Experimental Study on Chemical Polishing and Etching Solutions for Metallographic Testing of P91 Steel

Abstract

To meet the demand for rapid and high-quality metallographic specimen preparation of P91 steel, this study systematically optimized the chemical polishing and etching processes through full-factorial and single-factor experiments. The results indicate that the optimal formulation for the chemical polishing solution is 20 ml of hydrogen peroxide, 1.75 g of ammonium hydrogen fluoride, 2.0 g of oxalic acid, and 25 ml of deionized water. The optimal process parameters involve pre-grinding up to 600-grit, followed by chemical polishing at 25 °C for 120 s. Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDX) analyses demonstrate that this optimized process effectively disrupts temporary chromium-rich passivation films and promotes a dynamic oxidation-dissolution equilibrium. This mechanism helps to prevent localized over-corrosion and effectively retains critical secondary-phase precipitates, such as Cr-Mo-rich carbides. Furthermore, combining this polishing process with an optimized ethanol-buffered etchant (20 ml HCl + 100 ml anhydrous ethanol) clearly reveals the lamellar tempered martensite microstructure of P91 steel while mitigating severe matrix degradation. By avoiding the deformation layers typically introduced by mechanical polishing, this high-fidelity approach provides an efficient and reliable sample preparation methodology for on-site microstructural examination, thereby supporting the subsequent evaluation of critical pressure-bearing components.