A new look at molybdate

The investigation of the use of molybdate as a cooling water corrosion inhibitor has increased recently driven primarily by environmental considerations. There are several other reasons to consider molybdate-based treatments that have been overlooked because of performance evaluation techniques that do not reveal the best qualities of molybdate. Conventional thinking seems to be as follows: molybdate (as MoO{sub 4}) requires oxygen or an oxidizing agent to function as a corrosion inhibitor; conventional laboratory equipment used to evaluate corrosion inhibitors scores molybdate a weak anodic inhibitor; zinc improves the performance of molybdate; most technical data sources report performance based on measured general corrosion rates; molybdate has been found within the anodic area of concentration corrosion cells (tubercles); as a result of environmental acceptability, interest in molybdate has increased despite its expense. An evaluation of molybdate as a cooling water inhibitor has revealed some surprising performance characteristics. Hypotheses for these mechanisms are presented.