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Combination Corrosion Coupon Testing
Needed for Today’s Control Equipment
Accurate reliability analyses for electronic/electrical devices in pulp and paper
mills require silver and gold coupons in addition to copper
C.O. Muller
Purafil, Inc.
2654 Weaver Way
Doraville, Georgia 30340
INTRODUCTION
Copper, silver, and gold are important functional materials found in many of the electrical/electronic
devices in use in today's pulp and paper mill control rooms. The reliability of these devices may be
greatly affected by the presence of corrosive gases in the local environment. Even trace levels of
these gases can cause failures due to the formation of corrosion products in and on the circuitry and
connectors of these devices. And due to the nature of the processes involved in paper
manufacturing, it is almost a certainty that these devices will at sometime be exposed to corrosive
gases.
Atmospheric corrosion of copper has been studied extensively and tests have been devised to
measure the rates of copper corrosion. These corrosion rates are currently being used to gauge
electrical/electronic equipment reliability. The higher the rate of copper corrosion, the higher the
probability of equipment damage and/or failure (due to corrosion).
Studies of both laboratory and field data collected by Purafil, Inc. has shown that using copper
corrosion alone as a gauge for equipment reliability can seriously understate the corrosive potential
of the local environment. Examination of both silver and gold corrosion data have shown instances
of an environment which is noncorrosive to copper being extremely corrosive to silver and/or gold. It
is because of results such as these that any testing which attempts to predict electrical/electronic
equipment reliability should incorporate copper, silver, and gold corrosion as determinants.
CURRENT STANDARDS AND METHODOLOGIES
The Instrument Society of America (ISA) published a standard with which to classify airborne
1
contaminants that may affect process measurement and control systems. This presented the
manufacturers and users of electrical/electronic devices a way to predict the reliability of those
devices exposed to corrosive gases. One method involves the use of specially prepared copper
coupons. These coupons are placed in the local operating environment for a period of time, usually
thirty days, and then tested for the amount of corrosion film build-up. This provides a means of
2,3
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specifying the type and concentration of gaseous contaminants to which equipment may have been
exposed. The ISA classification of reactive environments for gaseous contaminants is shown in
Figure 1.
FIGURE 1 - Classification of Reactive Environments
Severity Level G1 G2 G3 GX
Mild Moderate Harsh Severe
Copper Reactivity Level
(In angstroms)a<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 humidity rate of change greater than 6% per hour.
Gas Concentrationb
Contaminant Gas Concentration
Reactive
Species c,d
Group A HS<3 <10 <50 $50
2
SO , SO <10 <50 <300 $300
2 3
Cl <1 <2 <10 $10
2
NO <50 <125 <1250 $1250
X
Group B HF <1 <2 <10 $10
NH <500 <10,000 <25,000 $25,000
3
O<2 <25 <100 $100
3
NOTES: a - Measured in angstroms after one month's exposure. See Reference #1, Appendix C, Item Numbers 2, 3.
b - mm /m (cubic millimeters per cubic meter) parts per billion average for test period for the gases in Groups A and B.
3 3
c - The Group A contaminants often occur together and the reactivity levels include the synergistic effects of these
contaminants.
d - The synergistic effects of Group B contaminants are not known at this time.
There are four severity levels in the standard; G1, G2, G3, and GX. Each level represents a more
corrosive environment, decreasing equipment reliability, and the higher probability of
corrosion-related failure than the one preceding it. The standard defines these contaminant severity
levels as outlined below.
CC Severity Level G1: Mild - An environment sufficiently well-controlled that corrosion is not a
factor in determining equipment reliability.
CSeverity Level G2: Moderate - An environment in which the effects of corrosion are
measurable and may be a factor in determining equipment reliability.
CSeverity Level G3: Harsh - An environment in which there is a high probability that corrosive
attack will occur. These harsh levels should prompt further evaluation resulting in
environmental controls or specially designed and packaged equipment.
CSeverity Level GX: Severe - An environment in which only specially designed and packaged
equipment would be expected to survive. Specifications for equipment in this class are a
matter of negotiation between user and supplier.
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Each Severity Level has a corresponding copper reactivity level which has a direct relation to the
amount of corrosion film build-up (Figure 1). This build-up, or thickness, is measured in angstroms.
In technical terms an angstrom (Å) is one ten-billionth of meter (39.37 inches). This is indeed a small
number. To help visualize this distance, the following example is offered. (Assume the amount of
rain to be equal to corrosion film thickness, the ability of the stack of paper to remain standing as a
measure of equipment reliability, and the stack of paper falling as equipment failure.)
Assume the thickness of a sheet of paper to be one angstrom. At 500 sheets per ream, one would
need twenty million reams of paper to equal one meter. If a standard ream is assumed to be
approximately two inches thick, this stack of paper would be over 630 miles tall! Assume the stack is
stable and freestanding and will not fall unless the bottom of the stack becomes wet...and then come
the rains!
With a Mild (G1) shower, the bottom 1¼ inches (300Å) of our stack could become wet without fear of
the stack falling. A Severe (GX) storm would wet the bottom 8 inches (2000Å) of the stack and
cause the entire stack to come tumbling down. Any amount of rain in between would cause the stack
to begin to sway and the possibility of the stack falling would increase as the amount of rain
increased. Therefore, to turn what was once a freestanding stack of paper over 630 miles tall into a
tremendous pile of trash, only the bottom eight inches would need to become wet. Just as minuscule
amounts of rain (relative to the height of the stack) could topple this stack, small amounts of
corrosion build-up on electrical/electronic equipment could cause the shutdown of a pulp and paper
mill due to equipment failure.
PURAFIL CORROSION CLASSIFICATION COUPONS
Purafil, Inc. has been performing corrosion testing for a number of years as a diagnostic tool in
response to customers' needs and requests. During this time more than five thousand Corrosion
Classification Coupons (CCC) have been analyzed with more than half of these being returned from
pulp and paper mills. One difference these CCC's have from the ISA standard is that they employ
both copper and silver coupons. Although silver is not a equipment reliability determinant in this
standard, it was included because of independent studies citing silver being a better indicator of
environmental chlorine than copper . The use of these two types of coupons presented, on
4
occasion, results that were quite surprising. Some environments that were non-corrosive to copper,
and thus considered harmless to equipment (by the ISA standard), were extremely corrosive to
silver. It was felt that using only the copper corrosion results from these CCC's would seriously
understate the potential for equipment failure in these environments. It was becoming apparent that
the standard being used to project the corrosive potential of an area was inadequate.
As mentioned in the introduction to this article, copper, silver, and gold are important functional
materials found in electrical/ electronic devices. Gold especially is being used more and more in
these devices because of its resistance to corrosive attack. Connectors on printed circuit boards,
data transmission cables, etc., are being plated with a thin film of gold to prevent corrosion from
occurring on these surfaces. These connectors are where corrosive attack is most likely to cause
equipment failure. Although the gold itself does not corrode, the thin gold film is very porous and
corrosive gases can attack the metal underneath through these pores. The corrosion film can then
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spread up to the surface of the gold layer and ultimately cause device failure. Purafil, Inc. has been
offering CCC's with copper, silver, and gold coupons for more than a year. Similar inconsistencies
as observed between copper and silver coupons have presented themselves upon examination of
field-exposed gold coupons. Testing was proposed to see if similar results could be obtained in a
controlled laboratory setting. It was hoped that the conditions under which the above occurred might
be determined.
LABORATORY-EXPOSED CORROSION CLASSIFICATION COUPON TEST RESULTS
The ISA Standard defines corrosion in terms of the corrosion film thickness which builds up on
specially prepared copper coupons after one month of exposure. It gives instructions for the
preparation, exposure, and sample analysis of Copper Reactivity Samples. Because silver coupons
were also used in this test, these same instructions were followed for the Silver Reactivity Samples.
(The classification method for the gold coupons will be described later.) The basic premise of the
test was to introduce known concentrations of gas(es) into environmentally controlled chambers
1,5,6
containing copper, silver, and gold coupons. Regression analyses were then performed on the
results of the copper/silver corrosion. These analyses showed corrosion trends for each type of
coupon as well as the effects of different gas combinations on each type of coupon . The gold
7
coupons were inspected for the presence or absence of pore corrosion.
Figure 1 lists two groups of gases designated as "Groups A and B". The Group A gases are those
most frequently encountered in pulp and paper mills and include inorganic chlorine compounds
(chlorine - Cl , chlorine dioxide - ClO , hydrogen chloride - HCl, etc.), active sulfur compounds
2 2
(hydrogen sulfide - H S, sulfur - S, etc.), sulfur oxides (sulfur dioxide - SO , sulfur trioxide - SO ), and
2 2 3
nitrogen oxides (nitric oxide - NO, nitrogen dioxide - NO , nitrogen tetroxide - N O). The four gases
2 2 4
which were used in this test were chlorine, hydrogen sulfide, nitrogen dioxide, and sulfur
dioxide.
The results of the copper and silver corrosion film analyses were recorded as total corrosion film
thickness. This follows the methodology of the ISA Standard. Results of the regression analyses
performed on the data are presented in Table 1 and used for the plots in Figures 2-5. Although gold
pore corrosion was observed, the results were inconclusive and will not be reported here.
After plotting the regression values for all the results obtained in the test, one significant trend
presented itself for the COPPER COUPONS. When H S is plotted along with the five combinations
2
that contained this gas, all six lines fall almost on top of each other (Figure 4). This suggests that,
for these four gases under the conditions of this test, COPPER CORROSION is dominated by H S.
2
Chlorine without H S produces little corrosion as do combinations of Cl , NO , and SO . SO even
2 2 2 2 2
appears to reduce the corrosion on copper when present with these other gases. Under these test
conditions, the magnitude of the H S-induced corrosion is greater than any observed synergistic
2
effects of different gas combinations.
While these gas combinations did not show any appreciable synergy on the amount of copper
corrosion, one combination did produce excessive corrosion over and above the expected levels on
the SILVER COUPONS. Although H S and NO were not particularly corrosive to the SILVER
2 2
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COUPONS by themselves, combining the two produced more than 4½ times the calculated expected
silver corrosion levels. All combinations which contained these two gases showed similar results
and plots of the regression values very closely tracked the H S+NO pair (Figure 5). This powerful
2 2
synergy overshadows any corrosive effects from any of the other gases. Where these two gases
were not in combination, the expected values closely matched the measured values.
The results of this testing indicate that for COPPER COUPONS the presence or absence of
hydrogen sulfide (H S) and for SILVER COUPONS the presence or absence of hydrogen sulfide in
2
combination with nitrogen dioxide (NO ) had the most profound effect on the amount of corrosion
2
produced. This leads to an interpretation that the presence or absence of H S or H S+NO when
2 2 2
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using COPPER COUPONS and SILVER COUPONS respectively would be the major factor in
determining equipment reliability.
FIELD-EXPOSED CORROSION CLASSIFICATION COUPON TEST RESULTS
The inconsistencies that presented themselves between the copper and silver coupons led to
concern over the true corrosive potential of some environments. These inconsistencies showed up
in both laboratory and field exposed coupons. With these significant differences between copper
and silver coupons, it was felt that the same may be true if gold coupons were included. Purafil, Inc.
has offered Corrosion Classification Coupons (CCC's) for more than five years with an optional
gold-plated coupon which shows how gold-plated contacts can be affected by the presence of
corrosive gases. Since their introduction, over 4000 "gold CCC's" have been delivered to the field
and more than 3800 have been returned to Purafil laboratories for analysis. The copper and silver
coupons were analyzed as per the ISA standard. The gold coupons were evaluated as described
below.
As stated above, the porous gold layer does not itself corrode. Rather, it is the corrosion of the
metal underneath the gold (nickel in this case) that corrodes and it is these corrosion products that
appear on the gold layer. This type of corrosion is known as pore corrosion. A microphotograph is
taken of the gold coupon upon receipt and it is inspected for the presence or absence of pore
corrosion. The amount of pore corrosion per unit area is used to determine the corrosive potential of
the environment to which the coupon was exposed.
Representative data obtained for the "gold CCC's" returned from the field is shown in Table 2 and
includes results for the three types of coupons. These results have been sorted by increasing
copper coupon corrosion - ISA classifications G1 through GX - to illustrate how an environment
which is noncorrosive to copper can be highly corrosive to other metals. The copper and silver
corrosion is presented as corrosion film thickness (in angstroms). The gold pore corrosion is
presented as a severity scale ranging from 1 to 5 with 1 showing no pore corrosion and 5 showing
the most severe pore corrosion.
Of the seven CCC's that were reported as ISA Class G1 - Mild, two (Nos. 1 and 3) showed significant
silver corrosion and two (Nos.2 and 3) showed significant gold pore corrosion. One silver coupon
(#9) and one gold coupon (#7) showed significant corrosion at the ISA Class G2 - Moderate level.
Although there was one silver and gold coupon (#15) which showed higher corrosion at the ISA
Class G3 - Harsh level, these rooms/areas are already compromised by the presence of high levels
of corrosive gases and should be treated to prevent further damage from corrosion. Premature
failure of any electrical/electronic equipment is almost certain otherwise.
It should be noted that currently there are no standards more widely used for the classification of
corrosive environments other than the one cited. The use of silver and/or gold coupons in addition to
the ISA standard's copper coupons can be used to get a more complete representation of the
corrosive potential of an environment which contains electrical/electronic equipment. The money
invested in this equipment and that which it controls makes it imperative that the most accurate
Page 7
determinations be made to protect these investments. This cannot be done by using copper
coupons alone. All functional materials must be considered.
One question in interpreting results from field-exposed CCC's is that (usually) neither the exact
contaminants and their concentrations nor the humidity and temperature of the local environment are
known. With a standard developed only for copper coupons, silver and gold corrosion data must be
interpreted carefully. The big advantage field-exposed coupons have over laboratory testing or any
standard interpretation is that the corrosion produced is due to the same environment to which
equipment is being exposed. This "real-time" picture of what is actually happening to these
functional materials can be an invaluable tool in determining equipment reliability. These field
results as well as some of the data which presented itself in the above laboratory testing presents a
case for the inclusion of silver and gold corrosion for the determination of equipment reliability.
CONCLUSION
The reliability of electrical/electronic equipment in corrosive environments must be accurately
gauged to avoid equipment failure. It has become apparent that using a standard which employs
copper-only testing is inadequate for this purpose. Field-exposed Corrosion Classification Coupons
(CCC's) have shown environments which would be considered noncorrosive by current (copper)
standards can be extremely corrosive to other functional materials. This was observed first on
CCC's with copper and silver coupons and, more recently, with copper, silver, and gold coupons.
Laboratory testing has produced similar results and has shown how the presence or absence or
certain corrosive gases affects the formation of corrosion on these metals.
The use of copper, silver, and gold coupons for assessing the corrosive potential of an environment
gives a more complete picture of what is actually occurring in that environment. By using the results
obtained from these CCC's, one can tell what type of contaminants were present and develop proper
control strategies. The probability of electrical/electronic equipment failure due to corrosive attack
can practically be eliminated by looking beyond copper-only environmental classifications.
REFERENCES
1. ISA Committee SP71, Standard ISA-S71.04 - Environmental Conditions for Process
Measurement and Control Systems: Airborne Contaminants, Instrument Society of America,
Research Triangle Park, North Carolina, 1985.
2. W.H. Abbott, Studies of Natural and Laboratory Environmental Reactions on Materials and
Components, Report to Environmental Studies Group, Battelle Columbus Laboratories, pp. 5-18,
July 26, 1978.
3. W.E. Campbell and U.B. Thomas, "Tarnish Studies," Bell Telephone System Technical
Publications, Monograph 13, 1170, 1939.
4. D.W. Rice, P. Peterson, E.B. Rigby, P.B.P. Phipps, R.J. Cappell, and R. Tremoureaux, Journal of
the Electrochemical Society: Electrochemical Science and Technology, Vol. 128, No. 2, pp.
275-284, February, 1981.
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5. W.H. Abbott, Studies of Natural and Laboratory Environmental Reactions on Materials and
Components, Report to Environmental Studies Group - Phase II, Battelle Columbus Laboratories,
pp. 19-20 & 39-47, August 1, 1986.
6. C. Fiaud, "Testing Methods for Indoor and Outdoor Atmospheric Corrosion," The Use of
Synthetic Environments for Corrosion Testing, ASTM STP 970, P.E. Francis and T.S. Lee, Eds.,
American Society for Testing and Materials, pp. 58-68, 1988.
7. C. Muller, "Multiple Contaminant Gas Effects on Electronic Equipment Corrosion", prepared for
CORROSION/90 Conference, Las Vegas, Nevada, April, 1990.
TABLE 1 - COPPER AND SILVER CORROSION (Regression Values)
Copper Coupon Test Data - Total Copper Corrosion
Coupon Cl ,H S, Cl , H S, Cl , NO ,HS, NO ,Cl , H S,
No. NO SO SO SO NO , SO
Cl H SNO SO Cl , H SCl , SO HS, NO NO , SO
2 2 2 2 2 2 2 2 2 2 2 2 2 2
222
2
2 2
2
2 2
2
2 2
22
3 108 330 153 149 57 129 366 170 317 152 250 283 309
6 127 431 173 153 236 137 525 197 349 273 354 393 367
9 147 531 193 157 414 146 649 223 403 385 434 503 433
12 167 631 212 161 592 155 754 249 479 491 501 612 512
15 187 732 232 165 771 166 845 276 576 593 561 722 606
18 207 832 252 169 949 179 931 302 694 692 614 832 717
21 226 932 271 173 1127 194 1009 329 834 788 663 941 848
24 246 1033 291 176 1306 212 1082 355 996 882 709 1051 1004
27 266 1133 311 180 1484 233 1151 382 1179 975 752 1161 1188
30 286 1233 330 184 1662 259 1215 408 1384 1065 793 1270 1406
ISA* 1000 1000 1000 450 2000 1450 2000 1450 3000 2450 2450 2450 3450
(+/-)% -71% 23% -67% -59% -17% -82% -39% -72% -54% -57% -68% -48% -59%
EXPECTED 1480 431 1517 476 1730 1620 654 1566 1766
DIFFERENCE (+/-)% 12% -40% -20% -14% -20% -34% 21% -19% -20%
Silver Coupon Test Data - Total Silver Corrosion
Coupon Cl ,H S, Cl , H S, Cl , NO ,HS, NO ,Cl , H S,
No. NO SO SO SO NO , SO
Cl H SNO SO Cl , H SCl , SO HS, NO NO , SO
2 2 2 2 2 2 2 2 2 2 2 2 2 2
222
2
2 2
2
2 2
2
2 2
22
3 23 39 33 38 33 8 173 25 162 57 41 206 161
6 30 60 35 41 68 18 314 33 273 89 51 349 305
9 37 80 36 43 104 29 446 41 384 115 58 475 423
12 44 100 38 46 140 40 571 49 495 139 63 590 577
15 51 121 40 48 175 52 692 57 606 160 68 699 709
18 58 141 41 51 211 65 810 65 717 181 72 802 839
21 65 162 43 54 247 77 925 73 829 199 76 902 967
24 72 182 44 56 282 90 1038 81 940 217 79 990 1093
27 79 203 46 59 318 103 1149 89 1051 235 82 1091 1219
30 86 223 47 61 354 116 1258 97 1162 251 85 1181 1343
EXPECTED 309 148 270 109 547 307 118 507 527
DIFFERENCE (+/-)% 14% -21% 365% -11% 112% -18% -28% 133% 155%
TABLE 2 - Field-exposed CCC's
COPPER - SILVER - GOLD COUPON DATA
(Sorted by ISA Class & Copper Corrosion)
CCC CCC ISA COPPER SILVER Ag/Cu GOLD PORE
#PANEL # CLASS CORROSION CORROSION RATIO CORROSION
1 3003 G1 151 846 5.6026 2
2 3004 G1 167 219 1.3114 4
3 8012 G1 186 2468 13.269 4
4 6001 G1 201 226 1.1244 1
5 5034 G1 216 95 0.4398 1
6 7003 G1 234 128 0.547 1
7 3002 G2 315 766 2.4317 4
8 4066 G2 320 285 0.8906 2
9 6000 G2 417 1556 3.7314 2
10 2005 G2 463 331 0.7149 2
11 6001 G2 470 935 1.9894 2
12 7015 G2 784 373 0.4758 3
13 7006 G2 791 590 0.7459 4
14 2605 G3 1096 1731 1.5794 2
15 1074 G3 1228 2848 2.3192 4
16 8000 G3 1243 598 0.4811 3
17 2603 G3 1349 1215 0.9007 2
18 6002 GX 2330 2897 1.2433 2
19 8010 GX 2845 1175 0.413 5
20 2602 GX 3308 4678 1.4141 5
21 4063 GX 3783 570 0.1507 5
22 6004 GX 3799 1523 0.4009 4
23 1055 GX 4278 3266 0.7634 2
24 1057 GX 4684 2221 0.4742 4
25 2409 GX 4912 1830 0.3726 1
26 2604 GX 4918 2160 0.4392 2
27 8086 GX 5372 2531 0.4711 5
28 4056 GX 7540 6075 0.8057 4
29 3001 GX 8533 3147 0.3688 5
30 2425 GX 11370 3191 0.2807 5
31 8011 GX 36244 10880 0.3002 5
32 4032 GX 51202 14047 0.2743 5
33 2006 GX 78656 12206 0.1552 5
34 8014 GX 80179 14920 0.1861 5
35 2016 GX 89275 22041 0.2469 5
36 8091 GX 102971 26850 0.2608 5
37 9070 GX 107666 12698 0.1179 5
38 1054 GX 109274 31894 0.2919 5
39 4057 GX 135060 14644 0.1084 5
40 9090 N/A 281 180 0.6406 1
41 9061 N/A 2019 980 0.4854 3
