Hydroxides affecting stress-corrosion cracking

The corrosion rate of nickel in sodium hydroxide is adversely affected by heat transfer by small amounts of oxidisable alkaline sulphur-containing salts, e.g. Na2SOj, NajS Oj, Na S and, at high temperatures, by alkaline oxidising agents, viz. NaClOj and NajOj. In the former circumstance Alloy 600 is more resistant than nickel, but not in the latter. When Alloy 600 is used for service in caustic alkalis, it should be stress relieved after fabrication to minimise the possibility of stress-corrosion cracking. 

Where a deposit contains an adequate concentration of sodium hydroxide and the affected area is stressed to a sufficiently high level, stress-corrosion cracking or caustic embrittlement (SCC) may occur. This type of caustic corrosion is different from caustic gouging, which does not require the presence of stress. 

A form of boiler waterside, caustic stress-corrosion cracking corrosion affecting carbon steels and austenitic stainless steels (300 series). Particularly associated with high localized concentrations of deposited sodium hydroxide (caustic soda). 

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