Corrosion of Chrome Plated Stainless Steel Part

[matt425]

I have a part that is still in development. It is made of a martensitic stainless. It is chrome plated. Some moderate percentage of these parts fails in the field. It is a type of M5 fastener. The parts that failed in the field showed visible corrosion on maybe 20% of the fracture surface, but none externally.

These parts fail salt spray testing at <96 hours. "Blushing" appears. We weren't overly concerned since the substrate is stainless so we didn't expect corrosion in actual use.

We sent the part out for analysis and one suggestion was that the chrome plating was causing galvanic corrosion in the part. The pinholes were providing an electrolytic path to the stainless steel substrate.

The service conditions would be outdoor winter use but stored inside. Some parts will be occasionally exposed to salty water from parking lots and roads.

I've asked the supplier for unplated, passivated parts to test.

Any suggestions or thoughts?

Thanks,
Matt

 

[TVP]

Most martensitic stainless steels are not very corrosion resistant in chloride-containing environments. Your salt spray testing confirms this. Chromium plating is not a good choice for resisting aqueous corrosion due to the defects inherent in the deposited coating, especially if it is of the "decorative" variety. The corrosion process at work is crevice corrosion, not galvanic corrosion.

If I were working on this type of project, I would investigate either an appropriate high strength stainless steel grade with good corrosion resistance (duplex or super-austenitic, possibly PH but not martensitic) or an alloy steel with a high performance coating (microlayer type such as Magni 565, Doerken Delta-Protekt, etc.). The following are some good sites for additional information:

http://www.fera.org.uk/pdf/Fera seminar 2 Feb 06 - Soren Andersson - Bulten.pdf

http://www.bufab-stainless.se/eng/index.htm

http://www.themagnigroup.com/images/PDFsheets/Magni565.pdf

http://www.doerkenusa.com/index1.php?id=100

 

[matt425]

Thanks for the suggestions, did I mention that the part is very cost sensitive? It is also difficult to forge.

And, its made in China. I did notice that one of the coating suppliers has a licensee in China. I'll give them a shout.

We are currently getting by with a zinc-plated medium carbon steel and we don't need a lot of improvement, but we do need some.

 

[TVP]

Definitely look into carbon or alloy steel with one of the microlayer coatings. The corrosion performance is substantially better than zinc + chromate conversion coating.