Passivation of stainless steels

Introduction

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.

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 steels 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 / hydrofluoric acid 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 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

Steel Types Condition Treatment
- - Code Temp. (C) Time (mins)
300 and 400 'series' and precipitation hardening types with 16% or more chromium annealed, work hardened or thermally hardened with dull, non-reflective surfaces F 50-70 10-30
20-40 30-60
annealed, work hardened or thermally hardened with machined or polished surfaces G 50-70 10-30
20-40 30-60
400 'series' and precipitation hardening types with 16% or less chromium annealed, or thermally hardened with dull, non-reflective surfaces F 40-55 20-30
20-40 60
annealed, or thermally hardened with machined or polished surfaces G 50-55 15-30
20-40 30-60
300 and 400 'series' free-machining types annealed, or thermally hardened with machined or polished surfaces G 20-50 25-40
K 50-60 10
L 50-60 10

ASTM A380 nitric acid solutions

Code Solution Composition (volume %)
F HNO3 20-50%
G HNO3 20-40%, Na2Cr2O7.2H2O, 2-6 wt %
K HNO3 1-2%, Na2Cr2O7.2H2O, 1-5 wt %
L HNO3 12%, CuSO4.5H2O, 4 wt %

Note: -

HNO3 - nitric acid

Na2Cr2O7.2H2O - sodium dichromate

CuSO4.5H2O - copper sulphate

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 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.
Specific treatments are however also specified.

Solution Composition (wt %) Treatment
Code Temp. (C) Time (mins)
4-10% citric acid 1 60-71 4 min
2 49-60 10 min
3 21-49 20 min

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

Process Class Stainless Steel Types
C1 or C2 Austenitics, austenitic precipitation hardening and duplex
C3 High chromium martensitics
C4 Ferritics, martensitics and martensitic precipitation hardening

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.

Process Class Solution Concentration g/l Temp. Range C Time mins. Anodic voltage V
HNO3 Na2Cr2O7.2H2O
C1 200-250 20-30 50-55 20-40 without
20-30 30-60  
C2 200-500 - 20-30 30-60 without
C3 200-250 20-30 50-55 2-3 3-5
- 40-60 60-70 30-40 without
C4 200-250 20-30 50-55 20-40 without
- 40-60 60-70 30-40 without

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