Optimization of inhibitive action of sodium molybdate (VI) for corrosion of carbon steel in saline water using response surface methodology

Abstract

The performance of sodium molybdate (Na₂MoO₄) (VI) as a corrosion inhibitor for medium carbon steel corrosion in saline water containing nitrate and chloride ions was studied at various inhibitor concentrations, temperatures, exposure times and rotational velocities. Mass loss and electrochemical techniques were used to evaluate the corrosion rates. The individual and interactive effects of these four parameters were optimized for minimum response of corrosion rate using central composite design (CCD) with response surface methodology (RSM). Nonlinear regression strategy in light of Gauss-Newton technique was utilized for modeling and optimization of the corrosion inhibition experiments. Second-order polynomial model was suggested to predict the corrosion rates as a function of four variables. The individual effect of temperature on corrosion rate was higher than the individual effects of inhibitor concentration, exposure time and rotational velocity, respectively. The interaction effects of independents variables were also addressed. Open circuit potential measurements were used as a significant way to gain information about the behavior of steel corrosion. Steady state potential was reached after one hour of immersion time. Mass loss results were in a good agreement with potentiodynamic polarization technique. Optimum inhibition efficiency was 95.9% at optimum operating conditions. Polarization plots revealed that the inhibitor acts as the anodic-type inhibitor.