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:

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.
Water rinse
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
Water rinse
Dry
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

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)
BS EN 2516 - Passivation of Corrosion Resisting Steels and Decontamination of Nickel Base Alloys

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ASTM A380 nitric acid based passivation treatments

Steel Types	Condition	Treatment
Steel Types	Condition	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
		F	20-40	30-60
	annealed, work hardened or thermally hardened with machined or polished surfaces	G	50-70	10-30
		G	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
		F	20-40	60
	annealed, or thermally hardened with machined or polished surfaces	G	50-55	15-30
		G	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
Solution Composition (wt %)	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
Process Class	HNO₃	Na₂Cr₂O₇.2H₂O	Temp. Range C	Time mins.	Anodic voltage V
C1	200-250	20-30	50-55	20-40	without
C1	200-250	20-30	20-30	30-60	without
C2	200-500	-	20-30	30-60	without
C3	200-250	20-30	50-55	2-3	3-5
C3	-	40-60	60-70	30-40	without
C4	200-250	20-30	50-55	20-40	without
C4	-	40-60	60-70	30-40	without

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