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R. A. POWERS and J. C. CESSNA National Carbon Research Laboratories, National Carbon Co., Cleveland, Ohio
How Polar-Type Oils Inhibit Corrosion If enough oxygen is present, a uniform layer of oil may give additional protection against corrosion by promoting true electrochemical passivation
POLAR-TYPE
OILS are used for rustproofing against atmospheric corrosion, protection against corrosion during machining, and as inhibitors for circulating water systems-e.g., in automotive antifreezes. They generally consist of a mineral oil base containing one or more polar organic materials such as fatty acids or their salts, petroleum sulfonates, or sulfated vegetable oils, together with dispersing and emulsifying agents. Polar molecules of oil adsorb on the metal surface in a hydrophobic, oleophilic monolayer which then anchors a thin uniform layer of the hydrocarbon carrier (7). The polar additive alone will not protect without the hydrocarbon carrier, and the oil will not protect without the additive ( 5 ) . This report shows that a uniform layer of inhibitor oil may give additional protection by promoting true electrochemical passivation of the surface.
Experimental T h e major item of the apparatus is a steel electrode cut from 3/4-inch S.A.E. 1030 bar stock and cast in a Bakelite epoxy resin, together with a connecting wire enclosed in a glass capillary tube. A bridge containing the solution to be tested or agar in distilled water connects the electrode with a saturated calomel reference half cell, and connections are provided for an aeration tube and reflux condenser. Potential was measured with a high input impedance p H indicator (Leeds & Northrup Catalog No. 7664), continuous output of the p H meter was fed into a standard 0- to 20-mv. strip-chart continuous recorder. The steel electrode was prepared by abrasion on a 150-grit Buehler belt
surfacer followed by scrubbing with wet, then dry, filter paper. T h e electrode was aged for 5 minutes or less i n air before use. The normal test conditions were 200 ml. of 33 volume yoof ethylene glycol in distilled water held at 77' C. with 0.028 cubic foot per minute aeration. A 300-ml. capacity, Berzelius tallform beaker contained the test solution. Experimental conditions and inhibitor oil used do not necessarily simulate those of use; they were chosen to illustrate the phenomena, using an oil of known composition and a polar material of known molecular weight which had been studied by other investigators ( 2 , 8). Discussion Adding sufficient polar-type oil to the corroding system produces a noble or passive potential and a decrease in corrosion rate (Figure 1). With insufficient oil, an intermediate potential is reached with isolated corrosion. If air is replaced by nitrogen, adding inhibitor oil does not cause a drop to a
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passive potential, but rather a change of only a few millivolts (Figure 2). However, a film must exist on the electrode surface; this inhibits even the low corrosion rate which occurs in the absence of oxygen. This situation is similar to that reported for adsorption-type inhibitors where passivation does not occur-Le., as in pickling inhibitors (74, 77). Uniform adsorption occurs over the surface but produces only a slight shift toward a more noble potential, although corrosion is definitely inhibited. Here the active potential does not mean that corrosion is taking place, but only that inhibition is not of the passivating type. After a layer of inhibitor oil has been established on the electrode surface in a nitrogen atmosphere, adding air causes almost immediate passivation (Figure 3). This shows again that the combined action of the polar-type oil and dissolved oxygen is necessary for a passive potential. Only a small amount of oxygen for a short time is required to induce pas-
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Figure 1. Adding enough p o l a r type oil produces a passive potential and a decrease in corrosion rate
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sivity which is retained for fairly long periods without adding more air. Thus, it is concluded that a polar-oil inhibitor film may exist on a steel surface and inhibit corrosion (as in a galvanic couple) in the absence of air without greatly shifting the potential in a noble direction. However, steel in contact with a hot aerated solution assumcs an active potential and corrodes. Steel in the presence of both oil inhibitor and oxygen does not corrode and exhibits the potential of a passive metal-i.e., the polar-oil film on the steel surface converts oxygen from a corrosive agent into an actual passivating agent. With oxygen but insufficient oil, the electrochemical and corrosive behavior of steel is similar to that of a passive metal under conditions of localized corrosion. That the inhibitor oil on the metal promotes electrochemical passivation by dissolved oxygen is not surprising. The passivating effects of high concentrations of oxygen are \+.ell documented ( 9 - 7 7 , 13, 79-22, 2 4 ) . T h e actual mechanism of promotion is probably twofold. First, because oxygen is more soluble in hydrocarbons than in water (23), its concentration at the metal surface is increased. Because of this, it is erroneous to assume that soluble oils protect by keeping oxygen away from the surface (5). Secondly, the hydrocarbon phase, through either its composition or its role as a diffusion barrier, may promote passivation by oxygen. Here the role of inhibitor oil is
similar to that of nonoxidizing inorganic inhibitors ( 2 1 ) . This mechanism is neither inconsistent with nor detracts from prior concepts of corrosion inhibition by polar-type oils. A physical diffusion barrier is still necessary before passivation can occur, and oxygen need not be present for effective inhibition. I n fact, this logic may be reversed-passivation can be used to measure the tendency of a physical barrier to form, and thus evaluate the adsorption tendency of polar additives. Polar-type inhibitor oils not only aid in forming a passive surface, but they also appear to enhance stability of this passivity. For example, 400 pap.m.of sodium chloride in the test solution does not destroy the passive potential of steel protected with 0.4y0 of a proprietary inhibitor oil which is used in glycol solutions. Similar results were obtained by adding sulfate and bicarbonate ions. Once passivation is attained, low concentrations of dissolved oxygen seem to maintain it. This proposed mechanism may also apply to other organic inhibitors employed in alkaline or neutral media exposed to air. For example, the electromotive force data reported for steel in tap water containing 0.047M diethylamine are similar to those for polar-type oils (75). Although attributed solely to adsorption-desorption, the potential changes are much greater than those produced by diethylamine in acid solution where passivation by oxygen is less
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Figure 3. When a layer of inhibitor oil is deposited on the electrode surface in a nitrogen atmosphere, adding air causes almost immediate pashivation. The electrode is still passive after 6 hours. When temperature i s reduced to 25” C., no corrosion i s visible after 1 week
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present in solution, a passive potential and l o ~ vcorrosion rate results.
Conc’usions Polar-type inhibitor oils may decrease corrosion of steel by promoting and maintaining a passive surface under conditions where it would not otherwise occur. The passivating agent is dissolved in oxygen rather than the oil, which provides a higher concentration of oxygen a t the metal interface. The barrier action of the oils permits passivity to be maintained under unfavorable conditions. This theory may perhaps be extended to explain the action of other organic adsorption inhibitors in neutral and alkaline media. References
(1) Baker, H. R., Jones, D. T., Zisman, W. A,, IND.ENG.CHEM.41, 137 (1949). (2) Baker, H. R., Singleterry, C. R., Solomon, E. M., Zbid., 46, 1035 (1954). (3) Baker, H. R., Zisman, W. -4., Zbid., 40, 2338 (1948). (4) ,Baker, H. R., Zisman, W. A,, Lubricatzon Engr. 7,117 (1951). (5) Barnum, E. R., Larsen, R. G., Wachter, A,, Corrosion 4, 423 (1948). (6) Bigelow, W. C., Pickett, D. L., Zisman, W. A,, J. ColZoidSci. 1,513 (1946). (7) Bondi, A., “Physical Chemistry of Lubricating Oils,” p. 156, Reinhold, New York, 1951. (8) Brockway, L. O., Karle, J., J. Colloid Sci.2, 277 (1947). (9) Cartledge, G. H., J. Phys. Chem. 60, 28 (1956). (10) Cartledge, G. H., Sympson, R. F., Zbid., 61, 973 (1957). (11) Evans, U. R., “Metallic Corrosion Passivity and Protection,” 2nd ed., p. 296, Edward Arnold and Co., London, 1948. (12) Greenhill, E. B., Trans. Faraday Soc. 45, 631 (1949). (13) Hackerman, N., Hurd, R. M., J . Electrochem. Soc. 98, 51 (1951). (14) Hackerman, N., Makrides, A. C., IND.ENG.CHEM.46, 523 (1954). (15) Hackerman, N., Sudbury, J. D., J . Electrochem. Sac. 97, 109 (1950). (16) Hamer, P., Powell, L., Colbeck E. W., J . Iron Steel Znst. (London) 151 109 (1945). (17) Hoar: T. P., Pittsburgh Intern. Conf on Surface Reactions, p. 127, Corrosion Publishing Co., Pittsburgh, 3 948. (18) Hang: V., Eisler, S.L., Bootzin, D.; Harrison, A,, Corrosion I O , 343 (1954). (19) Lochte, H. L., Paul, R. E., Trans. Efectrochem. SOC. 64, 155 (1933). (20) Pryor, M. J., Cohen, M., J . Electrochem. Soc. 98, 263 (1951). (21) Zbid., 100,203 (1953). (22) Roetheli, B. E., Brown, R. H., IND. ENG.CHEM.23, 1010 (1931). (23) Seidell, A , , “Solubilities of Inorganic and Metal Organic Compounds,” vol. 1, 3rd ed., p. 1359, Van Nostrand, New York, 1940. (24) Sympson, R. F., Cartledge, G. H., J . Phys. Chem. 60, 1037 (1956). RECEIVED for review July 19, 1957 ACCEPTED January 19, 1959
