Effectiveness of one-to-one phosphate to chloride molar ratio at different chloride and hydroxide concentrations for corrosion inhibition of carbon steel

•At a critical pH named passivation pH (pHp), the corrosion state changes from active to passive.•An increase in the amounts of chloride and phosphate at R = 1 causes a decrease in pHp.•At pH > pHp, resistance to pitting corrosion significantly increases.•The phosphate inhibition depends strongly on the amounts of both phosphate and chloride ions.•The inhibition mechanism of phosphate is processed by the formation a duplex layer passive film.