Stainless steel is not completely stainless in sea water. Protection comes from applying a passive oxide layer, especially at some places such as the bottom of the chain locker, where sea water can cause more corrosion.
Though, the best way to protect stainless steel equipment is to wash it with fresh water after each use.
If you cannot do that, and see dirty spots on your chain, you can easily clean it as described below;
The chain should now be clean.
You can use this solution to clean other stainless steel equipment, as well. To do that, you can apply the solution on to the rusted spots of your equipment and wait for around 10minutes, and then rub off by clean dust cloth. You can repeat this process if needed.
Oxalic acid is a weak acid. However, you should wash off immediately once it’s made contact with skin. Its appearance is alike with sugar and salt. You should pay attention not to ingest it by mistake.
Note: You can purchase oxalic acid from pharmaceutical warehouses.