Effect of Light Illumination on the Corrosion Behavior of Mg-RE Alloy EV31A in Chloride-Alkaline Solutions
Authors: Ninlachart Jakraphan , Raja Krishnan S. 
Published: February 2020

Mg-RE alloy EV31A is a Mg-Zn-Gd-Nd-Zr alloy used in aerospace and high performance automobile applications where creep and corrosion resistance are required. These alloys can also be used in Mg-air batteries as possible anode materials in alkaline electrolytes in order to minimize the negative difference effect. EV31A is an age hardenable alloy. The precipitation reactions occurring during the aging process follow the sequence of a-Mg →β'' → β' → β (Mg3RE) → Mg41RE5. The alloy showed excellent passivity in the 0.1 M NaOH solution. More than 80 ppm of chloride was required to induce passivity breakdown in any heat treatment condition. Peak aging at 200 °C for 16 h imparted better resistance for localized corrosion than the solution treated or over aged conditions[1].
Light illumination has been reported to increase the resistance to passivity breakdown of various metals and alloys such as iron[2], copper[3], aluminum[4], and stainless steel. During illumination, the band bending of the passive layer is modified by the electron-hole pair generation and the electric field that drives the transport of point defects across the passive layer is considerably decreased. This quenched electric field is considered to improve the localized corrosion resistance of the material. Light illumination also modifies the electronic properties of the passive film.
The effect of white light illumination on the corrosion behavior of peak aged EV31A specimens was investigated in 0.1 M NaOH solution containing 0, 80, 100 and 200 ppm chloride ions at room temperature. The passivation current density decreased upon illumination in the 0.1 NaOH solution without chloride addition. On the other hand, increased passive current densities were observed in the chloride containing solutions upon illumination. The white light illumination did not affect the open circuit potential of the EV31A. This result indicated that alloying additions did not significantly alter the band gap of the passive layer or induce band gap states in the passive layer in order to harvest the longer wave lengths of the white light. The incubation time for passivity perturbation decreased with illumination. The charge carrier density increased significantly upon illumination. The exchange current density for hydrogen evolution increased under the illuminated conditions. The Mott-Schottky results pointed out that light illumination induced surface defects of n-type character, possibly oxygen vacancies. These defects could have been potential sites for chloride adsorption that decreased the corrosion resistance. The white light illumination did not result in induced photo potential due to wide band gap of the passive layer. Since the band bending was not altered, the electric field remained the same in both illuminated and non-illuminated condition. Overall, illumination of white light was not beneficial to the localized corrosion resistance of the Mg-RE alloy in alkaline-chloride solutions.
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