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ISA Standard 71.04: Changes Required for Protection of Today's Process Control
Chris Muller, Purafil, Inc., Doraville, Georgia, USA
Grant Crosley, Visy Pulp & Paper, Campbellfield VIC Australia
Corrosion-induced failures remain frequent in electronics products used in industrial environments. The
International Society for Automation (ISA) Standard 71.04-1985 provides a classification system using corrosion (or
reactivity) monitoring to determine the corrosive potential of an environment towards electronic equipment.
Changes to electronic equipment mandated by the European Union directive 2002/95/EC “on the Restriction of the
use of certain Hazardous Substances in electrical and electronic equipment” (RoHS) required the elimination of lead
in electronic equipment. Recent research has shown that printed circuit boards made using lead-free materials can be
more susceptible to corrosion than their tin/lead counterparts. Now even environments previously considered
relatively benign concerning electronics corrosion are experiencing serious problems as a direct result of RoHS
With the passage of a number of RoHS regulations and the switch to lead-free finishes on printed circuit boards,
many are now questioning whether this type of environmental monitoring is adequate. Reactivity monitoring now
needs to provide a more complete environmental assessment than the monitoring techniques described in ISA
Standard 71.04. This standard is long overdue for a major revision to address issues described for and since the
implementation of RoHS. This paper will discuss changes that have been proposed for the current vision and what
changes may be anticipated in future revisions.
Ever since the pulp and paper industry began replacing pneumatic and hydraulic controls with computer control
systems, the reliability of these electronic and electrical devices has been challenged by attack from corrosive
gaseous contaminants present in the operating environment. In the context of electronic equipment, corrosion is
defined as the deterioration of a base metal resulting from a reaction with its environment. More specifically,
corrosive gases and water vapor coming into contact with a base metal result in the buildup of various chemical
reaction products. As the chemical reactions continue, these corrosion products can form insulating layers on circuits
which can lead to thermal failure or short-circuits. Pitting and metal loss can also occur.
Corrosion of metals is a chemical reaction caused (primarily) by attack of gaseous contaminants and is accelerated
by heat and moisture. Rapid shifts in either temperature or humidity cause parts of circuits to fall below the
dewpoint temperature, thereby facilitating condensation of contaminants. Relative humidity above 50% accelerates
corrosion by forming conductive solutions on a small scale on electronic components. Microscopic pools of
condensation then absorb contaminant gases to become electrolytes where crystal growth and electroplating occur.
Above 80% RH, electronic corrosive damage will occur regardless of the levels of contamination.
STANDARDS FOR AIR QUALITY ASSESSMENT
To address these concerns and to protect multi-million dollar investments in new control systems, a 10-year study
was performed by Battelle Laboratories underwritten by process control system manufacturers and by many of the
major pulp and paper companies. The goal was to develop the information necessary to establish a correlation
between electronic equipment reliability and environmental corrosion rates. This ultimately led to the publication of
standard S71.04-1985: "Environmental Conditions for Process Measurement and Control Systems: Airborne
Contaminants," by the Instrument Society of America (ISA, now known as the International Society of Automation)
This document was followed in 1987 by the International Electrotechnical Commission (IEC) Standard, IEC 60654-
4 (1987-07) “Operating Conditions for Industrial-Process Measurement and Control Equipment. Part 4: Corrosive
and Erosive Influences” . Japan's standard, JEIDA-29-1990, was revised in 1990 and published as the Japan
Electronic Industry Development Association’s (JEIDA) "Standard for Operating Conditions of Industrial Computer
Control System” . The goal of all three standards was the same – correlate equipment reliability to levels of
airborne corrosive contaminants.
ISA Standard 71.04-1985
Standard 71.04-1985, as it is known now, presents the manufacturers and users of electronic devices and computer
control systems a classification system to gauge the corrosive potential of an environment. It establishes
environmental classifications, and severity levels within each classification, according to the type of contaminant.
Although the standard also includes classifications for liquid and solid contaminants, only gaseous contaminants will
be considered in this paper.
This standard defines or characterizes environments in terms of their overall corrosion potential. By the use of
“reactivity monitoring,” a quantitative measure of this potential can be established. Reactivity monitoring involves
placing specially prepared metal strips into the environment. These “corrosion classification coupons,” or CCCs,
would be exposed for a period of time and then analyzed to determine the thickness of the corrosion films that had
formed. Copper reactivity was measured as the total corrosion film thickness (in angstroms) normalized to a 30-day
exposure. This analysis technique allows for the classification of the total amount of corrosion, as well as the
thicknesses individual corrosion films attributed to different classes of corrosive gases.
Four levels of corrosion severity have been established by Standard 71.04 (see TABLE I). The optimum severity
level is G1 - Mild. At this level, corrosion is not a factor in determining equipment reliability. As the corrosive
potential of an environment increases, the severity level may be classified as G2, G3 or GX (the most severe). The
effects of humidity and temperature are also quantified in this standard. High or variable relative humidity and
elevated temperatures may cause the acceleration of corrosion by gaseous contaminants. Relative humidity of less
than 50 percent is specified by the standard.
TABLE I - ISA Classification of reactive environments
Severity Level G1
Copper Reactivity Level
(in angstroms, Å)* < 300 < 1000 < 2000 2000
The gas concentration levels shown below are provided for reference purposes. They are believed to
approximate the Copper Reactivity Levels stated above, providing the relative humidity is less than
50%. For a given gas concentration, the Severity Level (and Copper Reactivity Level) can be
expected to be increased by one level for each 10% increase in relative humidity above 50% or for a
relative humidity rate of change greater than 6% per hour.
Gas Concentrations (in ppb)
Contaminant Gas Concentration
The synergistic effects of various combinations of corrosive gases make the determination of severity levels
complex. In addition to the contaminant gases themselves, temperature and humidity also have a major impact on
the corrosion rates. Therefore, the easiest method of measurement has been through the use of CCCs according to
the method prescribed in the ISA standard. This data is used to determine the severity level of the environment
which refers to the potential damage that corrosive gases could cause to electronics and electrical equipment and,
therefore, provides a method for determining equipment reliability.
* Measured in angstroms after one month’s exposure. See Appendix C, Item Numbers 2, 3.
† mm3/m3 (cubic millimeters per cubic meter) parts per billion average for test period for the gases in Groups A and B.
‡ The Group A contaminants often occur together and the reactivity levels include the synergistic effects of these contaminants.
§ The synergistic effects of Group B contaminants are not known at this time.
The standard actually describes two methods of environmental characterization – direct concentration monitoring
and reactivity monitoring. However, the method most commonly used since the standard was published is reactivity
monitoring. The standard provides instructions for the preparation, exposure, and analysis of copper reactivity
This standard had been developed with the goal of uniformity of equipment reliability in the field of industrial
process measurement and control instrumentation. However, to be of real value, this document cannot be static, but
must be periodically reviewed and updated to incorporate changes as necessary to remain relevant as a tool for
predicting equipment reliability.
CORROSION RESEARCH APPLICABLE TO STANDARD 71.04
Through the regular ISA review process, standards are subject to review at least once every five to seven years.
These reviews are intended to address any comments and criticism as well as any advances in technology, which
would enhance the standards application. In the twenty-five years since publication of S71.04, much research has
been performed in the area of environmental classification via corrosion monitoring.
From the research performed in support of developing Standard 71.04  and in research performed since its
publication, it was discovered that one of the main limitations in using copper alone for environmental monitoring
and assessment is that the presence or absence of environmental chlorine, a particularly damaging contaminant to
metals, cannot be accurately determined. Silver, on the other hand, is extremely sensitive to chlorine. For this reason,
silver coupons, in addition to copper coupons, have been used for almost since the standard was published to
provide what is known to be a more accurate assessment of the corrosion potential of a local environment.
Corrosion Coupon Research [5,6,7]
Corrosion reported per Standard S701.04 is actually the sum of individual corrosion films. Depending upon the type
of coupon used and the gases present, chloride, oxide, sulfide, and/or other films may be produced. Each coupon can
be examined for the type of film present and its relative contribution to the total corrosion. When using
electrolytic/cathodic reduction as an analysis technique, each individual film is dissolved, or “stripped away,” at its
own unique electrochemical potential. Corrosion potentials have been determined for the various corrosion products
which form of copper and silver. When standard S701.04 initially came up for review in 1990, new research was
available and applicable for inclusion into the standard. Some of the more important work is summarized below.
Some of the main findings from this corrosion coupon research are listed below.
• Single gas corrosion levels did not agree with the corresponding copper reactivity levels put forth in the
• The standard should be reviewed for inclusion of actual single gas corrosion levels for all severity levels and
their use in setting the reactivity levels.
• Using the copper reactivity levels put forth in the standard, one could significantly overstate the corrosive
potential of an environment when using gas concentrations as the sole determinant.
• The standard was shown not to be internally consistent with respect to the four main Group A contaminants.
• There was poor correlation with field-observed results.
• Copper corrosion was dominated by active sulfur contamination (e.g., H2S) when present.
• While H2S and NO2 were not particularly corrosive by themselves, this combination produced more than four
times the expected silver corrosion.
Initial testing only examined the total amount of corrosion formed on the coupons which was consistent with the
reactivity monitoring described in the standard. Data was subsequently examined in terms of the individual
corrosion films. Although electrochemical potentials have been determined for individual corrosion products when
analyzed by electrolytic/cathodic reduction, the complex nature of gas interactions can produce some unknown films
– even in a controlled laboratory environment. These unknown films, too, were used in the data described below.
• Examination of individual corrosion film data for copper and silver coupons further supported the assertion
that one cannot accurately determine the corrosive potential of an environment when following the
methodology in standard S701.04. The copper-only reactivity monitoring prescribed cannot conclusively
determine the presence or absence of Cl2 or SO2. By reporting only copper corrosion, even with the
breakdown of the individual films, one can and often does make incorrect assessments about the environment
in question. The most obvious example of this is where some continue to attribute an unknown copper film to
the presence of Cl2.
• The assumption that an unknown copper film could be attributed solely to the presence of chlorine was
proved false. Examination of silver corrosion results clearly showed only Cl2 was present. All testing which
included this contaminant produced a silver chloride (AgCl) corrosion film. With the standard giving Cl2 the
lowest tolerable concentration for a G1 environment, one must be able to accurately determine whether or not
this contaminant is present or not.
• Total copper corrosion, and particularly copper sulfide film (Cu2S) formation, is dominated by H2S. This film
was observed only when H2S was present in the test environment. SO2, by itself, produced only a copper
oxide (Cu2O) film. Therefore, the absence of Cu2S does not indicate a sulfur-free environment. This, too, can
be clarified by including silver coupons. Cu2O, without silver sulfide (Ag2S) confirms a sulfur-free
• A single copper film was produced for only one contaminant – SO2. This was an oxide film as opposed to a
sulfide film. However, on silver, each single contaminant produced a single film and were the only instances
where a single film was observed on silver. Except for SO2, a single silver film can be associated with a
specific contaminant and SO2 can be speciated when copper and silver films are compared.
Corrosion Coupon Field Results
This and other research allowed for examination of field-exposed corrosion coupons. One of the main criticisms
with reactivity monitoring has been that some were trying to read too much into what is actually only circumstantial
evidence that corrosive gases are present in the subject environment. There have been only a few studies with side-
by-side reactivity monitoring, gas concentration monitoring, and temperature and relative humidity monitoring.
Some have tried to extrapolate contaminant gases and concentrations from the electrolytic reduction analysis alone.
This has been shown to be inconclusive at best. However, examination of some large corrosion coupon databases
has produced some interesting observations and conclusions.
• For both copper and silver coupons, single film formation was confined predominantly to G1 and G2 coupons.
For the copper coupons almost all of these exhibited only Cu2O. Examination of the corresponding silver data
showed about a quarter of these exhibited only Ag2S corrosion. These coupons were concluded to have been
exposed to a SO2-only environment.
• Within this group, some copper coupons exhibited no Cu2S and were concluded to have been exposed in an
SO2-only environment. A few coupons similarly exhibited no Cu2O and were concluded to have been
exposed in H2S-only environment.
• Of those silver coupons which showed no silver chloride, only one third of the corresponding copper coupons
exhibited an unknown copper film. Conversely, about one third of all silver coupons, which did show silver
chloride showed no corresponding unknown copper films. This further illustrates how one could mistakenly
state the presence or absence of chlorine in the environment.
Standard 71.04 has been, and continues to be, a useful tool for characterizing the corrosive potential of an
environment. However, many have acknowledged the shortcomings of the standard by using combination
copper/silver corrosion classification coupons. New data for silver and gold (-plated) coupons established the need
to review the standard for the applicability and reliability of copper-only environmental reactivity monitoring. By
using copper and silver coupons for this monitoring, it has been shown that the subject environment can be more
accurately characterized as to the severity class(es) and type(s) of contaminants present. Any remedial actions
recommended as a result of this monitoring would be more concise, and consequently, less expensive. The addition
of gold coupons provides even further definition of the subject environment.
Real-time Corrosion Monitoring
These inconsistencies and shortcomings, coupled with the time it takes to obtain results from reactivity monitoring
using coupons, resulted in the development of real-time corrosion monitors using piezoelectric quartz crystal
microbalances (QCM) as sensors . A QCM is plated with copper, silver, or other reactive metals and circuit is
made incorporating an oscillator with the QCM so that the frequency at which the crystal is vibrating can be
measured. As corrosion films are formed, the resonance frequency of the crystal changes. These changes can be
correlated to the amount of corrosion which has built up over time.
Data from both laboratory and field-exposed copper and silver coupons produced close and reproducible correlation
between the passive CCCs and the real-time QCMs because both methods involve the measurement of weight-gain
caused by the buildup of corrosion films. The use of QCMs as corrosion monitors has been described in several
Corrosion-Indicating Bridge (CIB). Another method of monitoring corrosion is to measure the increase in
resistance of a metal film, accompanying a reduction in cross-sectional area by film growth. This type of
measurement is limited by the sensitivity of the instrumentation. However, in practice, even very slight changes in
temperature may produce resistance changes far greater than those due to corrosion alone. This technique appears
valid; however, the maximum measurable film thickness still needs to be determined.
These real-time corrosion monitors – especially those using QCMs – have been improved since their introduction in
the 1980s due to technology advances and the ability to make these devices smaller, more sensitive, and more
reliable. More recently, these devices have been modified due to changes mandated by new environmental laws
passed in Europe. These changes had nothing to do with product improvement; rather, they were required due to
restrictions on the use of various substances in electronic products.
RoHS AND ELECTRONIC EQUIPMENT RELIABIILTY
The European Union (EU) directive 2002/95/EC “on the Restriction of the use of certain Hazardous Substances in
electrical and electronic equipment” or RoHS was implemented in July 2006 (EU 2003), and was the first of many
RoHS (-like) regulations that have been passed . One of the purposes of these regulations has been to restrict the
use of hazardous substances in electrical and electronic equipment. The EU's RoHS Directive restricts the use of six
substances in electrical and electronic equipment: mercury (Hg), lead (Pb), hexavalent chromium (Cr(VI)), cadmium
(Cd), polybrominated biphenyls (PBB) and polybrominated diphenyl ethers (PBDE).
In order to comply with the EU legislation, all of these substances must either be removed, or must be reduced to
within maximum permitted concentrations, in any products containing electrical or electronic components that will
be sold within the EU. Manufacturers have made significant investments in new processes that will eliminate these
substances – especially lead.
In February 2006, China promulgated a law entitled “Administration on the Control of Pollution Caused by
Electronic Information Products.” The purpose of this law is similar to that of the EU’s RoHS Directive and the
Chinese law is simply called “China RoHS” in the industry. While there is some commonality between the RoHS
requirements in the EU and those in China, there are also significant differences that must be recognized and dealt
with. However, in both instances these RoHS regulations require the elimination of lead in electronic products and
manufacturers have to comply with RoHS if they want to continue in to do business in the EU and China.
A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components
using conductive pathways, or traces, laminated onto a non-conductive substrate. Alternative names are printed
wiring board (PWB), and etched wiring board. A PCB populated with electronic components is a printed circuit
assembly (PCA), also known as a printed circuit board assembly.
All PCBs have conducting layers on their surface typically made of thin copper foil. If the copper is left unprotected,
it will corrode and deteriorate. Traditionally, any exposed copper was plated with lead (-based) solder. Thus the
main issue for the electronics industry became the use of lead in the manufacture of components and circuit board
The hot air solder leveling (HASL) process worked well for many years, was the predominant surface finish used in
the industry, and was also the cheapest PCB available. Now RoHS essentially makes PCBs using the HASL process
obsolete. Failure modes on other common lead-free PCB finishes such as organic solder preservative (OSP) and
electroless-nickel immersion gold (ENIG) make these technologies undesirable. As a result, alternatives such as
immersion silver (ImmAg) and organically coated copper (OCC) are currently used as board finishes. Due to
inherent processing difficulties with OCC boards, ImmAg boards have become the standard PCB finish in the
electronics industry .
ImmAg is easy to apply to the boards, relatively inexpensive, and usually performs well. While ENIG presently has
a larger market share, since RoHS has been in effect, more ImmAg process lines have been installed in PCB
facilities than any other finish. However, some manufacturers have complained about issues with corrosion which
can lead to shorts and ultimate failure of the board.
RoHS AND ISA STANDARD 71.04
Research has shown that printed circuit boards made using lead-free materials can be more susceptible to corrosion
than their tin/lead counterparts . Two common chemical failure modes are copper creep corrosion on circuit
boards and the corrosion of silver metallization in miniature surface-mounted components. Creep corrosion failures
in high sulfur environments (ISA Class G2) have been reported on hard disk drives, graphic cards, DIMMs, and
motherboards in many types of systems.
Corrosion-induced failures are frequent in electronics products used in industrial environments. Now electronics in
environments previously considered benign with regards to corrosion are experiencing serious problems as a direct
result of RoHS. Table II shows CCC data from a number of pulp and paper mills around the world that met the
current definition of an ISA Class G1 environment but still experienced corrosion-related failures of RoHS-
compliant electronics. The common factor for all these locations was that the corresponding silver reactivity rates
averaged twice that of the copper rate with several locations showing silver corrosion 10-20 times higher.
Table II. Pulp and Paper Mill CCC Data 
Location Area/Room Cu2S Cu2O CuUnk Copper Total ISA Class AgCl Ag2S AgUnk Silver Total
AHU Room 0 180 0 180 G1 0 3142 0 3142
DCS Room 182 78 0 260 G1 0 3491 0 3491
Electrical Room 115 46 0 161 G1 0 1582 0 1582
Control Room 192 101 0 293 G1 0 1042 0 1042
Operations Room 205 88 0 293 G1 0 1234 0 1234
MCC 0 125 0 125 G1 0 948 0 948
DCS Room 182 104 0 286 G1 0 1022 0 1022
Wet End Control 182 78 0 260 G1 0 1118 0 1118
Computer Room 124 44 0 168 G1 0 5467 0 5467
DCS Cabinet 0 118 0 118 G1 0 1636 0 1636
MCC 0 106 0 106 G1 0 2634 0 2634
Control Room 0 150 0 150 G1 0 2796 0 2796
Mechanical Room 0 68 0 68 G1 0 2182 0 2182
Server Room 0 78 0 78 G1 0 1964 0 1964
Splicer Room 0 105 0 105 G1 0 1145 0 1145
Control Room 0 173 0 173 G1 0 1497 0 1497
IPC Room 0 231 0 231 G1 0 2145 0 2145
DCS Room 215 62 0 277 G1 0 1038 0 1038
DCS Panel 187 61 0 248 G1 0 1848 0 1848
MCC Room 204 56 0 260 G1 0 1810 0 1810
PLC Panel 102 50 0 160 G1 0 1155 0 1155
Electrical Room 172 34 0 206 G1 35 1315 0 1340
Wet End Drives 196 81 0 277 G1 0 1565 0 1565
Control Room 175 86 0 261 G1 0 5073 0 5073
Data centers in many urban locations have reported failures of servers and hard disk drives due to sulfur corrosion.
Desktop and laptop computers, servers, data communications (datacom) equipment and other information
technology (IT) equipment are now at risk due to RoHS. This is even being seen in personal computers and
Acidic (corrosive) gases and submicron particulates in urban environments come from motor vehicle exhaust,
emissions from other forms of transportation, heat and power generation, and industrial activity. In electronic
component corrosion, sulfur oxides, active sulfur compounds, and inorganic chlorides are of primary interest.
Although most data centers are protected against temperature and humidity variations, particulates and acidic gases
can be drawn in through the building's air handling system(s) causing corrosion of electronics – especially in
equipment produced since the passage of RoHS regulations.
In one study that looked at lead-free finishes , four alternate PCB finishes were subjected to an accelerated
mixed flowing gas corrosion test. Important findings can be summarized as follows:
• Immersion gold (ENIG) and immersion silver (ImmAg) surface finishes failed early in the testing. These
coatings are the most susceptible to corrosion failures and may make the PCB the weak link with regards to
the sensitivities of the electronic devices to corrosion.
• None of the coatings can be considered immune from failure in an ISA Class G3 environment.
• The gold and silver coatings could not be expected to survive a mid to high Class G2 environment based on
these test results.
The increasing number of hardware failures in data centers high in sulfur-bearing gases, highlighted by the number
of recent publications on the subject, led to the publication of a white paper on particulate and gaseous
contamination  by ASHRAE Technical Committee 9.9: Mission Critical Facilities, Technology Spaces, and
Electronic Equipment (ASHRAE 2009) recommending that gaseous contamination should be within the modified
severity level G1 which meets:
1. A copper reactivity rate of less than 300 Å/month and
2. A silver reactivity rate of less than 300 Å/month.
The word “modified” is used because Standard 71.04 uses copper corrosion alone to determine severity levels and
with the use of silver and/or silver alloys as a replacement for lead, it is felt by many that as mentioned in the studies
cited above, copper alone cannot provide as complete a picture of the corrosion risk as when using copper and silver.
The use of new PCB surface finishes – especially ImmAg – and this new susceptibility of electronic equipment to
environments previously considered benign has fostered a renewed interest in updating and improving ISA Standard
71.04 to protect RoHS-compliant products for use in both industrial and non-industrial settings.
PROPOSED CHANGES TO STANDARD 71.04
The primary consideration in updating the standard is incorporating silver as a quantifiable metric for corrosion risk
assessment. ASHRAE Technical Committee 9.9 has recommended that both copper and silver corrosion rates be
used with the higher of the two used to determine severity levels. Data is currently being by a number of
organizations to validate the silver reactivity rate. This and other proposed changes to Standard 71.04 are
Current: Environmental Conditions for Process Measurement and Control Systems: Airborne Contaminants
Proposed: Environmental Conditions for Electronic Equipment: Airborne Contaminants
Current: The purpose of this standard is to classify airborne contaminants that may affect process measurement and
The classification system provides users and manufacturers of instruments with a means of specifying the type and
concentration of airborne contaminants to which a specified instrument may be exposed.
This document is one of a series of standards on environmental conditions for process measurement and control
Proposed: The purpose of this standard is to classify airborne contaminants that may affect electronic hardware
such as process measurement and control equipment, as well as office electronic, networking and data center
The classification system provides users and manufacturers of electronic hardware with a means of specifying the
type and concentration of airborne contaminants to which a specified piece of electronic hardware may be exposed.
This document is one of a series of standards on environmental conditions for electronic equipment.
Current: §2.1 – This standard covers airborne contaminants and biological influences that affect industrial process
measurement and control equipment. Specifications for other environmental conditions, including nuclear radiation
and hazardous atmospheres, are beyond the scope of this standard.
Proposed: §2.1 – This standard covers airborne contaminants and biological influences that affect industrial
process measurement and control equipment, electronic office equipment, data centers, and network equipment.
Specific examples of electronic office equipment include: laptop computers, desktop computers, workstations,
servers, data storage hardware, terminals, displays, laser and inkjet printers, copiers, and faxes. Examples of data
center equipment include: servers, switches, routers, displays, keyboards, data storage hardware, power
distribution equipment, and climate control such as HVAC equipment. Some examples of networking equipment
include telecommunications hardware, switches, and routers.
§2.6 – CAUTION – Airborne or biological contaminants not listed in this document could cause equipment damage.
Caution should be used when a combination of factors approach or surpass class "X." Obtaining the guidance of a
chemical specialist is suggested when this condition occurs.
§2.6 – CAUTION – It is possible that airborne contaminants not listed in this document could cause equipment
damage. Caution should be used under this circumstance or when a combination of factors approach or surpass
class “X”. Obtaining the guidance of a chemical or biological specialist is suggested when this type of condition
Table 3 – Classification of reactive environments – Terminology
Keep only the Severity Levels and Copper and Silver Reactivity Levels in the normative part of the standard and
move the part of Table 3 referencing specific gases and concentrations to an informative appendix. This is for
several primary reasons: 1) the stated gas concentrations and corresponding copper reactivity levels generally do not
agree, 2) it has been generally agreed that a more reliable way to measure the potential for or the effects of corrosion
is to monitor corrosion directly via reactivity monitoring, and 3) similar data does not currently exist for silver
Section 6.1 - Reactivity
Recommended changes to this include the inclusion of silver reactivity monitoring as a metric along with copper.
The silver coupons will be used primarily for their ability to discern chlorine in the subject environment.
• It has been suggested that comparing the reaction rates between these metals can infer influences of relative
humidity and that this information be included as part of an informational appendix. Another appendix
section is proposed to provide further insight as to possible effects relative humidity may have on the
corrosion kinetics experienced.
• One recommendation is that the four severity levels described in Table 3 be expanded to five levels, in each
of two corrosion reactivity groups. These two groups would be used to indicate whether or not chlorine had
detected through silver film analysis.
• The concentration levels of individual gases currently in Table 3 that contribute to these reactivity rates will
be moved to a new table in an informative appendix. Because no data exist correlating all possible
contaminant of contaminant concentration combinations or the effects of temperature and relative humidity,
this table would be included as an aid in understanding the complexities involved in direct gas concentration
monitoring. This new table is described more fully in the following sections.
• A final appendix section is proposed, which will offer some idea of the reduction in mean time between
failures (MTBF) when equipment is continuously exposed to contaminants at various severity levels.
Section 6.2.1 – Relative Humidity
This section will be reworded for clarity and correctness. It will also include a reference to the appropriate sections
of the appendix pertaining to relative humidity.
It is known that the presence or absence of free moisture may accelerate or attenuate the corrosion reaction. A
proposal has been offered which correlates the ratio of the silver sulfide/cult of copper sulfide components to
humidity effects. These effects are described as low humidity attenuation, high humidity acceleration, or no
humidity effects. This is represented in Table III below.
Table III – Humidity Effects Corresponding to
Silver Sulfide/Copper Sulfide Ratios
Ag2S/Cu2S Ratio Effect
>0.5 and <1.5
High humidity acceleration (HHA)
No humidity effects (NHE)
Low humidity attenuation (LHA)
Section 6.3 – Explanation of Contaminant Severity Levels
Two additional corrosion reactivity groups have been proposed, which are differentiated by the absence or presence
of reactive chlorides in the reduced silver films. These two reactivity groups would be designated as the primary
sulfide group (GS), no silver chloride formed, and the chloride containing group (GC), were silver chloride is
If these new groups are accepted, one additional severity level may be considered. The former G1, G2, and G3
levels would remain the same as Mild, Moderate, and Harsh, respectively. The current GX – Severe level would be
redesignated as G4 – Severe and the new fifth level would be GX – Extreme. This new level would be defined as an
environment in which the operation of process and control equipment is not practical. Attempt to do so would result
in immediate catastrophic failure.
Section 7 – Biological influences
Remove this section and all references to “living” contaminants altogether.
New Appendix – Reduction of Mean Time Between Failure by Severity Level
Mean time between failure, or MTBF, is a measure of the time interval expected between faults from a
representative sampling of like components or systems. One proposed addition to an appendix is shown in the
second of two tables below.
Table IV – Reduction of MTBF
ISA Severity Class Percent Reduction
STATUS OF 71.04 REVISION
A final draft of the proposed revisions to Standard 71.04 is being prepared and will be discussed in the S71
Committee. Once approved, it will be submitted to the ISA Standards and Practices Board for consideration for
public review. Many comments, concerns, criticisms, etc. are anticipated due to the wide acceptance of the standard
in the processing control industry and a keen interest exhibited in the preparation of the draft revision. All written
reviews will be addressed by the committee and any changes (additions or deletions) agreed upon by all parties.
Once this has been accomplished, the standard will be submitted to the ISA for publication.
FUTURE ACTIVITY FOR STANDARD 71.04
Corrosion Classification Database
A corrosion classification database will be established and maintained by the ISA. This database will be set up with
data from paired copper and silver coupons** exposed in both industrial and non-industrial environments with an
indication of whether the equipment in the spaces monitored are RoHS-compliant. Solicitations are being made for
contribution to this database which by some accounts can consist of data from more than 70,000 pairs of coupons.
Over 4,000 of these pairs also have a corresponding gold coupon in anticipation that gold coupons may one day be
used along with copper and silver.
One of the main uses of this database is to establish and refine silver reactivity levels. The prevailing use of silver
maintains the same corrosion rates and severity levels as for copper coupons with the overall severity being
determined by the higher of the two. Because the mechanisms of copper and silver corrosion are different, statistical
analysis may lead to reformulation of these levels. At the very least, it is expected that review of this data will reveal
as of yet undiscovered trends with her between the different severity classes and coupon types.
Gold Pore Corrosion Monitoring
Some are using gold-plated corrosion classification coupons in addition to copper and silver coupons. This type of
corrosion monitoring has been examined by a number of organizations but little progress had been made in the
development of a “visual” gold coupon standards and work has apparently stalled.
Real-Time Corrosion Monitoring.
Whereas reactivity monitoring with CCCs may require exposure times of between 30 and 90 days before analysis
and reporting, electronic corrosion monitors capable of producing useful data almost immediately have been
developed and are being used in good success. The dominant monitor type on the market employs copper and silver-
plated quartz crystal microbalances (QCM) and have supporting data published that correlates weight gain on the
crystal caused by the build-up of corrosion products to changes in crystal frequency, which in turn is correlated to
corrosion film thicknesses. Good reproducibility of results has been demonstrated with this real-time monitor and
side-by-side comparisons of this methodology with both copper and silver coupon reactivity monitoring have shown
a high degree of correlation. QCM-based corrosion monitoring will be proposed for inclusion into the next revision
of the standard.
Whereas silver reactivity monitoring has been accepted as necessary for a number of years, and will be included in
the next revision of ISA Standard 71.04, it is real-time corrosion monitoring that has been deemed as vital by many
in the process control industries. When this form of reactivity monitoring is incorporated into the Standard and gains
even wider acceptance, reactivity monitoring with metal coupons appears to be headed for ancillary role as opposed
to its current everyday status. Although the real-time monitors will allow instant access to corrosion data, these
devices currently have some shortcomings when compared to coupon monitoring. The main trade-off is that
corrosion is reported only in terms of total corrosion. Specific corrosion films cannot be determined without
destructive testing of the QCM sensors.
** By paired coupons and is meant a copper and silver coupon exposed simultaneously in a subject environment for an equal period of time.
The passage of various “lead-free” regulations has resulted in a significant up-tick of corrosion-related failures of
new compliant electronic equipment. This is true for industrial application but even more so in non-industrial
applications which would have been considered benign had it not been for RoHS
Although having been accepted and in wide use, without revision, for more than 30 years, ISA Standard 71.04 is in
need of major revisions to incorporate the use of silver reactivity monitoring as a metric and to take advantage of
technology advances in real-time corrosion monitoring. Through the regular ISA standard review processes, the
findings described above, and ongoing research, significant changes have been proposed for standard S701.04.
From the examination of the available corrosion film data for copper and silver components, it is apparent that one
cannot accurately determine the corrosive potential of an environment when following the current methodology of
Standard 701.04. Specifically, copper-only reactivity monitoring will not or cannot conclusively determine the
presence or absence of environmental chlorine or sulfur oxides. By reporting only copper corrosion, even with a
breakdown of the individual films, one can and often does make an incorrect assessment of the environment in
While CCCs are good for the measurement of average corrosion rates, they do not allow for the measurement of any
variations that may have occurred during the exposure period. It is possible that all of the corrosion occurred on a
single day due to a chemical leak, spill, or some other upset condition common in the process industries. Therefore,
the ability to measure the short-term variations lasting several hours or days is desirable. Short and long-term
corrosion rate information, both incremental and cumulative is useful in predicting long-term equipment reliability.
The development of real-time corrosion monitors measuring both cumulative and incremental corrosion has
eliminated these limitations associated with corrosion classification coupons.
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 Standard: IEC 60654-4 (1987-07) “Operating Conditions for Industrial-Process Measurement and Control
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 Standard: JEIDA-29-1990, “Standard for Operating Conditions of Industrial Computer Control System,” Japan
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