Chemical Processing Method Applicability

Prior to machining, corrosion resistant (stainless) steels inherently have a thin protective oxide layer. Machining removes that protective layer and introduces metallic contaminants.

For machined stainless steel parts that undergo magnetic particle inspection (MPI), passivation will also remove MPI associated iron contaminants. Passivation of stainless steel parts is also commonly required following Fluorescent Penetrant Inspection (FPI) as a final clean.

Passivation is a chemical process to help restore the material surface to an optimal cleanliness. The resulting highly cleaned surface is then exposed to air to naturally reform the protective oxide layer. In the aerospace business, passivation is commonly required for the majority of machined stainless steel parts. Exceptions to that rule are parts that will be further processed via vapor blasting, acid etch, etc. that would again disturb the part’s protective layer.

Selecting the proper passivation treatment only removes iron contaminants and does not affect the base material. However, selecting the wrong passivation treatment can destroy the base material. See the list of acceptable materials below and Note 2 for MTC’s policy on this important issue.

There are a number of passivation methods, types, and testing as defined in industry standards.

MTC Passivation Laboratory Service

At Metals Testing Company, we have a complete in-house capability of equipment, chemicals, procedures, qualified personnel, waste treatment, environmental related permits, and industry certifications to accomplish high quality Anodic Etch processing. We also have the expertise to design associated fixtures, a critical element in the proper application of passivation processing.

Due to safety and environmental policies at MTC, we only provide AMS 2700 Passivation Method 1, Type 6 (Low Temperature Nitric Acid). We do not heat our acids or use chromates in our chemical processing and waste stream.

We also offer 2 of the 5 corrosion resistance test methods defined in AMS2700: Water Immersion, and Copper Sulfate Solution. Note 1: Water immersion is a 24 hour test.

The following are examples of Passivation related specifications that MTC is approved to perform:

  • AMS-QQ-P-35
  • AMS 2700
  • ASTM A-967

The following applicable materials trace back to the original QQ-P-035 recommendations for MTC’s Type VI passivation:

Austenitic Stainless Steels:

  • 304, 304L, 304H, XM-7, 304N
  • 305
  • 308
  • 309, 309S, 309Cb
  • 310, 310S
  • 311
  • 314
  • 315, 316, 316L, 316H
  • 321, 321H
  • 329
  • 347, 347H

Note 2: As the wrong passivation treatment can destroy parts, if a material is not on the above recommended list for Type VI processing, we require that samples be provided to confirm that the passivation treatment is acceptable.

Common types of parts that MTC passivates include bushings, sleeves, brackets, flanges, housings, fasteners, nuts, bolts, etc. – anything made out of 300 series stainless steel.

Copyright 2016 Delisle Inc. doing business as Metals Testing Company