Influence of treating frequency on microstructure and properties of Al ₂ O ₃ coating on 304 stainless steel by cathodic plasma electrolytic deposition

Alumina ceramic coatings were fabricated on 304 stainless steel by cathodic plasma electrolytic deposition (CPED). Influence of treating frequency of the power supply on the microstructure and properties of the coatings were studied. The results indicated that coatings obtained at various frequencies on 304 stainless steels were all composed of α-Al ₂ O ₃ and γ-Al ₂ O ₃ , and α-Al ₂ O ₃ was the dominant phase. The contents of α-Al ₂ O ₃ decreased gradually in a very small rate with increasing the frequency and γ-Al ₂ O ₃ gradually increased. The surface of alumina ceramic coating was porous. With increasing the frequency, the coating surface gradually became less rough and more compact, resulting in low surface roughness. The bonding strength of Al ₂ O ₃ coating was higher than 22 MPa and was not strongly affected by treating frequency. With increasing the frequency, the alumina coated steels showed better and gradually increasing corrosion resistance than the uncoated one in 3.5% NaCl solution. The coating steel with desirable corrosion resistance was obtained at 800 Hz whose corrosion current potential and corrosion density were -0.237 V and 7.367 × 10 ^-8 A/cm ² , respectively.