Interpretation and Use of Corrosion Test Results-Interpretation and Use of Corrosion Test Results
Introduction
IMPORTANT ASPECTS of any corrosion investigation are the credibility of the test results and user acceptance of the
test methodology. When commercially competitive materials are evaluated or when corrosion tests are used for material
acceptance, the testing techniques come under close scrutiny and quick criticism in the marketplace. Public safety and
producer liability are potent considerations in decisions involving material selection and the determination of safe
environments.
After the appropriate test (or tests) is completed, the data must be analyzed and presented in a manner that facilitates
proper decision making. To be useful, an expression of the test results must have engineering significance, must be
mechanistically sound, and must be comprehensible (Ref 1). Finally, there is the question of the amount of confidence
that can be placed in the decision.
Credibility of Test Results
The question of the reliability of corrosion test results is discussed in the first article in this Section, "Planning and
Preparation of Corrosion Testing." The following 13 articles are devoted to methods of generating reliable data. This
article will consider the cost effectiveness of corrosion testing. One of the conclusions reached in Ref 2 is that perhaps the
greatest problem is the misapplication of reliable laboratory corrosion data. An accelerated test can reliably indicate only
how a given material will behave under the actual conditions of testing. Confidence should not be placed in accelerated
corrosion test data when they are applied to other conditions, nor should a one-to-one correlation be expected. A
significant degree of confidence is justified only after an accumulation of sufficient experience in correlating test data
with actual performance behavior under known conditions (Ref 2).
Data that can be directly used in design or for control of operations will be obtained only in certain situations in which a
corrosion test is required to provide an answer to a specific corrosion problem in a specific environment. Most often, the
results must be interpreted in terms of comparisons or rankings. A criticism of such interpretations is that they are
subjective; that is, they depend too much on the judgment of the investigator. Therefore, it is advisable when planning a
corrosion-testing program to select techniques carefully in order to provide ranking parameters that are subject to minimal
influence from testing conditions and that have engineering significance. This is not possible in all cases. It is particularly
difficult in tests for stress-corrosion cracking (SCC) and corrosion fatigue cracking because there are so many confusing
variables in these tests.

Interpretation of corrosion test results is also complicated by the time factor and by the consequences in the event of a
corrosion failure. In addition, there are more difficult questions than ever before due to the increased use of toxic
materials--questions such as:
· Can service life be predicted, and with what level of confidence?
· Will serious corrosion occur? In what form? Is there an incubation period?
· What is the corrosion rate, and will it change with time?
· What are the consequences of corrosion failure in terms of public safety and inconvenience, loss of life
and property damage, and liability and cost?
· How can an acceptable degree of risk be defined in the selection of new, more corrosion-resistant
materials?
· Is supplemental corrosion protection needed? If so, what type is best, and how long will it be effective?
Therefore, credible interpretation in many cases of material and environmental evaluation is ultimately a shared
responsibility of the corrosion engineer, the material engineer/designer, and possibly a statistician. It should be recognized
in the planning stage that the corrosion engineer needs input from the equipment designer to help identify the corrosion
system correctly. A corrosion system consists of one or more metals and all parts of the environment that influence
corrosion during the expected lifetime of a structure.
User Acceptance
Reference 1 contains a discussion of the user acceptance problem, with emphasis on the rank-ordering concept for
interpreting material and environmental evaluation tests. A more critical approach to rank ordering is one way of
improving user acceptance of laboratory test results. A preferred ranking parameter would be a dominant factor in the
corrosion test and would be recognizable to the user/engineer as a type of data that would be useful.
Development of an appropriate corrosion-testing plan requires considerable research and engineering effort. Research
aspects involve defining the limits of the corrosion system, identifying the dominant corrosion factors, and
developing/selecting an appropriate corrosion test procedure. The engineering aspect involves collecting service
experience data and correlating it with laboratory corrosion test data to permit establishment of acceptability criteria. An
example of how this was done in the case of sulfide stress cracking is shown in Table 1. In this example, critical stress, Sc,
for 50% probability of failure in a standard laboratory test was the rank-ordering parameter for materials considered for
use in sour gas environments. Accumulated field experience with various materials was used to set an Sc acceptability
limit. The validity of this limit was confirmed by showing that specimens cut from actual field service failures had Sc
values below the acceptability criterion.

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