Climbing bolts were originally constructed of high carbon steel, a class of steel alloy that provided good tensile and shear strengths. However, carbon steel is very susceptible to the effects of oxidation, a process that can occur in a relatively short time when exposed to precipitation or humidity. To address this problem, engineers began designing bolts using other metals and alloys. Aluminum was found to be more corrosion resistant and was used in combination with carbon steel. However, aluminum is relatively weak and has a tendency to corrode when left in contact with steel.
In order to forestall the corrosion that can plague high carbon steel, many manufacturers use zinc plating, or galvanization. Zinc, or more specifically zinc carbonate, acts to coat and protect the surface of the steel bolt. By design, the zinc plating corrodes, acting as a sacrificial anode and protecting the underlying steel from the effects of oxidization. However, corrosion of the underlying steel is inevitable due to the effects of acid rain, salts, or minerals in seepage that affect the electrochemical balance between the zinc and steel. The rate at which the rust will appear and act to undermine the bolt will vary widely according to the environment. In general, zinc plated bolts are not acceptable for use in climbing, as they corrode even in arid environments.
Stainless steel was introduced as a strong metal alloy much more capable of resisting the effects of oxidation than carbon steel. 303 and 316 stainless are currently the most common stainless alloys used in bolts. Alloy 2205 can also be found in some bolt products and is more corrosion resistant than even 316 stainless. In general, these metals are able to survive far longer than carbon steel when exposed to the elements. However, stainless steel still corrodes very quickly in salty, high humidity environments, such as on coastal cliffs exposed to salt water spray.
Titanium was first developed into usable alloys in the 1930s using a small percentage of aluminum and vanadium. Titanium alloy (Ti64) bolts are expensive and rare, but they offer corrosion resistance vastly superior to even stainless steel in salty, marine environments. Currently, one piece, glue-in titanium bolts are popular among climbers in certain environments that are not conducive to steel hardware, such as Thailand and Cayman Brac. However, they are not mass-produced and must be custom ordered.