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White corrosion on 4130 Steel, Zn-Ni plated w/ Clear Chromate

Q. Material: 4130 Steel
Application: Aerospace
Issue: Looking for replacement for Cadmium Plating
Problem: Plated 4130 parts with Zn-Ni plating IAW SAE AMS2417 w/ supplementary chromate (clear) treatment. After 96-hr Salt Spray testing IAW RTCA/DO-160E, Section 14, Cat T parts exhibited significant white corrosion.

49578 Zn-Ni 49578-cad
Zn-Ni after 96 hours on left, Cadmium after 96 hours on right

Plater indicated this is normal in that clear chromate only protects against red corrosion. Is this correct? Are there alternative secondary treatments that will protect against both white and red corrosion over 96-hrs of Salt Spray?

Charles Richards
Project Engineer - Cleveland, Ohio, USA
August 5, 2008

A. I would first confirm that the plater is using a chromate rated at 96 hrs to white corrosion. This is a first step. If he says no, then ask him to find another product or you need to find another plater.

Clear chromates are available that offer protection from white salt corrosion (trivalent chromates) varying from 4 to 96+. Generally the ones that are above 96 are not truly clear and may exhibit some iridescence. Since the parts appear to have been barrel plated, I will point out that the corrosion resistance of the same chromate will be less when used in barrel applications due to the rub off from barrel and dryer rotations as well as from further part handling.

Gene Packman
- Great Neck, New York

A. Dear Richards,
It is possible to get up to 200-250 hrs WR.Clear chromate indicates white Rust.But Zn-Ni gives up to 600-700Hrs red rest.In Aerospace and Defence, Zn-Ni is considered replacement for cadmium.
regards

T.K. Mohan
plating process supplier - Mumbai, India

A. Chromate function is to protect only white corrosion.Enhance of white corrosion possible by sealant on top of it. Red corrosion depends on the thickness of zinc and its alloys deposits.

P. Gurumoorthi
electroplating process chemicals - Chennai, Tamilnadu, India

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