Page 199 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

CHEMICAL, PETROCHEMICAL, AND PHARMACEUTICAL INDUSTRIES 177

Figure 3.28 Stress corrosion cracking near a weld (7).

The National Bureau of Standards (NBS) performed a detailed investigation
consisting of chemical analyses, fracture mechanics analyses, SCC susceptibility
tests, and hydrogen cracking susceptibility tests. Preliminary NBS test results
indicated that the subject plate material (ASTM A516, Grade 70 carbon steel) of the
amine absorber was susceptible to hydrogen-induced cracking. The repair welds that
were done in the field and that had not been stress relieved were especially sensitive
to amine-induced corrosion and cracking. Figure 3.28 is an example of SCC both
parallel and perpendicular to the weld, but not in the weld. The propagation of
the crack clearly distinguishes SCC and reflects the different stresses along the
weld area.

3.25 CHEMICAL, PETROCHEMICAL, AND PHARMACEUTICAL
INDUSTRIES

The corrosion problems encountered are similar in the three industries (33). The
most common types of corrosion encountered are: (i) caustic and chloride cracking;
(ii) oxidation; (iii) sulfidation; (iv) corrosion under thermal insulation; (v) ammonia
cracking, and (vi) hydrogen-induced cracking. Corrosion in the chemical industries
is a well-known problem when chemicals such as hydrofluoric acid and hot sodium
hydroxide solutions are dealt with on an industrial scale. There are many examples
of how even a slight change in operating conditions, the presence of an unexpected
impurity, or change in the concentration of process chemicals can result in a dramatic
increase in the corrosion rate. The main causes of contamination in pharmaceutical
manufacturing are corrosion of embedded iron particles in vessel walls, failure of
glass linings, and corrosion under insulation.