I thought my 7800 series Dura Ace cassette cogs were made of titanium but, it turns out they are plated. When I bought my wife’s and my bicycles in 2004, I also purchased 12-21, 12-23 and 12-25 cassettes for each of them. Neither of us have ridden for the last ten years however I have gotten back on. When I went to dig out my 12-21 cassette, I discovered it had corrosion due to the drawer getting water in it. In cleaning it the plating flaked off in the corroded area. This confuses me because I thought titanium was pretty much corrosion proof. Under the flaked-off plating the base metal was badly pitted. Can anyone shed some light on this?

Which cogs were plated? Usually, only the largest cogs would be made of titanium to save weight. Smaller cogs wear faster and should not be made of a lighter metal that would wear faster still without saving significant weight

Any metal will corrode under certain conditions. Titanium will corrode rather quickly in the presence of cadmium and moisture for example. Cassettes should be stored individually in sealed bag with light oil film the cassette. Triflow spray https://www.amazon.com/Tri-Flow-Lube.../dp/B077GWZBPD is one choice, there are others.

On the cassettes that start with a 12t cog, the 4 largest cogs are Ti. The rest are plated steel.

done a magnet test? if anything its mixed, steel for the smaller , only Ti for the bigger ones because its too soft for the higher gears. so would wear out too fast ..
Ti for bigger just to save weight and being bigger chain engages a lot more teeth..

The 17 tooth cog is steel. Thanks.