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ASTM A967 Chemical Passivation Treatment for Stainless Steel

This covers several different types of chemical passivation treatment for stainless steel parts. The treatments are the following: immersion treatment using nitric acid solutions, immersion treatment using citric acid solution, and electrochemical treatment. Immediately after the removal from the passivating solution, the parts shall be thoroughly rinsed, using stagnant, countercurrent, or spray washes, singly or in combination, with or without a separate chemical treatment for neutralization of the passivation media.

The chemical reactions of the passivating media on the surface of the stainless steel shall be stopped by rinsing of the stainless steel part, with or without a separate neutralization treatment. A chemical treatment shall be applied which will accelerate the formation of the passive film on a chemically clean stainless steel surface. The passivated parts shall exhibit a chemically clean surface and shall, on visual inspection, show no etching, pitting, or frosting. The following tests shall be performed on each lot of stainless steel parts: water immersion test, high humidity test, salt spray test, copper sulfate test, and potassium ferricyanide-nitric acid test. A free iron test shall be used for the detection of free iron on the surface of stainless steel.

This covers several different types of chemical passivation heat treatment for stainless steel parts. It include recommendations and precautions for descaling, cleaning, and passivation of stainless steel parts. It includes several alternative tests, with acceptance criteria, for confirmation of effectiveness of such treatments for stainless steel parts.

Practices for the mechanical and chemical treatments of stainless steel surfaces are discussed more thoroughly in Practice ASTM A380.

Several alternative chemical treatments are defined for passivation of stainless steel parts. Appendix X1 gives some nonmandatory information and provides some general guidelines regarding the selection of passivation treatment appropriate to particular grades of stainless steel. It makes no recommendations regarding the suitability of any grade, treatment, or acceptance criteria for any particular application or class of applications.

The tests in this specification are intended to confirm the effectiveness of passivation, particularly with regard to the removal of free iron and other exogenous matter. These tests include the following practices:

Practice A—Water Immersion Test,

Practice B—High Humidity Test,

Practice C—Salt Spray Test,

Practice D—Copper Sulfate Test,

Practice E—Potassium Ferricyanide–Nitric Acid Test, and

Practice F—Free Iron Test.

The values stated in inch-pound units are to be regarded as the standard. The SI units given in parentheses are for information only.

The following precautionary caveat pertains only to the test method portions, Sections 14 through 18 of this specification: This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

2. Referenced Documents

ASTM Standards

ASTM A380 Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment, and Systems
B117 Practice for Operating Salt Spray (Fog) Apparatus
B254 Practice for Preparation of and Electroplating on Stainless Steel

Federal Specification

QQ-P-35C Passivation Treatments for Corrosion-Resistant Steels

cleaning of stainless steel; descaling; passivation; tests for cleanliness of stainless steels; Sulfur; Autopassivation; Chemical passivation treatment; Contamination--coatings; Descaling; Passivation treatments; Stainless steel--specifications; Sulfide removal; ICS Number Code 77.140.20 (Steels of high quality)

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 steel, such as 440C, are not suitable for acid passivation as they can be attacked or be subject to hydrogen embrittlement.

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

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