Influence of chloride ion concentration on the electrochemical corrosion behaviour of plasma electrolytic oxidation coated AM50 magnesium alloy

Abstract

The electrochemical degradation of a silicate-and a phosphate-based plasma electrolytic oxidation (PEO) coated AM50 magnesium alloy obtained using a pulsed DC power supply was investigated using potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) in NaCl solutions of different chloride ion concentrations viz., 0.01M, 0.1M, 0.5M and 1M. The surface of the PEO coated specimens after 50 h of immersion/EIS testing was examined by optical microscopy and scanning electron microscopy. The results showed that the corrosion deterioration of PEO coated magnesium alloy in NaCl solutions was significantly influenced by chloride ion concentration. The silicate-based coating was found to offer a superior corrosion resistance to the magnesium substrate than the phosphate based coatings in lower chloride ion concentration NaCl solutions (0.01 M and 0.1 M NaCl). On the other hand both these PEO coatings were found to be highly susceptible to localized damage, and could not provide an effective corrosion protection to Mg alloy substrate in solutions containing higher chloride concentrations (0.5 M and 1 M). The extent of localized damage was observed to be more with increase in chloride concentration in both the cases.

Influence of pre- and post-treatments on formation of a trivalent chromium conversion coating on AA2024 alloy

“…The cathodic current was reduced for the bare alloy by pre-treatment 2 in comparison with pre-treatment 1 and further reduced following application of the coating. The reduction in the cathodic activity by the coating is consistent with other reports [1,33]. At -0.75 V SCE , the cathodic current densities for coated specimens de-oxidized in nitric acid were ~1×10 -6 and 1×10 -7 A/cm 2 with pre-treatments 1 and 2, respectively.…”

“…The resistance due to the coating, R coat , is determined mainly by the influence of defects in the coating, such as pores and cracks [33]; the value R coat for the specimen with pre-treatment 2 was over an order of magnitude greater than that for the specimen with pre-treatment 1. The charge-transfer resistances, R p , were also greater by a factor of ~3.…”

“…The equivalent circuit used to fit the data is illustrated in Fig. 11 [32][33][34], where constant phase elements (CPEs)…”

An Optimized Trivalent Chromium Conversion Coating Process for AA2024-T351 Alloy

“…Figures 1a and 1b display EC models represented for TCC coatings without and with diffusion of species at the coating/substrate interface respectively. [14][15][16] The diffusion process in the latter EC model is due to the presence of coating cracks located at the boundary of coated intermetallic particles. The EC models included the following components: electrolyte resistance (R e ), coating resistance (R coat ), Warburg resistance (W s ), coating capacitance (C coat ), charge-transfer resistance (R p ) and capacitance of the coating/substrate interface (C dl ).…”

Mg Alloy Surface Treatment

“…Metal phosphate is insoluble and chemically stable, and heat resistance promises a chemical treatment method; however, a drawback is that the corrosion resistance is lower than the croissants mating in the water. As a way to improve this method, forming a composite film by a complex process that combines steam treatment and phosphate treatment of magnesium hydroxide (Mg(OH) 2 ) and phosphate (NH 4 Mg(PO 4 )⋯H 2 O) has been developed [28][29][30][31].…”