Boiler-Water Treatment for Corrosion Control

For corrosion control, the basic treatment consists of removal of dissolved gases, addition of alkali, and addition of inhibitors, as described in the following paragraphs. 

Deaeration is accompanied by some reduction of carbon dioxide content, particularly if the water is acidified before the deaeration process to liberate carbonic acid from the dissolved carbonates. Carbonic acid is corrosive to steel in the absence of dissolved oxygen and more so in its presence [18], but addition of alkali to boiler water limits any corrosion caused by carbon dioxide to the boiler itself by converting dissolved carbon dioxide to carbonates. At prevailing boiler temperatures, however, carbonates dissociate as follows  

Addition of Alkali. Alkali addition to boiler waters is standard practice for most high-pressure boilers in the United States and abroad. Feedwater for a high-pressure boiler is treated to a minimum pFI (measured at room temperature) of 8.5 to minimize corrosion of steel, with the optimal pFI range being 9.2-9.5. For copper alloys, the preferred pH range is 8.5-9.2. Since both steel and copper alloys are common in boiler systems, a compromise range of 8.8-9.2 is recommended [19]. 

Addition of Inhibitors. It is possible to add inhibitors for controlling two kinds of corrosion in boiler systems, namely, stress-corrosion cracking and return-line corrosion. The first can be minimized by addition of phosphates, as mentioned previously. 

Considerable progress has been made in elucidating the mechanisms and causes of boiler corrosion. Advances in this area have been assisted by the development 

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