Corrosion inhibitors types for closed cooling water systems

The three most reliable corrosion inhibitors for closed cooling water systems are chromate, molybdate, and nitrite materials. Generally, the chromate or molybdate types have proven to be superior treatments.
Product performance data developed in laboratory studies simulating a mixed-metallurgy closed cooling system identified steel and Admiralty corrosion rates for three closed system inhibitors at increasing treatment levels. As shown, the molybdate-based treatment provides the best overall steel and Admiralty protection. 

Nitrite-based treatment also provides effective steel protection, with results comparable to those obtained with molybdate; however, acceptable Admiralty corrosion inhibition is not achieved. Closed systems often require the addition of a suitable antifreeze.

To achieve similar inhibition with chromate, higher treatment concentrations are required. Nonchromate inhibitors are compatible with typical antifreeze compounds. Chromates may be used with alcohol antifreeze, but the pH of the circulating water should be maintained above 7.0 to prevent chromate reduction. Because glycol antifreezesare not compatible with chromate-based treatments, nonchromateinhibitors should be used.  chromate limits in brine treatments. Calcium brines are limited to 1250 ppm chromate, and sodium brines are limited to 2500 ppm chromate. The pH should be 7.0-8.5 with caustic adjustment only. Some success has also been recorded with nitrite-based treatment of closed brine systems at treatment levels of about 2000 ppm as

Molybdate treatments should not be used with brine-type antifreezes. In closed systems that continuously run at temperatures below 32°F (0°C), a closed brine system is often employed.