Corrosion is an electro-chemical process by which a metal such as mild steel, returns to its natural state,state,such as iron oxide or rust.

句中a metal何意？书上解释说a metal是泛指一切金属或合金，我没有理解明白？

metal是物质名词，通常不可数，a+物质名词，例如这里的a metal在句中究竟何意？通常，a + 物质名词又是何意呢？

谢谢！