In our previous papers, we proposed the adsorption mechanism of organic corrosion inhibitors as follows:
M:O-H-H+NR₃→(M:O-H-H…NR₃)→M+O-H-H…NR₃+NR₃→M:NR₃+O-H-H…NR₃
(I) (II) (III)
Water is originally adsorbed on metal from the air as vapor phase inhibitor, resulting in some corrosion inhibition. Organic corrosion inhibitor should take the place of the adsorbed water. If the water is equimolecular with added inhibitor in corrosive non-aqueous media as shown in (II), metal should be corroded more acceleratedly, as the corrosives can attack metal without any protective barrier. In our experiments, n-butyl bromide drastically corrodes iron and nickel in ether solution, when the amount of tert.-amine added was 10^-5mol/g-metal (powder). These reactions of metal with alkyl halides in the solution proceeded as well as Grignard's reaction on magnesium.
In aqueous alkali solution aluminum was acceleratedly corroded at 50°C, when the amount of N, N-dimethyloctylamine added was 10^-5mol/g-Al (plate). This showed that the water molecule could not be repaired from the aqueous solution, when the amine pulled off it from metallic surface. This might be caused by strong molecular association of H₂O in bulk solution. The association was often seen in the case of much amounts of pri.-amine addition to corrosive media, resulting in little protecting effect on metallic corrosion.
In conclusion, active sites on metal, where organic corrosion inhibitor can be adsorbed or corrosion can initiate, are covered with the water molecules which are adsorbed by donating electron-pair of its oxygen atom to the sites. When the water would depart from the sites, the sites should be strongly electron-accepting. For electron-donating substances, therefore, the sites should be active.