Critical aspects in the electrochemical study of unstable coated metallic substrates

Electrochemical methods are largely employed for the study of coated metallic substrates. Electrochemical techniques are very useful for investigation of coating performances also in the field of corrosion protection of aluminum alloys where replacement of chromate based coatings is an important issue. Application of sol–gel technique has recently made available thin inorganic protective coatings that can be considered as an alternative to chromate systems. Therefore, it is important to evaluate the electrochemical behaviour of defect free inorganic films which can be considered an inert material. As a consequence, investigation of this inert material might generate critical aspects related to rapid fluctuations of its open circuit potential and current density during open circuit potential and potentiodynamic polarization measurements. Moreover, the unstable nature of the sol–gel very thin films is a critical issue also in electrochemical impedance measurements carried out under potential control because variation of the open circuit potential of the system during data acquisition might lead to sample damage. Impedance measurements under current control is an alternative method for data acquisition. This technique is considered in the present paper for the investigation of AA2024 substrate coated with a thin sol–gel layer and for chromate conversion coated alloy.