Study On Corrosion Inhibition Effect And Inhibition Mechanism Of Polyaspartic Acid To Carbon Steel In Oxalic Acid Solution

Polyaspartic acid (PASP) is one of the well-known green chemicals. The current research on PASP is mainly focused on its usage in wastewater treatment and as additives. Few reports can be found on PASP's role as metal inhibitor under weak acid condition. Organic acid pickling to metal is an industry model system. This paper might be the first report in describing PASP as an inhibitor against carbon steel corrosion in oxalic acid solution. By taking the advantage of electrochemistry method, and combing the weight loss and SEM, this study illustrated PASP as an inhibitor on carbon steel corrosion; described PASP's coating process on carbon steel surface. The research also tested the synergistic effect between PASP and other chemicals; an initial discussion on its mechanism was also given. The primary results are as followed:Potentiodynamic polarization curves showed that: with the increasing of PASP concentration, E_corr moved to the negative zone; polarization resistances was also increasing. When the concentration reached 5.0g/L, the highest inhibition efficiency was 83.3% at room temperature. Tafel curve proved that PASP played as an adsorption inhibitor mainly restraining the cathode reaction. The corrosive currents (i_corr) were decreasing with the raising of PASP concentration, which indicated the adsorption film coated on the electrode surface became more completed and dense; the maximum inhibitor efficiency could be reached at 77.6% when the adding PASP concentration was 4.0g/L.Impendence measurement showed that:Under the experimental temperatures, E_corr charge transport resistance (Rt) increased with the increasing of PASP concentration. A stable inhibition efficiency could be observed when PASP concentration reached 4.0g/L; it was 70.1% at 25℃. No more significant increasing in inhibition efficiency could be detected when the PAP concentration was higher than 4.0g/L.PASP had a general good inhibition performance at the range of 5℃～35℃,when PASP concentration reached 4.0g/L, the best inhibition efficiency was more than 70.0% at this described temperature range. A drop in inhibition efficiency could be found when temperature was higher than 45℃. Carbon steel anode polarization curve showed a passivation trend in oxalic acid solution, Ima was -4.5*10-4, the passivation range was around -0.25v～1.3v. EIS showed the different pattern within the anode polarization area: impedance was relatively low in metal dissolving area; it changed to a high value in passivation area; the impedance dropped at the second current peak and Z'shrinking,Z''increasing at low frequency; in exceeding passivation area the value of impedance had a further decreasing.According to the chemistry industry standard—HG5/1526, loss weight experiments were conducted. The result indicated that adding PASP could compensate weight loss: corrosion rate dropped from 1.597mm·a^-1 to 0.426mm·a^-1 when the adding PASP reached 4.0g/L with the corresponding inhibition efficiency 73.3%. The data was consistence with which obtained from Electrochemistry method.SEM observation revealed that specimens treated in PASP solution had a layer coverage. The layer formed mainly by PASP accumulation at the short period treatment; the layer occurred by the process of Fe(C₂O₄) passivation and PASP adsorption under the longer time treatment condition or when carbon steel got polarized.The adsorption behavior of PASP on carbon steel follow the Temkin adsorption isotherm, its isotherm wasη=0.0461lnC+0.9943 in 5% oxalic acid at 25℃. The calculated apparent activation energy Ea was 35.75 kJ·mol^-1 in blank solution,it climbed up to 50.13 kJ·mol^-1when the added PASP achieved 4g/L.These data revealed that the formed PASP layer played a role in efficiently retarding the corrosion reaction on carbon steel and increasing the apparent activation energy.The positive synergistic effects were observed during the treatment process when combining PASP with Na₂MoO₄, TU, Na₂WoO₄ or SDS, respectively. The best performance group was PASP- Na₂MoO₄ at the ratio of 2:1, in which the inhibition efficiency reached 65.3%; while the inhibition efficiency was only 20.2% at 0.025g/L PASP and Na₂MoO₄ almost had no effect in preventing carbon steel corrosion at the low concentration range. These results were the evidence on the positive synergistic effects of the passivity Na₂MoO₄ layer and the PASP adsorption layer occurred on the carbon steel surface during the treatment process.