Extensive internal corrosion frequently occurs in boilers using water that has been passed through surface condensers over and over again. To prevent the corrosion add sufficient soda to the feed-water to make the water in the boiler alkaline, and place rolled zinc plates in good metallic and electrical connection with the inside of the boiler and under water, so that no part of the boiler is more than 6 ft. from the zinc, and renew the zinc when it is wasted.
To prevent corrosion in idle boilers fill them with water in which about 50 lb. of common soda has been dissolved to each 100 cub. ft. of water. If the water is sufficiently alkaline after this is done, a bright nail hung in the water will not rust.
The French Navy uses this system: The boilers are first completely filled with sufficient water, and a solution of milk-of-lime or soda is added to the water. The solution is made stronger if the tubes are large, and of less strength if they are small, in order to avoid the danger of contracting the effective area by deposit from the solution.
The outside of the steel or iron tubes is painted with red lead or tar as far as the parts are accessible. For those parts which are inaccessible a protective coating is obtained by burning tar under them.
In the American Navy, boilers not in use are thoroughly cleaned and painted with a mineral oil.
In the English Navy, after cleaning, boilers are thoroughly dried and a pan of charcoal burned in them to consume the oxygen of the air, and quicklime is used to absorb any moisture that may remain.
To prevent rust in unused boilers, it is advisable to keep them filled with water, and the exterior well painted. (W. W. Christie.)
External corrosion of boilers chiefly arises from damp in any brickwork in which they may be set, therefore it should be kept as dry as possible, and should be coated with a waterproof composition. Moisture may be communicated to the brickwork, in addition to the usual means, by leakage through rivet - holes, longitudinal seams, and other defects, or want of sufficient care in preventing water or damp air reaching it, particularly if a boiler be wholly or partly below ground level. Great width of bearing surface of a boiler on brickwork is liable to increase, if it be not the cause of, corrosion, for water will trickle towards it, and there remain.
Boilers placed on a broad wall perpendicular to their vertical centre are specially liable to corrode, as then the lowest portion of their whole circumference is seated on brickwork. Consistent with sufficient bearing, the bedded surfaces should be as small as possible, in order that but little of the entire circumference of the shell be covered. Material of a hygroscopic nature should not be used to join a boiler to the brickwork setting, and therefore fire-clay is considered far preferable to lime mortar. The longitudinal seams should be so arranged that they can be inspected and caulked, and therefore should not be covered. The joints and any flaws in the boilerplates and vulnerable places, and suffer most from heat and other causes.