Effects Of Sodium Molybdate And BTA On Corrosion Inhibition Of Q235 Carbon Steel

Pitting corrosion, the most harmful local corrosion, is small and deep, and very difficult to detect. The pitting corrosion on Q235 carbon steel widely applied in virous fields had caused serious losses. Adding inhibitors is the most common methods of corrosion inhibition. Therefore, it is significantly important to research the mechanism of the inhibitors on steels and find low level but high effective and non-toxic corrosion inhibition system.Potentiodynamic polarization, electrochemical impedance spectroscopy ?EIS?, X-ray photoelectron spectroscopy ?XPS? and scanning electron microscopy ?SEM? were applied to study the corrosion inhibition and mechanisms of Q235 carbon steel in the solution of 0.1 mol·L^-1 NaHCO₃+ 0.01 mol·L^-1 NaCl+Na₂MoO₄, BTA or mixed inhibitors of Na₂MoO₄ and BTA with final pH of 8.16. The different conditions such as concentration, pH, immersion time were also studied. Through analyzing and comparing different electrochemical parameters. The main conclusions are as follows:?1? In the solution of 0.1 mol·L^-1 NaHCO₃+0.01 mol·L^-1 NaCl, addition of Na₂MoO₄ or BTA can slow down the uniform corrosion. BTA-Na₂MoO₄ mixed system has good pitting corrosion inhibition ability for Q235 steel. Corrosion inhibition of Na₂MoO₄ on Q235 carbon steel increases with the increase of concentration, and the substrate is transformed from activation to passivation at 200 ppm concentration. Addition of 20-300 ppm BTA inhibitor can promote a secondary passivation. When ratio of Na₂MoO₄ and BTA is 4:1, the pitting corrosion inhibition is the best, and the corrosion inhibition efficiency is 97%.?2? With the increase of immersion time, the corrosion inhibition of Na₂MoO₄, BTA-Na₂MoO₄ mixed system on Q235 carbon steel was gradually increased. After immersion for 36 hours, oxygen reaction took place on Q235 steel, and pitting corrosion was completely inhibited.?3? The surface roughness only affects the passivation current density of Q235 steel in sodium molybdate corrosion inhibition system and corrosion potential of Q235 steel in BTA corrosion inhibition system. BTA-Na₂MoO₄ mixed system in high concentration of chloride ion can still maintain a good passivation state, the lowest chlorine ion concentration which can cause pitting corrosion is nearly 30 times of the concentration in the BTA corrosion inhibition system, nearly 3 times of the concentration in the Na₂MoO₄ corrosion inhibition system.?4? Corrosion inhibition mechanism of Na₂MoO₄ and BTA on carbon steel has been studied by many people, and the similar conclusion has been drawn in this paper. The corrosion inhibition mechanism of Na₂MoO₄ has two aspects:Firstly, promote the transformation of FeOOH in the passive film to the stable crystalline Fe₂O₃. Secondly, FeMoO4 deposited on the inner wall of pitting holes, and then promote the passivation to protect the substrate. The corrosion inhibition mechanism of BTA is mainly because of lone electron pair of N in BTA which can coordinate with D empty orbit of Fe, so that BTA can adsorb on the surface of the substrate. In addition to the above mechanism, we found that BTA molecules in the BTA-Na₂MoO₄ mixed system not only adsorbed on the surface of the substrate directly, but also form a protective film with MoO₄^2- together.