|
Molybdate Corosion Inhibitor
For mixed metallurgy systems, the molybdate inhibitors provide the best corrosion protection. Chromate treatments in the range of 500-1000 ppm as Cr4O2¯ are satisfactory unless bimetallic influences exist. When such bimetallic couples as steel and copper are present, chromate treatment levels should be increased to exceed 2000 ppm. Maximum inhibitor effectiveness can be achieved if the pH of these systems is kept between 7.5 and 9.5. In a closed system, it can be quite difficult to prevent corrosion ofaluminum and its alloys; the pH of the water must be maintained below 9.0. Aluminum is amphoteric-it dissolves in both acid and base, and its corrosion rate accelerates at pH levels higher than 9.0. The bimetallic couple that is most difficult to cope with is that of copper and aluminum, for which chromate concentrations even higher than 5000 ppm may not be adequate. Where circulating pumps are equipped with certain mechanical seals, such as graphite, chromate concentrations may not exceed 250 ppm. This is due to the fact that water leaking past the seals evaporates and leaves a high concentration of abrasive salts that can damage the seal. Another problem is encountered when chromate inhibitors are used in cooling systems serving compressors that handle sour gas. If sour gas leaks from the power cylinder into the water circuit, significant chromate reduction will occur, causing poor corrosion control and deposition of reduced chromate. In very high heat transfer rate applications, such as continuous caster mold cooling systems, chromate levels should be maintained at 100-150 ppm maximum. Under these extreme conditions, chromate can accumulate at the grain boundaries on the mold, causing enough insulation to create equipment reliability problems. The toxicity of high-chromate concentrations may restrict their use, particularly when a system must be drained frequently. Current legislation has significantly reduced the allowable discharge limits and the reportable quantity for the spill of chromate-based products. Depending on the type of closed system and the various factors of State/Federal laws limiting the use of chromate, a nonchromate alternative may be needed. Molybdate treatments provide effective corrosion protection and an environmentally acceptable alternative to chromate inhibitors. Nitrite- molybdate-azole blends inhibit corrosion in steel, copper, aluminum, and mixed-metallurgy systems. Molybdates are thermally stable and can provide excellent corrosion protection in both soft and hard water. System pH is normally controlled between 7.0 and 9.0. Recommended treatment control limits are 200-300 ppm molybdate as MoO42¯. Molybdate inhibitors should not be used with calcium levels greater than 500 ppm.
|
Nitrite Borax corrosion inhibitor
Nitrite is another widely accepted non chromate closed cooling water inhibitor. Nitrite Borax Corrosion inhibitor Liquid is highly effective corrosion inhibitor for the common ferrous and non-ferrous metals found in engine cooling water systems. Nitrite Borax Corrosion Inhibitor Liquid is one of popular corrosion inhibitor for Diesel engine cooling water systems. Compressor cooling water systems. Nitrite concentrations in the range of 600-1200 ppm as NO2- will suitably inhibit iron and steel corrosion when the pH is maintained above 7.0. Systems containing steel and copper couples require treatment levels in the 5000-7000 ppm range. If aluminium is also present, the corrosion problem is intensified, and a treatment level of 10,000 ppm may be required. In all cases, the pH of the circulating water should be maintained in the alkaline range, but below 9.0 when aluminum is present. When high nitrite levels are applied, acid feed may be required for pH control. One drawback to nitrite treatments is the fact that nitrites are oxidized by microorganisms. But this can be controlled by using BIO Guard RXSOL-40-4003-025 biological fouling liquid. 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. To achieve similar inhibition with chromate, higher treatment concentrations are required. 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. 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. 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. The American Society of Refrigeration Engineers has established 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
|
