Development of an advanced DC-polarisation/AC-impedance cyclic test to evaluate the corrosion of electrically-conductive coating/substrate systems

In this work, an advanced (AC)DC/OCP/AC cyclic test regime has been developed (and validated experimentally) as a new tool for the evaluation of the corrosion behaviour of metallic substrates and/or electrically-conductive coatings. Incorporation of concurrent solution pH measurement with the advanced (AC)DC/OCP/AC technique developed in this work, qualifies this approach to be applied in different industrial applications, such as in aircraft, nuclear and biomedical sectors. It provides opportunities to evaluate objectively the detailed corrosion behaviour in shorter exposure times (as little as 1 day) compared to the widely-used and accepted (but highly subjective) salt spray test (SST) and to conventional (periodically repeated but laborious) electrochemical impedance spectroscopy (EIS) evaluation at open circuit potential (EOCP).
The (AC)DC/OCP/AC cyclic test provides valuable information concerning the corrosion behaviour of uncoated Al 6082 alloy (repeat tests are performed, to eliminate test protocol variables and improve the robustness of the test). The three examples of prototype PVD Al-based nanostructured coatings (AlCr, AlCr(N) and AlCrTi), deposited on 17/4 PH steel, were then evaluated from a scientific perspective, to acquire a fundamental understanding of their performance and degradation with time. The results are shown in two main sets; i) electrochemical results obtained at EOCP and six repeated cycles of (AC)DC/OCP/AC cyclic testing and ii) solution pH results during continuous monitoring of the cyclic electrochemical test procedure. AlCrTi coating showed the best corrosion resistance after application of six successive DC/OCP cycles as a sacrificial protection for the substrate, followed by the AlCr(N) and AlCr coatings. However, the repassivation behaviour exhibited by the AlCr(N) coating after breakdown during extreme DC polarisation, may suggest an advantage to use this coating preferably in aerospace applications because of its strong ability to self-heal. This might be promising for future coating materials for corrosion protection where cadmium or hard chromium needs to be replaced. In addition, the structural and chemical composition of these samples was characterised using SEM and EDX analysis. Conventional open circuit potential (EOCP) and potentiodynamic polarisation scans were also employed, to determine the ‘as-received’ corrosion behaviour.