These circulated coins are by far the most corroded and damaged, and the hardest to properly clean.
The Steel Pennies shown below have been properly cleaned by PCCS.
In 1943, the US Mint produced these 1-cent pennies using steel in order to conserve copper that was needed for the war effort.
These coins are very susceptible to corrosion, mostly in the form of rust (due to the steel composition).
When in great condition or properly cleaned, these coins appear to be composed of a nickel alloy but are actually zinc-coated steel.
Your steel pennies should be stored separately from other coins in a dry non-humid location.