The Relative Corrosion of Iron and Steel Pipe as Found in Service
Journal of Industrial & Engineering Chemistry
While temperature has great influence on the development of rust in these experiments there are other influences which affect the results. The purity of the metal determines to a great extent the rate a n d amount of corrosion. This is definitely indicated in m y experiments. Of the impurities in steel, manganese seems to have the greatest influence on the rate of corrosion. My experiments are not exhaustive enough to warrant any sweeping conclusions regarding the influence of manganese, but as
... f manganese, but as far as they go they strongly indicate that this element is a n accelerator of corrosion. This is by no means new. For years, metallurgists and other scientists have held the opinion that manganese is the chief instigator of corrosion. Of this, however, there can be no doubt: the w o r e it will resist corrosion. I believe I am justified in drawing this conclusion from m y experiments. Indeed i t follows directly from the electrolytic theory of corrosion, and the great majority of scientists hold this opinion. Friend's apparatus is a n ingenious device for investigating the corrosion of iron and steel in a pure electrolyte, e. g., water containing only dissolved air, free from all traces of acids or bases. Since corrosion in a pure electrolyte is very probably the true starting point, much can be learned b y using this apparatus. The conditions of the experiment are ideal since only the three substances concerned-iron, air and wa,terare brought in contact; no soluble matter coming from glass vitiates the experiment. S o theory ever advanced t o explain the corrosion o f iron has been so satisfactory as the "electrolytic" theory, announced b y Dr. Whitney, and so ably championed by Drs. Walker and Cushman. Practically all known facts connected with corrosion are explained in the light of this theory. The "acid" theory explains many phenomena of corrosion, b u t it is narrow in its application and, after all, is only a special case under the general electrolytic theory. When carbonic acid enters the pure water electrolyte, i t greatly increases the number of hydrogen ions, and, in addition, pollutes the electrolyte with CO, ions. The solution pressure of the metal is thus greatly increased and corrosion is accelerated. Acids are only accelerators of corrosion. They are not the cause. The true 'starting point of corrosion is the solubility of iron in pure water, its electrolytic solution pressure. This property was given iron by nature, and with all our controversy we cannot take away that which nature gave. The purer the I . Part I of each experiment confirms Friend's statement that iron or steel will 7zot rust in pure water and air combined. The failure 01 the wzetals to rust, hoz,evcr, uras sittirely d u e to temperature cofiditiotzs awd to yapidly clha?zgiutg pure water. . Expts. No. I , 2 , 3, 6 and 8, Part 2 , prove conclusively that pure iron or steel will rust in pure water and air combined, provided the temperature of the metal and pure mater is not below 2 2 ' C., and pro-vided the same water remains for a sufficient time on the metals. 3. Exp. No. 4 , Part 2 , proves that rust is developed rapidly if the temperature of the metals and pure water is about j 5 O C. 4. Expts. Nos. 5 , 7 and 9 , Part 2 , prove that still further increase in temperature results in a decided increase in the rate of corrosion. j . Exp. No. I O , Part 2 , demonstrates that the same pure water may remain on the metal for an indefinite period, and no rusting takes place, provided the temperature of water and metal is sufficiently lo^. 6. With m y modification of Friend's apparatus, equally good results are obtained by using either barium hydroxide, or potassium hydroxide. Barium hydroxide is to be preferred since it is not as liable to render the metal passive. 7. I n general, pure iron or steel will rust in pure water and air combined, free from all traces of acids. The amount of rust produced is a function of the temperature and of the purity of the iron. 8. The "acid" theory of corrosion is untenable. 9. All phenomena observed in these experiments are in perfect harmony with the electrolytic theory. IO. The electrolytic theory of corrosion is further confirmed by these experiments. There are few subjects relating to the corrosion of metals which have received so much attention, or around which there has centered so spirited a discussion, as the relative merits of iron (meaning thereby wrought iron) and steel. The fact that this matter is one still receiving attention, notmithstanding the great volume of accumulated and available literature, is due t o a number of causes, among which may be mentioned: First, that although the words "iron" and "steel" carry with them a definite idea as to general methods of manufacture and some of the more easily discernible properties, they convey no idea as t o standards of value. It is possible to make very poor iron and very good steel, and i t is just as possible to make the reverse. Hence when an investigator compared the corrosion of a poor iron with a good steel, he obtained results which favored steel ; when the material under study was the reverse, iron w a s shown to be the more resistant metal. Second. there is a woful lack of uniformity of conditions obtaining in many, if not most of the experiments which have been carried on for the purpose of comparing resistance to corrosion. Some specimens were large, some small; some cleaned of scale, others not; some immersed in deep water, others in shallow water; etc., etc. The corrosion of iron is so sensitive to changing conditions of surface, oxygen concentration, salts in solution, and the like, that only when the most careful preparation is made to maintain all conditions 1 hbstract of a paper read before the New England \Vater TYorks Association, Dec. 13, 1911.