Hi Kathy,
Properly zinc plated and chromated part should last about 100 h to white
corrosion and 300-500 h to red rust (for iridescent chromate) in salt spray
chamber (5% NaCL mist at 120
°F). Properly chromated part should be resistant to
finger prints and no streaking should visible.
A few things to check:
1. Thickness of the zinc deposit
If the deposit is too thin and/or porous -- its corrosion resistance will
obviously be low. The chromate solution, which partially dissolves the zinc,
can even strip thin zinc down to the base metal.
2. Chromate
Iridescent chromate normally has fairly high corrosion resistance, while clear
or blue chromates are less corrosion and finger print resistant. The chromate
can be qualitatively checked with a drop of 5% lead acetate solution placed
directly on the coating. It should last at least 5 seconds without turning
black. The longer -- the better.
4. Clear seals applied over the chromate will further increase the resistance to
finger prints, but not necessarily corrosion resistance.
5. Zn alloys (Zn/Co, Zn/Ni and Zn/Fe) normally have much higher corrosion
resistance than pure zinc coatings. They will last at least 200+ h to white
corrosion in salt spray and up to 1000+ h to red rust.
Once the first traces of corrosion appear -- there is no proper way to save the
coating by tape lifting it or otherwise. It should be stripped, re-plated and
re-chromated.
Eric Yakobson
Alpha PC Fab
Kathy Stillings <[log in to unmask]> on 05/21/99 09:13:19 AM
To: [log in to unmask]
cc: (bcc: Eric Yakobson/AlphaPCFabUS/Cookson)
Subject: [TN] HELP - NEED INFO ON PLATING ISSUE
Hi Everyone,
Was wondering of I could get some input from any Guru on plating issues.
Need info ASAP!!!!!!!!!!!!!!!!!!!! Here's the problem: have a zinc plated
chassis with a base of stainless steel and the handle is brass plated with
nickel. We have finger prints that can't be removed and a wash-like
streaking across the surface of the material. This stuff "oxidizes" just
sitting in a room and transportation overseas only accelerates the process.
Desiccant agents don't even work.
Testing found iron oxidation randomly spaced along some finger print
patterns. Unsuccessful attempts were made to lift the deposits using tape.
Samples were then cut for testing evaluation. SEM/EDS analysis showed
trace elements of zinc, low levels of chrome, oxygen, aluminum, calcuim
chloride, phosphorus, carbon, chlorine, iron, potassium, silicon, and
sulfur. Higher levels of "corrosive" chlorine were found in fingerprint
areas with small definitive locations of very high calcium. Initiation of
the corrosion has been determined to be at the plating.
Is it possible the plating vendor is using a washing agent after plating
that contains the corrosives mentioned above and over time these elements
are leaching out and corroding the plating? Can someone shed some light in
this issue? We've been using the plating for 6 years and this is a new
occurrence.
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