Chromium has excellent adhesion to many materials because of its reactivity. Its affinity for oxygen forms a thin stable oxide coat on the outer surface, creating a passivation layer which prevents further oxidation of the chromium, and of the underlying metal (if any), even in corrosive environments. Chromium plating on steel for automotive use involves three diffusion barrier layers—copper, nickel, then chromium—to provide long term durability where there will be many large temperature changes. If chromium is plated directly onto the steel, then their different thermal expansion coefficients will cause the chrome plating to peel off the steel.