I.VDUSTRIAL .4SD ENGIIVEERING CHEMISTRY
Vol. 17, No. 4
The Practical Use of Laboratory Corrosion Tests By W. E. Ratt and J. A. Parsons THEDURIWNCo.. DAYTON. Onro
The oxides formed on copper and nickel are very c l w in composition to Cut0 and NO, respectively; yet the alloys do not oxidize to mixtures or solutions of them two alone. In Figure 14 experimentally determined oxygen - - ratios ~ c e n oxygen t of the oxides are given, and for compariper cent metal eon the line representing Cut0 NiO. It is, perhaps, hardly nemsary to say that these CUII’CB represent chemid cornpitions alone, and are independent of the physical aggregation of the components. The divergence between the two curves, which is in the direction of some higher oxide or nzidw, ia continuous throughout the wliole &ea, with nu discontinuity corresponding to the change in mechanics of oxidation.
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Lime for Manufacture of Bleachins Powder In the work of the Interdepartmental Conference on Chemical Lime on the preparation of specifications for lime to he used in the manufacture of bleaching powder, several investigations were undertaken to determine the properties that such a lime should possess. This work has been done by several agencies. At the Bureau of Standards the e5ect of varying percentages of water used in hydration was investigated.
In the bureau’s work definite quantities of finely ground C. P. quicklime were added to predetermined amounts of water. The temperature was watched carefully during the hydration process, and the samples were allowed to cool in air overnight, when the percentages of free water, calcium carbonate, free lime (CaOJ. and calcium hydroxide were determined, and the samples were bottled until used. Three special hydrates were also made: a crystalline form made by hydration with a very large excess of water; a hydrate made by drying a putty formed by slaking the lime with a considerable excess of water; and a product formed by slaking and cooling the lime directly with steam. These samples were all chlorinated by passing chlorine into a flask containing the dry hydrate for 21 hours. Available chlorine was then determined immediately, and a t the expiration of 30 days. The apparent density and rate of settling from water were Iso studied. Very satisfactory bleaches could he made by using percentages nf water, based on the dry quicklime, of from 32.5 to 55. The most satisfactory results, both as to keeping qualities and initial strength, were obtained when the percentage was from 15 to 50. Above 50 the strength of bleach falls 05rather rapidly, 50 that a percentage of 45 could well be recommended. The crystalline hydrate and that made from putty both gave good results. That made by steaming the quicklime, however, had apparently too high a percentage of free water for a good hleaching powder.
The type of corrosion data furnirhed on typical acid resisting metals is tabulated and conclusions are drawn as to the inadequacy of present iqformation and the desirability of additional work on tests and standardization of methods. The possibility and value of standardizina laboratory corrosion tests are rhown and a description is given of an apparatus and method of testing which have‘proved satisfactory. The use of accelerated testa in metallurgical control work is shown to be practical under certain conditions and the deviation of results of accelerated teatr from thaw of long experience is explained by the electrolytic theory. Typical corrosion curves are given, showing (a) initial losses at higher rate than conrtant loan after 24 hours; (6) error in predicting life of apparatus baaed on accelerated tests, (e) the use of conrtant losr rate in approximating depth of corrwion or penetration in inches per year, (d) the establirhment of a factor to interpret the short accelerated test to the rmultr of the conrtant lose rate of corrorion. Accelerated corrooion tcrtr are practically applied to show (a)the effect on corrosion by changing the physical properties of the metal, and (6)the effect on corrorion by changes in the chemical compoaition of the alloy.
HE fact that 80 much effort and time are being spent on the subject of corrosion testa, and indeed the d e sirability of ti& symposium on corrosion, is sufficient proof that data on laboratory tests, as commonly published am nnt entirely satisfactory. Cdcott and Whetxell in the r e p r t of their admirable work have suggeeted the desirability of extending their m m r c h on laboratory corrosion testa. Perhaps this paper serves somewhat as an extension of their work, but it is directed particularly toward the practical a p plication of the general principles of laboratory corrosion testa that are now quite universally accepted. In studying the available manufactured literature on corm i o n tests, and particularly the information given out on special acid-proof alloys, it is quite apparent that the user of acid-proof equipment must consider with some doubt all the claims about a special alloy or a new alloy. It should therefore be to the advantage of both ‘L -8er and the manufacturer of acid-proof equipment to have available methods of obtaining such reliable corrosion data on any acid-proof material that it can be accepted without question as a true index of performance over a reasonable period of time. A brief review of corrosion data available on a few acidproof alloys will illustrate this point. The information that has been set down in Table I was carefully taken from literature as published by the manufacturers concerning the suitability of the metal for sulfuric acid. The di!Terent alloys are designated as A, B, C, etc., instead of being mentioned by trade names. The importance of the data given in Table I may be briefly summariled as follows: a
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la-If per cent loss only is given, the size and shape are necessary to interpret the results. &Volume of acid should be sutlicient so that the coarontration will not be materially changed during the test. 2-Volume of acid should be sufficient so that the concentration will not be materially changed during the test.
