THE IMPROVED PASSIVATION OF IRON INDUCED BY ADDITIONS OF TUNGSTEN

Electrochemical and conventional immersion tests on pure iron and various low tungsten steels in 1 N H2SO4 under static and atmospheric conditions were performed. Similar experiments were also performed on pure iron passivated in 1 N H2SO4 containing various concentrations of WO42-. A steel alloyed with 3.02 wt% tungsten in 1 N H2SO4 containing various concentrations of Cl- was also studied. The results showed that alloyed tungsten increases the passive region. The results of pure iron in 1 N H2SO4 containing various concentrations of tungstate ion showed that the passivation current density decreases as tungstate ion increases; but it is limited at a critical concentration. The passive films of low tungsten steels do not offer a good resistance to Cl- ions. The corrosion products of low tungsten steels were examined by X-ray diffraction. WO3 and WO3 · 2H2O were identified for main corrosion products. These results indicated that oxidized tungsten ion remained on the metal surface, by interaction with water and insoluble oxide WO3 formed, which can enhance the stability of the oxide layer and improve passivation. © 1989.