Page 110 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

88 INTRODUCTION AND FORMS OF CORROSION

to be effective. Calcium and rare earth metals inhibit HIC by modifying the
morphology of inclusions (126). Calcium, La, and Ce spheroidize nonmetallic
inclusions and thus raising the Cth and hence preventing the HIC. Cobalt, Ca,
W, and Ni are also effective in the prevention of HIC. By reducing the sulfur
content from 0.002 to 0.0005%, the inclusions such as MnS can be reduced
and HIC can be prevented (127).
(iii) Hydrogen blisters may be avoided by using “clean” steel without voids such
as killed steels instead of rimmed steels. Manufacturing processes and treat-
ments affect MnS morphology and influence sensitivity; for example, rimmed
and Si-killed steels have low susceptibility. If the MnS inclusion content is low,
adverse effects of low-temperature rolling are minimized. Both quenching and
tempering reduce the susceptibility. Tempering eliminates Mn and P segrega-
tion when the Mn level is more than 1%; tempering reduces hardness around
inclusions and hence HIC susceptibility.
(iv) HE can be minimized by using dry conditions as well as low-hydrogen welding
rods.
(v) Inhibitors may be used to reduce corrosion and hence hydrogen. Baking after
processing reduces HE. Addition of inhibitors during pickling to avoid hydro-
gen pick up is recommended. The pH can be raised to reduce HIC.
(vi) Changing the environment can be very efficient. Blistering rarely occurs in pure
acid corrosives without hydrogen evolution poisons such as sulfides, arsenic
compounds, cyanides.
(vii) Coatings should be impervious to hydrogen penetration and resistant to a cor-
rosive medium. Metallic, inorganic, and organic coatings are often used to
prevent HIC and hydrogen blistering.

1.8.11.1 Corrosion Testing Objectives of SCC testing are: (i) determination of the
risk of SCC for a given application and comparison of alloys; (ii) examination of the
effects of chemical composition, metallurgical processing, fabrication practices for
structural components; (iii) evaluation of protective systems and prediction of service
life; (iv) development of new alloys, which are less expensive, and offer a longer,
safer, and efficient performance for chosen environments; (v) evaluation of claims
for SCC performance of improved mill products.
Prediction of corrosion performance can be done from published data and test-
ing. Accelerated testing should involve the same mode of failure and reflect a known
order of resistance of some alloys in service media (128). The common test objectives
of SCC are high stresses, slow continuous straining, precracked specimens, higher
concentration of corrosive agent than in service medium, higher temperature, and
electrochemical stimulation (129). For electrochemical corrosion, the properties of
the medium at the interface should be noted in accelerated tests.

1.8.11.2 Media Considerations SCC tests are conducted in: (i) natural atmo-
spheres; (ii) seawater immersion, and (iii) laboratory or other fabrication conditions.
Atmospheric exposure tests take a long exposure time, but are reliable as they