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Skin Fittings & Seacocks
Despite the findings from the UK Marine Accident Investigation Branch and ISO standards, brass skin fittings still form the vast majority on offer from many marine stores. Quoting from the Random Harvest Report No 28/2000..." Brass is not accepted for use in underwater through-hull fittings"....."because of its susceptibility to dezincification in seawater and the ready availability of superior alternatives". (See the full report here) Brass is commonly made of 60-70% copper and 30-40% Zinc, and we all know what happens to our Zinc anodes. Dezincification is the process where the zinc content of brass fittings slowly dissolves leaving a porous and fragile shell.
The solution is to use materials made for the job.
Bronze fittings are without doubt the very best you can buy, quality products could easily last the life of the boat, however they are not cheap, especially for valves, but taking into consideration the costs of lifting the boat, boat yard costs as well as the work involved in changing the fittings, then the difference overall might not be significant.
DZR Brass (Dezincification Resistant). Accepted for use in seawater, it is a low zinc content alloy which is heat treated after production. The resulting alloy is resistant to dezincification. It would be a good choice for those of us on a tighter budget.
Stainless Steel does not suffer from dezincification as it has no zinc in its composition however it is not a good choice for skin fittings and seacocks below the water line. Stainless requires oxygen to maintain its stainless properties. Areas starved from oxygen can corrode very quickly due to crevice corrosion.