Passivation and pH-Induced Precipitation during Anodic Polarization of Steel in Aluminate Electrolytes as a Precondition for Plasma Electrolytic Oxidation

Potentiodynamic and potentiostatic polarization tests in the potential range between open circuit potential (OCP) - 0.1 V and OCP + 4 V were carried out in aluminate-phosphate electrolytes with an aluminate concentration of 0.2 mol/L and varying phosphates contents between 0 and 0.1 mol/L. The pH was adjusted between 11.5 and 12.0 due to phosphate and optional KOH addition. A high-strength, dual-phase steel, which is relevant for lightweight construction, served as the substrate material. The layer microstructure was investigated by optical and scanning electron microscopy. Energy-dispersive X-ray spectroscopy and Raman spectroscopy were used for element and phase analyses. We found that iron hydroxides or oxides are initially formed independently of the electrolyte composition at low potentials. At around 1 V vs. standard hydrogen electrode (SHE), the current density suddenly increases as a result of oxygen evolution, which causes a significant reduction in the pH value. Precipitation leads to the formation of porous layers with thicknesses of 10 mu m to 20 mu m. In the case of a pure aluminate solution, the layer mainly consists of amorphous alumina. When adding phosphate to the electrolyte, the layer additionally contains the hydrous phosphate evansite. At the highest phosphate content in the electrolyte, the highest P content and the most pronounced crack network were observed.

Automated summary

Potentiodynamic and potentiostatic polarization tests were conducted in aluminate-phosphate electrolytes to investigate the layer formation on a high-strength, dual-phase steel substrate. The results showed that iron hydroxides or oxides were initially formed at low potentials regardless of the electrolyte composition. At around 1 V vs. standard hydrogen electrode (SHE), the current density increased significantly and led to the formation of porous layers. The composition of the layer varied with the addition of phosphate to the electrolyte, with the highest phosphate content leading to pronounced crack networks.