Engine Bolts

[RubyRod]

So I had to replace the Water Pump on my car and I got a shock when I discovered that the housing mounting bolts are high carbon steel. After 20 years I would have expected steel bolts to have fused solid to the aluminum engine parts but not so. The bolts can't be bare steel or they would have fused to the aluminum. They are not stainless either. They must be coated with something to prevent corrosion from fusing them to the aluminum. Does anyone know what the engine bolts are coated with? I'm sure at some point I'll have to replace one of the steel bolts somewhere on the engine and I'd like to know what type of coating I'll need to prevent corrosion.

Thanks