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This process takes place at pH between 6- 10, and Oxygen present. Leads to pitting. Very troublesome and can be due to ineffective feed treatment prevalent in idle boilers. Once started this type of corrosion cannot be stopped until the rust scab is removed , either by mechanical means or by acid cleaning. One special type is called deposit attack, the area under a deposit being deprived of oxygen become anodic. More common in horizontal than vertical boiler tubing and often associated with condensers. The ensuing pitting not only causes trouble due to the material loss but also acts as a stress raiser. The critical factors are: 1) the prescience of water or moisture; 2) prescience of dissolved oxygen; 3) unprotected metal surface. The corrosiveness of the water increases with temperature and dissolved solids and decreases with increased pH Aggressiveness generally increases with increased O2. The causes of unprotected metal surfaces are: 1) following acid cleaning; 2) surface covered by a marginally or non protective iron oxide such as Hematite (Fe2O3); 3) The metal surface is covered with a protective iron oxide such as magnetite (Fe3O4 , black); But holidays or cracks exist in the coating, this may be due to mechanical or thermal stressing. During normal operation the environment favours rapid repair of these cracks. However, with high O2 prescience then corrosion may commence before the crack is adequately repaired.

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