Anodizing Aluminum And Applying Black Chrome

I have been a bicycle enthusiast as far back as I can remember. My last century (of at least forty) was in 2011. I have always prefered a black finish to polished aluminum or chrome. In the eighties, black anodizing became popular on bike parts. I have black anodized parts that have done nothing but sit on a shelf for a few years that have faded to tan and where they have been handled the bare aluminum shows through. Anodizing is an excellent thermal conducttor - when it is applied thin. It expands more than aluminum so it cracks on parts that get hot (~300F). This is even worse if it is allowed to absorb water. So much for washing a hot engine. I learned that a little bit is a good thing from someone who used the "Report Errors" button. He sent me a technical article out of a respected journal and it lead to the correction about thin anodizing.

First, let's get one thing out of the way. Early on, I decided that I would have all metallic parts black (or hard) anodized. I foresaw my performance heads and cylinders having a "forever" finish. I shopped it around; did some research. I've come to find that the thick, glossy anodized layer acts as a thermal barrier and it does not conduct electricity. The engine heat spirals upward the oil circulating through non-anodized parts acts as the only source of cooling. Also, no sparks. I was pissed.

For a pleasant, inky black (or whatever color) the coating must be so thick that there are "border effects" -- essentially blocking the flow of heat from the base material into the ambient. Anodizing increases the oxide layer of the base material. When it occurs on a low-grade iron, it is known as "rust." It is used to create the electrolytic layers in capacitors. It does not permit the flow of electricity.

When my business was flourishing, I decided that a good trade indicator would be a black finish on my cylinders and heads. I called a nearby, largish plater and spoke to the (presumably educated) tank boss. I was told that it was not a good idea as the thermal barrier for a shiny finish and the lack of conductance would create nothing but problems. It would have been a three thousand dollar run.

More to come . . .

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