SODIUM MOLYBDATE AS A CORROSION-INHIBITOR OF MILD-STEEL IN NATURAL-WATERS .2. MOLYBDATE CONCENTRATION EFFECTS

Sodium molybdate (NaMoO4) is one of many proposed replacements for chr omate-based inhibitors of steel corrosion. However, its ability to pro tect steel in natural waters, especially flowing natural waters, has n ot been examined in detail. Part 1 of this study detailed effects of f low rate on the corrosion rate of rotating cylinder electrodes (RCE) o f ASTM A36 steel (UNS K02600) exposed to 125 ppm Na2MoO4 solutions und er different flow conditions. Part 1 also discussed limitations of ele ctrochemical impedance spectroscopy (EIS) as a method of examining ele ctrochemical behavior in low-conductivity environments. Part 2 examine d the behavior of A36 steel RCE exposed to various concentrations of N a2MoO4. For polished samples, a critical concentration of Na2MoO4 exis ted, beyond which the corrosion rate increased. However, no critical c oncentration existed,for samples that possessed a corrosion product pr ior to the addition of inhibitors. Oscillatory open-circuit potential behavior and EIS measurements suggested periodic active/passive transi tions on pre-corroded samples.