Dissimilar metal corrosion on highway signs.

[corrosionman]

We are refurbishing (in UK ) old highway signs - - 40 years old Cast iron arms with collars supported them from Mild steel columns. Generally at the interface the MS pole is seriously
corroded due to rain getting between the dissimilar metals. There is a very small differential between these two metals.

What staggers me is that in the UK the present motorway construction is for galvanised MS columns with the message board affixed by numerous Stainless steel bands.
Stainless and Galvanised metals are hugely different .
With the increase of acid rain I would imagine serious problems in near future.

Please has anyone any comments.?

We also find that generally, the 40 years old MS poles are still 100% original thickness where they have been buried, but then at ground level they instantly are corroded , sometimes zero metal left.

I have been told it is a well know phenomena. Please have you any comments ?

Thanks in advance.
ground level.