Stainless steel are designed to naturally self-passivate whenever a clean surface is exposed to an environment that can provide enough oxygen to form the chromium rich oxide surface layer, on which the corrosion resistance of these alloys depends. Naturally occurring conditions such air or aerated water will do this and so under many exposure conditions stainless steel will naturally self-passivate.
Pickling, passivation and removing iron contamination with nitric acid
Passivation treatments are sometimes specified, but it is important to consider whether this is strictly necessary or not. Stainless steel cannot be passivated unless the steel surface is clean and free from contamination and scale from welding operations.
Scale may need to be removed first by 'pickling' or mechanical abrasion and although the surface of freshly pickled stainless steel will normally be immediately passivated once the pickling acid has been washed off, it is important not to regard these two treatments as the same.
Pickling usually involves nitric acids / hydrofluoric acids mixtures, whereas, traditionally passivation has been done using only nitric acid. Nitric acid alone can be used to remove light surface iron contamination after which the acid facilitates the passivation of the cleaned steel surface.
Citric acid passivation as an alternative to nitric acid treatments
Citric acid treatments can also be considered as an alternative to nitric acid as both provide the oxidising conditions necessary for passivation. Citric acid is a less hazardous method and has environmental benefits in terms of 'NOx' fume emission and waste acid disposal. Solution strengths of 4-10% citric acid are specified for passivation treatments in ASTM A967.
Specifications for passivation treatments for stainless steel.
Traditionally the American standards have been used.
These include: -
ASTM A380 - Practice for Cleaning, Descaling and Passivating of Stainless Steel Parts, Equipment and Systems
ASTM A967 - Specification for Chemical Passivation Treatments for Stainless Steel Parts (based on US Defense Department standard QQ-P-35C)
In 1997 an alternative British Standard was published:
EN 2516 - Passivation of Corrosion Resisting Steels and Decontamination of Nickel Base Alloys