In previous article，the corrosion inhibition of sodium molybdate and sodium tungstate to stainless steel in acid medium has been introduced. This article will introduce the corrosion inhibition of sodium molybdate and sodium tungstate to carbon steel in weak alkaline medium. Carbon steel is unlike stainless steel. It refers to iron carbon alloy of carbon content less than 2.11%. Carbon steel generally contains small amounts of silicon, manganese, sulfur and phosphorus in addition to carbon. Sodium molybdate and sodium tungstate as green new corrosion inhibitors of low toxicity have important meaning of improving the corrosion inhibition effect of carbon steel and reducing the cost.

 

The experiment using polarization curve method and XPS surface analysis to study the corrosion inhibition of sodium molybdate and sodium tungstate to carbon steel in weak alkaline medium. The following conclusions can be drawn:

1. In 0.05mo1/L sodium bicarbonate solution, sodium molybdate and sodium tungstate can reduce critical passive current density of carbon steel and with the increase of its concentration, ∆ ipp also increases. And the anode second peak appears in the polarization curve. The peak value decreases with the increase of sodium tungstate and sodium molybdate concentration. 

2. After adding 3.30mol/L sodium chloride in 0.05mo1/L sodium bicarbonate solution, carbon anode polarization curve no longer has the properties of activation and passivation. This shows that chloride ion has strong destruction effect on the passivation film.

3. In 0.05mo1/L sodium bicarbonate and 3.30mol/L sodium chloride solution, only when the concentration of sodium molybdenum and sodium tungstate is more than a certain value, carbon anode polarization curve can show the properties of activation and passivation. With the increase of sodium molybdenum and sodium tungstate concentration, passivation range is wider.

4. In 0.05mo1/L sodium bicarbonate solution, sodium molybdate and sodium tungstate has depolarization effect to cathode polarization reaction of carbon steel. After adding the chloride ion, they have certain inhibitory effect on cathode.

5. XPS analysis shows that the carbon steel contains Mo (VI) or W (VI), Fe (III), O2-, C and a small amount of Fe (II) in 0.05/L sodium bicarbonate solution containing sodium molybdate and sodium tungstate. And CO32- also enters the passivation film.