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Passivity of high strengthen (HS) steel in pH 6.5 boric acid-borate buffer at 1.5 V(RHE) was investigated by electrochemical technique and scanning transmission electron microscopy (STEM). A larger passivity maintaining current flowed through the HS steel compared with S45C carbon steel. A charge transfer resistance of passive HS steel surface was also larger than that of S45C steel. It was indicating that the passive film formed on the HS steel has a resistance less than that on the S45C steel. STEM revealed that the passive film formed on the HS steel was relatively thick and varied depending on the substrate structure, especially retained austenite. It was concluded that micro-structure and/or chemical composition of the substrate structure strongly affect the formation of oxide film and the conductive property of passive film formed on the HS steel.

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