The metal parts of all spanners are given some form of coating to reduce the risk of corrosion.
A rust inhibitor solution is applied to all spanners towards the end of the manufacturing process.
Some spanners are plated with either nickel or chromium or both. Nickel and chromium are more corrosion-resistant than steel and the layer protects the iron from reacting with oxygen to produce rust. They are also used as they give the tool an attractive finish.
Black oxide is another coating that can be used as extra protection against corrosion.
The tool is immersed in a chemical bath which reacts with the iron in the outer layer of steel, creating a black layer. Because the iron has reacted with the chemicals, it won’t react with oxygen to form rust. This sort of coating is called a ‘conversion’ coating because the outside layer of steel has been converted to another material.