Corrosion inhibition effect of calcium gluconate on mild steel in artificial seawater

Abstract The corrosion inhibition effect of non-toxic calcium gluconate at 0.75%, 1.25%, 1.75% and 2.25% concentration on mild steel in artificial seawater (3.5% NaCl) was studied by potentiodynamic polarization, coupon measurement, optical microscopy characterization and open circuit potential measurement. Results obtained showed the inhibiting compound effectively suppressed the corrosion of mild steel with inhibition efficiency value generally above 80% from polarization test and 90% from coupon measurement after 0.75% concentration of calcium gluconate. Calcium gluconate exhibited mixed type inhibition property with dominant anodic inhibition effect. The inhibitor adsorbed onto the steel surface through physisorption mechanism according to Langmuir and Frumkin isotherm models. Calcium gluconate shifted open circuit corrosion potential plot of mild steel to electropositive values compared to the plot of the non-inhibited steel. Corrosion pits in addition to severe worn out morphology on the image of the non-inhibited steel were absent on the inhibited steel with relatively improved surface morphology.


PUBLIC INTEREST STATEMENTS
The economic impact and problems resulting from corrosion has drawn strong attention from scientists and engineers worldwide. Corrosion of mild steel in industrial environments is a major concern in chemical processing plants, oil and gas industry, manufacturing, automobile industry, marine operations, boiler plants and power generation plants due to the considerable cost involved in the replacement of metallic parts in their various applications. The consequence often leads to plant shutdowns, breakdown of industrial equipment, reduced efficiency, industrial downtime, high maintenance cost due to replacement of damaged part, wastage of valuable resources and expensive overdesign. Corrosion inhibition is of great practical importance, being extensively employed in curtailing wastage of engineering materials and minimizing costs of corrosion control. A great number of studies have being devoted to the subject of corrosion inhibitors. Identification of the functional groups in organic compounds responsible for corrosion inhibition is important in the development of organic corrosion inhibitors of mild steels.

Experimental procedure
MS of cylindrical dimension (0.7 cm diameter) was manually cut into five test coupons. Each coupon was mounted in acrylic paste which hardened after 5 min, grinded with emery abrasive papers (80, 120, 240, 320, 400, 600, 800 and 1,000 grits), polished with diamond polishing paste and subsequently washed with distilled H 2 O and acetone for potentiodynamic polarization technique and open circuit potential (OCP) measurement. Seven grams of recrystallized NaCl was added to 200 ml of distilled H2O to formulate to simulate artificial seawater. CGN obtained from Sigma Aldrich, USA was prepared in volumetric concentrations of 0.75%, 1.25%, 1.75% and 2.25%% per 2,000 ml of artificial seawater. The compound is non-toxic with molecular weight of 430.373 g/mol with molecular formular of C 12 H 22 CaO 14 . Its molecular structure is shown in Figure 1. Potentiodynamic polarization test was done with Digi-Ivy 2300 potentiostat at 25°C ambient temperature. Resin mounted MS electrodes (exposed surface area of 0.38 cm 2 ), Pt counter electrode and silver chloride reference electrode (Ag/AgCl) were immersed in artificial seawater solution, and connected to the Potentiostat-computer interface. Potentiodynamic measurement was performed between potentials of −1.5 V to 0.5 V at a scan rate of 0.0015 V/s. OCP measurement was performed for 9,000 s at 0.2 V step potential. Omax trinocular metallurgical microscope was used to study and capture images of MS surface before corrosion and after corrosion with and without CGN inhibitor.

Theory
System performance is limited by the precision and accuracy of the measuring instrument. The electrochemical system was checked for possible causes of systematic errors. Calibration of the instrument and hardware test was performed with the results shown in Table 1. Test for reproducibility of consistent results was also performed. Based on this assertion, the expected error in the experimentation from systemic hysteresis is less than 1%.
Corrosion current density (C I ) and corrosion potential (C J ) were calculated from the Tafel plots of potential versus log current. The corrosion rate (C R ) was determined from Equation (1).
where C I is the current density (µA/cm 2 ), D is the density (g/cm 3 ), E q is the specimen equivalent weight (g). 0.00327 is a constant for corrosion rate calculation. The percentage inhibition efficiency (η) was calculated from corrosion rate values using the equation below; where C R1 and C R2 are the corrosion rates with and without CGN inhibitor. Polarization resistance (R p , Ω) was calculated from Equation (3) below; where B a is the anodic Tafel slope and B c is the cathodic Tafel slope, both are measured as (V/dec). Optical microscopic images of control, non-inhibited and CGN inhibited MS surface were studied from captured images of MS surface before and after corrosion tests. Figure 2 shows the potentiodynamic polarization plots of MS corrosion in artificial seawater (3.5% NaCl) at 0% CGN to 2.25% CGN. Table 2 shows the data obtained from the polarization test. The optical microscopic images for MS surface before corrosion and after corrosion test in artificial seawater without CGN compound are shown in Figure 3(a,b). Figure 4(a,b) shows the optical microscopic images of MS surface after corrosion in artificial seawater at 0.75% CGN and 2.25% CGN concentration. The corrosion rate of MS at 0% CGN is 1.56 mm/y which corresponds to corrosion current density of 1.37 × 10 4 A/cm 2 due to the electrochemical action of Cl − anions within the electrolyte responsible for the oxidation of MS surface as shown in Figure 3(b). This mechanism results in increased anodic dissolution and hydrogen evolution reactions on MS surface. The image shows the presence of corrosion pits on MS surface in addition to general wear due to oxidation of the substrate Fe 2+ . The presence of chlorides significantly aggravates the conditions for formation and growth of the pits on MS through an autocatalytic oxidation process. The small size of Cl − ions enables localized deterioration of MS surface (Loto, 2013). Migration of Cl − ions into micro-pits is enhanced to maintain electrical neutrality and hydrolysis of the corrosion products inside pits causing acidification, and hence pit enlargement and penetration. The mechanism is autocatalytic because the increased acidity accelerates the dissolution rate inside pits.

Potentiodynamic polarization and microscopy characterization
The Fe 3+ and Cl − ions combine to form FeCl 3 and the overall balanced equation is FeCl 3 further dissociates according to the equations below Fe 3+ cation interacts with water shown below The reaction mechanisms lead to the continuous degradation of MS surface in the electrolyte. The corrosion rate results for MS in the presence of CGN significantly differed from the control solution due to the effective inhibition effect of CGN. Corrosion rate of 0.92 mm/y was obtained at 0.75%  CGN concentration, though this value is significantly lower than the value obtained at 0% CGN, its corresponding inhibition efficiency and corrosion current density are 40.72% and 8.10 × 10 5 A/cm 2 .
In the presence of CGN molecules at 0.75% concentration, Cl − ions from the electrolyte migrate through the partially CGN protected MS surface. There is the possibility of the Cl − ion complexing with protonated CGN molecules. However, the rate of corrosion is only partially and insufficiently inhibited resulting in the diffusion of dissolving metal cations from the pit interior. The optical image of MS (Figure 4(a)) is a slight improvement of the image at Figure 3( The released Ca 2+ forms Ca(OH) 2 on the cathodic portion of MS according to the equation below: The protective reaction product of Fe 2+ -gluconate precipitate and Ca(OH) 2 passivates on MS as shown in Figure 4(b) and the extended passivated region of the inhibited polarization plots in Figure 2. The passivated regions of the polarization plots in the presence of specific concentrations of CGN are much wider than the plot without CGN, which confirms anodic inhibition effect of CGN on MS surface. The passivated region confirms surface coverage of MS surface by CGN molecules. The extent of morphological wear in Figure 4(b) is significantly lower, the serrated edges are due to machining are quite visible and corrosion pits are completely absent.
Observation of the corrosion potential in Table 2 shows the maximum potential shift between the inhibited steel and control specimen is 75 mV in the anodic direction. This confirms the inhibitor to be mixed type with dominant anodic inhibition property (Amel Gharbi, Himour, Abderrahmane, & Abderrahim, 2018). This assertion is further confirmed from the variation of the anodic Tafel slope values, which shows the mechanism of anodic inhibition is through surface coverage and precipitation on anodic reaction sites on MS. This is further proven as

Coupon measurement
Graphical plots of MS corrosion rate versus exposure time are shown in Figure 5(a) while Figure 5(b) shows the plot of CGN inhibition efficiency versus exposure time.  Figure 5(a,b). The plots from 0.75% to 2.25% CGN were generally constant. The corrosion rate and inhibition efficiency plots at 0% CGN varied with exposure time due to the electrochemical action of Cl − ion. The effect of Cl − decreased with exposure time.

Adsorption isotherm studies
The mechanism of CGN interaction on MS surface was further studied through adsorption isotherms models, which shows the extent to which inhibitor molecules adsorb on metallic surfaces in relation to its concentration at constant temperature (Karthikaiselvi & Subhashini, 2014). Langmuir and Frumkin isotherm models produced the most significant plots with correlation coefficients close to unity. Langmuir isotherm states metallic surfaces fixed region for adsorption of adsorbates with uniform Gibbs free energy values irrespective of differences in surface coverage value and the lateral interaction effect (Karimi, Danaee, Eskandari, & RashvanAvei, 2016). Figures 6 and 7 depict the plot of C CGN θ versus C CGN with correlation coefficient of 0.9898 with respect to the Langmuir equation below.
θ is the sum of inhibitor molecules adsorbed per unit gram on MS surface at equilibrium. C inhibitor is inhibitor concentration and K inhibitor is the equilibrium constant of adsorption. Frumkin isotherm states that molecular coverage of metallic surface depends on electrode potential due to variation in energy of double-layer capacitor which itself results from replacement of H 2 O molecules by molecules of chemical compounds with a lower dielectric constant (El-Aila, Elsousy, & Hartany, 2016). The metallic surfaces are heterogeneous and the lateral interaction effect is not negligible according to the equation below: α is the lateral synergism parameter determined from the slope of the Frumkin isotherm plot. Plots of log θ = 1 À θ ð Þc

Thermodynamics of inhibitor adsorption
Results for Gibbs free energy presented in Table 4 were calculated from Equation (11) (Bobina, Kellenbergera, Millet, Muntean, & Vaszilcsin, 2013). The highest ΔG o ads value obtained for CG is −27.37 kJmol −1 while the lowest ΔG o ads value is −18.23 kJmol −1 . The highest and lowest ΔG o ads values obtained show the adsorption mechanism of CGN inhibitor compound is by physical adsorption whose attraction is through weak Van der waals forces. The non-linear relationship between the Gibbs free energy values and CGN concentration shows lateral interaction effect between CGN molecules is negligible.

Open circuit potential measurement
The OCP plots of MS corrosion in artificial seawater solution at 0%, 0.75% and 2.25% CGN is shown in Figure 8. The plot at 0% CGN is the most electronegative due to active corrosion reactions on MS surface resulting in the formation of porous oxides. At this concentration, the OCP plot initiated at −0.580 V and sharply decreased to values associated with increased corrosion reactions before achieving relative stability at −0.657 V (600 s). There was a gradual decrease in corrosion potential to −0.683 V at 9,000 s. At 0.75% CGN concentration, the corrosion potential plot shifted significantly to electropositive values due to suppression of the redox electrochemical processes on MS. The OCP plot initiated at −0.450 V (0 s) and sharply decreased to −0.612 V at 1000.01 s. However, between this potential and the final potential at −0.663 V (9,000 s), miniature potential transients are visible on the OCP plot at 0.75% CGN. These transients are due to thermodynamic instability on MS surface resulting from the active-passive reaction between Cl − anions and protonated CGN  Figure 8. Open circuit potential plot of MS corrosion in artificial seawater at 0%, 0.75% and 2.25% CGN.
molecules. The insufficient CGN molecules at 0.75% CGN were not able to completely hinder the diffusion of some Cl − anions unto the steel surface. At 2.25% CGN, the OCP plot shifted further to positive values signifying effective corrosion inhibition of MS surface. The OCP plot initiated at −0.421 V (0 s) and peaked at −0.630 V (9,000 s)

Conclusion
CGN effectively inhibited MS corrosion in artificial seawater with average inhibition efficiency above 80% and 90% from potentiodynamic polarization method and coupon measurement. The observation significantly contrasts the corrosion of MS in the electrolyte without inhibitor due to the presence of chlorides which significantly aggravates the conditions for formation and growth of the pits on the steel. At 0.75% inhibitor concentration, Cl − ions partially migrated to the steel surface resulting in the diffusion of dissolving metal cations and weak corrosion inhibition. The strong attraction and coordinated reaction of CGN onto the steel is responsible for its effective performance. The passivated regions of the polarization plots in the presence of specific concentrations of CGN are much wider than the plot without CGN. The presence of CGN stabilized the thermodynamic properties of MS compared to the non-inhibited steel. The thermodynamic tendency of MS to corrode decreased, with its corrosion potential shifting to electropositive values. CGN displayed mixed type inhibition with dominant anodic inhibiting effect. Thermodynamic calculations show the compound adsorbed through chemisorption mechanism according to Langmuir and Frumkin adsorption isotherms. Corrosion pits visible on the non-inhibited steel were absent on the inhibited steel.