Salt corrosion of welded 316L & 430

[dgallup]

I have a multi-component assembly of deep drawn and machined stainless steel parts that are YAG laser welded & finally partially over molded with nylon 612. I'm getting red rust staining from 2 of the welds that are encapsulated in the nylon over mold after 96 hours of salt spray. One of the welds is deep drawn 316L to 316L, the other is deep drawn 316L to deep drawn 430 with the carbon and manganese restricted.

There are 3 other welds between deep drawn 316L to machined 440C, stamped 304L to machined 440C and deep drawn 430 to machined 430F that don't rust. All of these 3 welds are outside the nylon over mold. All 5 welds are produced with the same YAG laser but with different weld schedules.

I expected more trouble from the 440C welds, not the 316L. Could it be caused by the nylon over molding?

I found this link but it doesn't really cover my problem:

http://www.eng-tips.com/viewthread.cfm?qid=252253&page=1

 

[stanier]

Have you pickled and passivated the welds prior to applying the nylon? If not the protective chromium oxide layer may not have reformed.

 

[dgallup]

No, kind of a sensitive assembly, don't want to expose the guts to acid. It could be done though with proper sealing.

 

[CoryPad]

Autogenous? Filler? If so, which alloy?

This is a head scratcher. Maybe the coating is porous and allows salt to concentrate underneath it? Maybe the nylon is absorbing salt water (usually a problem with nylon 6 and 6,6 but usually not with 6,12)? Maybe there is a corrosive component in the coating (plasticizer, heat stabilizer, etc.).

 

[matkenmd]

Nylon can be permeable does absorbs a lot of water compared to other materials. A lot more so than urethane or UHMW for instance. Most overmolding I've seen for subsea is done with urethane or gre.

 

[dgallup]

Autogenous. Is filler ever used with laser welding? I've not seen it done. Over moldings never seal water tight, there is always an infinitesimal gap that will wick in water & salt. But we have similar, older, larger products made with "less corrosion resistant" materials that do not do this.

 

[CoryPad]

Yes, laser welding is done with filler.

I would look at possible corrosive components within the nylon. Of course, full pickling and passivation is excellent advice and best practice.