Discussion o f Papers b y Evans and Thornhill W. H. J. VERNON AND F. WORMWELL Department of Scientific and hdustrial Research Teddington, h g h d
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ITHIN rxnall compass U. R. Evans has presented a lucid statement of general principles and R. S. Thornhill has communicated valuable data on a class of inhibitors that has been somewhat neglected. Both contributions refer in effect to neutral solutions (in which corrosion is essentially of the oxygen absorption type); Dr. Evans would doubtless wish this to be borne in mind in connection with his statement (with context) that cathodic inhibitors are less efficient than anodic inhibitors. I n the hydrogen evolution type of corrosion, greater importance naturally attaches to cathodic inhibitors-e.g., quinoline, pyridine, and other nitrogen-base organic compounds which appear to function by raising the hydrogen overpotential of the cathode surfaces. This aspect is fully discussed in Evans’ book (2). The efficiency obtainable from such inhibitors, not only in the acid pickling of iron and steel but also in the corrosion of magnesium under nonacid conditions, is sometimes spectacular. Our experience with corrosion inhibitors has been obtained largely in connection with various wartime problems; we havc examined the inhibitive or possibly inhibitive properties of a large number of substances, mainly in neutral solutions. Most of our results are embodied in reports to service departments and cannot be quoted in detail at this stage. As far as general principles are concerned, they confirm the findings of the two preceding papers. Our work has been concerned largely with couples or more complex assemblies of dissimilar metals. Such conditions are particularly difficult to meet since, as Dr. Evans pointed out, a reagent that is inhibitive toward one metal may be ineffective or positively inimical toward another metal in the same electrolyte, even in the absence of any contact between the two; in practice the position may be further complicated by the potentials set up between contiguous metals. A steel-duralumin couple in sea water provides an interesting example. I n the presence of chromates or borates, the duralumin appears to be anodic to the steel, the former being attacked and the latter protected. In the prevence of certain organic inhibitors the position is reversed, the attack being then thrown onto the steel and the duralumin protected. Solder provides an important example. Cases have been encountered in which the attack on a completely soldered surface has been almost entirely suppressed, whereas a soldered joint under precisely the same conditions has been corroded to destruction-for example, completely soldered brass and a brasssolder-brass joint in sodium nitrate solution. An important criterion in determining the value of an inhibitor is the extent to which it will eliminate localized attack; the latter ‘firm is used broadly to include both pitting of a single metal and excessive attack on one member of a metallic couple or complex system. Thus, in the case of an assembly of metals, it may bc necessary, in order to suppress a dangerous attack on one metal to increase the attack on another; this is frequently permissible as long as the increased attack remains reasonably uniform in character. It would appear, therefore, that the definition of a “safe” inhibitor must be extended to include a possibly increased amount of total corrosion, the usefulness of the inhibitor being determined by the shielding of some important part of the system from dangerous localized attack. I t cannot be too strongly emphasized that inhibition in respect to corrosion is a relative term, the mechanism being influenced by the nature of both the metal and the electrolyte. We should like strongly to endorse Dr. Evans’ comments in respect to the use of alkaline inhibitors for waters to which chlorides are likely to
have access. It is frequently overlooked that the amount of alkali that constitutes a %afe” inhibitor is intimately dependent on the amount of chloride present in solution. We have been interested in Dr. Evans’ remarks concerning localization of attack in boiler corrosion. Although it is true that the presence of mill scale may lead to pitting, it is also true that serious pitting can occur in tubes from which mill scale has previously been completely removed. The presence of a “scale” or oxide coating must constitute a no less essential part of the mechanism of pitting, but this scale is provided, a t boiler temperatures, by reaction with salt solutions in the boiler itself. We welcome Dr. Evans’ comments on the important part that magnesium salts (particularly magnesium sulfate) can play in this process. Dr. Evans’ explanation of the two-stage mechanism of corrosion of iron in distilled water is similar to the views expressed by Bengough, Lee, and Worpwell (1) in reporting earlier fundamental research on the immersed corrosion of iron and steel. It would be out of place to enter here into a more detailed discussion of the probable mechanism, but our views are essentially similar to those of Dr. Evans on this point, Dr. Thornhill makes the point that chromium salts, as distinct from alkali chromates, do not in general function as inhibitors. While we are in general agreement with this, at least one exception has come within our experience. The addition of sodium chromate greatly reduced the heavy attack on aluminum by a hot solution of sodium lactate, and the inhibition persisted even after the yellow chromate had become reduced to green chromic salt. It was then found that the attack was also greatly reduced by the addition of chromic sulfate in lieu of sodium chromate. Unfortunately the protection, although spectacular in the early stages, was not maintained indefinitely, and for long periods of immersion renewals of the inhibitor would probably be required. In earlier (unpublished) work we confirmed the beneficial effect of zinc salts in the corrosion of mild steel. For example, corrosion in 0.5 N zinc chloride solution was only about one fifth of that in 0.5 N sodium chloride under similar conditions (partial immersion at room temperature). The corrosion productcompletely covering the steel in the zinc chloride solution-was very compact, in contrast with the loose rust in the sodium chloride solution. We also observed that the rusting of mild steel in moving sea water, heated intermittently to 70” C., is reduced considerably by the presence of corrosion products derived from the corrosion of a separate zinc specimen in the same vessel, or from a zinc-rich paint. From these results it might be anticipated that protection of the steel could be achieved by the addition of ~ i n csalts, but in this we were not successful. The corrosion of mild steel in sea water (intermittently heated) was increased by about 30% by addition of either 10 grams per liter of ZnC&or by 1.0 gram per liter of ZnS0,.7Hz0. The corrosion of brass was trebled by the latter addition. It should be emphasized, however, that the conditions in these tests were very different from those in which Dr. Thornhill used zinc salts with success. It is evident that complications are introduced both by the higher temperatures and different electrolyte, and further study in this field should be worth while. LITERATURE CITED
(1) Bengough, Lee,and Wormwell, Proc. Roy. 800. (London), A134,
320 (1931). (2) Evans, U. R., “Metallic Corrosion, Passivity and Protection”. London, E. Arnold & Go., 1937.
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