Oxidation? Stainless should not rust.
Have you ever seen your stainless steel fork and spoon rusting? 
Eh, actually got. Stainless steel is not rust proof only rust resistant. Technical name for SS is CRES, Corrosion Resistant Steel.
SS is especially prone to corrosion if left in contact with water. Unlike normal steel, it does not react with water in the air but if continuous left in contact with water, it will corrode.
That's why there is some truth in drying your utensils. It keeps them rust free. Maintains hardness of SS knives as well.
Salt is another culprit. SS corrodes rapidly in salty environment. I stay in Pasir Ris. My leatherman rusts, my WMF knives rust, that's why have to make sure they are all dried before putting away.
So in short, I am not surprised the filter rusts.