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"Manganese Phosphate coating of 41XX Metals after Gas Nitriding"

A discussion started in 2016 but continuing through 2018

April 4, 2016

Q. Hello folks,

I have tried searching for some info on this but it has been fruitless.

We are working on a component made from 4140 120 KSI steel.
The design calls for gas nitriding, 0.007-0.0012" effective case depth, 58-65 HRc.

To add to that, standard practice here is to Manganese phosphate components for corrosion resistance (oil & gas industry)

The questions are:

1. Does a gas nitrided part even require any extra process to provide anti corrosion in similar levels to phosphating? Or is the nitriding itself enough?

2. If gas nitriding does nothing towards anti-corrosion, then would phosphating be an applicable process? I.e., can the phosphating process even work/react on a gas nitrided surface? Or doesn't the smoother/ higher lubricity nitrided surface lend itself to this?

3. If phosphate is not possible and gas nitriding is not enough in and of itself, then what would be proposed anti-corrosion coating (high durability and bonding similar to phosphating)?

Thank you for any replies or guidance.

Nick Tzallas
Design Engineer - Dubai, United Arab Emirates

June 19, 2018

A. Dear Nick,
Generally gas nitrided parts are not subjected to any further process such as manganese phosphating. The anti corrosion properties of gas nitriding is relatively unknown and also use of phosphating on nitriding surface is hardly ever used/ known.
If the requirement is for gas nitriding, you should strictly use gas nitriding and not phosphate. If your concern is more on anti corrosion, better option is QPQ, liquid nitriding, it will give you both surface hardness and excellent corrosion resistance. But the hardness depth is restricted to 0.003 - 0.006".

Udaya Kumar
- Dubai, UAE.

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