Hydrochloric acid is classed as reducing acid and lacks the oxidising properties that stainless steel need to maintain their ‘passive’ corrosion resistant surface layer.
The acid is formed as a product of the pitting / crevice corrosion mechanisms in stainless steel and so when hydrochloric acid is present in any ‘external environment’ corrosion is promoted. Commercially concentrated acid is around 37wt. %.
The common stainless steel types, 304 stainless steel and 316 stainless steel should be considered non-resistant to hydrochloric acid at any concentration and temperature.
Corrosion resistance of stainless steel
The iso-corrosion diagram 0.1mm/year lines are represented for stainless steel 316 types (red) and a 6% molybdenum austenitic type (green).
Higher grades of stainless steel can have limited resistance, up to around 3% maximum at ambient temperatures, but may suffer local attack mainly as crevice and pitting corrosion, even at such low concentrations. The steep curves for 316 and the higher alloyed grades on the iso-corrosion diagram illustrate their very limited resistance.
(The broken line represents the boiling point)
Any additional chlorides or chlorine in the acid can be expected to make attack more severe. Nickel based alloys, rather than stainless steels, should be considered for handling hydrochloric acid.
Contact between stainless steel and building mortar cleaners
The use of building mortar cleaners that contain hydrochloric acid can result in staining and pitting to nearby stainless steel items. Architectural metalwork and kitchen equipment has been reported with such problems resulting from either splashes or from the fumes given off from the acid.
Commercially available cleaning acids are around 16% hydrochloric acid. Using these either ‘neat’ or as a one to one (50%) dilution, which may be recommended by manufacturers / suppliers of these cleaners, makes these solutions extremely aggressive to most stainless steel grades.
It is advisable not to use such cleaners anywhere near stainless steel items.
What can be done if building mortar cleaners come into contact with stainless steels?
If damage has occurred it is usually evident as brown staining.
Provided visible pits have not been formed it may be possible to renovate the surface by routine cleaning methods.
If pitting has occurred then this must be bottomed out with abrasives (grinding). This is necessary to avoid further staining or corrosion in these areas.
If this is not acceptable then the affected parts may have to be replaced.
Source: Zhejiang Yaang Pipe Industry Co., Limited (www.yaang.com)