No abstract

Comprehending the corrosion mechanism of magnesium is of major interest in diverse fields. Typically, hydrogen evolution reaction is considered as the only cathodic reaction during Mg corrosion. However, recent works demonstrate importance of considering oxygen reduction reaction (ORR) as a second cathodic process at specific conditions. With oxygen micro-optode, we show that ORR rate was higher on slower corroding ultra-high-purity Mg (UHP-Mg), while lower on faster corroding commercially pure Mg (CP-Mg), where massive hydroxide layer impeded oxygen permeation. These findings shed light on yet another facet of complex mechanism of Mg corrosion.

The direct ZnAl layered double hydroxide growth on AA2024 is a fast-occurring reaction, yet is characterized by an inhomogeneous film thickness. It has been shown that at the periphery of Cu-rich intermetallic, the flakes tend to be larger and denser. A combination of in situ and ex situ measurements were used to monitor the changes in the layered double hydroxide film grown on the regions of intermetallics. Immediately after immersion, an activation of the intermetallics is observed due to the dealloying process with an almost immediate film growth. Dealloying is followed by trenching of the adjacent Al matrix leading to an excessive production of large and dense layered double hydroxide flakes at the periphery of the intermetallic. However, the scanning electron microscopy cross-section images revealed that the trenching process leads to defects in the area surrounding the intermetallic. This could weaken the corrosion resistance performance of the layered double hydroxide conversion coating and lead to adhesion failure of consecutive polymer coatings. Nevertheless, this work highlights a few advantages and drawbacks of the layered double hydroxide conversion coatings and pathways to its potential optimization and improvement.

No abstract