- Corrosion coupons exposed for longer periods (e.g. 9 months) showed lower corrosion levels than shorter exposures (e.g. 30-60 days), calling into question the ability to accurately pro-rate long exposures. - "Sister" coupons exposed together in the same location often showed significantly different corrosion results, suggesting lack of repeatability. Higher VOC levels may contribute to disparity between sister coupons. - For more consistent results, it is recommended to expose coupons for 90 days and use multiple coupons per location, as shorter or longer exposures and single coupons can produce unreliable data.

1. Written 1 June 1995. Presented as a poster paper at AIC, June 1995.
Spreadsheet with tabbed Graphs can be found at:
http://www.geocities.com/william.lull@verizon.net/COUPON6.xls
===============================================================================
RECENT EXPERIENCES WITH CORROSION CLASSIFICATION COUPONS
---------------------------------------------------------
William P. Lull, Garrison/Lull Inc.; and,
Hilary A. Kaplan, Georgia Department of Archives & History.
ABSTRACT: Recent experiences with Purafil Corrosion Classification
Coupons suggest some guidelines on their use and interpretation of
results for measuring the corrosive nature of preservation
environments. The coupons, rated for a 90-day exposure, are known to
be less reliable when exposed for a shorter period. However, a set
of Purafil corrosion coupons was left out for a nine-month period,
showing low levels of corrosion. Subsequent tests with 30- and 90-
day exposure periods showed much higher levels expected for the site.
This brings into question the ability of the current lab analysis to
accurately pro-rate long and possibly short exposures to the standard
90-day analysis level. Test variability was suggested in field tests
comparing a 90-day exposure with a series of concurrent 30- and 60-
day exposures under the same conditions. Repeatability is in
question from the results for "sister" coupons exposed together at
around 30 days.
PURAFIL CORROSION CLASSIFICATION COUPONS. This test is designed to
monitor contamination from inorganic corrosive gas conditions in
museums, libraries and archives. The coupons react with sulfur
dioxide, oxides of nitrogen, and chlorine. They should not react
significantly with volatile organic compounds (VOCs).
In this test, a prepared pure silver coupon is exposed for a
specified period of time, then returned to the lab. At the lab,
electrolytic stripping determines the amount of corrosion compounds
on the coupon. Corrosion results are listed for chlorine compounds
(AgCl), sulfur compounds (Ag2S), and all other compounds under the
category of "unknowns" (Ag-Unk). Corrosion from nitrogen oxides and
from silver oxides are included in the unknowns category. Regardless
of the exposure period, lab results are corrected to a normalized 90-
day exposure.
The Purafil coupon test is based on the gaseous corrosion "coupon
test" developed by the Instrument Society of America (ISA), detailed
in Reference 1. The Purafil test is, however, proprietary and is
designed to test for lower corrosion levels sought by museums,
libraries and archives. Costing roughly $500 for a set of six coupon
tests, it is substantially less expensive than the typical ISA test
at $300 per coupon. One coupon test is often inadequate to
characterize an environment.

2. At an average cost of $85 per coupon, a good shelf life, and the
ability to have individual coupons analyzed, the test is economical
for general survey work in environmental assessment. The test also
has the vested interest of a company committed to the conservation
field. Purafil is interested in the use of the test, freely
discussing its applications and problems.
NORMAL EXPOSURE PERIOD. Users who are interested in results as soon
as possible many expose the coupons for only 30 days. The average
exposure reported by Purafil for museum and archives applications is
about 80 days. In the lab analysis the results are always corrected
(pro-rated) to a standard 90-day exposure.
Purafil recommends an exposure period of 90 days to reduce the chance
of analytical error. They say this should give more accurate results
since the amount of corrosion being stripped will be more significant
and no correction multiplier is used. Some expose the coupons for a
period shorter than 30 days with expectedly unstable results. This
is due to the low amount of corrosion being stripped and the high
correction multiplier to normalize to a 90-day exposure.
NINE-MONTH EXPOSURE. A set of Purafil corrosion coupons was left out
for a nine-month period at the Georgia Department of Archives and
History. Despite clear corrosion and discoloration, lab analysis
showed only moderate to low levels of corrosion when corrected to a
90-day exposure. Purafil notes that many types of corrosion can be
very visible with only moderate levels of contamination. Subsequent
tests for periods closer to 30 days consistently showed much greater
corrosion levels, closer to the levels suspected for the site.
Graph 1 shows the corrosion levels for two areas tested. The first
set of three tests shows a higher total corrosion at 96 days than the
two 9-month exposures in the same area. The second set of six tests
show even shorter exposures give greater corrosion levels.

3. 0
50
100
150
200
250
300
TotalCorrosion(Angstroms@90-
day)
96 280 284 34 36 61 97 280 284
Exposure Period (days)
Nine-Month Exposure
Graph 1
Ag2S AgCl Ag-?
Graph 2 shows the results for four separate areas tested. In each
case the 96-day exposure is much higher than the 9-month exposure.
0
50
100
150
200
250
300
TotalCorrosion(Angstroms@90-
day)
96 280 96 286 280 96 284 96 284
Exposure Period (days)
Nine-Month Exposure
Graph 2
Ag2S AgCl Ag-?

4. In all these tests the variability seems to be greater in the unknown
silver compounds (Ag-Unk).
This brings into question the ability of the current lab analysis to
pro-rate very long exposures to the standard 90-day analysis. While
coupons are not usually exposed for nine months, there is some
concern over the regular use of 30-day and 60-day exposures, when
used in comparison with the rated 90-day exposures.
30-60-90 DAY EXPOSURES. The initial 9-month comparison led to a
second series of tests comparing 30-, 60- and 90-day exposures.
Since the 9-month test was compared to coupons exposed in the same
space but at different times, this second test was designed to expose
coupons concurrently. In this way any variations of contaminant over
the exposure period would be caught in the concurrent exposures.
With the help of Purafil, field tests were done to compare a 90-day
exposure with three (3) concurrent but sequential 30-day exposures
under the same conditions.
Graph 3 shows the results of these tests.
0
100
200
300
400
500
600
TotalCorrosion(Angstroms@90-
day)
36 34 61 97 35 34 61 96 34 61 96
Exposure Period (days)
Concurrent 30-60-90 Day Exposures
Graph 3
Ag2S AgCl Ag-?
The tests were exposed as follows, in the same order as the graph:
---- CORROSION ---- -----EXPOSURE-----------
LOCATION AgCl Ag2S Ag-? Total Days Period (1994)
------------- ---- ---- ---- ---- ---- -------------------

5. 4B Film Vault 0 183 27 210 36 7/6-8/11
0 120 111 231 34 8/11-9/14
0 123 48 171 61 8/11------10/11
0 69 9 78 97 7/6-----------10/11
7V Vault 0 177 81 258 35 7/7-8/11
0 96 84 180 34 8/11-9/14
0 81 63 144 61 8/11------10/11
54 87 30 171 96 7/7-----------10/11
Loft Roof 282 147 138 567 34 8/11-9/14
147 150 63 360 61 8/11------10/11
240 156 9 405 96 7/7-----------10/11
Since each corrosion level is normalized to a 90-day exposure, the
longer exposures should approximate the average of the other periods.
This is not the case. This shows a lack of repeatability for the
absolute numbers in the tests, and suggests the need to refine the
exposure period weighting system.
DUPLICATE "SISTER" COUPONS. Repeatability of the tests has always
been a concern ever since coupons used in the same general area were
observed to have significantly different results. Over the past year
duplicate "sister" coupons have been exposed by Garrison/Lull at
various sites. Sister coupons are from the same coupon batch, and
are handled, exposed and returned identically, never separated by
more than a foot for more than a few minutes. For the tenure of
their exposure they were exposed to essentially the same
contamination.
Graph 4 shows the results of six such tests.

6. 0
200
400
600
800
1000
1200
1400
TotalCorrosion(Angstroms@90-day)
Duplicate "Sister" Tests
Graph 4
Ag2S AgCl Ag-?
Each set of two coupons was exposed for exactly the same time in exactly the
same location. All but the first and last tests showed alarmingly different
levels of corrosion.
CROSS-INDICATION WITH VOCs. In some cases sister coupons were
exposed in environments where separate tests showed high total VOC
levels. This was the case for the fourth and fifth set of sister
coupons (Graph 4). In the fourth set, the total VOC levels measured
415 ppb, about three to four times a typical level. In the fifth
set, the total VOC levels were 1350 ppb, about ten times typical
levels. This may show a cross-indication on the coupons, suggesting
that the coupons may be reacting with VOCs in some unstable way.
This might contribute to the disparity between the sister coupons.
The first set of sister coupons was exposed with a VOC test that
showed very low levels, 6 ppb. The second set of sister coupons was
exposed in the same historic house with a total VOC monitor measuring
normal levels of total VOCs, 121 ppb. The third set of sister
coupons was exposed without the benefit of a total VOC test. The
last set had a VOC test showing 155 ppb.
In these tests VOC levels were measured with a 3M 3500 Organic Vapor
Monitor exposed for 4 to 7 days. This monitor will pick up most
large VOCs, but will not show exposure to smaller VOCs such as
formaldehyde and acetic acid.
CONCLUSIONS. From these tests several guidelines emerge for
meaningful use of these coupons:

7. 1. Longer exposure periods tend to show lower contamination levels.
2. For more consistent results try to exposed coupons for 90 days,
but avoid exposure periods significantly longer than 90 days.
3. When used for relative comparisons, coupons should be exposed for
the same period of time.
4. For more reliable results use two or three coupons per location.
Cluster the coupons so they are exposed to the same environment.
If there is a wide disparity between "sister" coupons, suspect
other contaminants, such as VOCs. If possible, use a total VOC
monitor concurrent with the coupon test, to see if high VOC
levels may skew the corrosion test results.
References:
1. Instrument Society of America, Standard ISA-S71.04-1985,
"Environmental Conditions for Process Measurement and Control
Systems: Airborne Contaminants." Instrument Society of America,
67 Alexander Drive, PO Box 12277, Research Triangle Park, North
Carolina 27709.
Prepared by: Mr. William P. Lull, Garrison/Lull Inc., PO Box 337,
Princeton Junction, NJ 08550, (609) 259-8050; and
Ms. Hilary A. Kaplan, Conservator, Georgia Department of
Archives & History, 330 Capitol Avenue, Atlanta, GA
30334, (404) 656-2374.
Reviewed by: Mr. Chris Muller, Manager of Gas Technology, Purafil,
2654 Weaver Way, Doraville, GA 30340, (404) 662-
8545.
CORROSION CLASSIFICATION COUPONS PAPER 1-Jun-95
SUMMARY OF DATA SHOWN IN GRAPHS
---- CORROSION ---- EXPOSURE
COUPON LOCATION AgCl Ag2S Ag-? Total Season Days Comments: Ser/No
----------------- ---- ---- ---- ---- ------- ---- -------------------
NINE-MONTH EXPOSURE
-------------------
Georgia Archives
1B Vault, N 0 102 48 150 Sum-Fa 96
1B Vault, N 6 84 9 99 Sm-Wnt 280
1B Vault, S 9 87 9 105 Sm-Wnt 284
4B Film Vault 0 120 111 231 Sum 34 392-1
4B Film Vault 0 183 27 210 Sum 36 381-2
4B Film Vault 0 123 48 171 Sum-Fa 61 392-2
4B Film Vault 0 69 9 78 Sum-Fa 97 381-1
4B Film Vault, N 6 42 21 69 Sm-Wnt 280
4B Film Vault, S 0 24 27 51 Sm-Wnt 284

8. 2V Vault, S 0 81 39 120 Sum-Fa 96
2V Vault, S 0 33 21 54 Sm-Wnt 280
6V Vault, S 0 111 21 132 Sum-Fa 96
6V Vault, Ctr 3 39 9 51 Sm-Wnt 286
6V Vault, N 18 51 9 78 Sm-Wnt 280
Lwr Roof Vault 0 102 108 210 Sum-Fa 96
Lwr Roof Vault 0 84 6 90 Sm-Wnt 284
Search Room 33 102 30 165 Sum-Fa 96
Search Room 6 48 21 75 Sm-Wnt 284
30-60-90 DAY EXPOSURES
----------------------
Georgia Archives
4B Film Vault 0 183 27 210 Sum 36 7/6 - 8/11/94 381-2
4B Film Vault 0 120 111 231 Sum 34 8/11- 9/14/94 392-1
4B Film Vault 0 123 48 171 Sum-Fa 61 8/11-10/11/94 392-2
4B Film Vault 0 69 9 78 Sum-Fa 97 7/6 -10/11/94 381-1
7V Vault 0 177 81 258 Sum 35 7/7 - 8/11/94 380-3
7V Vault 0 96 84 180 Sum 34 8/11- 9/14/94 392-3
7V Vault 0 81 63 144 Sum-Fa 61 8/11-10/11/94 392-4
7V Vault 54 87 30 171 Sum-Fa 96 7/7 -10/11/94 380-2
Loft Roof 282 147 138 567 Sum-Fa 34 8/11- 9/14/94 392-5
Loft Roof 147 150 63 360 Sum-Fa 61 8/11-10/11/94 392-6
Loft Roof 240 156 9 405 Sum-Fa 96 7/7 -10/11/94 381-5
---- CORROSION ---- EXPOSURE
COUPON LOCATION AgCl Ag2S Ag-? Total Season Days Comments: Ser/No
----------------- ---- ---- ---- ---- ------- ---- -------------------
DUPLICATE "SISTER" COUPONS
--------------------------
Fine Art Museum/Northeast (partial historic house)
Paintings Storage 0 72 42 114 Winter 6 ppb VOCs 341-5
Paintings Storage 0 90 30 120 Winter 341-6
Historic House/Southeast Coast
Lafayette Room 0 237 507 744 Wint 28 121 ppb VOCs 405-3
Lafayette Room 777 354 135 1266 Wint 28 405-4
Fine Art Museum/Northeast Suburban
Main Gallery/Entr 0 90 804 894 Wint 60 383-5
Main Gallery/Entr 0 69 126 195 Wint 60 383-6
Fine Art Museum/Southwest (partial historic house)
Print Storage 0 132 87 219 Spring 415 ppb VOCs 349-3
Print Storage 30 207 408 645 Spring 349-4
Paintings Storage 0 132 258 390 Spring 1350 ppb VOCs 349-2
Paintings Storage 30 123 408 561 Spring 349-1

9. Historical Society/Northwest
Textile Storage 0 93 39 132 Spring 63 390-4
Textile Storage 0 156 30 186 Spring 63 390-5

10. Historical Society/Northwest
Textile Storage 0 93 39 132 Spring 63 390-4
Textile Storage 0 156 30 186 Spring 63 390-5