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Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 1, 1978
The evidence of Figure 2 suggests that the surface polymerized layer has a constant S content to a: depth of about 300-350 A. Thereafter the uniformity of the layer deteriorates, though significant traces of sulfur persist to a depth of roughly 1000 A. This implies an average surface-layer fqrmation rate of about 1.5 8, per second of treatment. Reaction rates in lower-frequency plasmas are typically in the vicinity of 1 A/s (McTaggart, 1967; Hollahan and Bell, 1974). This calculation of deposition rates does not imply the existence of linear deposition rates. I t has been shown (Hall et al., 1972) that in low-frequency plasmas deposition rates may decay rapidly after the formation of films in the thickness range of 100-200
A.
The carbon concentration in this case (Figure 2) is essentially constant as expected, but the relative oxygen concentration increases significantly with increasing ion bombardment time. Though an explicit interpretation of this finding is not possible a t present, mild oxidation due to the elevated temperatures generated in the Auger analytic process is a suggested cause. (d) SEM. SEM was used in order to seek visual confirmation for the presence of a plasma-polymerized surface layer. Figure 3 gives typical results of this phase of work, all examples pertaining to Monel. An untreated control sample is shown in Figure 3A; a characteristic nodular surface structure is clearly discerned. This structure is virtually obscured in Figure 3B, which shows the surface appearance of a 3-min thiophene-treated Monel sample. The nature of the larger, coarse agglomerates is not known, though the presence of accretions cannot be ruled out considering that the samples used had not been cleansed prior to LMP treatment. An interesting surface appearance results from trichloroethylene LMP treatments, as displayed by Figure 3C (3-min exposure). The appearance of carbon steel surfaces treated in this vapor is entirely similar. Again, specific interpretation of this effect is not within the scope of the present work. A detailed study of LMP processes is clearly called for in order to clarify reaction mechanisms, to identify the structure of polymerized materials, and so to permit tailoring LMP processes toward the attainment of selected performance characteristics in polymerized films.
surfaces exposed to high temperature, alkaline environments. The results of the study hold out a significant promise of tailoring surface treatments by LMP for specific metal substrates, meeting preselected performance criteria. An intelligent application of the LMP technology along these lines will require, however, a basic understanding of the reaction mechanisms in plasma processes, their dependence on physical variables (e.g., monomer pressure and flow rate, plasma duration, reactor geometry, etc.) and on the chemical nature of the substrate being modified. Work intended to meet these needs is proceeding in our laboratories.
Acknowledgments We thank the National Research Council of Canada for financial support of this work. We gratefully acknowledge the assistance of Atomic Energy of Canada Ltd., Pinawa, Man., and particularly the cooperation of Dr. Param Tewari. Professor J. P. Bdlon, Department of Metallurgical Engineering, Ecole Polytechnique, kindly performed Auger spectroscopy, and Professor R. Bosisio, Department of Electrical Engineering, Ecole Polytechnique, generously made available elements of the plasma apparatus. Literature Cited Ashworth, V., Grant, W. A., Procter, R. P. M., Corros. Sci., 16, 661 (1976). Ashworth, V., Grant, W. A., Procter, R. P. M., Am. Chem. Soc., Div. Org. Coat. Plast. Pap., 37,(I), 40 (1977). Bialski, A., Manley, R. St. J., Wertheimer, M. R., Schreiber, H. P., J. Macromol. Sci. Chem., A-IO, 609 (1976). Bosisio, R. G., Wertheimer, M. R., Weissfloch, C. F., J. Microwave Power, 7,
325 (1972). Bosisio, R. G.. Wertheimer, M. R., Weissfloch, C. F., J. Phys., €6, 628
(1973). Carter, G., Colligon, G. S., "Ion Bombardment of Solids", H. E. B. Press, London,
1968. Dynes, P. J., Kaelble, D. H., J. Macromol. Sci. Chem., A-IO, 535 (1976). Goldberg, G., Am. Chem. Soc., Div. Org. Coat. Plast. Chem., Pap., 37 (I),157
1977). Hall, J. R., Westerdahi. C. A. L., Bodnar, M. J. Levi, D. W. J. Appl. Polym. Sci., 16, 1465 (1972). of Plasma Chemistry". Hollahan, J. R , Bell, A. T. "Techniaues and ADDliCatiOnS .. Wiley, New York, N. Y., 1974. McTaggart, F. K., "Plasma Chemistry in Electrical Discharges", Elsevier Publishing Co., Amsterdam, 1967. Millard, M., "Synthesis of Organic Polymer Films in Plasmas", in J. R. Hollahan and A. T. Bell, "Techniques and Applications of Plasma Chemistry", Wiley, New York. N.Y.. 1974. H. P., Schreiber, Y. B., Tewari, M. R., Wertheimer, J. Appl. Polym. Sci., 20,2663
(1976). Wertheimer. M. R . , Paquin, L., Schreiber, H. P., J. Appl. Polym. Sci., 20,2675
(1976).
Conclusion The present exploratory study has shown that the LMP process is capable of conferring substantial passivity to metal
Presented at the Division of Organic Coatings and Plastics Chemistry, 173rd National Meeting, of the American Chemical Society, New Orleans, La., March 1977.
Introductory Remarks-Tuesday Session C. A. Kumins Tremco, Incorporated, Cleveland, Ohio 44 104
Most ordinary metals corrode because they are thermodynamically unstable and in the presence of the ambient environment their oxides represent the lowest energy level of the systems. Organic coatings may slow down the corrosion or oxidation process (loss of electrons) temporarily by acting as a barrier to the gases, vapors, and ions in the environment of interest. Some special types of anticorrosive coatings may contain rust inhibitors as iron passivating compounds which may con0019-7890/78/1217-0030$01.00/0
tribute to the retardation of the process, but the former mode of action plays the primary role. Therefore it is worthwhile to consider the mechanisms and properties which are characteristic of organic coatings used as barriers against environmental intrusion. For corrosion of ferrous metals (and similarly active ones) protected by an organic coating to take place, water, oxygen, and generally ions must somehow arrive a t the metal surface. Water vapor and oxygen dissolve in the film surface and by
0 1978 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 1, 1978
a process of diffusion move through the film and arrive at the metal-polymer interphase (Kumins, 1965).There is no known polymer which will completely exclude water. Polymeric coating films have been shown to exhibit weak ion-exchange properties (Kumins and London, 1960) and are permselective. Thus as a result even mechanically perfect membranes will allow the critical or transportable ion which may be present in the exterior section to traverse through the film. Similarly, any metal ion (if the coating is cation selective) from the substrate will move in the opposite direction. A characteristic of permselective membranes is the loss of criticality if the activities of the ions surrounding them exceed their exchange capacities. Under these conditions the passage of the counterion which is normally excluded may take place thus giving rise to a higher concentration of salts a t the interphase (Kumins and London, 1962). The permselective properties of coating films are reduced by application of an electric field even if its magnitude is considerably less than the dielectric breakdown potential of the membrane. In this situation it becomes “leaky,” permitting the counterion to diffuse as well, resulting in a higher concentration of ions at the interface. This situation may be accelerated by the presence of the electric fields that exist between the cathodic and anodic areas that normally reside on the coated metal. The preceding has described the modes of transfer of those moieties which take part in the corrosion processes. However, in order for them to exert their influence they must penetrate a thick layer of polymer (ca. 5000-7000 A) which makes up the interphase and thence displace the coating molecules held on the metal surface by either physical or chemical bonds. This interphase exhibits properties similar to those associated with cross-linked macromolecules in that it displays reduced swelling in liquids in which the free molecules are normally completely soluble (Kumins and London, 1966; Kwei and Kumins, 1964). Significant and major changes are also measured by changes in T , and diffusion coefficients of gases and vapors through unsupported films of the macromolecule (Kumins and Roteman, 1961a,b). All these phenomena are caused by restrictions in the vibrational and oscillatory motions of the initially adsorbed molecule which are considered to be in a looped and/or coiled configuration (Roland et al., 1965) due
31
to the presence of the interacting radicals fixed on the surface sites. This configuration diminishes the free volume within which the adjacent polymer chains may vibrate and the restriction extends as much as 5000-7000 8, from the surface sites. However, it has been shown that if a sufficiently high concentration of molecules which have an affinity for the interface is present at that interface they eventually displace the originally adsorbed moities and are sorbed themselves. In other words, an equilibrium exists between the molecules originally on the surface and those off it. Depending on the appropriate equilibrium constants, the former may be substituted by other species whose affinity for the surface is greater if they are in a sufficiently high concentration (Gottlieb, 1960; Kumins, 1976). Therefore, in view of the above facts the molecules at the coating metal interphase may eventually displace the barrier to initiate and propagate the corrosion process. In summary, corrosion of an organic film protected metal will take place by gaseous and vapor solution and diffusion through the film to the interphase. Simultaneously, ions may be transported because of the ion-exchange characteristics of the film former. If the latter is not well adhered on a molecular basis to the metal, the adsorbed macromolecules may be displaced by the high concentration of the exterior moieties at that interface. The conditions for the initiation of the corrosion process are now present. Since the process, however, cannot take place if all the active metal sites are covered by polymer fragments, it is interesting to speculate that the primary factor in obtaining corrosion resistance by protective coatings is the degree of affinity of the polymer molecules for the metal surface so as to resist displacement-in other words, excellent adhesion under aqueous conditions primarily.
Literature Cited Gottlieb, M.,J. Phys. Chem., 64, 427 (1960). Kumins, C. A.. J. Polym. Sci., Part C, Polym. Symp., No. 10, l(1965). Kumins, C. A., J. Coating Techno/., 48, (622), 518 (1976). Kumins, C. A., London, A., J. Po/ym. Sci., 46, 395 (1960). Kumins, C. A., London, A., Off. Dig. Fed. SOC. Paint Techno/,, 34, 843 (1962). Kumins. C. A., London, A., Off.Dig. Fed. SOC.Paint Techno/., 38, l(1966). Kumins, C. A., Roteman, J., J. Polym. Sci., 55, 623 (1961a). Kumins, C. A., Roteman, J., J. Polym. Sci., 55, 699 (1961b). Kwei, T. K., Kumins, C. A., J. Appl. Polym. Sci., 8, 1483 (1964). Roland, F., Bulas, R., Rothstein, E., Eirich, F. R., I d . Eng. Chem, interface Symp., 57, (9), 46 (1965).
