# Effect of Cu2P2O7 on the Formation of Black Micro-Arc Oxidation Coating on AZ31 Magnesium Alloy

## TL;DR

Adding Cu2P2O7 to a silicate-based electrolyte creates a black micro-arc oxidation coating on magnesium alloy in one step, improving aesthetics but slightly reducing corrosion resistance.

## Contribution

This study introduces a one-step method to produce black micro-arc oxidation coatings on magnesium alloys using Cu2P2O7, simplifying the coloring process.

## Key findings

- Adding 4–5 g/L Cu2P2O7 changes the coating color to uniform black due to CuO formation.
- Optimal Cu2P2O7 concentration (4 g/L) increases coating density by thickening the dense layer and reducing pore size.
- CuO incorporation reduces corrosion resistance, as shown by increased corrosion current density.

## Abstract

What are the main findings?
Adding 4–5 g per liter of Cu2P2O7 can change the coating color from grayish white to uniform black.Copper is incorporated into the coating in the form of copper oxide (CuO), as confirmed by XPS/EDS, which results in the coating presenting a black color.The phase composition consists of MgO, MgSiO3, and Mg; however, the content of MgO decreases with increasing Cu2P2O7 concentration.The optimal dosage of 4 g/L enhances the coating density by thickening the dense layer and reducing the pore size.

Adding 4–5 g per liter of Cu2P2O7 can change the coating color from grayish white to uniform black.

Copper is incorporated into the coating in the form of copper oxide (CuO), as confirmed by XPS/EDS, which results in the coating presenting a black color.

The phase composition consists of MgO, MgSiO3, and Mg; however, the content of MgO decreases with increasing Cu2P2O7 concentration.

The optimal dosage of 4 g/L enhances the coating density by thickening the dense layer and reducing the pore size.

What are the implications of the main findings?
The one-step preparation of a black micro-arc oxidation coating has been achieved, simplifying the surface treatment process of magnesium alloys.Clarifying the coloring mechanism of Cu2P2O7 is conducive to the development of functional colored surfaces.Even if there are trade-offs in terms of corrosion, this approach can still support applications in the aerospace/automotive fields and offer aesthetically pleasing solutions.

The one-step preparation of a black micro-arc oxidation coating has been achieved, simplifying the surface treatment process of magnesium alloys.

Clarifying the coloring mechanism of Cu2P2O7 is conducive to the development of functional colored surfaces.

Even if there are trade-offs in terms of corrosion, this approach can still support applications in the aerospace/automotive fields and offer aesthetically pleasing solutions.

Magnesium alloys require protective surface coatings for widespread application, with micro-arc oxidation (MAO) being a prominent technique. However, conventional MAO coatings are typically gray or light-colored, necessitating secondary treatments for specific colors like black, which complicates the process. This study aims to develop a one-step method for fabricating black MAO coatings on AZ31 magnesium alloy by introducing cupric pyrophosphate (Cu2P2O7) as a colorant into a silicate-based electrolyte. As the Cu2P2O7 concentration increased from 0 to 5 g/L, the coating color transitioned from grayish-white to pink, then brownish-black, achieving a uniform black appearance at 4–5 g/L. XPS and EDS analyses confirmed the incorporation of copper as CuO, identified as the primary coloring agent. XRD indicated that the phase composition remained MgO, MgSiO3, and Mg, although the MgO content decreased. Microstructural analysis showed that an optimal concentration of 4 g/L enhanced coating compactness by thickening the dense layer and reducing pore size. However, electrochemical tests revealed that the incorporation of CuO significantly increased the corrosion current density, thereby reducing the coating’s corrosion resistance compared to the unmodified coating. This work successfully demonstrates the one-step fabrication of black MAO coatings, elucidates the coloration mechanism involving CuO formation, and provides insights into the trade-off between aesthetic functionalization and corrosion performance.