July, 1924
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
It is concluded that external physical stresses such as occur in practice, unless continued to the fatigue point, do not break down the natural resistance of the alloy to corrosion. AIR-LINECORROSION I n all the experiments the monel strips were cut long enough to allow a portion to extend above the surface of the solution in order to observe any air-line corrosion effects. In every case discoloration first appeared about 1 cm. above and below the air line. Subsequent scaling and deep pitting were noticed. The pits were not confined, except in a few instances, to the narrow line, a t the surface, but were scattered throughout an area extending about 1 cm. above and below the line. Fig. 12 shows the pits along the surface line of a strip of monel that was connected with solder and immersed in a normal acid fixing bath (Solution A) for 35 days a t 43’ C. (110’ F.). Fig. 13 is an example of scattered pitting as observed on a monel strip that was connected to solder arid immersed in an exhausted fixing bath (Solution C) for 35 days a t 43” C. (110’ F.). Opinions differ somewhat as to the direct cause of air-line corrosion. Bengough and Stuart18 state that “the attack at and around the water line is due, in practically all cases, to secondary reactions among the various products of corrosion.” Aerial oxidation has been considered bj7 many as the chief cause of this type of corrosion, but it has recently been shown by Watsonlg that convection currents similar to those set up by heat in a water line are formed by the moving away of oxidation products from the face of a strip of metal. The author states that there is a slow downward flow along the surface of the metal, thus bringing a fresh supply of the liquid at the upper surface. It was further shown that similar corrosion could be produced a t points below the surface where the air was excluded. Convection effects have also been shown by Bullock20 to exist in connection with photographic bathing operations in the absence of agitation. It is 18 Sixth Report of the Corrosion Committee of the Institute of Metals (British), edvance copy, 1933, p 46. 19 Trans A m . Electrochem. Soc., 44, 13 (1923). 20 Brit. J. Phot., 69, 110 (1922).
believed that the convection theory is one of the most plausible explanations of the air-line corrosion noted with monel, because of the similarity of the corrosion markings a t the air line with those found on the alloy below the surface of the solution. Incrustation produced by differential drying a t and above the air line also influences the rate of corrosion in this region. CONCLUSION 1-Under ordinary conditions of intermittent use, photograhic processing apparatus constructed of monel metal is quite satisfactory, but if the alloy is continuously exposed to strong acid fixing solutions, especially a t high temperatures and in electrical connection with other metals, serious corrosion takes place. 2-The corrosion first appears after about 2 weeks a t 43 O C. (110” F.) as dark brown spots or discolored areas on the surface, which after 6 to 8 weeks change to pit holes and scales accompanied by a general breaking down of the entire surface layer of the alloy. A hard surface film forms after about a week’s exposure to a fixing bath, and this deposit slows down the rate of corrosion to a slight extent, but does not completely prevent the progress of corrosive attack. 3-Silver-plating on monel metal produced by an exhausted fixing bath offers only a slight temporary resistance to the corrosive attack of acid fixing baths. 4-Physical strains produced by maltreating the alloy by bending and hammering in a vise do not cause localization of corrosion in the areas thus partially fatigued, and it is believed that the specific corrosion noted is due to small, localized, internal strains formed during rolling operations. These strained areas would not be detected by visual examination, but would be found when the alloy was subjected to preferential attack of a corrosive liquid. * 5-Excessive corrosion of the alloy occurs at the surface of the solution and is believed to be due largely to convection currents and to an increase in concentration of the solution a t the air surface.
Building on By-Products H E operation of natural economic laws, which tend to increase the efficiency of industry as a whole by pruning out those units which lack fundamental soundness, is sometimes considered a woeful hardship, when in reality it is a profound blessing however well disguised. Perhaps in no other industry are there more likely to be apparent anomalies than in that broadly designated the chemical industry. New developments in science, in technology, and in markets are continually rendering obsolete industries which have been established for generations. At times industries have just grown without particular regard for any natural advantages of location or process, only to be overturned later by a younger competitor, in the establishment of which the advantages of some natural element of success have been realized. During the past two years one of the oldest chemical industries has suffered through the operation of such economic principles. Bleaching power has been used by our textile industries from time immemorial, only to be replaced by liquid chlorine as soon as economical methods of transportation were devised. Differences in freight in favor of chlorine are too great a handicap for the manufacturer of bleaching powder to overcome, when even a moderately long haul is involved. This is gradually forcing the manufacturers of bleaching powder out of the market. The rise of our electrochemical industries. centered around
the great natural vantage points of cheap power, has provided other instances of this kind. The electrolysis of sodium chloride to form caustic soda, chlorine, and hydrogen-based on a large measure on the great demand for the first of the three-has resulted in serious changes in the alkali industry and in apparently undesired surpluses of chlorine and hydrogen. These, in turn, have upset the stetus quo of every industry into which they enter. First, their simple combination to form hydrochloric acid has made salt cake the major product and acid the byproduct of those manufacturers who used the old salt-sulfuric acid method. Excesses of chlorine have found their way into the market as liquid chlorine, as chloroform, as carbon tetrachloride, and as chlorine solvents of every description. Founded on the natural advantage of cheap chlorine, these products have worked havoc with industries already existing which possess no such advantages. The excesses of hydrogen have been converted into vegetable lard and thus have come into competition with still another ancient industry. It pays to watch one’s by-products. I n a figurative winking of an eye the dump heap or one’s greatest liability may become the foundation for a fortune. American business has been seriously criticized for the formation of the so-called “vertical trust,” but the building of sound and enduring business may demand it.
