Magnesium parts ruined -- was it by deoxidizing? If chemical etching 'deoxidizing'?
October 26, 2017
Q. I have just lost about $11K worth of parts because a local processor says deoxidizer attacked magnesium parts. Our PO specified "no chemical etching & no mechanical cleaning" on our precision motor end bells. All threaded holes on parts are now grossly oversized so that thread no-go gage spins in. Threads were gone in all tapped holes
after processing. Processor says we should have also specified "no deoxidizing" on PO. Process specified is MIL-DTL-5541F. All materials and prints were provided to processor and magnesium material was again noted in PO instruction.
Does processor have fault?
First time run on (11) part order. Is processor at fault? Should I send replacement order of parts to different processor? Knowledgeable people please comment. Thanks.
manufacturer - Mulvane, Kansas, USA
A. Sorry to hear about this Rodney. As we know, there's always plenty of blame to go around when things go wrong.
I think you might consider retaining a knowledgeable finishing consultant to come up with a full and complete description of the right way to process these parts before the next order and before apportioning blame. For example, MIL-DTL-5541F is for aluminum parts, not magnesium parts, which both the specifier and the processing shop should have known, so this should have been discussed and resolved before the first part was processed. And if this was the first run, sample parts should have been run instead of ruining the whole batch; you don't risk 11 expensive parts to untested theory, you test scrap that is generally similar, then you test one part.
Were the parts designed such that they could have been successfully processed? -- I don't know. Does the processing shop have any experience successfully processing magnesium parts? -- I don't know. Was it actually a 'de-oxidizing' step that ruined the parts? -- I don't know.
I'm an engineer rather than a lawyer, so to me it's rather a moot question of whether the parts should have been "de-oxidized" when the PO said they should not be "chemically etched". I think the processing should have been correctly specified, and the shop should have caught the fact that it wasn't; and samples should have been run. Yes, I think a different processor might have done a better job. Good luck.
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"
November 1, 2017
A. MIL-DTL-5541F is clearly intended only for aluminum and aluminum alloys. It says so in the title, paragraph 1.1 Scope, and paragraph 6.1 Intended use. An acid deoxidize pre-treatment would not be at all unusual for a chemical conversion coating on aluminum. However, this acid bath would aggressively attack magnesium. While verifying the correct material is the responsibility of the coater, I believe the specifier is equally responsible for laying this trap.
Luke Engineering & Mfg. Co. Inc.
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