Almost all bicycle chains are made out of steel which is a hard wearing and strong material which works well for a high use component such as a chain.
Some chains will have a treatment applied to them in order to improve corrosion resistance, durability, asthetics or reduce friction. Most brands have developed their own type of special treatment formula which they apply to their chains but they all have the same purpose.
One of the most common treatments is that of nickel plating; it greatly reduces the likelyhood of rust forming thanks to its corrosion resistant properties. A nickel plated chain still requires lubricant in order to function to it's fullest potential. Chain lube's main focus is to reduce friction which creates a smoother running chain; a byproduct of which is reducing the formation of rust.
Shimano's SIL-TEC surface treatment greatly reduces chain friction and improves wear resistance by imbedding flourine particals onto the surface similar to the slippery, non-stick surface that teflon coated frying pans provide.
A gold colored titanium nitride coating is also quite common since it's harder than steel so it greatly improves wear/abrasion resistance characteristics. Many drill-bits used with power tools have a similar coating to increase the expected lifespan.
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