Letter 37022

Red Cherry colored spots after cleaning and buffing of copper [Korea] 

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We are making copper fittings, are experiencing two major problems with changed color.1. red cherry color spots on surface 4-5 days after cleaning with sulfate solution, and rinsing with water, and drying under hot temp.
2. another problem is when the fittings are packaged in a box, it seems that the surface touching the box are being noticeably blackened after one month.
Please help on this.

Best regards

Chang S. lee
copper fitting - Seoul, Korea

The easy part is the box. Many cardboards contain a rather high amount of sulfur. The black is copper sulfide. The optimum answer is to wrap the parts in a fine sulfur free paper and then box. You might get by by putting lots of parts in a polyethylene bag and put that in the box. I suspect that the sharp edges of the parts will cut thru the plastic and touch the box, so you will possibly be back to where you started, only with less black. You might be able to get by with lining all 6 sides of the box with 1/4 inch styrofoam and then insert the plastic bag.

The sulfuric acid cleaning is leaving the part with different levels of activation, so it is oxidizing at different rates. The old school was to use a bright dip or passivation with a solution containing nitric acid and a dichromate. Since hexavalent chrome is no longer in fashion, check with several metal finishing vendors in your country to see what they have to offer as a replacement or a follow on finish for the copper parts. Whatever it is, you can not affect the solderability of the parts, so I would run some time trials in the lab before I set up the process.

James Watts
- FL

1. You are leaving some acid on the surface, rinsing is not complete, you need to neutralize, but any clean copper surface is going to tarnish in a few days unless protected with and organic or weak chrome wash. Try dipping in 0.2 oz/gal Chromic Acid or Sodium Dichromate

2. Sulfur in the paper packing of the box is reacting with the residual sulfuric acid on the surface and causing the black oxide of copper to form.

Robert H Probert Robert H Probert Technical Services Garner, NC, USA Editor's note:        Mr. Probert is the    author of Aluminum How-To

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