Corrosion studies on ancient metallic objects are performed with the aim of evaluating their stability for a very long term. They have crossed centuries and should be transmitted in good shape to future generations. They are generally protected from aggressive conditions by a layer formed by corrosion products and this layer stability has to be checked. This work intends to show that coupling an in situ technique like Raman spectroscopy to electrochemical measurements allows a mechanism of corrosion layer formation and stability to be proposed. For bronze the beneficial presence of tin IV oxide to slow down copper corrosion is evidenced. For steel, the hypothesis of lepidocrocite reduction to be coupled with iron corrosion is infirmed.