Tham khảo tài liệu 'central air conditioner units and refrigeration_7', kỹ thuật - công nghệ, hoá học - dầu khí phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Simpo PDF Merge and Split Unregistered Version - http BATTERY ACTION IONS IN MOTION START CORROSION HYDROGEN CORROSION PRODUCT CORROSION PROCEEDS WITH PITTING ft TUBERCULATION Figure 73. Causes and effects of corrosion. Chromates are anodic inhibitors but can intensify pitting if they are used in insufficient amounts. Field tests must be performed to be sure the required amount of chromate is in the water and to check the pH. Corrosion is greatest when the pH is between 0 to . 13. Chromate concentration is tested by color comparison. The color of the treated water is matched against a known chromate disc. For example if the sample of treated water matches a tube known to contain 200 . of chromate the sample would also contain 200 . of chromate. 14. Polyphosphates. Phosphates particularly the polyphosphates are used in cooling water treatment. The ability to prevent metal loss with polyphosphate treatment is inferior to the chromate treatment previously discussed. In addition pitting is more extensive with polyphosphates. Unlike chromate high polyphosphate concentrations are not practical because of the precipitation of calcium phosphate. 15. One advantage of using polyphosphates is that there is no yellow residue such as produced by chromates. This highly undesirable residue is often deposited on buildings automobiles and surrounding vegetation by the wind through cooling towers or evaporative condensers when the system is treated by chromates. Also polyphosphate treatment reduces corrosion products sludge and rust known as tuberculation. 16. A factor limiting the use of polyphosphates in cooling water systems is the reversion of polyphosphates to orthophosphates. Orthophosphates provide less protection than polyphosphates and orthophosphates react with the calcium content of the water and precipitate calcium phosphate. This precipitation forms deposits on heat exchanger surfaces. The reversion of polyphosphates is increased by .
