Fingerprint corrosion on and cleaning of zinc plated steel surfaces

Letter 18213

Fingerprint corrosion on and cleaning of zinc plated steel surfaces

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We design a large sheetmetal chassis for optical networking equipment. It is made from Type III SC2 zinc plated CRS. All external surfaces are specified cosmetic, so need to look good. To minimize fingerprints, our suppliers wear cotton gloves while assembling the chassis. However, a few fingerprints or handprints inevitably make their way onto the plated surfaces. I have heard that acids and sweat from handling can etch the plating and eventually show up as corrosion. Sure enough, after sealing the chassis in a plastic bag with a 1/2lb bag of desiccant and conducting environmental shipping testing (minus 40degC/72hrs:plus 70degC/72hrs; plus 40degC @90-95RH /72hrs) we witnessed handprints show up as dark brown corrosion or as white "powdery" residue). The result was pretty ugly.

Questions:

1) How would I determine if this is a pre or post-plating problem?
2) Would thicker plating help minimize/eliminate this problem, Type III SC4?
3) Also, what is recommended to clean fingerprints off of zinc plated surface? I can't find anything on the web and I have received 3 different answers. a) The plater recommends a 30% diluted solution of Windex. b) Sheetmetal vendor uses aerosol stainless steel polish/cleaner. c) Our contract manufacturer is used to using Isopropyl Alcohol [link is to product info at Amazon].
4) Finally, what else I am overlooking.

Thanks in advance for your wisdom.

Richard Huang
- San Jose, CA, USA

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I have worked extensively with the electronics and telecomunications chassis industry and have developed a sure fire 3 step program to solve your problems. 1. Wear clean cotton gloves when handling and assembling 2.Handle and assemble in an air conditioned environment and or provide sweatbands and towels for the handlers 3.Lower your expectations just a little. Problem solved!

Todd Osmolski
Charlotte Plating, Inc.

Charlotte, NC USA

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I'm not familiar with your spec's so this may already be covered by your question. What kind of surface treatment does your zinc plating get? I've worked with tin/zinc plating and it finger marks very easily. Do you have some kind of passivation in place? I would expect cleaning prior to plating to remove finger marks, and I can't imagine people handling objects between cleaning and plating, so I would suggest your blemishes occur post plating.

Ian Brooke
university - Glasgow, Scotland

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I agree with Todd. The handling and housekeeping at the plater and the assembler are critical. I would also like to point out that many trivalent chromates (blue bright) tend to be very soft when applied for maximum corrosion resistance and handling of any kind should be avoided for these and all other chromate films for a minimum of 24 hours to allow the gelatinous chromate film to reach full hardness. Also, remember the human factor. Gloves are fine but the workers need to change the gloves when they get damp or you will just leave smudge marks instead of distinct finger prints.

For Todd: I bet Dave remembers this from his years up North.

Gene Packman
- Syosset, NY

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As one who has some 30+ years of zinc plating on electronic cabinetry and some 15 years working on various spec writing committees including both ASTM and SAE, the problem looks all too familiar.

1) Go back to your plating house and insist they use a HEXAVALENT, clear chromate over the zinc plating. They've likely gone to a more environmentally friendly TRIVALENT or non-chrome conversion coating. These have far less protection.

2) Your burn-in temperature is beyond the upper limit for chromates. Around 140F ( 60C ) the hydrated gel-like chromate becomes dehydrated and the corrosion resistance of the chromated zinc drops off. All standard plating specs for chromates limit drying temperatures and final hot rinses to less than this.

3) Switch to a yellow chromate which offers more protection but probably won't meet aesthetic or electrical resistance requirements.

Lastly, and I can't believe Todd didn't suggest this, switch from zinc to electroless nickel. Nickel is FAR less prone to such staining and can more easily be cleaned, even with Windex!!

Milt Stevenson, Jr.
Anoplate Corp

Syracuse, NY, USA

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Now that Milt mentioned it electroless nickel does do very well and is used in that application. Especially when some of the panels are aluminum. The electroless nickel allows for RFI and conductivity that is not possible with anodize. The steel zinc plated panels of these cabinets have been switched to electroless nickel plating for cosmetic continuity.The electroless nickel plated panels are subject to the same pitfalls as zinc plating though.They will still stain , discolor, and fingerprint if mishandled.The EN plating can be restored by the customer if needed but the customer will likely be offended at this suggestion. High phos EN will somewhat alleviate the staining problem but the customer will still find fault with the plating.

Gene , Dave says hello.

Todd Osmolski
Charlotte Plating, Inc.

Charlotte, NC USA

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