After some research, I was able to locate one reference that mentions the porosity of electroplated tin on steel. Under exposure to atmosphere, the steel becomes more negative in potential (anodic), thus causing rust to form in pores thru the tin plate (older cans exposed to humid air would develop rust). When the cans were sealed during food processing, the tin would become more negative in potential because of oxygen deficiency and prevent corrosion of the steel can. The steel can would behave as the cathode.
Depending on the quality level of the tin coating, I found a reference that provides corrosion rates of tin (this would be tin metal) in a marine environment. The corrosion rate over a 10 year exposure period was 9 mils/year (page 166, "Corrosion and Corrosion Control", by UHLIG). Keep in mind this is for tin metal and if a coating could be applied that is solid tin, you could probably use the corrosion rate listed above. Otherwise, a tin plate that is not properly applied or contains small pores will probably suffer the same fate as the tin plated can.
The key I believe is to have a dense layer of tin coating so that tin acts as the anode and corrodes versus the substrate material.
