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Passivation of Stainless Steel
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Passivation of Stainless Steelposted on 2 April 2018 | posted in Stainless steel
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.
Passivation treatments are sometimes specified, but it is important to consider whether this is strictly necessary or not.
Passivation does not remove weld burn or Iron contamination
Pickling and removing iron contamination
Weldburn and scale may need to be removed first by 'pickling" (or mechanical abrasion) and although the surface of freshly pickled stainless steel will normally start to passivate 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 citric and nitric acids.
Phosphoric acid is the most commonly used to remove light surface iron contamination after which a process 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. The Citric Acid does not remove any material from the parent metal, such as thinning of pipework.
Specifications for passivation treatments for stainless steels
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:
BS EN 2516 - Passivation of Corrosion Resisting Steels and Decontamination of Nickel Base Alloys
ASTM A380 nitric acid based passivation treatments
ASTM A380 nitric acid solutions
HNO3 - nitric acid, Na2Cr2O7.2H2O- Sodium Dichromate, CuSO4.5H2O - copper sulphate
Sodium Dichromate is rarely used today as it has been classed as a carcinogen
ASTM A967 passivation treatments
This standard covers both nitric and citric acid treatments.
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.
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.
ASTM A967 tests for passivation
Practice A - Water Immersion Test
Practice B - High Humidity Test
Practice C - Salt Spray Test
Practice D - Copper Sulphate Test
Practice E - Potassium Ferricyanide-Nitric Acid Test
ASTM A967 citric acid passivation treatments
The standard also allows any combination of citric acid concentration, temperature and time, provided that the passivation test criteria can be met.
BS EN 2516 passivation treatments
This standard covers nitric acid and nitric acid / sodium dichromate solutions.
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.