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