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T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
EDITORIALS THE CORROSION OF IRON AND STEEL
It is now six years since the electrolytic theory in its developed form was offered as a basis for the explanation of the corrosion of iron and steel, and it seems worth while a t this time t o inquire regarding the accuracy of some of the conclusions t o which it has led, as viewed in the light of present experience. I n the first place we have found t h a t the factors controlling the rapidity or extent of corrosion are by no means so simple as they were a t first thought to be. Many conditions which were considered of little or no importance have been found t o exert a profound influence upon the reactions involved. For example, samples of iron and steel t h a t exhibit marked differences in corrosion exposed in the normal condition in which they come from the mill, fail to show any difference upon exposure when they are, first planed t o a uniform surface. Apparently the mechanical strain t o which the samples are subjected in the planer, masks or neutralizes the difference in corrosion inherent in the normal material. It is not surprising, therefore, that many conflicting results have been obtained and published from the investigators now interested in this work. Only those tests which have been carried on under identical conditions of surface finish, temperature, access of oxygen and moisture, general atmospheric conditions, etc., should be given any weight, and even when the greatest care is taken, generalizations must be drawn with caution. One of the conclusions reached b y a consideration of t h e electrolytic theory of corrosion which has proven misleading, is t h a t homogeneity in material insures protection, while heterogeneity leads t o rapid attack. While this is a corollary which may be logically drawn from the electrolytic theory and is doubtless in itself true, there are evidently other factors which superimpose themselves upon those due to differences in structure, producing a final effect contrary t o t h a t predicted. The iron of the old chain bridge a t Newburyport, Mass., has withstood corrosion in a truly remarkable manner for the last ninety-eight years ; and yet it is conspicuous for its heterogeneous structure. Large areas of perfectly pure iron, free from both carbon and slag, are mixed up with areas showing a t least two kinds of slag and very high carbon; yet all withstand atmospheric corrosion. On the other hand, Burgess has shown that iron free from all contaminating elements which could segregate or produce a lack of uniformity, does not withstand rusting so well as the same iron t o which has been added a little manganese, or copper, or nickel. This behavior is observed also in the case of the so-called pure irons made in a n open-hearth furnace, and which are relatively free from carbon, manganese, sulfur, and other constituents prone t o segregation, which have come t o the writer’s notice. While theoretically a very pure iron should withstand rust, there are apparently some factors present which more than offset any advantage
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inherent in purity. Obviously conditions affecting the surface of the material so soon as rusting has started are important causes which have largely been overlooked and which demand more thorough investigation. The most important advance in this field made in recent years is a knowledge of the effect of the addition of small amounts of copper t o normal open hearth or bessemer steel. This result is most strikingly brought out on the paper of D. M. Buck, published in full on page 447 of THISJ O U R N A L . The writer has, within a week, inspected with much interest the test roofs here described and can testify to the remarkable effect shown. While panels of bessemer and open hearth steel containing the ordinary amounts of metalloids have entirely failed, the corresponding panels made from these same heats of steel but t o which a small amount of copper was added, are in a remarkable state of preservation. A panel of the socalled pure iron very low in metalloids, but containing one-half the amount of copper carried b y the less pure steels is less attacked than those steels containing no copper, but is much less resistant than are those t o which two-tenths of one per cent copper was added. Had copper been omitted entirely from this iron it would doubtless have succumbed even earlier. While these tests do not show t h a t any steel however poorly made will, with the addition of copper, withstand atmospheric corrosion, they prove t h a t it is copper, and not the absence of manganese and the other Several ‘ I impurities” which is the controlling factor. hypotheses have suggested themselves as explaining this marked effect of copper in causing steel t o resist atmospheric corrosion, but as yet none are sufficiently tangible t o afford a working theory. Rapid progress has been made in acquiring that knowledge of the relation of pigments and finished paints t o corrosion which is necessary t o a better protection of iron and steel. Predictions founded on theory that a basic paint, or one containing a chromate pigment, would inhibit rusting, while one made up from lampblack or graphite would accelerate rusting, have, in the main, been found correct. The effect of the pigment upon the character of the oil film making up the paint, however, has shown itself also to be very important. Many basic pigments such as basic lead carbonate or zinc oxide which in themselves inhibit, do not withstand the weather; lampblack and graphite, on the other hand, make a very impervious and highly resistant paint film. The logical conclusion in protecting iron is, therefore, t o use a basic priming coat, a second coat of a mixture of a basic pigment and a little lampblack, and when well dried out to apply a lampblack or graphite finishing coat. Experience has shown also the importance of brushing the paint well onto the iron; a good paint may fail on account of poor application. Careful tests with galvanized work show that a n
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T H E J O G R S A L OF I N D U S T R I A L
even coating of zinc is the all-important factor. The common practice of clean wiping galvanized ware is fatal to durability, since the protecting layer is not metallic zinc, but a thin deposit of a zinc-iron alloy. While, therefore, much has been accomplished in the way of making a more resistant base, there is still the necessity of a uniform substantial coating of spelter over the surface. WM. H. WALKER.’ EFFICIENCY STUDIES IN THE HARDWOOD DISTILLATION INDUSTRY
VC’e hear a great deal about efficiency engineers and efficiency studies in business or in the mechanical side of various industries and about the great savings which have been obtained by the introduction of some new system of accounting or of some new method of handling a brick or a piece of iron. We have not heard so much about efficiency studies in purely chemical processes, probably because their application is not so direct and because they have been called by other names, such as “increasing the yield” or “improving the products.” But they are just as important and just as effective in chemical a s in mechanical processes, although they are usually more complicated in the former, since a change in one part of a chemical process is likely t o have a n effect in several other parts of the process. The hardwood distillation industry has some particularly interesting problems which might be made the subjects of efficiency studies, and a few of these will be briefly outlined. Some are well recognized problems and are given only as examples of the application of efficiency studies, while others are not so generally recognized as important. Variatio.tt iut Y i e l d s jrovn. Different Raw 1Waterials.The question of the variation in yields due t o the kind of raw material distilled has never had proper consideration. I n this country the usual raw material consists of mixtures of different species of hardwoods in various unknown proportions, depending upon the natural occurrence of these species in the forests from which the wood supply is obtained; beech, birch and maple predominate, but there are also small amounts of elm, ash, oak, ironwood, etc. Some recent work of the Forest Service* has indicated the possible variation in yields due to species and has shown the importance of considering this factor in judging the value of a raw material. Of course in a plant already established this question would probably not be of importance because i t is likely t o have only one certain mixture of species available and it is doubtful whether sorting of the species would be practicable. But in establishing new plants a consideration of the species available and their relative costs is very important. For instance, the question might arise, 1 Abstract of paper presented before the Piew York Section of the Society of Chemical Industry, Chemists’ Club, April 2 5 , 1913. 2 “Yields Obtained by the Destructive Distillation of Different Forms and Species of Hardwood,” L. F. Hawley and R. C. Palmer, Eighth Int. Congr. App. Chem., Vol. 6, p. 138, and THISJOURNAL. 4, 789. This work has been extended to include another species and check determinations on several of the previous species; it will be published in more complete form as a Forest Service Bulletin.
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“Which is the most favorable source of ram material, tupelo a t $ 2 per cord or birch a t $ 2 . s o ? ” The solution of this question would depend almost entirely upon the value of the products per cord from the two classes of material, since the cost of distilling the wood and refining the products would differ only slightly. I t would therefore be necessary only to determine the yields per cord from each of the two species and find whether the difference in the value of the products made up for the difference in cost. However, in comparing two species which differ considerably in weight per cord, i t would be necessary to consider also the difference in cost of distilling the wood and refining the distillate. For instance, in comparing oak (weight per cord with 1 5 per cent moisture about 4300 lbs.) with red gum (weight per cord with 15 per cent moisture about 3300 lbs.) it can be seen that more heat would be required to distil a cord of the former and, assuming the same percentage of pyroligneous acid, 40 per cent, in both cases, there would be 400 lbs. more pyroligneous acid per cord t o refine in the case of the oak. I n distilling the wood the extra fuel would be the only added expense, but in refining the distillate there would be required a n increase in capacity of the refining apparatus as well as extra fuel. I n comparing the values of two species which differ in weight it is therefore necessary to take into account other things beside just the yield of products and the cost per cord. A very similar study is the determination of the relative values of different forms of material from the same species. This question has never been completely solved, and various opinions have been held concerning the relative values of different forms. I t has lately been shown’ t h a t there may be considerable variation in the yields from different parts of the same tree and in some species the slabs, which, on account of the bark, are usually considered inferior, have given higher yields of both alcohol and acetic acid than the heartwood without bark. This difference in yields from different forms is not the same in all species and therefore a separate study is required for each species. A more complicated study is the determination of the minimum size of material which can be economically used, since there are several effects of variation in size which must be combined into the final results. Let us consider a case where a wood distillation plant is installed to use the mill and forest waste from a hardwood lumbering operation ; what shall be the limit in size of limbs and slabs to be distilled? There are four effects of size which must be taken into account in this problem: ( I ) The effect of size on composition of material and therefore on yields is very important. A small slab naturally means a slab with a large proportion of bark and in the same way the size of a limb effects the proportion of bark and sapwood; therefore, the yields of the products from different sizes of slabs and limbs must be determined. ( 2 ) The cost of collection and handling the material also varies with the size of the piece; it is very evident 1
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