Book - School of Science and Technology

Corrosion 231forming or both, some level of treatment will be necessary. The symptoms of a problemrelate to evidence of:. corrosion at exposed parts. deposition of salts at valve glands etc.. gassing at any tested air vents (hydrogen). sludge formation in the system (magnetite). bacteriological growth (anaerobic).Corrosion, deposition and gassing, although apparently diverse, may well all resultfrom the decomposition of the calcium bicarbonate (Ca(HCO 3 ) 2 ), common in all rawwaters, to produce calcium carbonate (CaCO 3 ) and carbon dioxide (CO 2 ). As has beennoted under the previous heading, the presence of air within a hot water system isinevitable at some time: the oxygen content of that air is a significant source of corrosionproblems, in combination with the differing electrical potentials of the various metalsinvolved. The formation of magnetite sludge (Fe 3 O 4 ) results from oxygen combinationwith ferrous components. The deposition of a hard calcium carbonate scale on theinternal surfaces of pipes may provide some measure of protection against corrosionbut where such a coating breaks down due to thermal movement, attack may then beconcentrated upon a small area of the pipe wall thus exposed. The presence of anaerobicbacteria within the quiet dark warmth of a pressurising vessel, where they ingest sulphatesto produce corrosive sulphides, may generally be disturbed by raising the temperature ofthe water content to as far above atmospheric boiling temperature as the design of thesystem will permit.As far as most conventional domestic and commercial systems are concerned, failinga detailed expert analysis of the situation by an independent professional adviser, theapplication of one of the commercially available corrosion inhibitors must be recommended.These take the form of chemical additives to the water content of the system at acalculated concentration. In the case of an existing system which may already have beenso treated, it is imperative that details of any previous inhibitors be identified since thevarious specialist suppliers approach the subject in different ways and a disastrouscolloidal interaction might result if two separate chemical compounds, however dilute,were to be mixed. Additives having reliable antecedents are available, based upon aselection from the processes listed in Table 8.2. or their equivalents.