Oxidizing Agents for Waste

In contrast to reducing agents, oxidizers either react directly with the metal surface (chromate and nitrite) or work in conjunction with oxygen (molybdate) to achieve a passive film on the metal. The standard inhibitor in this category, nitrite, has been used to inhibit corrosion of mild steel for many years1, 2 in neutral or alkaline aqueous solutions. Nitrite is the only remaining anodic oxidizing inhibitor that can still be used. Unlike molybdate, another anodic inhibitor, nitrite does not need oxygen.3 For this reason, it is very effective in closed systems. It has been proposed that nitrite protects ferrous metal by an oxidation-reduction process where ferrous hydroxide forms a passive magnetite layer.

The overall reaction is:

9 Fe (OH)2 + NO2- ---------> 3 Fe3O4 + NH4+ + 2OH- + 6 H2O

Whether nitrite is used alone or in conjunction with pH buffering agents, relatively high concentrations are needed to establish a protective film, usually on the order of 700 to 1200 mg/L to completely inhibit pitting corrosion. Once the protective film has been established, the nitrite concentration can be lowered slightly to 700 to 1000 mg/L. Some sources have stated that the required nitrite level is influenced by the amount of chloride and sulfate present in the water5, 6 because they can affect the stability of the magnetite layer. As with all anodic inhibitors, severe pitting can occur at low concentrations (<500 mg/L as NaNO2). In other words, too little nitrite is actually worse than none at all because low levels of nitrite will speed up the corrosion process. Loss of nitrite can occur via electrochemical and biological processes. In the former case, if corrosion continues, nitrite can be reduced at the cathode to form ammonia7 according to the equation:

NO2– + 5H+ + 6e- -------> NH3 + 2 OH-

In chilled water loops (or hot water systems that are not in operation) exposure to bacteria has the potential to oxidize nitrite to nitrate or reduce it to ammonia or nitrogen. Controlling biological activity is difficult because oxidizing biocides (like chlorine) will oxidize the nitrite to nitrate, and the efficacy of non-oxidizing biocides tends to be less certain. Difficulty in preventing biological degradation of nitrite has always been a serious limitation.

The use of molybdate for corrosion protection in cooling water, either open recalculating or closed loop. While molybdate is not as strong an oxidizing agent as chromate, it can function in this role in the presence of oxygen. In the presence of oxygen, molybdate will convert hematite (Fe2O3 or red rust) to magnetite (Fe3O4 or magnetic black rust). This process is quite visible as boilers (either hot water or steam) change from a reddish color to black when treated with molybdate.

This mechanism predominates at higher concentrations (>50 mg/L as Mo). By contrast, molybdate’s efficacy as an anodic (or pitting) inhibitor is related to its ability to accumulate within the acidic part of a pit and block the corrosion process. Use of molybdate alone at <20>50 mg/L as Mo), molybdate (in the presence of oxygen) is capable of passivating boiler metal. The only concern regarding molybdate use is related to its accumulation in sludge from waste treatment plants if the sludge is spread on agricultural land. However, based on its low human and aquatic toxicity, molybdate is not severely restricted in most areas of North America. The last oxidizer is hexavalent chromium, or chromate, which, while a very effective corrosion inhibitor is seldom used due to concerns of health/environmental effects. Being a strong oxidizing agent, chromate is capable of converting hematite to magnetite. The reduced chromium becomes incorporated into the resultant oxide layer.