Page 497 - Corrosion Engineering Principles and Practice

464 C h a p t e r 1 1 M a t e r i a l s S e l e c t i o n , Te s t i n g , a n d D e s i g n C o n s i d e r a t i o n s 465

often determine the choice of a laboratory test. In immersion testing,
acceleration is achieved principally by
• Lengthening the exposure to the critical conditions that are
suspected to cause corrosion damage. For example, if a vessel
is to be batch-processed with a chemical for 24 hours, then
laboratory corrosion exposure of 240 hours should be
considered.
• By intensifying the conditions in order to increase corrosion
rates, that is, increasing solution acidity, salt concentration,
temperature or pressure, and so forth.
• By provoking localized environmental changes that can result
in the formation of acidic/high chloride conditions furthering
the initiation and propagation of crevice corrosion of
susceptible alloys, for example, ASTM G78 [20] (Fig. 11.25).
Once the environmental conditions have been determined, and
the test designed, then it should be repeated a sufficient number of
times to determine whether it meets the desired standard for
reproducibility.
With such a long list of considerations and options, it is important
to simplify the design of test plans by adopting a testing strategy that
relates requirements to the main test parameters. For example, the
decision tree presented in Fig. 11.26 has been developed to facilitate
the selection of tests designed to verify the susceptibility of steels to
various forms of SCC. The strategy would be to start with the most
severe and least expensive SCC test, that is, the slow strain-rate test,
in which a bar made from the relevant material is exposed to the
environment of interest and slowly monotonically strained to fracture
[21]. When cracks are found, the susceptibility of the material should
then be further evaluated by performing a battery of other tests
designed to differentiate among the various mechanisms leading to
SCC and hydrogen embrittlement.
Test methods for determining corrosion resistance are quite
specific and must be based on the conditions prevailing and the
materials to be used, including coatings and other protective measures
planned for a given application. All these details, including the
specification ranges for significant variables, must be determined
from individually formulated tests based on the desired service life
and other requirements of an application.
11.6.2 Test Standards
There are a multitude of organizations around the world dealing with
the production of test methods and standards related to the overall
behavior and performance of materials. Organized in 1898, the Amer-
ican Society for Testing and Materials (ASTM) has grown into one
of the largest institutions of this kind. ASTM is a not-for-profit