Real-time electrolyte conductivity and coating response during sequential micro-arc oxidation of TC4 titanium alloy

Conductivity measurements before and after micro-arc oxidation (MAO) cannot readily distinguish changes in electrolyte formulation from transient process fluctuations. We assembled a real-time monitoring system for MAO and used it during treatment of TC4 titanium alloy. Four independently prepared electrolytes, A15, A13, A11 and A9, had initial electrical conductivity (EC) values of approximately 15, 13, 11 and 9 mS cm−1. Each formulation batch was used for two or three sequential treatments. In two reuse experiments, a single electrolyte batch was used sequentially to treat six or nine fresh specimens, while EC, pH and temperature were recorded. Across the formulation series, Si in selected areas decreased from approximately 13 to 6 at.%, Ti/(Si+P) increased, and TiO2 diffraction weakened relative to the Ti substrate. During reuse, EC fluctuated or decreased and then increased to different extents, but coating phase assemblage and selected-area composition did not follow the sequence of EC responses. Differences were confined mainly to selected fields, including local pore morphology, surface structure and elemental distribution. Real-time EC can identify process responses that depart from preceding treatments and guide specimen selection. It cannot alone determine phase assemblage, selected-area composition or local morphology, or establish electrolyte ageing.