INDUSTRIAL AND ENGINEERING CHEMISTRY
650
Vol. 23, No.6
Products of Corrosion of Steel' Factors Determining Their Composition and Its Influence on Rate of Corrosion in Oxygenated Water H. 0. Forrest,2B. E. Roetheli,r and R. H. Brown8 RESEARCH LABORATORY OF APPLIEDCHEMISTRY, DEPARTMENT OB CHEMICAL ENGINEERING, MASSACHUSETTS INSTITUTE CAMBRIDGE, MASS.
OF
TECHNOLOGY,
The composition and uniformity of products formed in precipitated a reaction takes place between it and ferric the corrosion of steel in oxygenated water are shown t o be hydroxide to form the black granular magnetic oxide of important factors in determining the ultimate rate of iron. corrosion. The corrosion products having a major inUnder conditions of agitation such t h a t the pH in the fluence on ultimate corrosion rates are gelatinous ferric liquid film adjacent t o the metal is below the value neceshydroxide and granular magnetic oxide of iron (FeoOl); sary for precipitation of ferrous hydroxide, the red gelatithe former, under the conditions which it is formed, is nous ferric hydroxide will be formed, which effectively very resistant to the diffusion of oxygen and the various reduces the corrosion rate to a very low value. When the ions present, while the latter is essentially non-resistant. pH in the liquid film is allowed t o build u p because of Precipitation of ferrous hydroxide on the metal surface slow diffusion of hydroxyl ions from the liquid film to a t ferrous-ion concentrations encountered in neutral the main body of the liquid (due t o poor agitation), ferric water is possibly only a t pH values higher t h a n 8 (approxi- hydroxide will be precipitated and subsequently magmately), while ferric hydroxide may be precipitated a t netic oxide of iron will be formed. When the film is much lower pH values. However, the ferrous ions initially composed primarily of magnetic oxide of iron the corrosion formed must k s t be oxidized t o ferric ions by the oxygen rate approaches t h a t of a film-free surface. present in the water. Once ferrous hydroxide has been
OR some time films have been considered of major
F
importance in determining the corrosion rates of metals in oxygenated water. Previous papers have shown the effectiveness of films in preventing the corrosion of stainless steel (4) and other metals (1). The work of many investigators has indicated the presence of films even though they are invisible on the metal surface (2, a). The purpose of the present paper is to call,attention to the fact that oxide or hydrous oxide films are of importance in the corrosion of ordinary steels, and to discuss some of the more theoretical aspects of the factors governing film formation. Many investigations have shown that the rust films formed on a steel specimen decrease the rate of corrosion. The effectiveness of such films depends on their specific resistance to owgen diffusion and to their uniformity over the metal surface. The specific resistance of films is determined by the compounds which form the films and by the amounts and the distribution of these compounds from the metal surfaces outward. The uniformity of films is determined by many factors, the most important of which are the original condition of the metals and the distribution of dissolved oxygen and hydrogen ions. A number of previous investigations have shown that the products of corrosion forming the films are oxides or hydroxides, The compounds which usually form the films are ferrous hydroxide, magnetic ferric oxide (Fe304),and ferric hydroxide, and it is clear that a consideration of these compounds will be involved in explaining the mechanism of film formation. I n order to advance any theory for the formation of films, it will be necessary to assume a series of possible reactions. Fortunately, thermodynamic calculations may be made t o ascertain if the assumed reactions are possible. The thermodynamic property of a substance which measures its stability is its free energy, and therefore the best indication of the Received March 23, 1931. Present address, M. W. Kellogg Co., Jersey City, N. J. a Research associates, Massachusetts Institute of Technology.
2
tendency for a reaction to proceed is the decrease in free energy accompanying a chemical reaction. The free-energy decreases accompanying reactions involving compounds which might compose the films are given in Table I. These values are calculated from data on chemical equilibria (6), solubilities (Y),thermal properties (6, 12), and the free-energy contents (8) quoted in the literature. Table I-Free-Energy
Decreases Accompanying Poseible Corrosion Reactions in W a t e r AFZ~OC./ATOMIC WEIGHTFe
-
+ +
Calories
-
Fe(s) H:O(l) f 1/202(0.21 atm.) = Fe(OH)r(s) 2/a02(0.21 atrn.) L/rFesO4(s) Pe(s) Fe(s) 4- a/tHzO(I) 5/40z(o.21 atm.) = Fe(OHla(s) Fe(OH)z(s) 4- l/zHzO(l) '/4Oz(O.21 atm.) Fe(OH)s(s) l/sFeaOi(s) :/zHzO(l) 1/1202(0.21 atrn.) = Fe(OH)a(s) '/aFe(OH)ds) $ z/aFe(OH)a(s) 1/1FaOi(s) '/aH10(1)
+
+
+ +
-
-
+
+58880 +79716
+SI855 f24975 $2140 $8270
Table I indicates that ferric hydroxide is thermodynamically the most stable compound formed. However, the table also indicates that, under certain conditions, it is thermodynamically possible for it to react with ferrous hydroxide to form magnetic oxide of iron, and furthermore that it should be possible to oxidize this compound further to ferric hydroxide. illthough the free-energy decrease affords a true measure of chemical affinity, and therefore makes it possible to predict in which direction and to what extent the corrosion process can ultimately proceed, the mechanism of the processes and the rates a t which the reactions take place cannot be predicted in advance. I n order to show whether or not reactions proceed a t an appreciable rate, experimental facts must be obtained. Precipitation Studies When iron goes into solution as ferrous ion, an equivalent amount of hydrogen must plate out and the solution a t some point must increase in hydroxyl-ion concentration. If the points of increased concentrations of hydroxyl ion and ferrous
tire adjtrceiit,, precipitation of ferrous hydroxide may takr place, whereas, if the region of high ferroiw-ion concentration is distant from the region of high hydroxy-ion concentratinn; the iron will remain in solution. It may, howcvcr, lie oxidizeit to ferric inn and precipit.ated as ferric hydrrixido a t a much loww hydroxy\-ion conceiltrat'Lon. iizii
Loc c
log,, CF&*
__
= logK KFdoH)r - 3(pH)
K
h
(2)
where Krc(os). = solubility product of Fe(O€I), KF*(OE)~ = solubility product of Fe(OH), K m = ionixation constant of water
I t is obvious that at a given pII the coneent.ration of ferrous ion reqiiired for precipitation is far greater than that of the ferric ion and that ferric hydroxido may I x rasily precipitated from solntion having a pII below 7. I n order to study the precipitation of ~ W I Y ~ ~ad I S ferric hydroxide individunlly arid t o g e t h e r ,
The experiment with oxidation of the ferrous-ion solution by dissolved oxygen indicates tliat the ferrous ion is rapidly oxidized to ferric and that precipitation of ferric hydroxide t,akes place in prefercnce to the formation of ferrous hydroxide i i i solut,irmsof IOW pH, as would he predicted from tlic sduidity products. This indicates that in tlic corrosion of iron in iixygonated distilled water tlic first product precipitated rwruld Ire ferric hydroxide, provided the pll of the solution near to tho film was maintained a t a low value and tliat I I X ~ C C I Ihail access to ilre solution. Obviously, if the pIf at any point werc high, ferrous hydroxide might be preeipit:~tt.il. In addibion, if oxygen were prevented from reaching t1:r solution of ferrous ions, the concentration of ferrous ions might build up to a point where ferrous hydroxide woiild precipitate even a t relatively low pN. In view of tlle high rate of oxidation, and of the high concentration of ferrous ions iiecessary for precipitation at a p B below 8, ail increase of liydroxylion concentration near tire film seems the most import.ant factor in the precipitation of ferrous hydroxide. Whcn the precipitated ferrous and ferric hydroxides were rnixcd, a black substance was formed which upon drying in a vacuum was attracted by a magnet. An x-ray spectrograph of the black oxide showed the same spectrum hands as a material wliicb was known to he magnetic oxide of iron. 111 P'i&wre2 arc shown the spectra of magnetic oxide of irorr arid of tire black granular precipitate. This black compound \VIS also formed by aeration of a slurry of ferrous hydroxide i n water. Obviously under bot11 conditions the pEI of the sdut.ion containing the mixed precipitates and tire ferrous hydroxide was high (8-9). The fact that the magnetic oxide W:LS formed on aeration of a precipitate of ferrous hydroxide irdicates its partia,l oxidation to ferric hydroxide anti snbsequcnt combination of tho latter precipitate with tlie fmrous lrydroxide according to Equation 6, Table I. It is evident, therefore, that in regions in the corrosion film where ferrous hydroxide is formed, and where oxygen has access to the film, part of ttre ferrous hydroxidc will be oxidized t o ferric hydroxide and will then combine with ferrous hydroxide to form tlie granular magnetic oxide. The tliird experiment was conducted to determinc more ciefinitely the mechanism of formation of tho inagoetie ?xi&. I k m i Figure 1 it, may hc seen t.liat, a sdution 0.01 riiolal i n f r r r i w ion s 1 ~ ~ 1 lstart d prmipiistc f(5rroos hy-
(1) A 0.01 molal mlotion of ferrous Flaure 2~ 3pecfra of Mitgneflc Oxide of Iron ( A ) and Dried Black tkecipliilfs (8) ammo'lium '*Iate, has a pH Of Although t h e intensity of the linea ia the ~ p e c l i u mof the precipitate is somewhat less tlian that of less than 7, was aerated, and in a few the limeain the oxide spectrum. the ~ i m i i a ~ofi tthe ~ t w o materia)^ i s dearly i n d i c e d nnd minutes a reddish gelatinous precipitate the difference in intensity is probshiy due LO differences in p ~ r t i d es i z e was farmed. (2) Sodium hydroxide was added slowly to a solution which droxide a t a pH of about 8. Actually the ferrio hydroxide was 0.01 molal in ferrous ammonium sulfate and 0.02 molal in wBs formed (up to a p~ of 8.5) &8 red ferric chloride. CJp to a pH of about 7.5 there was formed only a reddish gelatinous predpitate; at ,,H of 8.5 the red gelatinous precipitate from a solution containing both ions. precipitate started to turn black; and at a pII of 11 appeared However, even at the pH of 8.5 enough ferrous hydroxide to be totally black. was precipitated to react with the ferric hydroxide to produce (3) When gelatinous precipitates of ferrous and ferric hy- mme magnetic oxide. When a PIX of 11 was reached, esscndroxJdcs suspended in w a t ~Were mixed, *ere res,lted immetialiy all of tho ferrous ion was preoipitated and the product diatdy a black granular precipitate. A sul,stance was formed in a few minutes when a ferrous hydroxide precipitate in waF ilf*finit(xlyLiarh.
652
INDUSTRIAL AND ENGINEERING CHEMISTRY
It may be concluded from these preliminary studies that: (1) A corrosion product of precipitated ferrous hydroxide will be formed a t ferrous-ion concentrations such as are found in corrosion processes, only when the p H in the film is high. (2) Ferric hydroxide may be precipitated from oxidized ferrous-ion solutions a t pH values much lower than those required for precipitation of ferrous hydroxide. (3) .Ferrous hydroxide, when formed, may react with ferric hydroxide to form a granular magnetic oxide of iron.
I n a previous investigation (9) it was found that, when an oxide-free iron surface was subjected to the action of oxygenated water, the surface first became gradually gray, but by the end of a 40-minute period it was completely covered with gelatinous ferric hydroxide. This ferric hydroxide film was extremely thin, revealing the sheen of the metal beneath. However, when the specimen, covered with this red film,
TIME-HOURS
was allowed to stand in unagitated water for several days, large, loosely adhering patches of black granular substance were formed on the surface. I n view of the phenomena observed in agitated and unagitated water and the preliminary work on the precipitates, it was decided to allow a film to form on an oxidefree iron surface in water under constant conditions of agitation, then to treat it with water in a quiescent state, and finally to treat the specimen again in agitated water. I n this manner apparently the effect of various films formed on the metal surface on the rate of corrosion could be studied. Experimental Procedure The experimental procedure used was essentially that previously described ( 2 ) . Rotating cylinders of low-carbon annealed steel were rendered oxide-free by treatment with 10 per cent hydrochloric acid in a glass jar in the absence of oxygen. The adhering acid was removed by washing with a stream of oxygen-free water until the effluent was neutral. The specimen was then subjected to the action of oxygenated water, but under somewhat different conditions from that in the previous paper ( 2 ) . The specimen was treated with oxygenated water flowing through the jar a t approximately 1cc. per second. During the first phase of the experiment the specimen was rotated a t 228 r. p. m. After it had been rotated for different lengths of time, the oxygenated water was flushed out of the jar with nitrogen and the jar filled with water of known oxygen concentration. It was rotated until a measurable oxygen drop occurred, the decrease in oxygen concentration being taken as a measure of the corrosion rate. After the oxygen
Vol. 23, No. 6
decrease had been determined, the specimen was treated with oxygenated water for another interval and the drop determined as above. The same procedure was continued for nearly 80 hours. I n the second phase of experiment the specimen was not rotated, except during those times when the oxygen drop was being determined. Agitation of the specimen was deemed necessary in order to insure the same liquid film thickness during those portions of the runs a t which corrosion rates were being determined. In the third phase of experiment the procedure was exactly the same as that during the first phase. Results The results of the determinations of corrosion rates during all three phases of the experiment are presented in Figure 3. I n plotting the corrosion rates correction was made for differences in oxygen concentration by dividing by the arithmetic average oxygen concentration of oxygen existing in the corrosion cell during the oxygen-drop determination. At the end of each phase visual observations on the nature of films gave the results recorded in Table 11. Discussion of Results Figure 3 and Table I1 show t h a t marked changes not only in the corrosion rates, but also in the character of the films formed, occurred on passing from one phase of the experiment to another. For a satisfactory explanation of the phenomena observed, the electrochemical theory of corrosion as proposed by Whitney (11) and modified by Walker (9, 10) affords only a starting p o i n t . In its generally accepted form, the theory postulates the dissolution of a metal with the formation of a positively charged metal ion and the simultaneous displacement of hydrogen ion from the solution. Furthermore, the points a t which the metal goes into solution are assumed to be finite distances from the points a t which the hydrogen ions are deposited. Corrosion proceeds because of the removal of hydrogen from the metal surface by reaction with dissolved oxygen. The rate of corrosion in water containing dissolved oxygen is therefore dependent upon the speed with which oxygen reaches the metal surface, and consequently upon the oxygen concentration and the resistance to diffusion offered by the liquid and the corrosion-product films. This theory considers only the initial and final states and is not concerned with the factors influencing the diffusion resistance of films. Table 11-Condition
of Film after Each Phase
E N D OF
CONDITION OF FILM PHA~B Uniform film of red gelatinous ferric hydroxide 1 2 Loose granular black film Red gelatinous ferric hydrovide film formed over the black oxide 3 film of phase 2; less uniform than in phase 1
Since the oxygen-drop determinations were made under the same conditions of agitation, the liquid-film thickness would, according to the general principles correlating velocities and film thickness, be essentially constant and hence the rate of diffusion of oxygen and of. various ions through it would be practically the same. Therefore, changes in corrosion rate which a t any time manifested themselves could be attributed only to the instantaneous diffusion resistance of the film of corrosion products present a t different times during the three phases of the experiment.
June, 1931
INDUSTRIAL A N D ENGINEERING CHEMISTRY
The observed facts cannot be explained by the electrochemical theory alone. However, by supplenienting this theory with other principles a suitable explanation is possible. According to the electrochemical theory, iron will pass into solution as ferrous ion a t some point (Figure 4) as A , and hydrogen ions will leave the solution a t another point as B. If the liquid film adjacent to the metal remains quiescent, the solution a t A will become richer in ferrous ions and that a t B richer in hydroxyl ions. The deposited hydrogen at point B will then be removed by oxygen if the character of the film is such as to allow the oxygen to diffuse through it. Because of the difference in concentration of ferrous and hydroxyl ions in different parts of the film, diffusion of these ions from regions of high concentration may then take place readily; thus some of the ions will diffuse outward toward the main body of the liquid and others in a direction parallel to the plane of the iron surface. It may be seen from the above that the hydrogenion concentration will be different a t different port i o n s of the liquid film near the metal; and from what has been indicated previously, different corrosion products may be e x p e c t e d which may in turn influence the corrosion rates. Because of the diffusion of ferrous and hydroxyl ions parallel to the metal surface, the conc e n t r a t i o n of these ions will tend to build up a t some area between A and B. I n the relatively staFIGURE 4 tionary layer a c o i c e n tration of ferrous and hydroxyl ions may thus be attained which is high enough to permit the precipitation of ferrous hydroxide. If, however, the rate of diffusion of ferrous and hydroxyl ions outward is aided by a high state of agitation of the liquid, the pH of the solution adjacent to the metal will approach that of the main body of the solution and no ferrous hydroxide precipitation will result. In addition, the ferrous ions will be oxidized by the dissolved oxygen to ferric ions. I n the event of the outward diffusion of these ions into the liquid, there will probably result the precipitation of ferric hydroxide, since i t can be precipitated a t a much lower p H than ferrous hydroxide. If, however, for any reason both ferrous and ferric hydroxide are precipitated, as may be the case when the solution is quiescent (or moving slowly), a reaction may take place between these two hydroxides to form the magnetic oxide of iron, which has a markedly different physical structure than the hydroxides. This picture of the mechanism of film formation apparently affords a satisfactory explanation of the phenomena observed during the experiment. Throughout the first phase of the run the rapid rotation of the metal cylinder evidently aided the diffusion of ions outward in the direction of the main body of the liquid. I n this manner the concentrations of ferrous and hydroxyl ions never became high enough to permit the precipitation of ferrous hydroxide. However, in the presence of oxygen the ferrous ions were oxidized to the ferric state and ferric hydroxide precipitated. The precipitate was uniformly distributed over the metal surface and the measured rate of corrosion was low, indicating that the uniform ferric hydroxide film greatly retarded diffusion.
653
During the second phase of the run the agitation afforded by convection currents was apparently insufficient to cause much outward diffusion of ions, and it became possible for the hydroxyl and ferrous ions to attain, in some portions of the film, concentrations which were high enough t o permit the precipitation of ferrous hydroxide. Furthermore, since the liquid film was thicker, the possibility of oxidation of ferrous ions to the ferric state was decreased, but not entirely eliminated, so that both ferrous and ferric hydroxide were precipitated. The two hydroxides reacted to form the magnetic oxide of iron, which was loose and granular in structure. The measured corrosion rate became higher and, since it very nearly reached the value observed a t the beginning of the first phase, indicated that the black oxide afforded little resistance to the diffusion of oxygen and of the various ions. During the third phase of the run the specimen was again rapidly rotated and therefore conditions, in so far as the liquid film was concerned, were essentially the same as in the first phase. Accordingly, the corrosion rate would be expected to decrease and the corrosion product covering the surface film would be composed largely of ferric hydroxide (since the diffusion resistance of the black oxide was such that the outward movement of ferrous ions would not be greatly impeded). The corrosion rate did decrease, but it did not reach so low a value as was observed during the first phase of the experiment. Since operating conditions were the same during the third phase as during the first, except for the presence of the loose granular layer of magnetic oxide, the increased rate of corrosion during the third phase was probably due to the poorer diffusion resistance of ferric hydroxide, which was precipitated in an irregular manner upon the underlying magnetic oxide. Conclusions
1-The ultimate corrosion rate of iron in oxygenated water is dependent largely upon the character, composition, and thickness of the corrosion film. 2-The composition of the film is dependent upon the pH of the liquid near the metal surface and the uniformity of ionic concentrations in the liquid film on the metal surface. 3-The product formed in the submerged corrosion of iron in oxygenated distilled water is composed largely of the magnetic oxide of iron when the diffusion of ions from the liquid film to the main body of the solution proceeds slowly or non-uniformly. &The corrosion product formed under conditions of uniform and rapid ionic diffusion from the liquid film is composed largely of gelatinous ferric hydroxide. &The granular magnetic oxide formed is not very resistant to the diffusion of the various ions or of oxygen. &The gelatinous ferric hydroxide greatly retards the diffusion of oxygen and ions and thus prevents the formation of high concentrations of hydroxyl ions a t the metal surface and the subsequent formation of non-resistant films. Literature Cited Brown, Roetheli, and Forrest, IND.ENG.CHEX.,23, 350 (1931). Evans, J . Chem. Soc., 1937, 1020. Evans, I b i d . , 1929, 2651. Forrest, Roetheli, and Brown, I b i d . , 22, 1197 (1930). International Critical Tables, Vol. V, pp. 93, 97,98. Latimer and Hildebrand, “Reference Book of Inorganic Chemistry,” p. 307. I b i d . , p . 389. Lewis and Randall, “Thermodynamics,” p. 608. Walker, J . A m . Chem. SOC.,29, 1251-64 (1907). Walker, I b i d . , 30, 473-4 (1908). Whitney, I b i d . , 25, 394-406 (1903). Wiist, Meuthen, and Durrer, “Selbstverlag Ver. deut. Ing.,” p. 17
(1918).
