Electrochemical behavior of AZ91D magnesium alloy in phosphate medium—part I. Effect of pH

Abstract

The influence of pH on the corrosion behavior of Mg-based AZ91D alloy was investigated in a constant composition phosphate medium using various electrochemical techniques, complemented with surface analysis data. The studied solutions were 0.1 M H 3 PO 4 , NaH 2 PO 4 , Na 2 HPO 4 and Na 3 PO 4 having pH values of 1.8, 4.5, 9.1 and 11.8, respectively. Spontaneous passivation was substantiated from monitoring the continuous positive shift of the open circuit corrosion potential with both immersion time and solution pH. The impedance data indicated more improvement in the insulating properties of the corrosion products formed on the alloy surface with increase in pH. The electrolyte pH plays a determinant influence on surface film properties, as films formed in phosphate solutions with higher pH values are thicker, thus affording better protection for the alloy than those formed in acidic solutions. Good agreement was observed between the results obtained from electrochemical techniques and those from EDX and XRD examinations. The alloy is more susceptible to corrosion in acidic phosphate solutions than in the alkaline ones. Crystalline magnesium (Mg), magnesium hydride (MgH 2 ) and magnesium oxide (MgO) were found to be the main constituents of the surface film after holding for 2 h in the acidic phosphate medium.

Electrochemical behavior of AZ91D magnesium alloy in phosphate medium: Part II. Induced passivation

“…The working electrodes with a fixed surface area of 0.2 cm 2 were made from the as-cast AZ91D alloy as described elsewhere [23]. The alloy had a chemical composition in wt% of: 9.0 Al, 0.67 Zn, 0.33 Mn, 0.03 Cu, 0.01 Si, 0.005 Fe, 0.002 Ni, 0.0008 Be and balance Mg.…”

“…This behavior suggests that high potential is essential for forming a good anodic film with excellent insulating properties which exhibits better stability at higher pH. The outer layer of the passive film incorporates PO 4 3À species from the electrolyte in its structure during anodizing, hence, higher R 1 values were recorded for this layer compared to R 2 for the inner one [23].…”

“…Such a diffusion process may indicate that the corrosion mechanism is controlled not only by charge-transfer but also by diffusion. In order to compensate the errors arising when using capacitance for the LF time constant (R 2 C 2 ), a constant phase element (CPE) was considered in the EC, whose impedance takes the form: 1/C (jx) -1 = 1/Y o (jx) -a , where j 2 = -1 and a has a value between 0.5 for a porous electrode and unity for an ideal flat one [23,26]. It can be deduced that the frequency independent constant (Y o ) of the CPE will be equal to the idealized capacitance (C) at x = 1, x being the angular frequency (x = 2pf rad s -1 ).…”

Corrosion resistance of Ti modified by chitosan–gold nanoparticles for orthopedic implantation

“…The corrosion potential and the corrosion current density were obtained through the linear analysis of Tafel approximation. For linear polarization (i/E analysis) we use Thales software [34][35][36]. To avoid the presence of some degree of nonlinearity in the Tafel slope region of the obtained polarization curves, the Tafel constants were calculated as the slope of the points after E corr by 50 mV, using a computer least squares analysis.…”

Electrochemical behavior of 304L stainless steel in high saline and sulphate solutions containing alga Dunaliella Salina and β-carotene

“…The role of ␤-carotene in inhibiting the corrosion process seems to be via an adsorption process for which the degree of surface coverage (Â R ) can be estimated using the following relation [38]:…”