Acoustic emission activity had been noted previously on corroding steel structures. This work identified the acoustic outputs from the various potential oxides formed and from paint fracture. By an algebraic prcoess it proved possible to determine when corrosion commenced and what oxide species were involved. High correlation co-efficients were determined throughout the whole work and it was finally shown that corrosion films as thin as 10 μm or as light as 3.5mg could be detected.