CORROSION-CAUSES AND PREVENTIVE MEASURES AR S the life of a boiler mainly depends upon the rate of progress of the corrosion of its pressure parts, the prevention or delay of this destructive action is one of the most important duties of the intelligent engineer. Not only should the subject be studied in its various aspects, but the greatest care and watchfulness are necessary in order to successfully stay the advances of this subtle force. The principal causes of corrosion of iron and steel boilers, in sea-going vessels, can be classified as follows: 2d. Acidity-the use of animal or vegetable oils in the steam cylinder. 3d. Admixture of air with the feed water. Each of these causes of corrosion, and means of preventing or remedying them, will be considered separately. USE OF SEA WATER Salt water is known to be a solvent of iron or steel, and when boiled under high pressure the magnesium chloride, about 250 grains of which are contained in every gallon, becomes highly corrosive. Under certain conditions, particularly in the process of corrosion, the water becomes acid by the dissociation of magnesium chloride into hydrochloric acid and magnesia; the acid, in contact with iron not protected by scale, forms an iron salt which, at the very moment of formation, is neutralized by the free magnesia in the water, thereby precipitating oxide of iron and reforming magnesium chloride. Thus it is easily seen that free iron is never found in solution in boiler water. The black and red deposits formed in boilers which have had an excess of sea water in them are generally iron oxides. The red is found when there is much air allowed to get into the boiler; the black when little or no air is present. Just here comes in one of the most astonishing neglects of marine engineering. It is the neglect of modernizing the condensers of sea-going ships. To deliberately install an expensive and well-constructed boiler, and as deliberately permit the use, in connection therewith, of condensers known to be subject to leakage, and constructed so as to make quick and efficient repair extremely difficult, is at least commercially criminal. There is far more room for improvement in design and construction of the condensers than in marine boilers, and the great importance of the former is most obvious when the first cause of corrosion is properly considered. OWNERS: SMITH LUMBER Co. BABCOCK & WILCOX BOILERS, 1800 INDICATED HORSE-POWER PREVENTIVE. To prevent salt feed, the condensers must be tight, and an ample provision made for fresh water "make-up" either by carrying a supply in bulk or by installing an adequate evaporating plant, designed and located so as to operate without priming. If salt feed does enter the boiler, the quantity must not be increased by "blowing off" water from the boiler, at least not until the saturation has reached. A high saturation is preferable to a continuous renewal of salt feed, aside from the heat loss of blowing off. A light scale will reduce the evaporative efficiency of a boiler, in spite of statements to the contrary, and a heavy scale will induce the burning out of parts exposed to the flames. REMEDY.-A small amount of salt water is bound to get into the boilers, even under favorable conditions, through priming in the evaporator and slight leakage from the condenser, and it is an excellent plan to constantly use a small quantity of milk of lime to neutralize it. One or two pounds per 1000 indicated horse-power fed per day, in the manner below mentioned, may suffice. The lime used is the ordinary unslaked lime of commerce, and it should be finely powdered and kept in a dry place; for instance, on the up-take gratings. Milk of lime is a mixture of about one pound of lime to a gallon of water and should be added at times to the water in the filter box. THE USE OF LIME.-When starting with new boilers on a voyage for the first time, ten pounds of lime should be put into the boilers for every 1000 horse-power (dissolve in water and put in through man hole); and four to six pounds of lime per day for every 1000 horse-power should be passed through the hot well (as milk of lime) for about six days. At the end of the voyage the boilers should be examined to see if they have a thin coating of lime scale on their interior surface. If this is not the case and the water shows an improper color, the use of the lime should be continued. The rationale of the use of lime is the conversion of magnesium chloride, which is corrosive in effect on iron and steel, into magnesia and chloride of calcium neither of which is corrosive; and the light scale on the surface also prevents the corrosive elements from coming into contact with the iron. Further precautionary methods must be employed by the marine engineer in order to conquer corrosion. The boiler water should be tested daily, and if found to be acid or to contain a larger amount than 50 grains of chlorine per gallon, a remedy must be applied. ACIDITY This cause of corrosion may arise from salt feed, or from the introduction of animal or vegetable oil with the feed water by reason of using such oils in the steam cylinders, the exhaust steam entraining much of it to the condensers. This oil, containing fatty acids, will decompose and cause pitting wherever the sludgy deposit can find a resting place in the boilers. PREVENTIVE. Next in importance to the total exclusion of sea water, is the necessity of keeping oil out of the boiler. Only the highest grade of hydrocarbon oil should ever be used in the steam cylinders, and of this the least possible amount. Also, in lubricating piston rods and valve stems, this same precaution should be observed. For, apart from the evil effects of acidity, the hydrocarbon deposited upon the heating surfaces is most harmful, as a thin film of this deposit forms a complete non-conductor, thereby preventing the heat from passing through into the water, and causing the surfaces to burn, blister and crack. Where surface condensers are used, the feed water should be purified on its way to the boiler by passing it through a cartridge filter, which must be kept clean. A large amount of impurities is thereby caught, and the condition of the feed water materially improved. REMEDY. If the boiler water is strongly acid, a solution of carbonate of soda should be added to the feed at the rate of a bucket of soda solution per hour until the water just turns red litmus paper blue, after which daily additions of soda will suffice to keep the water in a safe or alkaline state. Carbonate of soda has also been found effective in cases. where scale of sulphate of lime is formed, as it possesses the property of changing the sulphate of lime to sulphate of soda, which is soluble, and therefore, harmless. Carbonate of lime, which is also formed, may be easily blown or washed out. To sum up, oil and salt water should never be allowed to enter any kind of a steam generator, and, where surface condensers are used, the feed water should be purified as much as possible before entering the boiler. Graphite can be used in place of oil as a cylinder lubricant with equally satisfactory results. In fact, graphite is superior to oil when the steam pressure carried is from 200 to 275 pounds, corresponding to a temperature in the neighborhood of 400° F. Oils containing animal fats produce rapid corrosion and should never be used in the cylinder of a steam engine. Many steam vessels are running without a particle of oil ever being injected into either their main or auxiliary cylinders, the slushing of the piston rods being found ample for piston lubrication. ADMIXTURE OF AIR WITH FEED WATER Air has been a well-recognized cause of corrosion for many years, and instances of rapid corrosion have been proved to have been caused by the feed pumps sucking air from the hot well, and the feed being delivered at a level considerably below the water line. The boilers that have been most free from this kind of corrosion are those in which the best means have been adopted to keep out air. Small bubbles of air expelled from the water on boiling, attach themselves tenaciously to the heating surfaces. The oxygen in this air at once begins war on the iron or steel and forms iron rust; making a thin crust or excrescence which, when washed away by the circulation or dislodged by expansion and contraction leaves beneath a small hole or pit. Pitting, once started, progresses rapidly, as the indentations form ideal resting places for the bubbles of air, and at the same time present increased surfaces to be attacked. *Thorpe states that "nearly all natural waters contain oxygen in solution, and can only be freed therefrom by prolonged boiling in vacuo." *Spenmath states that water absorbs oxygen as follows: *"Corrosion of Boiler Tubes in U. S. Navy, " Lt. Com. Walter F. Worthington, U. S. N., Journal of the American Society of Naval Engineers, Vol. XII. |