Critical aspects in the electrochemical study of unstable coated metallic substrates

Abstract

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. The measurements under current control enable to avoid sample damage due to system polarization that might occur when the open circuit potential significantly changes during the measurement. Moreover, the current control approach guarantees the stability of the system during the impedance measurement. However, measurements under current control carried out using the equipment and the software available in our laboratory have some drawbacks. The frequency range available for the measurements is reduced for data acquisition under current control. In addition, the output signal/noise ratio can be altered for measurements at high frequency. Moreover, input parameters must be carefully selected depending on the system investigated for measurements under current control. Nevertheless, impedance measurements under current control might be very useful for the study of unstable systems.