On 20 Aug, 08:50, Huge wrote:

The first time you use them the zinc galvanising melts and runs off and within
a few months it rusts away to nothing.

The zinc doesn't melt, it evaporates (this is one reason why zinc
metal wasn't smelted in the West until the mid 18th century). If the
zinc merely melts, surface tension holds it in place and the coating
isn't destroyed. The problem arises if the steel gets too hot, hot
enough to boil the zinc.

So to keep a galvanised incinerator galvanized, you need to keep down
the peak temperature of the steelwork. Don't overheat it. Allow a
layer of wood ash (great insulator) to sit inside it (although clean
this out afterwards before leaving it to get damp). Don't overheat
the edges of sheets, or the edges of the vent holes.

For a "dustbin" incinerator I wouldn't pay too much heed to this. Just
keep it dry over winter and it will last OK. For a box incinerator
made of galv sheet, then I would try to arrange stainless steel (all
obviously made from scrap) heat shields on the inside of things,
around major hot spots like the flue outlet.