Nearly all waters contain foreign substances in greater or less degree, and though there may be a small amount in each gallon, they become of importance where large quantities of water are evaporated. Naturally, when water is evaporated and turned into steam, salts held originally in solution must become deposited.

The accumulation of incrustation is of course objectionable in every form of boiler, for, irrespective of the greater wear and tear it entails, it is the cause of loss of efficiency.

Outside corrosion which is found upon the surface of the fire tube by contact of the water depends upon the boiler being fed with bad water. Generally this corrosion is not being properly considered, and it is found that salt deposits are formed upon the surface of the plate. The plate is thus in those places exposed to a higher temperature than in its other parts, and it is also being chemically acted upon by the salt.

These corrosions can appear upon the whole surface, but generally they are limited to a width of 10 to 20 m.m., and run along the whole length of the tube above the grate bars. This is the place where the tube is exposed to the greatest heat, and where therefore the salt deposits are most easily formed. Such corrosion is often very deep, and the engineer who only can look over a comparatively small surface attaches little importance to them; besides, it is very difficult, if not altogether impossible, to make a correct examination by simply ocular inspection.

In cases where the corrosion has been freely exposed by scraping off the salt deposits, the true nature and extent of the dangerous state of the tubes will be apparent, the corrosion having reduced the thickness of the plates to that of a few millimetres. The corrosions which appear as a straight band along and on both sides of the tubes are very dangerous, being just on those places where an inspection or control is almost impossible, and produce thereby a considerable reduction in the safety of the boiler. For a long time no remedy could be found, and it was necessary to exchange the damaged tube for a new one.

Internal Corrosion. - Many and various have been the explanations offered for the phenomena of internal corrosion in marine boilers. Professor Vivian B. Lewes - a recognised authority on the subject of marine boiler deterioration - states that in the presence of moisture carbonic acid and oxygen simultaneously attack iron and steel, forming a thin layer of carbonate of iron. This is a very unstable salt, which almost immediately breaks down into iron oxide and ferric hydrate, liberating the carbonic acid, which, with a further supply of atmospheric oxygen, continues the process of corrosion or rusting. This process is further hastened by a certain degree of electrolytic activity between the iron and the electro-negative hydrated iron oxide. Inasmuch as the layer of oxide, or, as it is commonly known, rust, is highly porous, the action progresses without interruption as long as the conditions are favourable. The above general conditions obtain when any iron or steel is exposed to the action of oxygen, carbonic acid, and moisture.

Cracks. - The question of the formation of cracks in iron and steel by heat stresses has been widely discussed, especially in the case of steam boiler construction. There is frequently doubt concerning the nature of the origin and the action of such stresses and concerning the true reason for the formation of the cracks. Lacking any other satisfactory explanation, one is easily inclined to ascribe the cause either to the material and its chemical composition or to the design of the boiler, or perhaps to its construction. Without doubt one or the other of these reasons enters into a great many cases in a greater or less degree, but it is also certain that cracks have been found where no known reason will suffice for an explanation, where material has failed which fulfils all specifications, where the design of the boiler is above criticism, and where its construction has been proved excellent.

The furnaces and combustion chambers of boilers suffer generally through overheating, caused principally through the presence of scale, the lavish use of oil, or shortness of water.

The special interest attached to the application of autogenous welding to repairs in marine boilers is based upon the following reasons : -

1. Repairs may be carried out at once, avoiding thereby extra delay in harbour.

2. Defects, caused, for instance, by corrosion, leakage, cracks, may be repaired which otherwise would necessitate the replacing of the damaged pieces by new ones.

3. Defects may be repaired almost as soon as they are detected, and at any place. A boiler which could thus be repaired as and when required would naturally add to its durability. Sometimes it happens that a boiler has to be removed and replaced by a new one even after the first breakdown; it may now be possible to repair it, saving thereby considerable expense. For instance, where the tube plates are destroyed more or less by corrosion, it is impossible to take them out without first removing the boiler, and it is generally preferred, in such cases, to replace it by a new one. By means of the autogenous welding it may be possible to make a proper repair.

4. It happens often that the skin of the shell is unevenly affected, particularly on old vessels; it may be rectified, and thus extend the life of the vessel.

The following paper by Mr. Harry Ruck-Keene, read at the Engineering and Machinery Exhibition, Olympia, London, before the members of the Institute of Marine Engineers, on 28th September, 1907, is reprinted here with kind permission of the said Institute and the editor of The Marine Engineer and Naval Architect: -

"The repairing of boilers is a subject which must always be of interest to marine engineers, and I propose in this paper to describe two processes of effecting repairs, by welding in place, which have so far given satisfactory results, and at the same time have effected repairs at probably less cost and in many cases in less time than by the ordinary methods of welding. These processes are the oxy-acetylene and electric systems of welding, whereby cracks in plates may be welded up in place patches may be fitted and welded in place without forming new seams, as would be necessary if they were riveted, and wasted plates and landing edges may be built up to their required thicknesses. Now the ordinary form of welding can certainly not be called a new process, for though I have been unable to find who was the first discoverer of the art of welding, yet on referring to the fourth chapter of Genesis I find that Tubal Cain (who lived about 3,950 years b.c), is there described as 'an instructor of every artificer in brass and iron,' and so we may fairly conclude that the ordinary form of welding was known in those days. And by the ordinary form of welding in wrought iron or steel I mean that which consists of the parts to be united being heated to a suitable temperature at which they become plastic, but not actually fused, and are then united by hammering, squeezing or rolling. Although the metal itself does not become fused at this temperature, yet it becomes rapidly oxidised, but the oxide formed is liquid at this temperature, and in properly made welds it is entirely squeezed out from between the surfaces to be welded. To render the oxide still more liquid and, therefore, more easily expelled from the weld, a flux of white sand (silica) is sometimes used ; this forms with the oxide a silicate of iron which has a lower melting point than oxide of iron, and although when a flux is used the iron or steel is probably less adhesive than it is at the temperature at which the oxide melts, yet the importance of using every means of getting rid of the scale between the surfaces to be welded justifies the use of a flux in most cases. But to come down from the days of Tubal Cain to more modern times, it was the practice of several well-known firms when making iron boilers to weld the longitudinal seams of the shell plates of boilers instead of riveting them, and in 1874 some exhaustive tests then made proved the efficiency of these welded seams to be about 70 per cent. of the solid plate. And I have only heard of one case in which the weld gave way, and that was in 1889, when a boiler, eight years old, was subjected to hydraulic test, after undergoing repairs, and the longitudinal seam cracked through the weld for a length of about 6 ins. When steel took the place of iron in the manufacture of boilers this practice of welding longitudinal seams was discontinued. But many firms still continue to weld the furnaces to the tube plates in steel boilers; and it is the universal practice nowadays to weld the longitudinal seams of furnaces, no matter whether they be of the plain, corrugated, or ribbed type. So that it will be seen that welding, though decried by many engineers, is still extensively used in the manufacture of boilers. In the oxy-acetylene and electric processes of welding, though the surfaces of the metal to be welded are heated up to practically the same temperature as in the ordinary methods of welding, yet the subsequent hammering, squeezing, or rolling is dispensed with, except in that process of electric welding which I propose to describe where a certain amount of hammering is still used in making the weld. For the purpose of repairing boilers by the oxy-acetylene process the necessary apparatus practically consists of a steel cylinder containing oxygen gas and another containing dissolved acetylene, both under pressure, a special blowpipe, flexible tubes for transmitting the gases from the cylinders to the blowpipe, and small bars or rods of iron or mild steel about 3/16 in. diameter, which are fused and attach themselves to the parts to be united. The oxygen and acetylene gases in these cylinders are led to the blowpipe by means of the before-mentioned pipes and there ignited at the nozzle, the resultant flame giving out an intense heat. Where plates are wasted away by corrosion or otherwise, the wasted parts are first thoroughly cleansed to remove any dirt or grease, and are then heated to a welding heat by means of the flame from the blowpipe; the iron or steel bar is in the meantime held in this flame until a small portion at the end of the bar is melted off and attached to the part to be repaired, and this process is continued until by the addition of drop after drop sufficient metal has been added to bring the plate up to its required thickness. When a crack in a plate has to be welded up, the metal on either side of the crack is cut away to form a V-shaped groove, and thus enable the flame to penetrate to the bottom of the crack and heat the surrounding metal to the required temperature, metal being at the same time added from the small bar to fill up the groove, in the same way as the wasted plate was built up. In a similar manner, by chamfering away the edges, two plates can be welded together. Naturally in all these cases great care must be taken to see that each and every piece of metal added becomes firmly attached before adding more metal to it. This process has been very satisfactorily employed in this country for many purposes, and more especially for welding flanges and branches on iron and steel pipes (which have to withstand high pressure), but so far it has been little used for effecting boiler repairs. In Marseilles and Genoa quite a considerable number of boiler repairs have, however, been carried out in the last few years by this process with satisfactory results. Among other repairs I may mention those carried out to two marine boilers, where the bottom plating of the combustion chambers and the lower part of the combustion chamber back plating, and also parts of the furnaces (19/32 in. thick), were considerably wasted by corrosion. The defective parts were cut out, patches made to suit, and instead of riveting them on, they were welded in place by this process, thus avoiding the making of additional riveted seams in the furnaces and combustion chambers, which often give so much trouble in boilers.