Passivation

Passivation is a critical step in maximizing the essential corrosion resistance of parts and components machined from stainless steels. Passivation is not a scale removal treatment, nor is it like a coat of paint. Passivation provides a protective oxide film on the surface of the stainless steel. This invisible film is considered to be extremely thin, less than 0.0000001 inch thick, which is about 1/100,000 the thickness of a human hair.

During the machining process, a microscopic amount of free iron may be worn off the cutting tool and transferred to the surface of the stainless steel part. This may cause a thin coating of rust to appear on the part. This is actually corrosion of the steel from the tool and not the corrosion from the substrate. The embedded particle of steel from the cutting tool or its corrosion products may cause an attack of the part itself.

Similarly, small particles of iron-containing shop dirt may adhere to the part surface. Although the metal may appear shiny in the as-machined condition, the invisible particles of free iron can lead to rusting on the surface after exposure to air.

Exposed sulfides also can be a problem. They come from the addition of sulfur to stainless steels to improve machinability. Sulfides improve the alloy’s ability to form chips that break away cleanly from the cutting tool during the machining process. Unless the part is properly passivated, sulfides can act as initiation sites for corrosion on the surface of the fabricated product.

In all cases, passivation is used to maximize the natural corrosion resistance of the stainless steel. It can remove surface contamination, such as particles of iron-containing shop dirt and iron particles from cutting tools that can form rust or act as initiation sites for corrosion. Passivation also can remove sulfides exposed on the surface of free-machining stainless alloys.

Types of Passivation:

After thorough cleaning, the stainless steel part is ready for immersion in a passivating acid bath. Any one of three approaches can be used—nitric acid passivation, nitric acid with sodium dichromate passivation and citric acid passivation. Which approach to use depends on the grade of stainless steel and prescribed acceptance criteria.

More resistant chromium-nickel grades can be passivated in a 20 percent-by-volume nitric acid bath. Less resistant stainless grades can be passivated by adding sodium dichromate to the nitric acid bath to make the solution more oxidizing and capable of forming a passive film on the surface. Another option, used in place of nitric acid plus sodium dichromate, is to increase the concentration of nitric acid to 50 percent-by-volume. The sodium dichromate addition and the higher nitric acid concentration both reduce the chance of undesirable flash attack.

The procedure for passivating free-machining stainless steels is somewhat different from that used with the non-free-machining stainless grades. That is because the sulfides of sulfur-containing free-machining grades are partially or totally removed during passivation in a typical nitric acid bath, creating microscopic voids in the surface of the machined part. Even normally efficient water rinses can leave residual acid trapped in these voids after passivation. This acid can then attack the surface of the part unless it is neutralized or removed.

Citric acid passivation has become increasingly popular with manufacturers who want to avoid the use of mineral acids or solutions containing sodium dichromate, along with the disposal problems and greater safety concerns associated with their use. Citric acid is considered environmentally friendly in every respect.

Passivation treatment in citric acid baths has been found useful for a large number of stainless steel families, including several individual stainless grades.

The ultimate choice of passivation method will depend on the acceptance criteria of the end user.

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)

ASTM A380 nitric acid based passivation treatments

ASTM A380 nitric acid solutions

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.