correlation of corrosion test results

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Accelerated Corrosion Tests - Service Life Correlation:

Ideally, accelerated corrosion tests would provide data that could be correlated to actual in-service performance. However, by the very nature of accelerated corrosion tests, these procedures can rarely (if ever) be used to confidently predict service life.

In general, actual corrosive environments are more complex and less carefully controlled than accelerated laboratory tests. Therefore, at best, the latter can be used to measure the relative field performance in terms of a particular corrosion mechanism/mode. Some factors that make correlation of accelerated laboratory test results to actual in-service performance difficult include:

By their very nature, accelerated corrosion tests are designed to deviate from actual service exposure to produce results in significantly shortened time frames.
In general, laboratory tests are more simplistic and standardized, with fewer variables than actual service conditions.
Multiple failure mechanisms/modes and their interaction in the field are not easily reproduced in the laboratory. Artificially accelerating one corrosion mode may retard another mode.
Small coupon size, sample geometries, finite number of samples can affect laboratory studies compared to the "real world".
The "acceleration" of corrosion by adjusting a few selected variables in laboratory tests usually does not represent the complex interplay of multiple variables under actual service conditions.
Laboratory specimens usually deviate from production parts/components used in actual service, especially when details pertaining to the surface condition are considered.

It has been pointed out that there is no "magical" accelerated corrosion test that will accurately correlate to in-service use. However, for specific applications (such as cosmetic automotive corrosion) some relatively "sophisticated" tests (such as cyclic corrosion tests) have been shown to correlate better to service performance than overly simplistic tests (such as conventional salt spray tests). Furthermore, the development of data bases comparing accelerated corrosion test results to actual in-service performance results can increase the confidence in the former.

References/Literature:

ASTM G31: Standard Practice for Laboratory Immersion Corrosion Testing of Metals.

"Which Accelerated Test is Best", Problem Solving Forum, JPCL, August 2000, pp.17-28. (comments by various experts)

J. Guthrie, B. Battat and C. Grethlein: "Accelerated Corrosion Testing", The AMPTIAC Quarterly, Volume 6, No.3., Fall 2002.

D.A. Claydon: "Performance Testing of Anti-Corrosive Coatings", Akzo Nobel, England.

Links:

ASTM International
www.astm.org