Sometimes I should not clean
 

Saturday, I was trying to work outside but at times it was coming down so hard that I had to come inside to let the heaviest rain pass. I used that time to start sorting a bin of parts that I had been ignoring for quite some time.
Came across a crankset that I had saved for what I hoped was a good chainring. It was so grease caked that I took it off, threaded the chainwheel bolts back in so I wouldn’t lose them and put them filthy part in a pan filled with Zep to soak.
The pan was aluminum and had briefly held cooked shrimp but had been washed. I forgot about the chainring until the next day when I pulled it out and saw this..

https://cimg4.ibsrv.net/gimg/bikefor...9d9550a23.jpeg
Chrome plating had come off the chainring bolt and the chainring itself had become dark. Chrome loss was greatest on the bolt that was in contact with the tray.
So I won’t do that again but can anyone explain why this happened? Zep? Lemon front shrimp? Contact with aluminum tray? All of the above?

Next time thread the chainring bolts unscrewed with a plastic zip tie retaining them all.

my guess you made a weak battery and that set off the cascade. Seen worse with Simple Green.

I would go plastic tray, aluminum and steel parts segregated, a mixed metal component?
monitor closely, no extended soaking.

Zep is a cleaning supply house. Listing what you actually used can help (Heavy-duty degreaser, etc). The biggest risk I see here is an electrolytic solution that was able to pull the chromium ions from the base metal. IME, this is a greater risk with Iron chelating agents, since they can pull off nickel plate and zinc, but generally don't affect intact chrome. It's also possible that you managed to deposit a film of aluminum on the chrome, which will look like tarnish, but should buff away. Try ultra-fine steel wool or a metal polish and see what happens?

I'll bet cyccommute will be along shortly with his vast chemical knowledge to help.

Zep heavy duty citrus degreaser. A few warnings on label about use on stone and plastic but no cautions on metal

This has happened to me before when I left some alloy parts in a degreaser for too long. The degreaser will stain aluminum a dark grey colour. I use dawn soap now.

I've seen interesting reactions with CLR, a fairly mild cleaner for removing calcium, lime and rust. I accidentally got some on a stainless steel knife. It permanently frosted the metal along the edge where it contacted the CLR overnight.

For gunky bike stuff I usually spray on WD-40 and let it soak overnight, then clean with hot water and Dawn or similar soap. Seems okay.

I've also used various "green" degreasers, especially good cheap stuff from the dollar store called "LA's Totally Awesome Oven and Grill Cleaner." But for bikes I use it only during a complete wash down and follow up immediately with hot water and Dawn, then clean water. So far, so good, no change in appearance of aluminum rims, chainrings, bolts, chains, paint, etc. I wouldn't leave any bike stuff soaking overnight in the stuff though.