)+13.5 log Ni (§) 26while these depend mainly on NaCl concentration (Type B) for the other alloys from ferritic to austenitic stainless steels. These differences could be explained from the temperature dependence of repassivation potentials for the two type of alloys.
This paper elucidates some problems on electrochemical aspects of the pitting corrosion in stainless steels. It describes a new method used for the investigation of pitting corrosion. This method makes it possible to determine at any arbitrary period of time the currents flowing from the actually operating pits, the true current densities through the pits, as well as the distribution of anode and cathode currents along the metal surface subject to pitting corrosion. The results are used as a basis for discussion of main relationships characterizing the origin and development of pitting corrosion. Attention is paid to the process mechanism.
The site of crevice corrosion initiation on stainless steels has been investigated. It has been found that the initiation takes place at sulfide inclusions situated at the junction between a covering layer and the free metal surface with a portion of the inclusion covered and the rest exposed to the solution. A mechanism is suggested based on electrochemical dissolution of the sulfide inclusions.
Potentiodynamic polarization curves in and in were obtained for 25% Cr ferritic stainless steels containing 0–5% Mo and 0–4% Ni. High‐purity alloys and alloys containing the usual commercial levels of C,N, Si, and Mn were investigated. The critical current density was decreased by molybdenum and molybdenum‐nickel additions in both acids. All the high‐purity 25% Cr steels containing 3.5 or 5% Mo were immune to pitting corrosion and highly resistant to crevice corrosion at 25°C in and in . Alloys containing commercial levels of impurities, on the other hand, were all subject to pitting attack in solution under potentiodynamic conditions and in immersion testing in ferric chloride solution. The relative pitting resistance, however, was still improved by molybdenum additions.
The protection of crevices poses an important problem in industrial applications of anodic protection. Experimental studies with a special crevice assembly have shown that the interiors of crevices often remain active and corrode at a rapid rate. These experiments, together with theoretical analyses, demonstrate that the ability to passivate crevices during anodic protection is controlled by electrolyte characteristics, crevice geometry, and the electrochemical behavior of the protected metal. Of these, critical anodic current density, ic, is the most important parameter.
A mathematical model for the initiation of crevice corrosion on 304 stainless steel in
was developed. The central concept is that thiosulfate ions, formed by dissolution of
inclusions and trapped within the crevice, act in concert with chloride ions to cause passivity breakdown when their concentrations
exceed critical values. The model includes consideration of migration and diffusion of the supporting electrolyte, as well
as ions produced by dissolution of metal and
inclusions. Experimental data reported separately in Part I were compared with theoretical predictions for the critical
geometry at which initiation occurred, and for the degree of cathodic protection required to prevent crevice corrosion. A
simplified model was developed to predict conditions under which breakdown would be expected to occur for various cases of
crevice geometry, potential at the edge of the crevice, and
inclusion density.
A mathematical model of the initiation stage of crevice corrosion has been developed, based on the generally accepted mechanism of oxygen depletion within the crevice, followed by a pH fall and eventually permanent breakdown of the passive film and the onset of rapid corrosion. The model highlights the importance of the critical crevice solution (CCS) and the current flowing when the alloy is in the passive condition (passive current), which characterise a given alloy's resistance to crevice corrosion, the crevice geometry and the Cl− level of the bulk solution. The model predicts how the pH within the crevice falls with time until it reaches either the limiting value, as a result of mass transfer considerations, or the critical pH value which causes breakdown and the onset of rapid corrosion.Potential uses of the model lie in the areas of materials selection, setting design criteria and defining environment limitationsfor the use of given alloys.
Electron microprobe analysis has shown that the corrosion pits in Fe13Cr and 25Cr20Ni commercial stainless steels nucleate only at the sulphide inclusions. These are probably also the main sites for the Pit nucleation In Fe16Cr crystals of high purity. The role of the sulphide inclusions in pit nucleation phenomena is discussed.
The susceptibility of stainless steels and high nickel alloys to crevice corrosion severely restricts their use for marine engineering applications. The development of crevice corrosion test procedures for alloy evaluation and development purposes is reviewed. As susceptibility to crevice corrosion is an inherent weakness of this type of alloy, it is suggested that it might in future be more rewarding to determine the design parameters required for the prevention of crevice corrosion by galvanic protection, rather than to pursue the continuing present investment in the quest for the development of an immune alloy.
Experimental studies have been carried out using crevices of Type 316 stainless steel freely exposed in chloride solutions. The characteristic behaviour of corrosion potential with time, together with morphological evidence, indicates a mechanism in which an induction period precedes the permanent breakdown of passivity and the initiation of active dissolution within the crevice.On this basis electrochemical techniques have been developed for determining two parameters which, it is considered, characterise an alloy's resistance to the initiation of crevice corrosion. A mathematical model of crevice corrosion has been developed in Part I; when the electrochemical data are fed into this model encouraging agreement is found between the predictions of the model and the actual observations of crevice corrosion behaviour.The potential use of these techniques as a test method for assessing the resistance of alloys to crevice corrosion is discussed.
Test methods currently available for determining the resistance of stainless steels and related alloys to pitting and crevice corrosion in chloride environments are assessed. The present understanding of the mechanisms of pitting and crevice corrosion are examined, and the major factors affecting the processes are noted. Accelerated and exposure test techniques are considered in relation to their ability both to provide an accurate ranking of materials and to relate to service conditions. All tests reviewed had some drawbacks.
S.C.C. of mild steel in passivating nitrate solutions and crevice corrosion of passive austenitic stainless steels in acid solutions have been electrochemically investigated. Besides the known interconnection between localized attack and s.c.c. of passivable alloys in regard to the initiating circumstances, certain strong evidence has resulted from the experimental analysis of some strict geometrical and electrochemical analogies between the mechanisms of these two forms of corrosion. In this connection a theoretical treatment of a possible common mechanism is outlined on an idealized model in terms of active-passive cells.
Rotating disc assemblies are employed to study the cathodic electrode process and its inhibition by cerium implantation into UNS S31603 stainless steel in a solution of 0.6 M NaCl + 0.1 M Na2SO4. The reduction of oxygen and protons on both gold and untreated steel are shown to be controlled by the mass transport processes in solution. Cerium implantation effectively inhibits the cathodic reduction, reducing the cathodic current by more than two orders of magnitude. The cathodic reduction of oxygen and protons on ion-implanted stainless steel is thus controlled primarily by charge transfer at the electrode. Thermodynamic data suggests that the highly stable cerium oxide may be responsible for blocking the active sites for both cathodic and anodic reactions.
The mechanism of crevice corrosion of stainless Cr steels in neutral 3%NaCl solution has been studied in a cell constructed for this purpose. Immediately after the start of an experiment, both pH and potential of the steel surface increase as a result of alkali formation (O2 reduction) during corrosion of the steel in the passive state and thickening of the air-formed oxide film. After the oxygen available within the crevice has been consumed, the cathode process is concentrated to the outer surface whereas on the crevice surface only metal dissolution takes place. By hydrolysis of metal ions (Fe2+ and Cr3+) the crevice solution is now acidified. After having passed a maximum value of about 10, the pH in the crevice is lowered to a steady state value of about 4. The crevice surface is first activated locally (pitting) and finally in its entire extent and ecrev. and eout. are both lowered. At the anode (crevice) surface, the Flade potential is raised whereas it is lowered on the outer surface where the pH is raised due to the cathode reaction. The crevice must therefore be active and the outer surface passive even if their potential difference is quite small (150 mV for 13%Cr, 50 mV for 17%Cr).
- Degerbeck