Passivation of Stainless Steel

Introduction

Stainless steels are designed to naturally self-passivate whenever a clean surface is exposed to an environment that can provide enough clean oxygen to form the chromium rich oxide (Cr O) surface layer, on which the corrosion resistance of these alloys depends. In good clean air or aerated areas and under many exposure conditions stainless steels will naturally self-passivate.

Passivation treatments are sometimes specified, to ensure the formation of this protective layer faster than normal 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.

Passivation will not remove weld-burn or Iron contamination

Pickling and removing iron contamination

Scale/Weldburn may need to be removed first by 'pickling" (or mechanical abrasion) and although the surface of freshly pickled stainless steel will normally naturally start to passivate it requires time to form completely, 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. Pickling can also be carried out with an electro-chemical cleaning machine.

Iron contamination is removed using Phosphoric Acid based solutions (95E)

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 treatment is normally carried out at higher temperatures than Nitric acid to achieve similar results.

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.

Citric Acid is normally preferred in food, pharmaceutical and medical applications.

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

Note: - HNO₃ - nitric acid, Na₂Cr₂O₇.2H₂O- Sodium Dichromate*, CuSO₄.5H₂O - Copper Sulphate

*Sodium Dichromate use is banned in EU under REACH regulations

ASTM A967 passivation treatments

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.

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.

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 ImmersionTest

Practice B - High HumidityTest

Practice C - Salt Spray Test

Practice D - Copper SulphateTest

Practice E - PotassiumFerricyanide-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.

Specific treatments are however also specified.

BS EN 2516 passivation treatments

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

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.

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