Scale—Scale is formed on boiler heating surfaces by the depositing of impurities in the feed water in the form of a more or less hard adherent crust. Such deposits are due to the fact that water loses its soluble power at high temperatures or because the concentration becomes so high, due to evaporation, that the impurities crystallize and adhere to the boiler surfaces. The opportunity for formation of scale in a boiler will be apparent when it is realized that during a month’s operation of a 100 horse-power boiler, 300 pounds of solid matter may be deposited from water containing only 7 grains per gallon, while some spring and well waters contain sufficient to cause a deposit of as high as 2000 pounds.
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The salts usually responsible for such incrustation are the carbonates and sulphates of lime and magnesia, and boiler feed treatment in general deals with the getting rid of these salts more or less completely.
[Table 15] gives the solubility of these mineral salts in water at various temperatures in grains per U. S. gallon (58,381 grains). It will be seen from [this table] that the carbonates of lime and magnesium are not soluble above 212 degrees, and calcium sulphate while somewhat insoluble above 212 degrees becomes more greatly so as the temperature increases.
Scale is also formed by the settling of mud and sediment carried in suspension in water. This may bake or be cemented to a hard scale when mixed with other scale-forming ingredients.
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Corrosion—Corrosion, or a chemical action leading to the actual destruction of the boiler metal, is due to the solvent or oxidizing properties of the feed water. It [Pg 102] results from the presence of acid, either free or developed[15] in the feed, the admixture of air with the feed water, or as a result of a galvanic action. In boilers it takes several forms:
1st. Pitting, which consists of isolated spots of active corrosion which does not attack the boiler as a whole.
2nd. General corrosion, produced by naturally acid waters and where the amount is so even and continuous that no accurate estimate of the metal eaten away may be made.
3rd. Grooving, which, while largely a mechanical action which may occur in neutral waters, is intensified by acidity.