Boiler-Water Treatment for Corrosion Control

18.3.2 Boiler-Water Treatment for Corrosion Control

Feedwaters of boilers are chemically treated both to reduce corrosion of the boiler and auxiliary equipment and to reduce formation of inorganic deposits on the boiler tubes (scaling), which interfere with heat transfer. If steam is used for power production, concentrations of silica and silicates in feedwaters must also

be reduced in order to minimize volatilization of SiO 2 with steam, causing forma- tion of damaging deposits on turbine blades. Control of scale formation usually requires removing all calcium and magnesium salts by various means, including use of ion - exchange resins or adding substances to the water that favor precipita- tion of sludges rather than adherent continuous scales on the metal surface. Details are discussed in standard references on boiler - water treatment.

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.

1. Removal of Dissolved Oxygen and Carbon Dioxide. For high - pressure boilers, any remaining dissolved oxygen in the feedwater combines quantitatively with the metals of the boiler system, usually causing pitting of the boiler tubes and general attack elsewhere. Removal of oxygen is accomplished by steam deaeration of the water, followed by addition of a scavenger, such as sodium sulfi te or hydrazine (see Section 18.1 ). Final oxygen concentration is usually held

below values ( < 0.005 ppm O 2 ) analyzable by chemical methods (e.g., the Winkler method). Deaeration is accompanied by some reduction of carbon dioxide content, particularly if the water is acidifi ed 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:

BOILER-WATER TREATMENT

Na CO 2 3 + HO 2 → CO 2 + 2 NaOH (18.9) bringing hot carbonic acid into contact with the condenser and return - line

systems. Steel return - line systems suffer serious corrosion, therefore, if the carbon dioxide content of boiler water is high. Soluble FeCO 3 is formed, and it returns with the condensate to the boiler, where it decomposes into Fe(OH) 2 + CO 2 , with the carbon dioxide again being available for further corrosion. The copper - alloy condenser system also suffers corrosion should dissolved oxygen be present together with carbon dioxide, but attack of copper - base alloys is negligible in the absence of oxygen. Since carbon dioxide is not used up in the corrosion process, it accumulates increasingly in the boiler with each addition of feedwater unless an occasional blowdown (intentional release of some boiler water) is arranged.

2. 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 pH (measured at room tempera- ture) of 8.5 to minimize corrosion of steel, with the optimal pH 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] .

It is apparent from Figure 18.4 that excess alkali can be damaging to a boiler in that the corrosion rate increases rapidly as pH is increased above 13. The

Figure 18.4. Corrosion of iron by water at 310 ° C (590 ° F) at various values of pH measured at 25 ° C [E. Partridge and R. Hall, Trans. Am. Soc. Mech. Eng. 61 , 597 (1939)].

328 TREATMENT OF WATER AND STEAM SYSTEMS

Figure 18.5. Structural formulas for three neutralizing - type amines.

danger is not so much that the initial pH of the boiler water may be too high as that accidental concentration of an alkaline boiler water at a crevice, such as is formed between riveted plates, at a weld, beneath a cracked oxide scale, or at a

hot spot on a scaled tube surface, may reach OH − concentration levels above the safe range. For this reason, it is held advisable to add buffer ions, such as PO 3 4 − (Na 3 PO 4 ), which limit the increase in pH that a water can achieve no matter how concentrated it becomes. Such ions are also useful in avoiding similar high OH − concentrations that lead to stress - corrosion cracking of any portion of the boiler under high residual or applied stress. Goldstein and Burton [17] reported that

5 – 10 ppm phosphate at pH 9.5 – 10.0 was more effective in reducing corrosion of high - pressure boiler tubes under a variety of operating conditions than either NaOH or NH 3 treatment.

3. 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 fi rst can be minimized by addition of phosphates, as men- tioned previously. Corrosion caused by dissolved carbon dioxide in steam condensate is mini- mized by adding a volatile amine to the boiler water. There are two categories of volatile amines used for this purpose: (1) neutralizing amines and (2) fi lming amines. The fi rst group includes cyclohexylamine, benzylamine, and morpholine (Fig. 18.5 ). When any of these is added to boiler water in suffi cient amount, it neutralizes carbonic acid and raises the pH of steam condensate to an alkaline value, thereby making the condensate less corrosive. Operating by a different principle, volatile octadecylamine, hexadecylamine, and dioctadecylamine are typical of the fi lming - type inhibitors, which protect against corrosion by building up a protective organic fi lm on the condenser surface. The fi lming amines more nearly fi t the defi nition of an inhibitor, whereas the other amines are actually, for the most part, neutralizers.