Dissimilar metal corrosion

The corroded areas are due to the zinc being more noble than the stainless and trying to protect it, by sacrificing itself. You will notice the white area adjacent to the rusted area that indicates the zinc is corroding. This example was taken from a seaside area and so would have occurred within a year of being installed. You need to correct this by paint repair and by insulating the two metals by using a plastic washer. To prevent it occurring it is best to insert an insulation layer between the two metals.  Other common metals are aluminium and copper.

white stainless rust corrosion

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