A method of determining the service life of a gas turbine engine component comprises providing, within a gas turbine engine, a component having an air washed surface, the component comprises a metal, wherein the air washed surface receives a supply of cooling air from a chamber within the gas turbine engine. The method also comprises mounting a metallic test sample within the chamber so as to be located in the supply of air which is delivered to the air washed surface. The metallic test sample is constructed of the same metal as the component. The method further comprises removing the metallic test sample and determining the degree of corrosion of the metallic test sample.