[arucano]

The hat is an aluminum alloy and the "whitish" deposit is the result of corrosion. You will need to apply some sort of barrier coating to prevent that corrosion ( or move to somewhere that they do not have to salt the roads ) Here in Texas we are fortunate that we do not have to deal with that. BTW, cleaning it just accelerates the corrosion unless you protect the freshly exposed aluminum alloy.