BOILER ACCESSORIES: CORROSION AND INCRUSTATION There are several causes which tend to shorten and destroy the life of every boiler. These may be divided into two general classes, chemical and mechanical, and are usually the result of improper feedwater or of improper care. Pure water, free from air and carbon dioxide, has no evil effect on the iron; but all natural waters, whether from rain, lake, river, or sea, contain air and a little carbon dioxide in solution, and such water will cause iron to corrode, even though no other impurities are present.
Sea water, heated under a steam pressure of 30 lbs., even if it contains no air, will liberate a small amount of hydrochloric acid, which instantly attacks the iron of the boiler unless counteracted by some chemical agent.
External Corrosion. There are two forms of corrosion, external and internal. External may be clue to faulty setting, to improper care, or to moisture from external sources or from leakage from joints and valves. A large amount of external corrosion is the result of setting boilers in a mass of brickwork, which readily absorbs moisture, and which, when not under fire, is likely to keep the boiler-plates damp. The exterior of a boiler encased in brickwork, moreover, is not so easily accessible, and a considerable amount of deterioration may take place without being readily detected. The leakage from a joint, although slight, may, if long continued, badly corrode the boiler.
Internally-fired boilers are supported on saddles and are easily accessible; and the magnesia or asbestos lagging with which they are usually covered will tend to absorb a certain amount of moisture,which will be given off when hot, thus helping to keep the boiler dry. If a leak occurs of appreciable size, the covering will become softened and its presence will be detected at once, and repairs can be made before any serious damage is done. The exterior of an internally-fired boiler, being at all times accessible, can be properly taken care of, which is not true of a boiler set in brickwork. Rivets and riveted joints should as far as possible be kept out of contact with the fire.
Internal Corrosion. This is the result of the chemical action of impure feed-water. It may occur in the form of a general corrosion or wasting-away of the boiler-plates, or in the form of pitting or grooving, the effects of which are likely to be local. Pitting and
general corrosion are entirely the result of chemical action, while grooving is the result of chemical and mechanical action combined.
It is not easy to discover general corrosion, because it acts more or less uniformly over a large surface. Sometimes the rivet-heads rust in proportion to the plates, so that the wasting-away of the plates is not easily noticeable. A uniform corrosion is the hardest to detect, and can usually be discovered only by drilling the boiler and gauging the thickness of the plate. If the thickness of the plate is found to be materially reduced, the working pressure of the boiler should be lowered in proportion.
Sometimes the water will attack the plates only in the vicinity of the water-line, in some instances confining the damage to a belt 6 inches or 8 inches wide. Sometimes a few rivets below water-level will be corroded, the rest remaining in a comparatively good condition. Often the stays are weakened more rapidly than the plates, and the screw-threads of a stay may be badly corroded while the shank of the stay remains uninjured.
Pitting. Fatty acids, which are likely to come over in the feedwater if vegetable oils are used to lubricate the cylinder, are especially active in the production of small pits throughout the interior of the boiler. Pitting appears in the form of small holes or in patches from inch to 1 inch in diameter, or even as irregularly shaped depressions. If the holes are small and close together, the plate is said to be honeycombed. It is generally believed that this phenomenon, the result of chemical action, is due to a lack of homogeneity in the material of the boiler, although an entirely satisfactory explanation has not yet been given. Pitting may also be caused by galvanic action, which may take place especially if sea water is used. As pitting occurs when there is no cause whatever for galvanic action, this can be only a secondary cause at nest. It is reasonable to suppose that acids will attack the most susceptible portions of the plate; and if there is any lack of homogeneity in the iron, it is probable that the places or spots most favorable to chemical attack will suffer first.