Study On Corrosion Of Carbon Steel And Corrosion Inhibition Effect Of Na₂MoO₄ To Carbon Steel In NH₄Cl Solution

Ammonium chloride system is used chemistry, chemical industry, and pharmaceutical. But this kind of chemical also corrodes seriously metal equipment. Therefore, there is very impotent for industrial field about the corrosion and protection research of ammonium chloride system. There are few reports about corrosion research of ammonium chloride system with sodium molybdate. In the paper, The corrosion, inhibition function and mechanism of 45" carbon steel in the ammonium chloride solution are studied with steady state polarization method and electrochemical impedance method . The experiment result is as follows:1. The test of carbon steel corrosion in ammonium chloride system.1) The whole corrosion and pitting corrosion of carbon steel are promoted with increasing the CI'density. The concentration of ammonium chloride solution impacts the critical potential when C_ci^-≤ 0.1 mol/L. The passivation potential gradually moves to positive when the CI" concentration increases. The pitting corrosion critical potential and protective potential will move to negative obviously, and both of them show linear relationship with the CI^- concentration.2) When the temperature going up, the corrosion reaction is going faster. The passivation film is going to instability, and making it easy to be eroded by active Cl^-. When the passivation film is partly destroyed, than pitting corrosion will happen. There is not Warburg characteristic straight line in resistance diagram as lower frequency comparing at 10°C with other temperatures. It indicates that concentration polarization could be neglect at lower temperature. It is controlled by charge transfer systems.3) As pH increase within the range from 3.5 to 8.5, the polarization resistance, Rp, increases too. That is to say that carbon steel is more easily corrode in acid environment. When pH = 8.5, Rp reaches maximum value, and the corrosion resistance is maximum too. When pH increased at the range from 8.5 to 11.5, the polarization resistance (Rp) will decrease, and the corrosion is seriously. But Rp is still high value compares with acid environment. Carbon steel is more easily eroded inacid environment compares with that in natural and alkaline system. 2 The corrosion inhibiting function in ammonium chloride solution1) In 0.1 mol /L ammonium chloride system, sodium molybdate has the main function of anodic inhibition to carbon steel.2) At 20°C, corrosion inhibiting efficiency of sodium molybdate is increasing sharply to carbon steel as concentration increase. When the concentration is 0.2g/L, the corrosion inhibiting efficiency is increasing slowly. As the concentration is 0.8g/L, the corrosion inhibiting efficiency could be up to more than 76% which is measured by methods of polarization resistance and polarization curve.3) Sodium molybdate could effetely inhibit the pitting corrosion occurrence. Following sodium molybdate concentration rising, the whole passivation area is wider, the peak current decreases obviously, and the corrosion speed reduces greatly.4) In this experimental, the inhibiting function of sodium molybdate will decrease when temperature going up. The corrosion efficiency increases along with the increase of the temperature. When at 50 °C, the inhibiting efficiency of sodium molybdate is up to more than 83%. It is suitable to be the corrosion inhibiting reaction at high temperature.5) sodium molybdate could be absorbed by the carbon steel which follows the Temkin absorption isotherm.3. The corresponding corrosion inhibiting efficiency in ammonium chloride system.In the experiment, authors employ the different concentration sodium molybdate solutions, 0.05 g/L, 0.1 g/L, 0.2 g/L, with different concentration of 1-Dodecane sulfonic acid sodium.When sodium molybdate is certain concentration solution, the inhibiting efficiency is increasing at beginning, and then it will decrease following when the concentration of 1-Dodecane sulfonic acid sodium increase. The corresponding effect is better as the solutionwhich has 0.05g/L sodium molybdate and 0.06 g/L 1-Dodecane sulfonic acid sodium salt. The corrosion inhibiting efficiency could be 69.78%, it is better than the sum of using the two solutions separately.