Corrosion Inhibition Of Soybean Extract On A3 Steel In Hydrochloric Acid Solution

Corrosion phenomena of metal materials were ubiquitous and the economic losses caused by metal corrosion were 2-4%of the gross national product each year.Researches showed that corrosion inhibitor was one of the effective ways to solve the corrosion problems in corrosive environment.Because of the nontoxicity and the degradation natural product extracts havebecome research hotspot in corrosion inhibitors fields.In this paper,weight loss method,open circuit potential,potentiodynamic polarization curve,electrochemical impedance spectroscopy,scanning electron microscopy(SEM),EDAX energy spectrum and Fourier transform infrared spectroscopy were used to investigate the corrosion inhibition effect of soybean extract(SE),soybean protein isolate(SPI)and their hydrolysates(SEH,SPIH)by papain for A3 steel in hydrochloric acid solution.The four corrosion inhibitors were good corrosion inhibitors for A3 steel in hydrochloric acid solutionand the adsorption on A3 steel surface was well represented by Langmuir adsorption isotherm.The corrosion rate of A3 steel decreases gradually until it remained unchanged with inhibitor concentration and the degree of hydrolysis rising in the range of20-60?,6-72 h and 1.0-3.0 mol/L hydrochloric acid solution.The corrosion rate of A3 steel increaseed with temperature,HCl concentration and reaction time rising.When A3 steel immersed in 1 mol/L hydrochloric acid solution with 400 mg/L SE,SPI,SHE and SPIH for 6h at 20?,corrosion inhibition efficiency of SE,SEH,SPI,SPIH were 84.3%,91.5%,94.4%and 97.5%,respectively.The soybean protein isolate hydrolysate was the best under the same conditions.In the hydrochloric acid solutionwith four inhibitors the corrosion potential varied small,the corrosion current density decreased obviously,and the Tafel slope of cathode and anode increased.They acted as mixed type inhibitors.The electrochemical impedance spectroscopy(EIS)composed of only one capacitive reactance arc,which means one time constant.The radius of capacitive arc increased gradually with the corrosion inhibitor.In low frequency with the increase of inhibitor concentration and hydrolysis degree the impedance modulus and the phase angle extremum increased obviously,but the corrosion rate of A3 steel decreased and the corrosion resistance increased obviously.The results of surface analysis showed that the corrosion of A3 steel was obviously reduced in hydrochloric acid solution with inhibitors.The polishing traces were clearly visibleand there were no local corrosion marks.The surface composition of A3 steel consisted of C,O and Fe elements.The infrared spectra of corrosion inhibitors showed that the functional groups containing-NH₂,-COOH and C-N in the four inhibitors.The inhibitors adsorped on the A3 steel by chemisorption and physisorption.The chemisorption came from coordination bond formed between lone pair electrons or p electons of functional group and the empty d orbital of Fe.The physisorption came from intermolecular forces.The interface structure of steel/solution was changed by adsorption which enhanced the corrosion resistance of A3 steel.