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"Chrome" finish on aluminum casting
5

"Chrome" finish on aluminum casting

"Chrome" finish on aluminum casting

(OP)
I have an aluminum (300 series) casting with a bright chrome type finish.  It is mainly an aesthetic coating, although the corrosion protection it provides is a plus.  Analyzing the coating I've found it to have two layers: a copper layer and a chromium + nickel layer.  The two layers are equal thickness (~5 microns).

Is this normal?  I thought that the underlying layer would be much thinner than the top layer.  Also, why is the top layer made up of both nickel and chromium?  I thought that a single-element chemistry was standard industry practice.

RE: "Chrome" finish on aluminum casting

Chrome finishes for aesthetic/decorative purposes, instead of hard chromium plating for wear resistance, are always a multilayer coating, usually consisting of a first layer of copper, followed by some type of nickel and chromium layers.  The copper layer has good levelling capabilities, which will create a smooth surface, free of pits, etc.  The chromium layer is usually quite small compared to the nickel and copper layers.

RE: "Chrome" finish on aluminum casting

On aluminum, a zincate treatment would typically be applied before the copper layer, but this would be too thin to be seen , assuming you measured the plating thicknesses with a metallographic cross section.

RE: "Chrome" finish on aluminum casting

(OP)
Thank you for your replies.  Further, I found that the chromium layer is very, very thin (cannot be seen on the metallograph), probably 1 micron or less.  In fact, an EDS map of a cross-section did not pick up the chromium at first.  I had to increase my magnification quite a bit before I could detect it.  The layering is: substrate, copper, nickel, then chromium.  

RE: "Chrome" finish on aluminum casting

3
Re “ the chromium layer is very, very thin.”
Decorative chrome is only 0.08-0.8 microns thick. ASTM B456 requires only 0.13 microns for 'mild' conditions.  The white color, luster and corrosion resistance* comes mostly from the nickel.  
The plating is probably 5 layers:  Zincate, Cu strike (usually from Cu cyanide solution), ductile Cu (usually from acid Cu solution), Ni plating (probably bright), Cr (conventional bright).

Finding the thin zinc immersion coating can be very difficult as mentioned by swall (more so using EDS if the Al alloy contains any Zn) or impossible (if the strike coating solution dissolves it).  Copper is used for leveling, as mentioned by TVP, and especially for polishing prior to bright plating.  With regard to metallography, differentiating between the Cu strike coating and the ductile Cu plating is ~impossible.  

*In ASTM B456-03 Standard Specification for Electrodeposited Coatings of Copper Plus Nickel Plus Chromium and Nickel Plus Chromium, the corrosion resistance ('Service Condition') is based primarily on the nickel thickness (the types of the nickel & chromium deposits also matter). Duplex, even triple Ni coatings of slightly different compositions may be used when more thickness and corrosion resistance are required.  If something happens to the nickel, the copper can cause bad galvanic corrosion of the aluminum.  For better corrosion resistance, omit the copper:  use an alkaline electroless nickel strike, electroless nickel primary coat and chromium topcoat.

RE: "Chrome" finish on aluminum casting

An informative webpage on Decorative Copper-Nickel-Chromium is at http://www.techplate.com/chrome2.htm
Gives details on design, aplications, specification and testing the finish, etc.

It seems based upon an older version of ASTM B456, as it doesn't mention ASTM B253-87(2005)e1 Standard Guide for Preparation of Aluminum Alloys for Electroplating
http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/REDLINE_PAGES/B253.htm?L+mystore+cmiv4567+1141180664

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