Best Passivation Method?

[dchu129]

Here I have at my work place the passivation process.

It would work for most of the time with new solution but only last for a month or so and parts would start failing and the phosphoric acid would need to be change.

For more complicated parts (bigger and more edges/holes made from NC), it would fail very often even with new solution.

Wondering what would be the problem.

Here is general process

Electro Polish with current through 5 mins
Rinse
Phopsphoric Acid 5 mins
Rinse
Type 2 Passivate 30 mins
Rinse
Dichromate Treatment 30 mins
Rinse
Hot Water Seal 1 mins
Spin Dry

 

[HEC]

Dchu129,
In the description of the process you do not mention any cleaning step. What cutting fluids are used and are they removed prior to the electropolish and passivation. Also what material are you working with.

Mark Hutton

 

[dchu129]

Working with stainless steel, mainly 17-7, 17-4, and 15-5

The only cleaning processes are Electro polish with current through part to remove excess oil; and soak in phosphoric acid to let it react and clean the rest of the part.

However problem we have is the process could be running well today and tomorrow parts will fail with same procedure. (FYI the chemical solution remain the same for both day with same % of acid).

 

[Compositepro]

Current does not pass through oil film very well. Parts should be caustic or detergent washed before they enter the tank. Where do you think the oils goes? And you wonder why the bath does not last long!

 

[dchu129]

The electro polish has a metal belt that gets the oil out of the tank and the phosphoric acid has a filter. Thanks for the input and I will try to clean it before it enters the tank

 

[EdStainless]

How reliable is your temperature control?
How soon after processing are you testing?
Give the parts overnight before you test.

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Plymouth Tube

 

[dchu129]

Lately, we changed replaced electro polish soap solution and passivation solution and the process run fine again.

However we are never sure what happen when things started failing again.

Generally we test right away in salt spray chamber.
What is the reason behind testing after a night?

Thanks