Stained Steel

This week I was taking a frying pan out of our dish rack, and it caught the sun, showing a surprising array of colors:

I wondered where these might be coming from, so I started reading about the makeup of stainless steel. I knew it was an alloy of iron and another metal – I thought maybe aluminum or tin, but it turns out it's chromium. The chromium reacts with oxygen from the air to form chromium (III) oxide, which protects the iron from forming iron oxide, or rust. If this layer gets damaged, a new one quickly forms. Heat makes the reactions involved happen more easily, so I wondered whether the color differences were due to greater heat in the center of the pan creating a buildup of the chromium.

I was able to find a paper analyzing the compositions that lead to different colors:

Figure 8

Using macOS's color meter, I was able to read the RGB values of the samples given. I plotted those to see if there were any obvious trends:

The blue seemed relatively linear, so I decided to use that as a link between my photo of the pan, and the composition data. I took the average of concentric circles outward from the center of the pan:

There does appear to be a downward trend in the chromium and oxygen fractions, which would be consistent with the heating idea, but there's an awful lot of noise, and the upturn at the edge. Part of the problem may be that lighting can have a big effect on color perception (I'm reminded of the dress from several years ago). The colors on our pan don't seem to match well to the scale used in the paper. In any case though, it was interesting to learn more about stainless steel, and know the colors are a sign it's working, not degrading!