Passivation ASTM A380, ASTM A967, QQ-P-35C, BS EN 2516
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Passivation ASTM A380, ASTM A967, QQ-P-35C, BS EN 2516

The process of passivation, is a post-fabrication acid treatment that removes "free iron" contamination left behind on the surface of the stainless steel wire from machining and fabricating. It is not a scale removal treatment, nor is it like a coat of paint.

Advantages of Passivation

  • Improved Corrosion Resistance
  • Uniform, Smooth Appearance and Finish
  • Deburring (Polished Surface)
  • Cleanliness
  • Improved and Extended Life of Product

When passivation is ordered, the following process is performed:

  1. Cleaning: a commercial degreaser may be used to clean grease, coolant or other debris from the wire surface to obtain the best possible corrosion resistance.
  2. Water rinse
  3. Passivating process: wire is ready for an immersion in a passivating acid bath. There are three treatments are available: nitric acid passivation, nitric acid with sodium dichromate passivation and citric acid passivation
  4. Water rinse
  5. Dry
  6. Testing: a copper sulfate test is performed to check if the passivation process was performed correctly. If so, there will be no iron contamination and corrosion resistance will be improved

At Optimum Spring, we offer on-site passivation process on all types of metals with a focus on stainless steel, Inconel, Hastelloy, and other nickel alloys.

Specifications for passivation treatments for stainless steels

  1. ASTM A380 - Practice for Cleaning, Descaling and Passivating of Stainless Steel Parts, Equipment and Systems
  2. ASTM A967 - Specification for Chemical Passivation Treatments for Stainless Steel Parts (based on US Defense Department standard QQ-P-35C)
  3. 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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