THE wastage of metals in World Wars I and I1 has brought home to us the fact that depletion of our mineral resources i s becoming a serious problem which demands earnest study and conservation. Widespread practicGl interest in corrosion problems resulted in the organization of the American Coordinating Committee on Corrosion i n 19381 it n o w includes representatives from nearly a l l the large national technical societies and i s doing useful work in coordinating research on this problem. During the past ten years or so, many chemists and metallurgists have found full-time employment as “corrosion engineers”. A national association of this group has recently been organized. The time seems to be near when w e will need a technical periodical to keep us informed of developments in this field. Corrosion may be defined as the result of chemical or electrochemical reaction of a metal with its environment1 an inhibitor i s a substance which, when added to a corrosive media in relatively small amounts, retards or stops this reaction. Generally speaking, there are three ways by which the useful life of a metal may be extended: (1) by the use of a more durable metal or alloy, usually at d considerably higher cost-for example, stainless steel in place of carbon steel. (2) By interposing a protective layer of some inert material between the metal and its environmentl this type of protection includes coating materials such as bituminous compounds, paints, and portland cement. (3) By changing the environment so that there will be less tendency for i t to react with and damage the metal. Electric currents, impressed on the
metal as cathode, n o w often used for protecting the exterior of underground pipe lines and the interior of water tanks, may b e included in the last group) deaeration of water i s another example. Each of these protective measures has limitations. The papers in this symposium discuss inhibitors for industrial water supplies and for neutral aqueous solutions, and therefore come within the third category. The materials known as acid inhibitors, which are designed to prevent undue wastage of metals in pickling, are not included. O n l y one method of inhibiting corrosion in gasoline pipe lines w a s mentioned in the discussions) this was the basis of an article (page 749) immediately following this symposium. Inhibitors for the latter purposes which are not covered i n this symposium deserve consideration a t a future time.’ The authors were requested to discuss the efficiency of inhibitors particularly on bimetal systems and concentration cells. Chemists have rendered conspicuous service in the creation of “better things for better living through chemistry”. The papers presented here indicate not only what has been done to combat destructive corrosion i n aqueous media by means of inhibitors, but more particularly the limitations of these materials. It is unlikely that a general panacea will be discovered for corrosion, but w e trust that research chemists will take this discussion as a challenge to develop more effective inhibitors, particularly for domestic water supplies.
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FUNDAMENTAL PRlNClPLES 0. R. EVANS Cambridge University, England
FACTORS DECIDING between corrosion and inhibition are discussed. When the immediate corrosion product is sparingly soluble, attack is likely to be stifled. However, when the immediate product is soluble but a secondary product is only sparingly soluble, attack will uaually continue. Anodic inhibitors function by forming a sparingly soluble anodic product. When the corrosion reaction is under cathodic control, anodic inhibitors are likely to be “dangerous” since, when added in insufficient concentrations, they tend to localize attack but may result in intensified attack at those areas which continue to corrode. Cathodic inhibiton form sparingly soluble producta on the cathode areas. This type of inhibitors i s likely to be “safe”, since they do not cause intenaification of attack. However, they are generelly leas effective In reducing the total corrosion reaction than are anodic inhibitors. Several examples of both types of inhibitors are given in this paper.
thrown down out of physical contact with the metal, will not, in general, stifle attack. Thus iron suffering electrochemical corrosion in sodium chloride solution will yield ferrous chloride and sodium hydroxide as anodic and eathodic products, respectively, and these will interact a t a short distance from the actual site of attack to give, with further oxygen, hydrated ferric oxide (yellow rust); the precipitation of this relatively loose rust will not put a stop to further sttack. An electrochemical mechanism is, however, not necessary for the production of a nonprotective precipitate a t a distance from the metallic surface. Iron placed below distilled water to which oxygen has slow accws will pass into solution as the relatively soluble ferrous hydroxide, which will be converted to the less soluble hydrated ferric oxide (rust) at a slight distance from the metal where oxygen is present in excess; this constant removal of ferrous hydroxide will prevent the maintenance of a protective film on the metal itself and thus allow the attack to proceed. If the oxygen supply is sufficient to cause the formation of the hydrated ferric oxide in physical contact with the metal, corrosion will be stifled (8). In these cases of “two-stage attack” the secondary precipitate need not be an oxide or hydroxide. Lead placed in distilled water below air containing a trace of carbon dioxide may behave in two ways. A film of basic lead carbonate may be formed over
M E metals are attacked by most reagents except those which would lead to a sparingly soluble compound as the immediate corrosion product. The relative resistance of lea& to sulfuric acid, of silver to hydrochloric acid, and of magnesium to hydrofluoricacid, is connected with the low solubilities of lead sulfate, silver chloride, and magnesium fluoride, respectively. The formation of a sparingly soluble body as secondary product,
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the lead surface and thus stifle further attack. Alternatively, the lead may enter the water as relatively soluble lead hydroxide, and the basic carbonate may be precipitated a t a distance, usually as a scum on the water surface; this change uses up the l a d hydroxide from the water, which locally becomes unsaturated with the hydroxide so that corrosion continues. Thus the trace of carbon dioxide may either inhibit or stimulate corrosion according to circumstances, and the decision between the two possible occurrences depends both on the previous history of the lead, and also on the area of the free surface of the water ( I ) . It is evident that the same substance may promote or inhibit corrosion according to circumstances. Frequently the cause is not in doubt; but the complexity of the situation is distressing to the practical man, who would like to classify every substance as definitely dangerous or definitely beneficial, and is perturbed to find that many so-called inhibitors are really “wolves in sheep’s clothing”. CLASSIFICATION OF INHIBITORS
I n order to render water noncorrosive, it is necessary to introduce some substance which will ensure that incipient corrosion leads to a sparingly soluble body and thus stifles itself. Since most dangerous corrosion processes follow an electrochemical mechanism, i t is possible to choose a substance which will yield either a sparingly soluble anodic body or a sparingly soluble cathodic body; these substances are known as anodic inhibitors and cathodic inhibitors, respectively. Probably, however, any substance which is strongly adsorbed at a metallic surface will tend to shield the metal and retard attack; even a porous sheath of adsorbed substance may diminish the rate of corrosion processes by interfering with the replenishment of the corrosive substances. The retardation of many types of attack by colloidal substances or by the complex organic bodies used as pickling inhibitors may be connected with general adsorption, It should be remembered, however, that colloidal particles usually carry electric charges, and will consequently tend to move preferentially toward either the anodic or the cathodic areas-generally (at least in acid solutions) toward the latter. Most of the inhibitors used today for the treatment of cooling waters appcw to act specifically by smothering either the anodic or the cathodic reaction. Anodic Inhibitors. If a water is made sufficiently alkaline to ensure that the principal anion is (OH)-, the attack on iron is likely to be smothered, since the immediate anodic product is ferrous hydroxide which, although appreciably soluble in neutral water, is very sparingly soluble in an alkaline liquid. Most of the anodic inhibitors used today are alkaline bodies such as sodium hydroxide, sodium carbonate, sodium silicate, and sodium phosphate, but the special value of the last two is probably due to the formation of sparingly soluble films of iron silicate or phosphate. The chromates which are particularly efficient as anodic inhibitors, owe their value to the fact that any ferrous salts momentarily formed will be precipitated as a mixture of hydrated ferric and chromic oxide (6). An iron surface which has been exposed to air will already carry a film of feiric oxide, but this will generally be too discontinuous to prevent rusting by untreated water. If the water contains a chromate, the discontinuities will be repaired with the mixture of ferric and chromic hydroxides thrown down in physical contact with the metal, and thus prevent the formation of loose rust. The highly soluble chromates of so-’ dium or potassium are used to render waters noncorrosive, while less soluble chromates (those of zinc, lead, barium, and strontium) are used as inhibitive pigments in paints. Anodic inhibitors are extremely effective when added in sufficient quantity; iron will remain bright and free from rust for long periods in water containing a chromate or an alkali. If the inhibitor is present in insufficient quantity, corrosion will set in locally and may be more intense than if no inhibitor had been added. This intensification of attack can easily be explained (6).
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The addition of a n anodic inhibitor in insufficient amount will stifle corrosion over a large part of the iron surface, but will still allow i t to proceed at the most susceptible places. If the corrosion rate is controlled solely by the anodic reaction, the rate of attack will be diminished in proportion to the reduction in the area undergoing anodic attack; in other words, the intensity of corrosion &he attack per unit area of the part affected) will be unchanged. But if the control is even in part cathodic, the diminution of the total attack will be less than the diminution of the area suffering attack, and the intensity will therefoie be increased. For reasons discussed elsewhere ( I I ) , this intense local attack tends to be concentrated at the water line or a t the margin of drops. This intense attack a t a certain critical concentration of inhibitor may occur even when no substance is present except the inhibitor and water. Thus local corrosion of steel has been encountered along the margins of drops (3) of sodium carbonate solution at concentrations about 0.05 N , and at the water line of partly immersed specimens a t concentrations between 0.03 and 0.16 N (11). If, however, the water contains chlorides, which militate against the action of inhibitors, probably owing to the power of chlorine ions to penetrate protective films, the dangerous intensified attack will occur at much higher concentrations where the corrosion would, in any case, be more rapid. Consequently, a mixture of chlorides and carbonates in unfavorable proportions may produce much more rapid grooving, pitting, or intensified corrosion than can be produced by carbonate alone. This fact should be borne in mind by those who advocate the use of alkaline inhibitors for waters to which chlorides are likely to have access. Cathodic Inhibiton. It has long been known that drops of zinc sulfate qotution usually produce slower corrosion than the distilled water from which the solution was made (3). This is due to the fact that the cathodic product, zinc hydroxide, is a sparinqly soluble body, Zinc salts have, however, been used little in industry as inhibitors. The particular cathodic inhibitor which makw it, possible for steel pipes to convey many natural waters is calcirlm bicarbonate. A hard water containing calcium bioarbonatv will tend to throw down a film of calcium carbonate on an iron pipe, owing to the rise in pI-l value connected with the cathodic reaction, provided the water does not contain carbonic acid in excess of the amount needed to stabilize the calcium bicarbonate. The film of chalk precipitated on the steel surface will usually bec9me converted to a clinging form of rust through interaction with anodically produced iron salts. The layer of chalky rust formed in pipes through whieh hard water runs may be inconvenient in reducinq carrying capacity, but at least i t tends to check rorrosion by cold water. Althouqh less liable to produce intensification of attack when added in insufficient quantities, cathodic inhibitors are less effici mt than anodic inhibitors, since they do not restrain attack until a film of visible thickness has been formed; even then the corrosion, although slowed down, is not arrested altogether. It is not possible to keep iron bright in a water dosed with a cathodic inhibitor alone. On the other hand, treatment of a water with a cathodic inhibitor may leave a film which will continue t o retard corrosion even after dosage of the water with the inhibitor has been discontinued. This is important in the case bf iron which has been galvanized or coated with zinc-rich paint. In recent experiments a t Crtmhridge (7) steel received a single coat of a zinc-pigmented paint, and a scratch was made through the paint exposing the steel; the specimen was then patltly immersed in sea water, and no loose rust has appeared even after twenty months, notwithstanding the fact that the zinc was found to be no longer in electrical ronnection with the steel. This favorable result was undoubtedly due to the fact that, in the opening stages when the zinc and steel were in contact, a film of magnesium hydroxide, calcium carbonate, or possibly zinc hydroxide was deposited
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o n the steel exposed at the scratch line; this film afforded protection even when the cathodic treatment had ceased. The value of galvanized coatings in protection against cold water may be partly due to the films thrown down by cathodic action (9)by operation of the cell zinc iron. The polarity of this cell reverses after several days in water at ordinary temperature (9) and much more rapidly at 70” C. (10). The reversal does not necessarily interfere with protqction at discontinuities in the coat, at least against cold water, since the film formed during the period before reversal may continue t o give protection.
pale brown rust still containing magnesia (hence the unusual paleness); but under boiler conditions i t would probably s d e r dehydration and give a Conducting layer of anhydrous magnetite, which would act in the same way as mill scale in producing pitting. This seems to afford a satisfactory explanation of the fact that magnesium salts, which undoubtedly diminish corrosion a t ordinary temperatures under half-immersed conditions where the adherent product is unsuited to act as a cathode, may, under boiler conditions where the product is compact and probably conducting, give rise t o dangerous pitting.
SAFE AND D A N G E R O U S INHIBITION SYSTEMS
LITERATURE CITED
I
From the foregoing it apljeara that, in general, anodic inhibitors are efficient but dangerous, while cathodic inhibitors are inefficient but safe. Attempts made to obtain a system which would be both safe and efficient, by using cathodic and anodic inhibitors concurrently, have been only partially successful (11). Later attempts have been made to obtain the desired result by using anodic and cathodic inhibitors alternately; the results are more encouraging, but a pronouncement on the matter would be premature. Circumstances are, however, known where even a oathodic inhibitor may intensify attack. One such case is discuseed in thiu symposium by Thornhill (page 706),who shows that certain salts bring the attack t o the water line-i.e., near the source of oxygen, where otherwise it would be well below the water line; this shift in the site of attack is accompanied by greater local inteiisity But there are situations where any inhibitor-cathodic, anodic, or general-which covers the greater part of the surface with a protective layer may, in time, lead to intensified attack. Consider two closed iron boxes completely filled with water, which remains until all the oxygen, carbonic acid, and other corrosive substances present have become exhausted; in the first box the whole of the metallic surface is exposed to the water, but in the second, 90% is covered with a protective layer. The final corrosion produced will be the same in both boxes, although in the second box i t may require a longer time for completion; but since the corrosion in the second box is confined to one tenth of the original area, it will ixi the end be more intense than that in the 6rst box, whatever the mechanism of attack or the nature of the protective film may be. Localization of attack possesses considerable importance in connection with the corrosion of boilers. Turner (18)reports that the preaence of mill scale on the surface of boiler tubes tends to produce pitting a t breaks in the scale. The reason is clear. The scale will constitute a large cathode, and the steel exposed at the discontinuities in the scale will be a small anode. Any oxygen reaching the large scale-covered surface will act as cathodic stimulator, and the current flowing will produce anodic attack a t the breaks in the scale. Since this attack is concentrated on a small area, it is liable to be intense and lead to pitting. Vernon and Wormwell (19)point out that the presence of magnesium and calcium wits in the water (particularly sulfates) can lead t o pitting, apparently by producing an oxide scale not unlike mill scale and capable of acting in the same way. The fact that such salts lead t o the production of a compact oxide scale may perhaps be explained as follows: I n the author’s research (4) on the corrosion of steel specimens partly immersed in magnesium sulfate solution at ordinary temperatures, it was noticed that a bright green clinging deposit was formed a t places which apparently had been cathodic but were becoming anodic. This was probably a hydrated magnetite (with some ferrous ions replaced by magnesium ions, which explains the unusually bright green color) formed in the presence of a restricted quantity of oxygen by the action of ferrous sulfate, the anodic product, upon the layer of magnesium hydroxide which had previously been deposited by cathodic action. Under the conditi6ns of the author’s experimenta. the green substance was subsequently oxidized to
(1)
Bauer, O.,and Wetsel, E., Mitt. deut. Materialpr.llfungsansstalt., 34, 347 (1916); Bauer, O.,and Schikorr, G., Zbid., Sonderhefte 28,69(1936); Schikorr, G.,Korrosionu. MBtallschzrtz, 16,
181 (1940). (2) Britton, 9. C.,J. SOC.Chem. I d . , 65, 19T (1936). (3) Evans, U. R.,Ibid., 43,316 (1924). (4) Zbid., 47,67T (1928). (5) Evans, U.R.,Tram. Electrochem. Soc., 69,213(1930); Chymwski, E., and Evans, U. It., Zbid., 76,215 (1939). (6) Hoar, T. P.,and Evans, U. R., J . Chem. Soc., 1932,2476. (7) Mayne, J. E. O., and Evans, U. R., Chmistry & Industry, 1944, 109. ( 8 ) Mears, R. B., and Evans, U. R., Trans. Faraday Soc., 31, 627 (1936). (9) Roters, H.,and Eisenstecken, F., Arch. EisenhUttenw., 15, 59 (1941). . (10) Schikorr, G.,Trans. Electrochdm. Soc., 76,247 (1939). (11) Thornhill, R. S.,and Evans, U. R., J. Zron Steel Znst. (London), 146,73 (1939). (12) Turner, T. H., Proc. Znst. Mech. Eng. (London), 149,77 (1943). (13) Vernon, W. H I., and Wormwell, F., Zbid., 150, 111 (1943).
Discussion o f Paper by U. R. Evans D. S. MCKINNEY AND J. C. WARNER Grnegie Institute of fechnology, Pittsburgh, Pa.
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EVERAL questions will be raised t o make clearer the interpretation of Dr. Evans’ paper. I n the first place, the interpretation of corrosion phenomena would be improved by treating all anodic and cathodic reactions, as well as the so-called secondary reactions, from the ionic standpoint. For example, the author states: “Iron suffering electrochemical corrosion in sodium chloride solution will yield ferrous chloride and sodium hydroxide as anodic and cathodic products, respectively, and these will interact a short distance from the site of attack.” We prefer to say: “Iron suffering electrochemical corrosion will yield ferrous ion and hydroxyl ion as anodic and cathodic products, respectively, and these will interact at a short distance from the actual site of attack.” The author defines anodic inhibitors and cathodic inhibitors as substances which will yield either a sparingly soluble anodic body or a sparingly soluble cathodic body, respectively. He does admit that inhibition may be caused by adsorbed substances such as those which are effective in the pickling of iron and steel. We would prefer a broader definition-namely, that any substance be considered an inhibitor which serves t o increase anodic or cathodic polarization. Further amplification of the author’s explanation of the action of chromatea as anodic inhibitors seems desirable. When one considers the fact that chromic salts are relatively poor inhibitors, his explanation of the action of chromates through the formation of a hydrated ferric-chromic oxide film appears to be inadequate. We would also like t o call attention to the fact that the alkaline
