Evaluating the Corrosion Behaviour of PVD Al-Based Coatings By Characterising the Relaxation Time of the Open Circuit Potential in AC-DC-AC cyclic tests

A recently developed AC-DC-AC cyclic test method is used for the first time to evaluate the corrosion behaviour of PVD Al-based coatings. The technique can induce relatively rapid coating degradation as the cathodic reactions at the coating/metal interface produce OH^- ions and H₂ gas. The AC-DC-AC cyclic test steps involve characterising the coating properties using electrochemical impedance spectroscopy (EIS), a DC cathodic potential step (this is applied for given period of time to induce the electrochemical reactions that cause coating damage) and a potential relaxation stage (when the open circuit potential (E_oc) is monitored over a given time for the coating to reach a stable E_oc condition). This relaxation time defines when cathodic reactions stop and the processes such as electrolyte/ion travel from the coating, pore formation, corrosion reactions and double layer stabilisation that were accelerated (by DC polarisation) reach the steady state. The analysis of Eoc during AC-DC-AC cyclic test involves the regression of an expression describing the potential relaxation which includes two summed terms attributed to i) the end of cathodic reaction and ii) to expulsion of electrolyte ions from the coating. The new AC-DC-AC test technique provides a rapid evaluation of corrosion degradation behaviour of PVD AlCr(Ti) coatings in a shorter time (≤ 24hrs) than traditional EIS and SST tests (days or weeks). The influence of applied DC was determined by subjecting constant voltages of -2V -4V, and -8V to the PVD Al-based coating relative to the stable E_oc value.