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INDUSTRIAL A N D ENGINEERING CHEMISTRY
character of many anodic inhibitors is more or less incidental. The important point seems t o be that the p H must be sufficiently high to give a reasonable concentration of the inhibiting anion. In the author’s example to show that corrosion would be more intense in the one of two iron boxcs which had 90% of its surface covered with protective coating, other conditions being equal, i t is implied that corrosion will cease when all oxygen and carbonic
Vol. 37, No. 8
acid have become exhausted. It seems important to emphasize that the corrosion of iron in contact with pure water to form either ferrous hydroxide or magnetite is spontaneous, the formation of the latter corrosion product corresponding t o thc more spontaneous reaction’. 1 Corey. R. C., and Finnegan, T. J., Proc. A m . SOC.Testing Materials. 3Y; 1257 (1939); Thompson, M. deK., Trans. Electrochem. SOC.,78, 251 (1940), Warner, J. C., Trans. Electrochem. SOC.,83, 319 (1943).
Reply to Discussion U. R. EVANS
IT
I S equally correct to regard the corrosion products as either ( a ) ferrous chloride and sodium hydroxide or ( 6 ) ferrous ions and hydroxyl ions; it is well known that sodium hydroxide dissociates in solution into sodium and hydroxyl ions, and one sort of ion cannot exist without the other. Which form of expression is most helpful depends on the nature of the argument and, perhaps, the temperament of the audience being addressed. I n the present case it is more informative t o call the anodic product ferrous chloride than ferrous ions. The course of attack may sometimes be affected by the nature of the anion with which the metallic cation is paired; for instance, ferrous sulfate is more likely t o become supersaturated and separate out than is ferrous chloride, and, as shown by W. J. Miiller, such separation may be the first stage of passivity. The fact that chromic salts are relatively poor inhibitors is not inconsistent with the view that the good inhibition by chromates is due t o the precipitation of hydrated ferric-chromic oxide over anodic points. The obstructive action of a precipitate depends, not so much on what is precipitated, as on where it is precipitated. I n the case of chromic salts, chromic hydroxide is thrown down on the cathodic zone and fails seriously t o hinder attack; in the
case of the chromate, the mixed hydroxide is precipitated locally at places where anodic attack would otherwise set in and prevents it from developing. The barrier needed t o prevent anodic action is essentially a type impervious to ions, whereas that needed t o prevent cathodic action must be impervious to electrons. Cases are known where films of considerable thickness have formed over the cathodic zone, which, although possessing electronic conductivities, provide no hindrance t o the electrochemical action. That corrosion of iroq by water may continue even when oxygen and carbonic acid are absent is generally admitted, and credit is due J. C. Warner for pointing out that this is thermodynamically possible, as well as to the other authors quoted for showing that it actually does occur. However, the hydrogenevolution type of attack produced by neutral water is frequently slow compared to the corrosion produced in the presence of oxygen or acid, and the hypothetical example used in showing that intensification is sometimes possible with any film-forming inhibitor, whatever its mechanism, appears t o be legitimate even if it represents an oversimplification of the state of afiairs commonly encountered in practice.
R. S. THORNHILL’ Cambridge University, England
DATA are presented showing that the rate of corrosion of steel In tap water is reduced b y adding small quantities of zinc and manganese salts, chromic salts bring about a certain measure of inhibition at low, but not high concentrations. A t relatively high concentrations of zinc and chromium, marked intensification occurs in the water-line zone, manganese salts are free from this defect, since the water-line zone is not attacked. Zinc and chromic salts must, under some circumstances, be regarded as “dangerous” inhibitors. HE addition to corrosive waters of small quantities of certain chemicals to minimize corrosion has been common industrial practice for many years. Probably the first reference to the beneficial effect of certain metals in solution was that of Parker (7) in 1881 who, in the course of experiments on the prevention of corrosion in boilers by zinc protectors, came t6 the
T
1 Present address, Imperial Chemical Industries, Ltd., Winnington, Northwloh, Cheshire, England.
conclusion that much of the success must be ascribed to zinc salts. The beneficial effect of zinc salts was later recorded by Evans (B) in experiments on drop corrosion; protective treatments for magnesium and steel depending on the precipitation of a film of zinc hydroxide have also been developed (5,9). I n some of these cases the deposition of zinc hydroxide arises through the formation of alkali at the corrosion cathodes; presumably the hydroxides of other metals should behave in a similar manner. A further important class of inhibitor relies upon the formation of insoluble bodies with the anodic products of corrosion. I n this class are the chromates, phosphates, and silicates. I n choosing an inhibitor from one of these groups, it is important to remember that inhibition may result in (a) diminution in the corroded area and (b) reduction in the corrosion rate; if effect a exceeds b, there will be an intensification of attack; thus certain inhibitors may be “dangerous”. Evans (3)concludes that inhibitors which function by the deposition of material on the cathode are likely to be “safe”, in that no intensification should
Augucrt, 1945
INDUSTRIAL AND ENGINEERING CHEMISTRY
result from a n i n s d c i e n t amount of inhibitor. Substances, such
as the chromates, which stifle the anodic reaction may prove dangerous if the rate of corrosion is determined by the cathodic reaction. It is therefore important in investigations on inhibitors to ascertain particularly if there are certain concentrations of inhibitor which lead to intensification of attack. Recently (8) the systems chromate tap water, chromate I potassium chloride have been examined in greater detail, with measurements both of weight loss due t o corrosion and of maximum depth of penetration. The chromate was added in the form of the potassium or magnesium salt, and a wide range of concentration was studied. It waa thought that magnesium chromate might be free from the defect of intensified attack which attends the use of potassium chromate, but this proved to be incorrect, a t least in potassium chloride solutions. Evidently, in partly inhibited solutions the rate of corrosion is governed by the anodic reaction, and the presence of magnesium ions is without much influence.
I
Table I.
Iron Content of the Corrosion Product after Immersion of Steel Strips for 60 Days in Tap Water Containing Zinc Sulfate, Manganese Sulfate, and Chrome A l u m Iron in Corrosion Produat, Mg. Tap water Molar Concn. Cambridge aontainin Inhibitor of Inhibitor tap water additional 0.010 22.1 17.9 Zinc sulfate 12.9 96.2 0.0033 0.0010 61.3 68.6 0.00033 83.4 96.2 107.2 109.6 0 Manganem sulfate 0.010 25.8 29.1 0.0033 45.4 40.6 0.0010 80.9 87.0 0.00083 87.1 79.6 0 110.4 110.9 Chrome alum 0.010 106.7 101.6 0.0033 44.4 50.7 35.8 20.7 0.0010 0.00033 57.2 61.6 0 102.0
80s
...
As a consequence of this lack of success, it was thought desirable to return to a study of purely cathodic inhibitors, and examine a number of salts of metals possessing insoluble hydroxides, some of which had already been studied by Borgmann (1). The conditions of test were purposely made as severe as possible by the use of scratched, tinted specimens, so that corrosion would tend to be localized a t the scratch line. A preliminary set of experiments with the cations Fe--, Mg--, Cam-, Co--, Sn--, Ba--, Zn--, Mn--, and Cr--- served to show that the three latter metals merited further study. CORROSION IN TAP WATER CONTAINING ZINC, MANGANESE, AND CHROMIC SALTS
The zinc and manganese were added as sulfates, and chromium as tho double sulfate with potassium (chrome alum). The corroding solutions were: (a) Cambridge tap water, a chalk water partially softened by base exchange and containing 37.5 p:p.m. Ca--, 48.7 Na-, 120 COa--, 14 SO,--, 13 C1-, 25 Nos-, and also 14 p,p.m. SiOl (pH 7.3); and (a) a water prepared by mixing nine volumes of tap water with one volume of tap water saturated with carbon dioxide. Enough salts were added to 100 cc. of each of these waters to give solutions which were, respectively, 0.01, 0.0033, 0.001, and 0.00033 molar. Blanks consisting of water containing no inhibitor were included in the experiments. The specimens were strips of a type of steel used for tin plate, and contained 0.13% carbon, 0.35% magnanese, 0.01% silicon, 0.061% sulfur, 0.060% phosphorus, and 0.080% copper. The strips, measuring 0.75 X 2.5 inches, were abraded on both sides with John Oakey No. 0 emery cloth and degreaaed with redis-
707
tilled acetone. They were heated in an oxidizing flame until the thirdorder interference colors had come and gone; before immersion, they received a vertical scratch line with the Mears-Ward scratch machine (6). The specimens were half immersed a t an angle of about 60" to the horizontal in small jars, which held about 40 cc. of solution. The jars were not covered but were kept together in a closed cabinet. I n this way losses due to evaporation were found to be negligible. The experiments were concluded after 60 days, when the rust was removed from the specimens by gentle scrubbing and, together with that already in the corrosion vessels, was estimated in the usual way with potassium di-chromate. The figures obtained are expressed as milligrams of iron in the corrosion product and should be equivalent to the weight loss of the specimen. Table I gives data for the three salts of different concentrations. Considerable protection has been furnished by zinc and manganese sulfates, but with chrome alum the amount of corrosion passes through a minimum as the concentration increases. Additional carbon dioxide renders the solution more corrosive when ainc salts are present, and less in the presence of manganese. Regarding the'distribution of corrosion the following remarks may be noted: 1. When zinc salts are added to tap water, the corrosion is gradually spread out from the scratch line t o include more and more of the immersed area. When the concentration is of the order of 0.0033-0.01 molar, the immersed area is covered with a rust film; below it, however, and next to the metal is a gray film of, presumably, zinc hydroxide. At lower concentrations the water-line zone remains free from attack, and crystals of calcium carbonate may be seen adhering t o what were probably the cathodic spots. The greatest amount of penetration is t o be found in the waterline zone in 0.01 M zinc sulfate; in dilute solutions the deepest penetration is around the margin of the corroded and uncorroded areas but, in fact, is no greater than that found in uninhibited solutions. 2. Manganese salts also spread out the corroded area but iiot t o the same extent as with zinc. Even in the strongest solutions the water line remains immune, and the greatest penetration, which again takes place a t the margin of the corroded and uncorroded zones, is similar to that found on the blank specimen. 3. Incrertsing concentrations of chrome alum progressively spread out the corroded area until the whole of the immersed area, up t o the head of the meniscus, becomes attacked. I n these concentrated solutions the lower half of the specimen is severely etched, whereas the upper half is covered with a lightly adherent yellow corrosion product. The greatest penetration is again along the water line. From these results it follows that the considerable penetration found on specimens in ordinary tap water is not eliminated, despite the amount of extension in the area of corrosion. Further, the actual site of the greatest penetration may be displaced from the boundary between the corroded and uncorroded zones t o the water line; this was observed with salts of zinc and chrome alum. Corrosion at the water line is usually intense; consequently i t cannot be said that zinc salts are %afe", unless the concentration can be kept low; chrome alum cannot be classed as inhibitor at any but low concentrations. CORROSION IN SOLUTIONS CONTAININCS CHROMiC CHLORIDE
The fact that high concentrations of chrome alum exert no inhibiting action has already been noted, but it must be remembered that by adding chromium in this form an extra quantity of a corrosive material--e.g., potassium sulfate-is being supplied. Experiments have therefore been carried out with chromic chloride, added as the green variety, [Cr(H*O)&12]Cl.2H20; the liquids employed were the two tap waters and also sodium chloride solution.
I N D U S T R I A L A N D E N G IN’E E R I N G C H E M I S T R Y
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Chrome Chloride, Tap Water Series. Sufficient chromic chloride to give concentrations of 0.01, 0.0033, 0.0010,and 0.00033M were weighed out and dissolved in samples of the two waters. The specimens were tinted and scratched, and the period of immersion was again 60 days. The results of these tests are es follows: Molar Concn. of Chromic Chloride 0.010 0.0033 0.0010 0.00033 0
Iron in Corrosion Product, Mg. Cambridge Tap water containtap water ing additional CO1 104.7 85.1 52.1 44.6 52.6 64.2 83.8 78.7 101.9
111.3
As in the case of chrome alum, these figures again show a minimum in the region of 0.0033 AI, and the distribution is also similar. At low concentrations the corrosion is confined to the lower half of the specimen, whereas the water-line zone remains free from corrosion. White crystals indicate that soluble calcium bicarbonate has been converted to calcium carbonate by cathodically formed alkali. At higher concentrations the whole of the immersed area is covered with a dense deposit, which is yellow on the outside and black next to the metal. I n the water line itself severe corrosion occurred as a result of the formation of a “box”, a thin membrane of rust, k e d t o the metal and adhering to the air-water interface. One may conclude that the chromium hydroxide is not protective; the reason is that, in the main, i t is not formed on the metallic surface but as a loose precipitate which settles to the bottom of the vessel. Chromic Chloride-Sodium Chloride Series. As most of the previous work had concerned tap water, it was thought of interest to observe the effect of high chloride concentrations. The solutions were 1, 0.33,0.1,and 0.033iM sodium chloride, and for each concentration two strengths of chromic chloride were investigated-e.g., 0.01 and 0.0033 M . Unscrat,ched abraded specimens were employed. The results of these experiments, which lasted 70 days, follow: Molar Concn. of NaCl 1 0.33 0.10 0.033 0
Iron in Corrosion Product, M g . 0.0033M CrCls 0.010M CrCls 105.1 106.7 131.7 91.7 132.0 78.0 82.8 153.6
71.1 80.6
145.7 169.5
I n the presence of 0.0033 -11 chromic chloritle, increasinq the concentration of chloride ion increasen the rate of corrosion; with 0.01 M chromic chloride the reverse happens. A study of the data suggests the reason: In molar sodium c*hloridethe inhibiting effect of trivalent chromium is negligible, but the actual figure for the corrosion rate lies between the values given by the two strengths of chromic chloride. In these chloride solutions the corrosion is so genein1 that thwe is little or no localization. DISCUSSION
Corrosion by tap water is dimiiiished by sniall additions of zinc, manganese, and chromir salts; but whereas larger amounth of zinc and manganese proerrqhively reduce thca total rorrosion still farther, this is not the caw with large dosagev o f chromium. Further, despite this improvement in the corrosion rate, zinc sulfate, which had previously been regarded as a “safe” inhibitor, has been shown at high concentrations to lead to intensification at the water-line zone. The formation of zinc and chromium hydroxides can readily be seen in the corrosion product, and it is probable that they have been produced by cathodirally formed alkali. When the con-
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centration of inhibitor is low, there is always sufficient alkali,
from the normal corrosion process, to keep the water line and the upper half of the surface free from corrosion; hence there is no danger from water-line attack. There may, of course, be enhanced penetration at other places. The reason corrosion is reduced, even a t low concentrations, can be ascribed with a fair degree of certainty to the deposition of hydroxide or hydrated oxide films, at least in the case of zinc. I n the case of manganese, visible films, presumably of a hydrated manganese oxide, have been observed; but whether they interfere with the cathodic reaction is not known and is a matter for future experiment. Herzog and Chaudron (4) give high place to manganese oxidc ax a n inhibitor of the oxygen-depolarization reaction. The behavior of chromium is peculiar, since chromium hydroxide is largely precipitated at a distance and presumably does not greatly interfere with the cathodic reaction. As to the behavior in stronger solutions, i t is obvious that there is sufficient metallic salt there to remove the alkali as soon as it is formed. Consequently, the protected area shrinks to very small proportions, and there is no immune water-line zone in solutions containing zinc and chromic ions. But since zinc hydroxide is precipitated in contact with the steel, the inhibitive properties are retained, in contrast to the lack of protection noted with chromic salts, although the surface is covered with a thick layvr of corrosion product. The behavior of manganese is of interest, since corrosion does not extend to the water-line zone, and protection is still maintained by cathodic alkali; the separation of calcium carbonate is proof of this. When corrosion occurs in or near the water line, there is probability that soluble corrosion products will diffuse outward to the air-water interface and be precipitated there as a membrane (9). If this membrane adhercs to the metal a t the head of the meniscus, conditions are set up which lead to intense attack in the meniscus zone, and this is evidently the reason for the excessive penetration noted with both zinc and chromium salts. SUMMARY
Experiments on the addition of zinc, chromium, and manganese salts to Cambridge tap water indicate that zinc and manganese reduce the corrosion to about 20-30%. While zinc sulfate provides the more efficient inhibition, its use is associated with excessive penetration along the water line; manganese sulfate is free from this defect. Chromic salts are not inhibitive except at low concentrations. Four conditions are suggested which decide whether a metallic salt may function effectively as a cathodic inhibitor: 1. The 3alt 4iould possesr it11 imoluble hydroxide or hydrated oxide. 2. The hydroxide should b v procipittttecl on the metal and not a t a distance. 3. The deposit so formed qliould interiere with the cathodic reaction. Thus thick films o f iron hydroxide formed on the cathodic area do not necessarily inhibit the corrosion process. 4. The geometry of the situation in which the inhibitor is being ubed should be such as to avoid the deposition of corrosion prodiwts at air-liquid interface\. LITERATURE CITED
Borginann, C . W.,IND.ENO.CHEW.,39,814(1937). Evans, U. R.,J . Soc. Chem. Ind., 43,315T (1924). Evans, U. R.,TTUW.Electrochem. SOC.,49,213 (1936). Herzog and Chaudron, Compt. Rend., 190, 1189; 192,837(1930); 193,587 (1931): Trans. Electrochem. SOC.,44,87 (1933). Lewis, K.G., arid Evms, U. R., J . Inst. Metals, 57, 231 (1935). Mears, R. B.,and Ward, E. D., J . Soc. Chem. I d . , 53, 382T (1934).
Parker, W., J.Iron Steel Inst. (London), 1881,39. Thornhill, R.S.,and Evans, U. R., Ibid., 146,73 (1942). Thornhill, R.9..and Evans, U. R . . 5th llept. Corrosion Comln. Iron Steel Inst., 21,381 (1988).
