Page 111 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

REFERENCES 89

represent the true conditions of projected use. The standard practice is to use 3.5%
NaCl, pH 6.5, and the standard tests are AST B-117, 2003, and the conditions of the
test AST G44 (130).

1.8.11.3 Test Samples Commonly used specimens under elastic stress range are
bend-beam specimens, C-ring samples, O-ring specimens, tension specimens, and
tuning fork specimens. Plastic strain specimens and residual strain specimens have
also been used. Static loading of precracked specimens as well as slow-strain-rate
testing are also used. Stressed O-rings as well are used for protective treatments for
SCC prevention. The samples are subjected to various loading conditions such as con-
stant load, constant strain, or monotonically increasing strain to total failure in some
of the slow-strain-rate tests. Other tests are cyclic loading as well as slow straining
over a limited stress range (131).

1.8.11.4 Stressors Corrosion accelerators for testing alloys is achieved through the
use of “stressors,” such as cold work of the material, high concentration of aggressive
ion, low pH, high temperature, higher stress. Externally applied stresses are known
and easy to control, but residual stresses are the ones that are normally responsi-
ble for stress-corrosion failures under service conditions. Although laboratory tests
are useful in encouraging conservative design of the equipment, results of long-term
atmospheric tests of tensile loaded specimens are considered to be more reliable.
Constant load SCC tests are more severe than constant deflection tests.

1.8.11.5 Slow strain test The strain rate chosen frequently for tests indicates a
s
value of about 2 × 10 −6 −1 for cracking of steels, aluminum, and magnesium alloys.
This value for cracking refers to open-circuit conditions, and the stain rate sensitivity
of cracking depends on the potential and the solution composition. Where neces-
sary, the potentiostat may be used to control the potential of the samples during
slow-strain-rate tensile testing (132). The reduction in area is a simple and reliable
way to quantify the susceptibility to SCC. Both AC and DC potential drop measuring
methods are proven techniques for obtaining information on the onset of stable crack
growth.

REFERENCES

1. LL Shreir, RA Jarman, GT Burstein (eds.), Principles of Corrosion and Oxidation,But-
terworth Heinemann, 1994.
nd
2. LL Shreir, Corrosion, Metal/Environment Reactions,Vol. 1,2 ed., Butterworths, Lon-
don, 1976, 1:169–174; 8:1–129.
3. CP Dillon, Introduction in Forms of Corrosion, NACE Handbook, Vol. 1,NACEInter-
national, Houston, TX, 1982, pp. 1–4; ED Verink, pp 5–18.
4. ASM Handbook Vol. 13A: Corrosion: Fundamentals, Testing and Protection, Frankel, pp.
236–241, 257; Kolman, pp. 381; Phull p. 568; Jones pp. 346–366; Craig pp. 367–380,
American Society of Metals, 2003.