The reasons for the corrosion of aluminum alloy surface by anodizing

The reasons for the corrosion of aluminum alloy surface by anodizing

After anodizing treatment for a period of time, aluminum alloys exhibit various corrosion defects on the surface, among which spot corrosion is more common. In order to reasonably improve the surface quality of aluminum castings and achieve the goal of controlling surface spot corrosion, today precision casting manufacturer Tianyao Metal analyzed the spot defects as follows.

1、 The essential analysis of spot corrosion

In the anodizing process, the anode phase Mg2Si is the pitting source of the alloy. During anodic oxidation alkaline washing, Mg2Si particles preferentially dissolve and form corrosion pits. Magnesium dissolves in the solution while silicon remains on the aluminum alloy. When the corrosion pits gather on the grains, the color of the grains becomes darker. Silicon is not easily removed in the sulfuric acid neutralization process, so the silicon content at the bottom of the pitting corrosion pit is higher than in other areas.

2、 Analysis of the Causes of Spot Corrosion

The main factors affecting spot corrosion include the alkaline washing temperature and time during the pretreatment process, as well as the content of Zn, Fe, Si elements in the alloy composition and the extrusion state of the alloy. Among many factors, the squeezing state plays a crucial role, which relates to the distribution of elements such as Zn, Fe, Si that have a significant impact on corrosion performance, as well as the precipitation positions of particles such as intermetallic compounds. In the coarser extrusion stripe area, the distribution of spot corrosion has obvious directionality, because the resistance is high during extrusion in this area, and stress is concentrated here. The lattice of the metal in this area undergoes severe distortion, becoming a local high free energy zone. In the subsequent recrystallization process, nucleation is preferentially formed. In order to reduce interface energy and maintain a stable state, the grains here not only grow abnormally, but also the Mg2Si anode phase, free Si, FeSiAl, FeAl3 and other cathode phases preferentially precipitate, creating conditions for subsequent spot corrosion.

3、 Conclusion

Spot corrosion is caused by the segregation and coarsening of the anode phase Mg2Si in aluminum alloys, and the impurity element Zn in the alloy, as well as Cl - and pH values in the solution, accelerate the occurrence and development of spot corrosion. The addition of elements such as silicon and copper in aluminum silicon alloys increases the corrosion sensitivity of the alloy, making it prone to intergranular corrosion and damage in atmospheric environments. When performing anodizing treatment, silicon elements are prone to form surface ash; Copper will accumulate during oxidation and release hydrogen gas, making oxidation difficult. In summary, the formation mechanism of the anodic oxide film on aluminum is very complex, and a completely consistent view has not yet been reached. With the continuous evolution of research methods, new breakthroughs will be made in the mechanism of aluminum anodizing in the future.

The above is the analysis of the reasons for the corrosion of aluminum alloy surface by anodizing, hoping to be helpful to everyone.

magnesium alloy