15-5 PH Heat Treated After Passivation - Corrosion Risk?

Question

15-5 PH Heat Treated After Passivation - Corrosion Risk?

pt87 (Aerospace)

(OP)

11 Mar 14 20:01

Hi All,

I have an aerospace product made from 15-5 PH Stainless Steel per AMS 5659, Precipitation Heat Treated per AMS-H-6875, Condition H1025. The part is then passivated per AMS 2700, Method I, Type 2 or 8, Class 4, tested with Water Immersion or High Humidity.

For a certain lot, we had to re-Heat Treat the parts because we had used a non-Customer approved processor for the Heat Treat. Thus, we re-Vacuum Heat Treated the parts after they had already undergone Passivation, and subsequently cleaned the surfaces of our parts with Cratex to remove the discoloration. No Re-Passivation after this process was performed.

My question:
Can this process remove the protective oxide layer on the part surface from Passivation? We always Passivate after Heat Treat during the fabrication of our parts, so this was somewhat unfamiliar territory for us.

Note: These parts will see humid and severe environments typical in aerospace, so I want to be sure we have not increased the risk of corrosion by doing this.

Thank you.

tbuelna (Aerospace) · Answer 1 · 2014-03-11 22:27:57

pt87,

AMS 2700 section 3.1.2 requires passivation to be accomplished after completion of all manufacturing operations including heat treatment.

Regards,
Terry

EdStainless (Materials) · Answer 2 · 2014-03-12 18:18:55

You reheated, so you must re-passivate.
Passivation on stainless steels is a cleaning operation.

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Ripz (Mechanical) · Answer 3 · 2014-03-14 10:34:00

Doesn't Stainless Steels self-passivate on exposure to air?

EdStainless (Materials) · Answer 4 · 2014-03-14 22:25:01

It will become passive, but it may not be clean.
Passivation of stainless steel is a cleaning operation.
If there is an impurity, say a small particle of imbedded iron then a continuous chrome oxide passive film cannot form.
The iron will begin to corrode, the corrosion products will then start to attack the base metal and you will have a corrosion pit.

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tbuelna (Aerospace) · Answer 5 · 2014-03-15 21:53:33

I agree with EdStainless' comment about the purpose of passivating stainless steel surfaces. But going back to the question asked in the OP, it doesn't really matter whether any of us think it is actually necessary, or even helpful, to perform a final passivation of the parts. If you are providing a CofC, FAI, etc. that shows the processing of the parts conforms to AMS 2700, then you must perform a final passivation process on the parts.

The modern aerospace industry requires strict compliance with processing of components and the QA activities that verify compliance. If your company is AS9100 certified, then there should be documentation of this batch of parts being rejected for a non-conforming heat treatment. There should also be documentation of how to disposition the discrepant parts and/or any corrective action required. With this kind of record keeping, eventually someone will figure out that the passivation requirements were not met. And even though skipping the additional passivation operation might seem like no big deal to most people, if your manufacturing records don't accurately reflect how the parts were processed, then it can jeopardize your AS9100 certification. And that would be a big problem.

In short, passivating parts is cheap and quick. It would be silly to risk the company's reputation by trying to save a buck or two skipping a passivation operation on a few parts.

Good luck to you.
Terry

Ripz (Mechanical) · Answer 6 · 2014-03-20 13:08:44

Pickling is your cleaning process. After this is done, Stainless Steels self-passivate on exposure to air.

EdStainless (Materials) · Answer 7 · 2014-03-20 21:12:25

No Ripz, picking is oxide removal.
In stainless steel vernacular passivation does not refer to the material forming a passive film, it refers to cleaning so that it can form an optimum passive film.

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tbuelna (Aerospace) · Answer 8 · 2014-03-20 22:01:24

Per section 1.1 of AMS 2700, "This specification covers the requirements for a process to assure removal of free iron or other less noble contaminants from the surface of corrosion resistant steel parts".

Per section 1.2 of AMS 2700, "The processes defined in this specification have been used to remove metallic contaminants from the surfaces of corrosion resistant steels using chemically oxidizing methods to prevent injury to the basis metals, but usage is not limited to such applications".

Hope that helps.

Ripz (Mechanical) · Answer 9 · 2014-03-25 09:07:52

EdStainless, I was always under the impression that Pickling was the cleaning process. And Passivation being either chemically induced or self passivation, as in the case of Austenetic Stainless Steels.

For my knowledge, What cleaning does passivation entail?

EdStainless (Materials) · Answer 10 · 2014-03-25 11:05:12

Ripz, that may be how it works sometimes, but in reality the pickling process will remove oxides and the Cr depleted layer of metal. If you leave it in too long it will simply keep removing metal. So it does clean, but the real purpose is to remove oxides since there is no other way to remove them.
As others have pointed out, the reason that stainless steels are stainless is that they naturally repassivate in air. If the surface has any contamination (iron being the most common) then you will get corrosion of the foreign material that often leads to corrosion of the SS. The worst cases are often architectural. They put up a beautiful SS facade, lifting it with steel clamps and chains. Then they wonder why they develop rust streaks within a few weeks.

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15-5 PH Heat Treated After Passivation - Corrosion Risk?