18-8 stainless steel carriage bolts have good chemical resistance and may be mildly magnetic. 

316 stainless steel carriage bolts are more corrosion resistant than 18-8 stainless steel carriage bolts, these bolts have excellent resistance to chemicals and salt water.

They're a good choice for ACQ-treated (arsenic-free) lumber. A square neck keeps them from turning when tightening a nut. A smooth, rounded head provides a finished look.

DACROMET® is an aqueous solution comprising zinc in flake phase, aluminium metal powder and inorganic compound. It is normally treated by dip and spin coating of this aqueous solution followed by heat-treatment at a relatively high, that is, about 300°C temperature. As a result, 7-8 μm thick coating is formed by drying and curing during the hot air drying process. Metal zinc and aluminium, which constitute the coated film, can protect steel via self-sacrificing, namely, by inhibiting the corrosion of a given steel due to the electric activity of highly ionisable metal zinc. Here, the chromic acid in a polymerized state can control and inhibit the self-sacrificing of metal zinc thus serving the role of a binder while also establishing the passivation of coating, which then leads to inhibition of chemical reaction between zinc and iron and accordingly reduces the speed of corrosive progress, thus exhibiting high corrosion resistance. In addition, the zinc, aluminium flake and metal oxide, which are of multi-layered structures, can protect the surface of iron alternatively thus functioning as a barrier against external corrosive factors thereby exhibiting superiority in corrosion resistance.