1350 Korean J. Chem. Eng., 36(8), 1350-1359 (2019) DOI: 10.1007/s11814-019-0291-1 INVITED REVIEW PAPER pISSN: 0256-1115 eISSN: 1975-7220 INVITED REVIEW PAPER † To whom correspondence should be addressed. E-mail: aneesdr@gmail.com Copyright by The Korean Institute of Chemical Engineers. Optimization of inhibitive action of sodium molybdate (VI) for corrosion of carbon steel in saline water using response surface methodology Khalid Hamid Rashid * and Anees Abdullah Khadom ** ,† *Department of Chemical Engineering, University of Technology, Baghdad, Iraq **Department of Chemical Engineering, College of Engineering, University of Diyala, Diyala, Iraq (Received 25 January 2019 • accepted 1 May 2019) Abstract-The performance of sodium molybdate (Na 2 MoO 4 ) (VI) as a corrosion inhibitor for medium carbon steel corrosion in saline water containing nitrate and chloride ions was studied at various inhibitor concentrations, tempera- tures, exposure times and rotational velocities. Mass loss and electrochemical techniques were used to evaluate the cor- rosion 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 vari- ables. The individual effect of temperature on corrosion rate was higher than the individual effects of inhibitor concen- tration, 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. Keywords: Corrosion, Optimization, Inhibition, Mass Loss, Saline Water INTRODUCTION Medium carbon steel (MC-steel), because of its great mechani- cal properties and low production costs, is in wide usage in vari- ous fields of industry [1,2]. Unfortunately, MC-steel suffers from corrosion problems when contacted with aggressive solutions [3]. Saline solutions are one of the harmful environments that affect steel corrosion resistance. Besides the high salts content in saline solu- tions there are other common factors that influence the metallic corrosion rates. Temperature, flow velocity and time of exposure are some examples of issues that may affect the corrosion rate of a given metal in saline solutions. The changes in temperature have a significant effect on the corrosion process. Corrosion rate in diffu- sion control process is doubled for a certain increase in tempera- ture, while in activation process may be increased by 10-100 times depending on the magnitude of the activation energy [4]. Increas- ing solution temperature leads to decreasing viscosity of solution with a consequent increase in oxygen diffusivity, which leads to in- crease the rate of mass transfer of dissolved oxygen to the cathode surface, then increasing the corrosion rate. On the other hand, the decrease in saline water viscosity with increasing temperature im- proves the saline water conductivity with a consequent increase in the rate of corrosion [5]. Generally, the dissolution rate of steel in- creases with the increasing of the flow rate. This may be attributed to a decrease in the thickness of hydrodynamic boundary layer and diffusion layer across which dissolved oxygen diffuses and other corrosive species to the metal surface. With the consequent increase in the diffusion rate of oxygen, the resistance of surface film nearly vanishes, the corrosion products, depolarization of oxygen, and pro- tective film are continuously swept away and continuous corrosion happens [6]. The exposure time is another important factor and the corrosion rate may increase with time [5]. Corrosion of metals can be controlled using different methods. Inhibitors represent one of these corrosion control techniques that can be classified according to chemical structure and mechanism of action [7-10]. Synthesis and natural organic inhibitors are widely used as safe, cheap and environmentally friendly materials. In con- trast, most inorganic inhibitors are dangerous and poisoning, such as nitrites, arsenates, chromates [11]. Although molybdate is one of the inorganic corrosion inhibitors, it can be used with nonhazard- ous effects [12]. The inhibition impact of sodium molybdate (SM) for the carbon steel protection corrosion in saline solution has been reported in many research works [13-15]. Most of literature works in the field of corrosion control focused on corrosion mechanism, type and performance of inhibitor, kinetics studies, etc. Optimiza- tion, statistical and mathematical studies received less attention. In chemistry, the optimization procedure is a standout amongst the most well-known applications. The principal target of optimization is to decide the levels of free factors that prompt a minimum (or a maximum) estimation of an outcome. This methodology can be achieved in two diverse means: multivariate and univariate. Multi- variate procedures are statistical techniques that measure connec- tions among factors. In customary univariate techniques, the analyses are led keeping every one of the factors consistent with the excep-