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Sodium Hydroxide (Caustic Soda) is a strong base.
It is used in metal degreasing and cleaning processes in a wide range of industry applications.
Stainless steels types 304 and 316 can be considered resistant below 80 oC, up to the limit of solubility.
The iso-corrosion diagram 0.1mm/year lines for 304 and 316 types coincide (purple).
(The broken line represents the boiling point)
See Corrosion Handbook for source data.
Both 304 and 316 stainless steel types are resistant to a wide range of concentration and temperature.
Below 80 oC they can be considered resistant to any concentration of sodium hydroxide, up to the limit of solubility.
There can be a risk of stress corrosion cracking (SCC) attack at higher temperatures, which is common to both the 304 and 316 types.
This should not be an issue if service temperatures are limited to a 95 oC maximum.
‘Caustic stress corrosion cracking’ occurs at higher temperatures than chloride stress corrosion cracking (which can occur at temperatures as low as 60 oC).
The area of risk in sodium hydroxide is shown on the iso-corrosion diagram by the area bounded by a green line.
Chlorides should not pose as great a pitting and crevice corrosion attack threat in sodium hydroxide as they do in acid solutions.
The high pH values of the ‘basic’ sodium hydroxide helps arrest the normal mechanisms of attack.
Care may be necessary when selecting 304 types for sodium hydroxide cleaning systems or tanks, where ‘carry-over’ of chlorides could occur from prior treatment stages.
It may be better to consider 316 or the 316L types if this could occur.
The 316L type may be a marginally better choice where the steel may have been sensitised in the heat-affected zone (HAZ) of welds and post weld softening / stress relief is not practical.