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 / hydrofluoric acid mixtures, whereas, traditionally passivation has been done using only nitric acid.

How topassivate stainlesssteel at home

Stainless steels 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 steels will naturally self-passivate.

Passivation ofstainlesssteel pdf

The standard also allows any combination of citric acid concentration, temperature and time, provided that the passivation test criteria can be met. Specific treatments are however also specified.

The standard notes that the high carbon martensitic stainless steels, such as 440C, are not suitable for acid passivation as they can be attacked or be subject to hydrogen embrittlement.

Nitric acid alone can be used to remove light surface iron contamination after which the acid facilitates the passivation of the cleaned steel surface.

Passivation chemicals forstainlesssteel

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This standard covers nitric acid and nitric acid / sodium dichromate solutions. Like ASTM A380 it classifies types of stainless steels as suitable for specific treatment conditions according to EN 2032-1 – Aerospace series – Metallic materials – Part1:Designation

Passivate stainlesssteel

Parts treated however must pass specific tests to confirm the effectiveness of the passivation, although in practice the tests are for the detection of the effects of residual iron contamination on the surface of the parts. Unlike ASTM A380, the standard does not require specific solutions for particular stainless steel grades or types, although 3 specific treatments are identified.

Passivation treatments are sometimes specified, but it is important to consider whether this is strictly necessary or not. Stainless steels cannot be passivated unless the steel surface is clean and free from contamination and scale from welding operations.

Passivate stainlessvsstainlesssteel

The treatments are then defined by the process classes. In the case of classes C3 and C4, a two step process is defined, with a clean water rinse between the two steps, shown in the table below.

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.

ASTM A967 – Specification for Chemical Passivation Treatments for Stainless Steel Parts, (based on US Defense Department standard QQ-P-35C)

This standard covers both nitric and citric acid treatments. The nitric acid treatments are similar to those identified in ASTM A380. In addition, this standard also includes citric acid treatments.