The Determination Of The Endpoint In Pickling And Performance Of Environmentally Friendly Compound Inhibitor System In Strong Acid

This paper studied on the pattern of potential change of pickling to removing the rust, as well as the determination of the endpoint in pickling, the influence of pickling time when the inhibiter was added and the synergistic corrosion inhibition of Polyepoxysuccinic acid(PESA) and Polysorbate 80 (Tween-80) compound to Q235 carbon steel in 18% HCl were investigated with weight loss and electrochemical technique. The adsorption model of corrosion inhibitors on the surface of carbon steel were studied systematically according to the adsorption theory in 18% hydrochloric acid. To expound the adsorption of inhibitors to the carbon steel and the effect of corrosion inhibition, the thermodynamic parameters (adsorption enthalpyΔH0, adsorption free energyΔG0, adsorption entropyΔS0) and the kinetic parameters (apparent activation energy Ea, pre-exponential factor A) were calculated and the corrosion morphologies were observed by Scanning Electron Microscope (SEM).The experimental results indicated that there were 3 steps of the pickling process, and the pickling time accorded to the theory analysis. The endpoint of pickling occurred when the derivative value of the potential was zero. The results measuring by eyes and observation to the surface of carbon steel can also confirm that. When the mass fraction of acid liquor was higher and the temperature of pickling was higher, the pickling time was shorter. It can be got that the optimum pickling temperature was 60℃, and the optimum mass fraction of HC1 which was used to pickling was 18% based on the experimental results.The inhibition efficiency of separate PESA and Tween-80 increased with the increasing concentration, decreased with the increasing temperature, but both substances'inhibition efficiency were not high, which were unable to meet the requirement of practical application. Under the same conditions, the inhibition performance was below to that of commonly used corrosion inhibitor Urotropinum (HA). The inhibition efficiency was 93% after the PESA combined with Tween-80. This showed that there were existed obviously synergistic effects after mixing.PESA and the complex inhibitor of PESA and Tween-80 were accorded to Langmuir adsorption isotherm. The linear slope of C/θand C were very close to 1.The adsorption on Q235 carbon steel surface was a spontaneous and exothermic process. The adsorption energy of PESA and complex systems on Q235 carbon steel were-13.4687 kJmol-1 and -15.9257 kJmol-1 respectively, which both were belonged to mainly chemical adsorption. At the same temperature,ΔG0 (PESA+Tween-80)<ΔG0 (PESA), which suggested that the adsorption tendency of complex systems on the surface of Q235 carbon steel were superior to the separate PESA, corrosion inhibition performance were higher than that of the former. The apparent activation energy of them was bigger than that of separate agent. The confusion of the whole solution system increased after combination.The outer layer of the protective film formed by complex of PESA andTween-80 was more compacted and thicker than the layer formed by single of inhibitor.Comparing the results of potential-time curves including adding and not adding corrosion inhibitor, the addition of corrosion inhibitor will prolong the pickling time. The impact of each inhibitor was similar.