Just make sure that the screw, washer and nut are of the same material. I find that stainless steel (as Qyota just said) has better corrosion properties than the usual zinc plated stuff. Since you are going to paint the hole to protect it, you will be fine as far as galvanic corrosion goes.
i thought zinc plated stuff, which is still better than black oxy, is more like a Sacrificial rod and once it starts to corrode it goes fast. (of course all of the zinc has to be eaten away first)
"A common example of galvanic corrosion is the rusting of corrugated iron sheet, which becomes widespread when the protective zinc coating is broken and the underlying steel is attacked. The zinc is attacked preferentially because it is less noble, but when consumed, rusting will occur in earnest. With a tin can, the opposite is true because the tin is more noble than the underlying steel, so when the coating is broken, the steel is attacked preferentially."
http://en.wikipedia.org/wiki/Sacrificial_anode
here is a list in order of least to most corrosive of materials
http://en.wikipedia.org/wiki/Galvanic_series
"Zinc plating
Main article: Galvanization
Zinc coatings prevent oxidation of the protected metal by forming a barrier and by acting as a sacrificial anode if this barrier is damaged. Zinc oxide is a fine white dust that (in contrast to iron oxide) does not cause a breakdown of the substrate's surface integrity as it is formed. Indeed the zinc oxide, if undisturbed, can act as a barrier to further oxidation, in a way similar to the protection afforded to aluminum and stainless steels by their oxide layers. The majority of hardware parts are zinc plated, rather than cadmium plated.[2]"
and as stated above it is a fact that SS is a better material against corrosion than zinc,brass,black oxcy, anodized ect ect.
personally i'd go with a mil-2531 grade bolt
cad plated
http://www.google.com/products?hl=e...admium plated bolts&um=1&ie=UTF-8&sa=N&tab=wf
Cadmium plating
Cadmium plating is under scrutiny because of the environmental toxicity of the cadmium metal. However, cadmium plating is still widely used in some applications such as aerospace fasteners and it remains in military and aviation specs.[3] Cadmium plating (or "cad plating") has offers a long list of technical advantages such as excellent corrosion resistance even at relatively low thickness and in salt atmospheres, softness and malleability, freedom from sticky and/or bulky corrosion products, galvanic compatibility with aluminum, freedom from stick-slip thus allowing reliable torqueing of plated threads, can be dyed to many colors and clear, has good lubricity and solderability, and works well either as a final finish or as a paint base. [2][4]
