Page 109 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

ENVIRONMENTALLY INDUCED CRACKING (EIC) 87

(iv) Cathodic polarization may reduce or prevent SCC but may only be used with
care for alloys that resist all types of critical hydrogen damage.
(v) Avoiding temperature range that causes SCC, for example, SCC of steel in
∘
caustic solutions at >105 C will also help.
Metallurgical Considerations:

(i) Selection of a resistant alloy with desirable microstructure and suitable chem-
ical composition and microstructure is preferrable.
(ii) Shot peening and other mechanical processes that create compressive residual
stresses at the surface are desirable.
(iii) Stress-relief at low temperature is used to lower corrosion susceptibility. Ten-
sile residual stresses from welding are dangerous and a low-temperature ther-
mal stress-relief treatment is desirable for welded assemblies.
(iv) Alloy composition and/or alloy microstructure may be changed as in changing
stainless steel grade 304 by nickel base superalloy.

Surface Treatments:
(i) Surface modification by oxidation, phosphating ,and anodizing is helpful for
certain alloys when these treatments are followed by sealing with suitable
inhibitors or coatings.
(ii) Coatings can extend life but do not totally prevent SCC as defects in the coating
lower the protection.
(iii) Cladding a susceptible alloy with a resistant alloy is also an effective method.
(iv) Carrying out nondestructive testing, inspection, and maintenance programs to
avoid SCC precursors such as concentration of stresses by localized corrosion
will help. In the case of coatings, routine inspection is essential as scratches
could be favorable sites for the initiation of SCC.

1.8.11 Prevention of Hydrogen Damage

Metallurgical Considerations:

(i) Modifying the design to lower stresses as well as choice of materials resistant
to HIC such as nickel-containing and nickel-based alloys because of their low
hydrogen diffusion rates will be an advantage. Hydrogen damage can often be
prevented by using more resistant material such as alloying high-strength steels
with nickel or molybdenum that reduces susceptibility to HE.
(ii) Susceptibility of pipeline steels to HIC depends on nonmetallic inclusions and
anomalous structures because of P and Mn segregation in steel. Lowering the
sulphur content or modifying the morphology of segregation by the addition
of Ca is useful in avoiding HIC. Tempering is effective in the elimination of
low-temperature anomalous structure. Addition of Cu (>0.2%) has been found