This is one of the more common questions that we are asked. Customers are primarily concerned with adhesion and surface resistance. Does the surface resistance increase with increased tarnish levels? Will epoxy adhere to a tarnished copper surface? To try and provide an answer, we undertook some elementary evaluations of copper samples with varying levels of tarnish identified using the L*a*b* color space coordinates. For those unfamiliar with the L*a*b* system, the short answer is that every color can be defined in terms of 3 numbers. The first number, “L”, is for lightness which ranges from 0-100, with 0 being black and 100 being white. Everything else is shades of gray. The second number, “a”, is either positive (Red) or negative (green). The third number, “b”, is either positive (yellow) or negative (blue). That is it in a nutshell. All shades of copper and its tarnish can be defined in this way.
To achieve the various tarnish levels was a simple matter of heating the samples of solid copper strip in an oven with normal atmosphere. This gave us a variety of colors from dark copper to a brilliant red and some “bluish” tints.
The adhesion test was done with a MAS epoxy using their SLOW hardener. This is an excellent epoxy with a very low level of VOC’s that makes it an attractive product in these days of work environment regulations. It also adheres well to a variety of substrates. The test was done with a crosshatch carbide tool with a pattern of etched lines intersecting at approximately 40 degrees. There was only the most minor of non-adhesion at a couple of intersections on the plain untarnished copper which is pretty good performance. All of the samples with tarnish showed no signs of failure at all. Conclusion: The tarnish did not adversely affect the adhesion of the epoxy to the substrate copper.
While this was somewhat a surprise, that material that was tarnished would show improved adhesion, it was not entirely unexpected. I suspect that the tarnish created a slightly rougher surface for the epoxy with the addition of the oxygen molecules.
A similar test was done using copper mesh and evaluating the surface resistance of clean and various levels of tarnished mesh. The samples were all tested in the clean state and then subsequently exposed to ever increasing temperature levels with normal atmosphere. The results of this test were most unexpected. The general trend was that samples with increased tarnish demonstrated reduced surface resistance. The change was minimal and not statistically significant, but there was a pretty consistent trend. The conclusion: levels of tarnish appear to have little or no effect on surface resistance, and may result in some improvement. That last point, improvement, may be taken with a grain of skepticism. It was certainly not any worse.
NASA Corrosion Control and Treatment Manual TM-584C (Revision C) had indicated that a green patina would inhibit further corrosion, so the same would also apply to tarnish, one could deduce. Testing this theory proved inconclusive and will require a new approach. We were unable to produce a green corrosion on any of the samples evaluated regardless of level of tarnish.
Kirt Griffin, Quality Assurance Manager, Dexmet Corporation
Kirt can be reached at firstname.lastname@example.org