The goal of this study is to investigate corrosion behavior of Ni-Si intermetallic coatings produced by HVOF (high velocity oxy-fuel) spraying of ? 1 -Ni 3 Si and ?-Ni 2 Si intermetallic powders. In this regard, 420 stainless steel was used as substrate. The results showed that the phase composition of coating produced from ? 1 -Ni 3 Si was composed of Ni(Si) supersaturated solid solution and Ni 5 Si 2 phase while ?-Ni 2 Si phase was preserved during spraying. The phase evolutions were discussed in relation to congruent and non-congruent melting of intermetallic compounds. Tafel polarization tests were performed in 5 and 70% H 2 SO 4 solution at 25 and 75°C. All coatings exhibited an active-passive behavior. Here, ?-Ni 2 Si coatings showed lower corrosion rate. In 70% sulphuric and at 75°C, corrosion potential and cathodic Tafel slope values decreased due to hydrogen depolarization (for all samples). Corrosion behavior of Ni 2 Si coatings and bulk materials was investigated in 5 and 65% nitric acid at 25 and 75°C. In 5% nitric acid, Ni 2 Si coatings and bulk materials showed an active-passive behavior while in 65% nitric acid a spontaneous passivation was observed (the passivation occurred at open circuit potential). By the way, as nitric acid concentration increased, the corrosion potential value significantly increased while corrosion rate decreased. The change in polarization behavior was discussed using mixed-potential theory. The active behavior of coatings was related to Ni dissolution while passive behavior was due to SiO 2 passive film. Keywords: Corrosion, Coating, HVOF thermal spraying, Ni-Si intermetallic compounds, Sulphuric acid, Nitirc acid
