January, 1933
ISDUSTRIAL AKD ENGINEERING CHEMISTRY
alcohol, volatile acid, nitrogen, tsnnin, and sugar are not significant. LITERATURE CITED (1) .Inonymous, Bull. assoc. intern. d u froid, 9-10, 62. 63 (1918-19). (2) .Inonymous, Inst. intern. du froid, Monthlg Bull. of Information o n Refrigeration, I , 38 (1920). (3) Anonymous, Italia vinicola agraria. 24 (11, 9 (1934). (4) Assoc. Official h g r . Chern., Methods of Analysis, 3rd ed., pp. 136-43. Washington. 1930. (5) Bioletti,’F. T., Calif. h g r . Expt. Sta., Bull. 167, 1-63 (1905); 213, 396-442 (1911). (6) Cabane, Y . , Proc. 4th I n t e r n . Congr. of Refrigeration, London, 11. 1248-58 (1924). (7) Cabane, Y . ,Rev. g e n . froid, 12, 199 (1931). (8) Cabane, I-., Rev. vit., 75, 277-82 (1931). (9) Carles, 1st Intern. Cong. of Refrigerating Ind., summaries in English of papers to the congress, pp. 158-60, Paris, 1908. (10) Casale, Luigi, Italia vinicola agraria, 24 ( l ) , 3-6 (1934). (11) Dumolin, L., Rev. gen. froid, 13 (5), 106 (1932); Intern. Bull. Information Refrigeratzon, 13 (4), 911-4 (1932). (12) Haas, Bruno, 1st Intern. Congr. of Refrigerating Ind., summaries in English of papers t o the congress, pp. 160-1, Paris, 1908.
35
(13) Joslyn, M. A., and Tucker, D. A , IND.ENQ.CHEM.,22, 614 (1930). (14) Laborde, J., Rev. vit., 40, 369 (1914). (15) La Grassa, Filippo, A\rolit. chim. ind., 1, 248, 285, 315 (1926): 2, 131 (1927). (16) Lallie, N., “Le froid industrielle et les machines frigorifiques,” pp. 300-10, Paris, J. B. Bailliere et Fils, 1912. (17) Lathrop, C. P., and Walde, W. L., Canning A g e , 9 , 139 (1928). (18) Marcelet, H., and Bespaloff, Schoulim, Ann. fals., 24,353 (1931). (19) Mathieu, L., Chimie et industrie, 28, 175 (1931). (20) Miroir, Virgil, Ibid., Special number, 805-8 (March, 1931). (21) Pacottet, P., ”Vinification,” 5th ed., pp. 375-6, Paris, J. B. Baillisre et Fils, 1926. (22) Rousseau, Eugenet, Ann. sci. agron., 2, 420 (1909); Bull. sci. pharmacolog., 14, 254 (1909). (23) Sebastian, V., Rev. g e n . f r o i d , 13, 200 (1932). (24) l e n t r e , J., ”Trait6 de vinification,” Val. 2, pp. 319, 422-33, Montpellier, Librairie Coulet, 1931. (25) Vergnette-Lamotte, A. de, ”Le vin,” 2nd ed., pp. 181-203, Librairie dgricole de la Maison Rustique, 1868. RECEIVED September 25, 1934. Presented before the Division of Agrioultural and Food Chemistry a t the 88th Meeting of the American Chemical Society, Cleveland, Ohio, September 10 to 14,1934.
Metal Priming Paints Inhibiting Qualities and Influence of Reactions within the Paint Film HARLEY A. NELSON, The Yew Jersey Zinc Company, Palmerton, Pa. t!ie center of the s t a g e u n t i l HEMISTS charged with 4letal priming paints are considered as pigWhitney (31) proposed the electhe r e s p o n s i b i l i t y of ment-aehicle combinations subject to chemical and trolytic theory in 1903. Accordstudying metal protecphysical changes. Theories of corrosion and the ing to the acid theory, the prestive paints and metal painting external agents that each theory would hold acence of acid material is necessary problems will do well to follow countable f o r the progress of corrosion are disand carbon dioxide with moisthe literature on corrosion and ture is sufficient to start and the electrochemistry related to cussed. The essentials for corrosion-water, maintain the corrosion process the s u b j e c t . It is true that carbon dioxide, acids, and hydrogen peroxide (a on iron. Carbon dioxide in parmuch of it deals with the corrodepolarizing agent)-are supplied during the ticular, is considered important sion of unprotected metal, but decomposition of natural drying oils and resins. in the process of normal atmosthere have been worthwhile atPigments that give effective service in metal pheric corrosion, because the retempts (10, 14) to tie in theory with practical results in the presactions with it can be pictured as priming paints apparently are those which neuence of organic protective coatinvolving i t s r e g e n e r a t i o n . tralize acids and reduce or otherwise decompose ings, and increasing recognition Moody (ZO), for example, mainhydrogen peroxide without the formation of of the need for considering intained that the effectiveness of corrosive reaction products. The study of the hibition as related to metal surprotective coatings is dependent decomposition products formed in organic bindfaces under organic binder syson exclusion of carbon dioxide (and other acid materials) and tems rather t h a n exposed ing media offers a promising angle from which to directly to the atmosphere or that inhibitors serve primarily attack the problem of formulating still better metal to water solutions. Some investo depress t h e s o l u b i l i t y of priming paints. tigators have considered paint carbon dioxide in the medium as a combination of Diment and next to the metal surface. vehicle in its relatio; t o the metal surface, but there has as yet Dunstan (9) contended that rusting will proceed in the been comparatively little said about this combination as a complete absence of carbon dioxide and that only oxygen changing material and the influence these changes may have and water are essential, although he later admitted that on the results. What follows represents an effort, to carry this carbon dioxide (and other acids) might enter into the process phase of the subject a step farther, without aspiring to dis- by destroying the protective (oxide) film that renders the cuss it completely. The discussion will be practically limited surface “passive” in the presence of inhibitors. He and to priming paints on iron and steel, although the basic ideas others pointed out that hydrogen peroxide may be detected will undoubtedly apply in the cases of other alloys. a t the surface during the corrosion of metals, notably in the case of zinc. Presumably, the only reason i t could not be THEORIES OF CORROSION OF METAL detected in the case of iron was because hydrogen peroxide A brief restatement of the theories advanced to explain cor- oxidized iron so rapidly. Later, however, Keiser and Macrosion of exposed metal is desirable, both as a review and as Master (8) are credited with having detected the presence of a basis for considering the problem from the point of view hydrogen peroxide during the corrosion of iron. The reof the paint chemist. action is pictured: About thirty years ago the acid or chemical theory of corFe HzO O2 = FeO HZOZ rosion, supplemented by the hydrogen peroxide theory, held 2Fe0 Hz02 = Fe20z(OH)a (rust)
C
++ +
+
36
INDUSTRIAL AND ENGINEERING CHEMISTRY
Dunstan analyzed several samples of rust and found essentially the above reaction product. He also observed that hydrogen peroxide attacks iron directly to produce a red basic ferric hydroxide. He maintained that the favorable effect of alkalies and chromates, etc., that had been attributed to withdrawal of carbon dioxide, is due to establishment of conditions that inhibit hydrogen peroxide formation, and that this is true especially on iron, zinc, and lead. Moody (20) and Divers (7‘) took issue with Dunstan; Divers in particular rejected the idea that cold water can be endothermically oxidized to hydrogen peroxide by iron, and maintained that lack of rusting under alkaline conditions need not involve inhibition of hydrogen peroxide formation. It is useful to keep these early controversies before us, for they may have some bearing on what can happen under a paint film, as we shall see later. The electrolytic theory has been widely discussed since it was first announced by Whitney ( S l ) and developed with relation to pigments and painting problems by Cushman (4) and Cushman and Gardner ( 6 ) ,and now receives general support as a basic theory. Watts (SO) has restated it very succintly when he says that (according to Nernst) “the dissolving of metals by acids or other corroding media takes place by ions (electrically charged atoms) leaving the surface of the metal. All that is needed for atoms of a metal to go into solution is that they be provided with tickets of admission, a number of positive charges equal to the valence of the metal in that particular solution.” He summarizes electrolytic corrosion, in so far as we need be concerned with it for this discussion, as follows: Corrosion by invisible displacement of hydrogen is by far the most common type of corrosion, yet it is least understood. This occurs only with those metals and in those solutions in which the discharge potential of hydrogen exceeds the (solution) potential of the metal. Corrosion by invisible displacement of hydrogen proceeds as follows: When the metal is immersed in the solution it begins t o dissolve, displacing its equivalent in hydrogen. This continues until the counter e. m. f. set up by hydrogen equals the t.hrust of the metal to go into solution, when dissolving the metal and displacement of hydrogen cease. In the case of metals whose otentials lie far below the discharge potential of hydrogen on tfem, the contact surfaces of metal and electrolyte must be far from saturated with hydrogen. Oxygen from the air dissolved in the electrolyte removes hydrogen by formation of water, thus lessening the force which prevented further displacement of hydrogen, so that dissolving of metal begins again. The speed of solution of metal is limited to the rate at which oxygen reaches its surface . . . . . . The two major factors which affect the magnitude of the discharge potential of hydrogen are the metal of the cathode and the hydrogen-ion concentration of the electrolyte. Substitution of normal sodium sulfate for normal sulfuric acid causes a rise of 0.5 to 0.7 volt in discharge potential, depending upon what metal constitutes the cathode. The rapid and visible attack of iron and commercial zinc by hydrochloric acid and the lack of visible action by sea water is due, not to a change in potentials of the metals, but t o a rise in the discharge potential of hydrogen resulting from the lower hydrogen-ion concentration in sea water. Increases in solution pressure (e. m. f.) can be brought about (1) b y heterogeneous structure, which characterizes all steels
and other alloys, as well as stresses and strains due to improper cooling, etc., or (2) by differences in environment on adjacent areas of the metal-for example, differential aeration. The electrolytic theory, as presented, provided an obvious explanation for cathodic corrosion, and it was soon generally conceded that the type of corrosion usually encountered is of the cathodic type. I t was recognized, however, that corrosion occurred under condit,ions that apparently could not be explained on this basis; this led to some confusion and continued attack on the electrolytic theory as insufficient.
Vol. 27, No. 1
The more obscure facts concerning anodic attack and the relationships between cathodic and anodic corrosion have since been brought to light largely through the work of Evans (11, 12) and his co-workers, who presented the first logical explanation for the effects of differential aeration and showed that under these conditions the corrosion instead of being a t the cathode (point of access of oxygen) is at the anode, or the point of least accessibility of oxygen. For example, on an iron panel immersed in sodium chloride solution, corrosion would take place near the bottom, since a t the middle area there would be, if it remained undisturbed, a protective accumulation of ferric and ferrous hydroxides. Vernon (28) maintains that the differential aeration effect is usually caused by unequal alkali distribution and will rarely be observed in the absence of alkali salts. I t seems, however, that it might still be of importance in the industrial paint problem, because such surfaces usually cannot be cleaned entirely free from adhering dust, scale, corrosion products, or alkali salts. The importance of accumulations of dust, etc., has been aptly demonstrated b y Vernon (28) in a series of experiments that should be of interest to the paint chemist: Surfaces placed in a muslin-screened box immediately after cleaning accumulated a uniform film of oxide which protected them after exposure to the ordinary atmosphere. Similar surfaces exposed to a dusty atmosphere immediately after cleaning rusted from the outset a t an exponentially decreasing rate, when maintained a t a relative humidity below about 65 per cent. Above this humidity (but considerably below 100 per cent) the contaminated surfaces corroded a t a greatly increased rate, whereas the uncontaminated but uniformly oxidized surfaces were relatively unaffected. Vernon points out that this might be explained either on the basis of differential aeration caused by local protection offered by the dust particle and the corrosion product, or on the basis of depolarization being promoted by the corrosion product under the dust particle, because of moisture carried through a porous structure of the corrosion product. In either case the corrosion will occur under the dust particle, but in the case of differential aeration the corrosion is anodic and in the other case cathodic. Howeve-, Vernon gives the impression that he favors the viewpoint that the corrosion is cathodic on the basis of a hypothesis advanced by Patterson and Hebbs (22) based on Zsigmondy’s theory of gel structure. I n this the assumption is made that ‘(rust has a gel structure, in which case a large proportion of the water content is strongly held in the capillaries of the gel and is not free to participate in the furtherance of corrosion . . . . . . . When the gel is maintained above the critical humidity, the capillaries commence to fill with water, the state of compression is released, and water may be free to pass from the surface of the rust to the surface of the metal. Thus an excessively small increment in the actual water content of the rust may lead t o a very much greater increase in the rate of rusting.” As mentioned above, this critical relative humidity was a t some point near 65 per cent relative humidity. Many paint chemists have no doubt puzzled over the progress of corrosion under a priming paint that is further protected by “moisture-resistant” finish coats (asphalts), when it is known that the film is only occasionally exposed to saturated humidities and it is not likely that alkali salts are present. The idea that a gel structure of the corrosion product makes moisture (and oxygen) available for depolarization a t humidities so far below saturation might explain some such paint failures. It also suggests that the inclusion of other elements, as in alloy steels, is effective in part because the porous structure of the corrosion product is modified. Paint tests are reported ( 6 , d l ) which show definitely the advantages of certain of these irons and steels.
January, 1933
INDUSTRIAL AKD ENGINEERING CHEMISTRY
This discussion pertaining to the theories' of corrosion may not be continued farther except to point out for future reference t h a t t h e e x t e r n a l agents essential for a normal corrosion p r o c e s s , according to the different theories, are:
37
TABLEI. DECOMPOSITION PRODUCTS FROM RAWLINSEEDOIL AND DRIER (ACCORDIXG T O LONG,REINECK, .4ND BALL) (In grams per 10 grams of oil)
---H20---.
I
IN DRY AIR
I N DRY AIR
coz
7
ACIDS
,
I N WET AIR
IK D R Y A I R
I N W E T AIR
10 dayo 30 days 10 days 30 days 10 days 30 days 10 days 30 days 10 days 30 days0 0.35 0.95 0.25 0.45 0.55 0.95 0.20 0.27 0.08b 0.08 Films exposed in wet air were still in an unsolidified state after 30 days. b Acid content reached a maximum in 10 days and did not increase; it should be noted t h a t volatile acids are being determined.
Acid or so-called chemical theorycarbon dioxide, mater, oxygen (and miscellaneous acid materials). Hydrogen peroxide theory-water, oxygen, hydrogen peroxide (and acid conditions favorable t o the presence of hydrogen . peroxide), Electrolytic theory-witer, oxygen, and electrolytes (acid favorable to cathodic attack; alkali favorable t o anodic attack).
DECOXPOSITIOX OF DRYING OIL FILMS Cushman and Gardner ( 5 ) , and before them Walker (29), have pointed out that linseed oil is an active stimulator of corrosion when applied to iron, particularly if the oil film is abraded in the least degree. Walker thought that this was due (1) to the porous state of the film, which permitted the passage of depolarizers, from without and/or (2) to an unsaturated state of the oil which permitted it to absorb nascent hydrogen and thus in itself act as depolarizer. He pictures areas exposed by abrasion as becoming actively anodic with the cathodic areas under the remaining film, where depolarization take. place due to either of the above mentioned causes. According t o recent discussion by Evans and Hoar ( I S ) , it is now appreciated that the surface of metal may be subject to both cathodic and anodic attack and that the velocity of corrosion may be determined by either anodic or cathodic control: In special cases the velocity will be controlled mainly by the cathodic reaction (cathodic control), being stimulated by oxygen, a cathodic depolarizer, or by those foreign impurities which assist the liberation of hydrogen; in other special cases it is controlled mainly by the anodic reaction (anodic control), being then usually stimulated by bodies which dissolve, peptize, or penetrate the metallic hydroxide. In the general case (probably more common) both anodic and cathodic polarization curves will affect the resultant corrosion velocity, which must be equivalent t o the value of the current that reduces the e. m. f. to a value just capable of forcing the current through the resistance of the corrosion circuit. It is evident that conditions under a drying oil film are sufficiently complicated to offer plenty of possibilities bet'ween anodic and cathodic control. For example, oxidizable matter favors anodic depolarization (3) (reduction) which should be unfavorable to the stability of any protective oxide or hydroxide film a t the surface of the metal. From this standpoint a drying oil film in the first stages of oxidation should be a powerful promoter of anodic action, particularly if any accumulation of alkali exists under the film. This is not in accord with the idea of Walker as quoted by Cushman and Gardner (5), who would ascribe the cathodic depolarization (ability t o oxidize) to the oil being an "unsaturated hydrocarbon.', However, it is now generally accepted that this period of intensive oxidation of the film is limited, and that, subsequent hardening and aging is largely accomplished by polymerizing reactions, when this reducing power probably would be limited. Any such reducing action that would disturb the prot'ective oxide film during the early stages of oxidation might, of course, serve to start a process that would be more easily continued under the reverse condition later on. In any case, the situation certainly must be influenced by a number of factors which can be studied only by considering as many as possible of the decomposition products that are formed as the film solidifies and ages. The nature of the decomposition products from drying oil 1 In addition, the reader is referred to the excellent summary of the situation up to 1924 by Hancroft [ J . P h y x . Chem., 28, 785 (1924)].
films has been discussed by several investigators but the number of complete analyses available is limited. The results by Long, Reineck, and Ball (16) for volatile decomposition products are quoted in Table I. Merzbacher (19) gives the following complete analysis of a 9-month linseed oil film (in per cent) : Ash 1Tater Glycerol Formic acid Propionic acid Capronic (caproic) acid Pelargonic acid
0,4 9.0 9.0 1.0 1,O 0.3 1.6
Azelaic ,acid Satd. higher acids Unsatd. higher acids Rater-insol. oxidized fatty acids Water-sol. oxidized f a t t y acids Undetd. water-aol. organic acids Lossea (COz, etc.)
9.0 9 5 9.6 26.0 8.0 3.0 12.6
The water-soluble portion of this film, which should be the only portion involved in any electrolytic process, amounted to 20.6 per cent, distributed as follows (in per cent) : B.kSED O S
BASED ON FILM
Formic acid (HCOOH) Propionic acid CHaCHzCOOH) Caprqic acid (dHs(CH2)rCOOH) Azelaic acid (COOH(CHz);COOH) Oxidized fatty acids Glycerol Undetermined
1.0 1.0 0.15 3.4 3.5 9.0 2.6
WATERSOL.
PORTTOY .~. 5 ~~
5
0.5 16.5
li
45.0 11.0
Other investigators have also reported the presence of acetic acid, as well as aldehyde. According to Wegscheider and Shudder, as quoted by Creighton and Fink (S), the dissociation constants for wine of these acids are as follows: K Formic Acetic Propionic
x 10-0 21.4 1.8 1.34
K X 10-a Caproic Azelaic (sebacic) Oxalic
1.45 2.76 10,000 (approx.)
Stieglitz (26) gives the following limited data on organic acids, compared with hydrochloric acid: Hydrochloric Acetic Formic
ACTIVITY 100 0.3 1.3
CONDUCTIVITY 100 0.4 1.7
Although these acids are only slightly ionized as compared x i t h oxalic acid, which attacks iron a t a very rapid rate, it is probable that, in the presence of the water also formed, any of them would more or less fulfil the requirements either as electrolytes for the relatively slow process of normal cathodic corrosion, particularly in the presence of an active depolarizing agent, or possibly break the continuity of or dissolve (peptize) a protective (passivating) film of oxide existing on the surface of the metal. Azelaic acid, for example, which is one of the major wat,er-soluble constituents and is formed by the splitting of oleic acid by oxidation, is dibasic and is reported as having a solubility of about 1 part in 100 of cold wat,er.2 It has already been mentioned that Dunstan (9) 2 The ionization characteristics of the products of reactions between these acids and basic pigments must also be considered in order to make the picture complete. The soluble salts would normally be quite highly ionized and would reduce the ionization of the weak acids. This could be one benefit derived from the presence of pigments t h a t react to form neutral salts of the acids.
38
I K D U S T R I A L AIVD E N G I X E E R I N G C H E M I S T R Y
admitted that even a weak acid such as carbonic might enter into the corrosion process on iron by disturbing this oxide layer. It is obvious from the above that, by placing a drying oil film next to the metal, an active potential source of essentials for corrosion, such as water, carbon dioxide, and acids, is supplied. The fact that one other essential, oxygen, is also supplied in very active form should not be overlooked. Russel (26), as long ago as 1899, discovered that oils, among other organic materials such as resins from vegetable sources, give off hydrogen peroxide. Baughman and Jamieson (1) studied the matter further and found that saturated oils and acids give off appreciable amounts of hydrogen peroxide, which are greatly increased upon exposure to sunlight. Stutz, Xelson, and Schmutz (27) in this laboratory, considered the phenomenon from the standpoint of the effect of light on paint vehicles, both in liquid form and as normal dried films, with and without driers, and found that drying oils differ considerably. Raw tung oil and raw linseed oil are about on a par, with other raw oils showing much greater evolution in the following increasing order: soy-bean oil, sunflowerseed oil, chia oil, lumbang oil, poppy-seed oil. Films of raw linseed oil and drier evolved appreciable amounts of hydrogen peroxide after 8 months of aging. Ahbodied linseed oil after 2.5 years evolved even slightly more. KO quantitative data are available. It is not difficult to explain the above observations, since the oxidation of drying oils is accompanied by the formation of peroxides. Light (particularly ultraviolet) greatly speeds the process, and the presence of driers in normal amounts also accelerates peroxide formation (23). Obviously, peroxides in the presence of water and acids would favor the formation of more or less hydrogen peroxide. Considering these facts, it is not a t all strange that a clear coat of a drying oil proves to be an ineffectual primer, for it, in itself, supplies the essential conditions required for starting corrosion according to any theory that one might care to choose. It would be interesting, if possible, to determine to what extent anodic and cathodic reactions control the corrosion taking place under such films, but, when one considers the complicated reactions that occur simultaneously and successively, there is reason for questioning our ability to do so. It would seem, however, that, since the prevailing environment is acid, a predominance of cathodic attack would be favored, except possibly shortly after the film is applied and is passing through the stage of most rapid oxidation.
PIGMENTS Cushman and Gardner (5) state that, when linseed oil contains pigments, there is a marked decrease in the power of oil to remove hydrogen and act as a depolarizer. I n a general way, the improvement in imperviousness of the film to moisture (with oxygen), acids, etc., is also considered by other writers as a most important reason for the better results. Not much consideration has been given to the disposition of the moisture formed within the film, which is probably far more important, since it is well known that the better priming pigments produce excellent results in spite of the fact that the films are still relatively permeable to the passage of moisture from without. I n this connection one might also ask why one primer should prove more effective than another when h i s h coats used over them (such as asphalts) may be quite impermeable to moisture, acids, and electrolytes. With moisture present in the film, the most obvious property required of the pigment is ability to neutralize acid decomposition products, and it has long been recognized that priming pigments should preferably be able to establish
Vol. 27, No. 1
neutral or basic conditions within the film-hence, the general popularity of the oxides of the metals. However, the investigations of Stutz, Nelson, and Schmutz (27) offer evidence of other reactions that probably have a n equally important bearing on the effectiveness of pigments in priming paints. The observation in question is the unique behavior of different metallic soaps as to hydrogen peroxide evolution. It was noted that soaps of metals that act as driers (lead, cobalt, and manganese) show no evidence of hydrogen peroxide evolution, even after intense exposure to ultraviolet light, whereas the soaps of metals known to have little or no drying action (zinc, copper, and aluminum) give off hydrogen peroxide freely. Of the zinc soaps, the tungateq give off the least hydrogen peroxide, with the linoleates about ten times and the resinates about thirteen times as active. According to the theory of driers originally proposed by Jlackey and Ingle (17), the drying metals are those which in their oil-soluble form exist in more than one state of oxidation and of which the lower oxides are more stable than the higher. The rating given several metals for drying power is as follows (Mackey and Ingle worked with resinates of the metals): Cobalt has at least two well-defined oxides; the salts of the lower are more stable. Manganese has a large number of oxides; salts of the lowest are most stable. Lead has two readily formed oxides; salts of the lower are more stable. Chromium has a considerable number of oxides; salts of the lowest (CrO) are readily oxidized, but salts of the intermediate (Crp03)are stable. The higher oxide (CrOa) readily reverts to Cr20s. Iron forms a number of oxides; salts of the intermediate (Fe20B)are most stable. Hence, it is a weak drier. The addition of drying metals to oils in the small quantities necessary to promote drying (less than 1 per cent) does not inhibit formation of hydrogen peroxide, since their presence actually promotes the hydrogen-peroxide-forming reactions. The addition of 70 per cent by weight of a pigment of particle size of the order of 1 micron or less, and with a specific surface ranging from 0.5 to 1.0 square meter per gram, naturally offers opportunity for very complete distribution of oilsoluble soaps throughout the mass. Stutz, Nelson, and Schmutz (27) found that a 70 per cent lead resinate showed only very slight evidences of hydrogen peroxide evolution after exposure for 30 minutes to intense ultraviolet light from a mercury arc. The mechanism whereby lead compounds, for example, might remove hydrogen peroxide from the scene of action would obviously be oxidation of the lower valence compound by hydrogen peroxide to the higher form, which, in turn, could be reduced by the remaining unsaturated double bonds and decomposition products of a reducing nature (aldehydes). According to this, a red lead of high lead monoxide content should form a more inhibitive priming paint than a red lead of low lead monoxide content. Purdy and Fasig (24) came to the conclusion that as much as 10 per cent lead monoxide might profitably be present in red leads if only the reactions that result in thickening of the paint in the container could be controlled, and that lead monoxide by itself rates high as an inhibitor. It would be unusual if apparent discrepancies did not crop up a t once, the most obvious case probably being metallic zinc powder (zinc dust). However, if the properties of zinc are considered , it also satisfies conditions demanded by the theory. Zinc in the presence of mater and acids forms hydrogen, which can reduce the hydrogen peroxide formed from peroxides, as well as increase the concentration of hydrogen near or a t the metal surface. That this reactivity of the
January, 1935
39
I N D U S T R I A L A N D E N G I N E E R I IC‘ G C H E M I S T R Y
TABLE11. APPAREKT RESISTANCE OF PAINTS TO EXPOSURE (Results of laboratory tests with ultraviolet light, water spray, high humidity, and ozonized air). E O . OF
PIGMENTS
STUDIED Metallic zinc powder Red lead Iron oxide (Spanish)
BESTTYPEO F VEHICLEa Raw linseed oil of low thinner content Raw linseed oil of low thinner content Raw linseed oil of low thinner content
oxide Basic lead chromate
Raw linseed oil of low thinner content High bodied oils and varnishes with high thinner content
Blue lead
High spar-varnish content
Graphite
Intermediate bodied-oil content with rather high volatile content High spar-varnish content
Hlack magnetic iron
Aluminum
RELATION OF MOISTURE PERME.4BILITY OF FILMTO RUSTING TEXDEXCIES Most permeable film the best Most permeable film the best Most permeable film the best Most permeable film the best
VEHICLE
c OMBINA TIONS
REMARKS TESTED 10 Othern-isc no definite order of relationship between permeability and durability 13 Films with bodied oil3 rank close second 10 Films with bodied oils and varnish tended to fail by blistering and lack of adherence 10 Differences in vehicle had little effect; adherence 0. K. No blistering tendency or evidence 11 of poor adherence
Less permeable film the best (no direct permeability measurements) Films showed bad tendency to pinLeast permeable best, most perhole meable poorest (others not in exact order) Permeability data not conclusive Films tended to fail by blistering and poor adherence Least permeable films the best
Films tended to fail ultimately by blistering
Tested in combinations ranging from 100% raw linseed to high bodied oil or high china wood oil-linseel oil-ester gum varnish. with low (5%) to high (25cI,) added volatile content. (1
pigment with acids can be carried too far is evidenced by the fact that under highly acid exposure conditiorii metallic zinc powder primers should be protected by leqs reactive finih coats (as is generally the case with good priming paints), although under normal atmospheric conditions they are ,erviceable both as primers and finish coats. Reaction products derived from oxidation of the pigment ibelf, which might contribute either favorable or unfavorable conditions from a corrosion standpoint, must also be con-idered. For example, in the work of Stutz, Nelson, and Schmutz (27), it was shown that reactions that stop the evolution 01 hydrogen peroxide from paint films can also take place in the presence of pigments that are not rated as inhibiting pigments, probably because in such cases detrimental oxidation products are formed. From this it follows that we must avoid the assumption that the hydrogen peroxide evolving characteristics of a pigmented film are in themselves an indication of the merit of the pigment. The author wiihes to emphasize that further study of priming paint films for hydrogen peroxide evolution is not suggested with any such idea in view. The data given in Table I1 are useful in this discussion. These exposures were made in the laboratory under a special accelerated cycle consisting of continuous ultraviolet light (modified mercury arc) with periodic water spray for 2 minutes of each half-hour and with an ozonized atmosphere about 80 per cent of the time, which conditions certainly offer every opportunity for oxygen or hydrogen peroxide to come into contact with the surface of the metal. In Table I1 an attempt has been made to classify the paints according t o the apparent resistance to the exposure conditions. Here again is evidence, that in the cases of the pigments which might be more or less readily involved in oxidation-reduction reactions, the permeability of the paint film to moisture is no handicap. Below lead chromate, in the scale of results, the benefits derived from additional imperviousness of the paint film through changes in the vehicle become quite evident. Considering the situation when drying oils are in direct contact with the metal, it seems reasonable to suggest that a probable reason for the disadvantageous showing of aluminum in priming paints on iron or steel surfaces, aside from the
11
10 11
Also
very insoluble nature of its oxide and the stability of its soaps, is the fact that this pigment tends to float to the surface of the film, thus leaving a relatively free pigment layer of vehicle of appreciable thickness next to the metal surface. One prerequisite for a good priming paint of the drying oil type is to have the pigment (and its soluble and oxidizable soaps) a t or very near the metal surface. Unfortunately, zinc chromate did not happen to be included in this test. Dunstan (9) long ago pointed out that soluble chromates decompose hydrogen peroxide. The chemistry of the chromntes is very complicated; to some the position of lead chromate in Table I1 may seem wrong, although the results happen to be in accord with the results of several series of outdoor exposure tests a t Palmerton, Pa., during the last 10 years that included many commercial lead chrornates of different degrees of basicity. Lewis and Evans (15) in their recent classification also put lead chromate in the noninhibitive class. It is significant that, according to Mellor (18), lead chromate is hydrolyzed by water so that a t 25’ C. the water contains 0.02 millimole of chromic acid per liter. The depolarizing efficiency of chromic acid is, of course, very great, particularly in the presence of other acids, and Dunstan (9) has shown that it attacks iron directly. Further, it is noted that lead chromate primers generally benefit from the presence of a basic pigment; therefore, it may be suggested that these pigments are usually not sufficiently basic. Commercial zinc chromate pigments, on the other hand, are generally so actively ba4c that the possibility of the existence of chromium trioxide in their presence is questionable. Britton and Evans (2) have also stated that “chromates appear excellent under some conditions but may fail when the rain is acid; this falls in line with laboratory work which shows that, while chromates prevent corrosion in neutral solutions, they stimulate corrosion in acid by depolarizing hydrogen.” The use of basic pigments with chromates to overcome this defect was suggested. Experiments by this laboratory indicate no particular benefit from adding zinc oxide as such to zinc chromate when the exposure is to normal atmospheres, but, when the exposure was made in the irnmediate vicinity of a sulfuric acid plant, some advantage
I NDUSTR IA L AN D EKGIS EER ING CH EM ISTR Y
40
Vol. 27, No. 1
TABLE111. EFFECTOF AMOCST OF Basic PIGMENT (Exposure conditions the same as in Table 11) BEST
NO. O F
OF
PIGMEXI COMBINA-
PIGMESTS TIOKS
i
I
Zinc dust Zinc oxide Zinc dust Iron oxide Zinc oxide Red lead Zinc oxide Red leacl Iron oxide Red lead Graphit e
Red lead SO; Blue lead 20~’ Iron oxide 70b Zinc oxide 30 Iron oxide 80 Graphite 20 ,f Magnetic 70-SOb j iron oxide 30-20 }
1
1
Iron oxide
40
Zinc oxide
30
PIGMENT COMBINA-
TYPEOF
TIONS
T-EHICLE~
RUST-IXHIBITIVE QUALITIES
TESTED
Raw linseed oil (low thinner)
Rest combination is slightly better than 100yc zinc dust
Raw linseed oil (low thinner)
Best combination is about as good as SOYo zinc dust-20% zinc oxide
5
Raiv linseed oil (low thinner)
Bast conibination is better than 100% red lead
9
Raw linseed oil (low thinner) Medium bodied-oil content with moderately high thinner Loiy-spar varnish content
Best combination is slightly better than 100yored lead Combination is not as good as 10070 red lead (100% graphite blisters and rusts under film, but, added t o red lead, the film integrity of exposed red lead is improved)
6
All combinations are poorer than lO0vGred lead
6
Raiv linseed oil (lorn thinner)
All combinations up to 40% zinc oxide are better than looc> iron oxide Best conibination is not as good as 100% iron oxide: 60 to 100% graphite showed poor adherence
9
Low bodied-oil content Raw linseed oil
(107~-
thinner)
All coinbinations are better than 10070 magnetic iron oxide
11
6
0 11
High varnish
All combinations are better than 100% blue lead
0
Sfoderate bodied-oil content
None of the coinbinations are as good as 100% blue lead
G
Moderate varnish content
b l l combinations are better than l O O 7 , of either pigment
0
High bodied-oil content
A11 combinations are better than 10070 lead chromate
Moderate bodied-oil content
High varnish content
Rust inhibitive qualities of the combinations and of 100% lead chromate 6 and 100% iron oxide seemed to be about the same Best combination is better than lOOYc graphite (tendency to blister and 9 rust under the film) All combinations are better than 10070 aluminum 11
High varnish content
Best combination is not as good as 10070 aluminum
Moderate bodied-oil content
11
6
The vehicle used represented a compromise of the vehicles that produced the best results with the individual pigments (Table 11). These results have been verified from time t o time by exposures under different service conditions.
from adding zinc oxide has been el-ident where certain synthetic resin vehicles are involved. Continuing on the subject of the value of basic pigments, the data in Table I11 shorn that even the more definitely inhibitive pigments can benefit from the presence of additional basic material in the film, and that the need for it increases as we go down the scale of inhibitive qualities of the other pigment component, being quite helpful in the case of lead chromate. It is evident that acid decomposition products do play a role in the corrosion p r a e s s which can be modified by the neutralizing effects of basic pigments to establish a more favorable environment. As was pointed out a t the beginning, it is obviously beyond the scope of this paper t o discuss every phase of a subject as complicated as corrosion under the innumerable special conditions that can exist. The recent papers by Evans and Hoar ( I S ) and Lewis and Evans (15) deserve careful reading. The latter, especially, seeks t o winniarize the situation and concludes that “the inhibiting action of pigments rests on the same principles as inhibition by soluble metals used in water treatment (such as sodium hydroxide or potassium chromate). When the products of incipient corrosion are precipitated in physical contact with the metal, the attack stifles itself (sometimes before it produces any physical change) and the metal remains immune.” An acid or a strongly reducing state next t o this film might disturb it, as
pointed out above in the discussion of the decomposition of oil films. The method proposed by Lewis and Evans (16) for testing inhibiting qualities of pigments in paints by placing drops of sodium chloride solution on a painted steel surface carrying a single scratch to expose the metal, still seems questionable as a general basis on which to formulate priming paints, since it establishes a special condition. The author of this paper, a number of years ago, tried a similar scheme on panels exposed to salt spray equipment similar to that used a t the V. S.Bureau of Standards and found that the specific effects of salt solutions on the organic binder were often quite contrary to the results of normal exposures, minus the salt conditions. However, the ratings given to several pigments by Lewis and Evans, according to results by the “scratch test,” are generally in line with what would be expected from their probable neutralizing efficiency (basic nature) and probable ability to stifle the evolution of hydrogen peroxide. It is particularly interesting, for example, in view of the previous discussion, to find that lead chromate is placed in the noninhibitive class by Lewis and Evans, whereas metallic zinc powder and litharge are given high ratings. If, as Walker (29)maintains, the scratched area becomes anodic, the ability of the remaining film to prevent access of a cathodic depolarizer (oxygen, hydrogen peroxide) and maintain a neutral or slightly basic state, is, of course, of prime importance.
January, 1935
INDUSTRIAL AND ENGINEERING CHEMISTRY
VEHICLES No specific data on the relative merits of vehicles, other than already mentioned, are available for discussion, although the author has seen exposure tests that indicate considerable differences in some recently developed synthetic resin varnishes when used with the same pigments. One of the investigators of hydrogen peroxide evolution from oils found that foots, etc., promoted its formation. This is in line with statements that oils free from foots are superior for use in metal primers. A closer study of the decomposition products formed under different conditions of exposure, and their reactionswith pigments, is in order for organic binding media intended for use in metal priming paint.. It is also worth while considering the gel structure of the resulting film and the effect this might have in making moisture and other constituents available for the corroqion process above or below certain limits of concentration, as suggested by Vernon (28) in the discussion of the rapid corrosion that occurred under dust particles at humidities above 65 per cent and below saturation.
SUMMARY Considering the voluminous literature on corrosion, there has been comparatively little said about metal priming paints as pigment-vehicle combinations that are subject to chemical and physical change, and the influence these changes may have on the results. This paper represents a n effort to carry the discussion of this phase of the subject a step further. The different theories of corrosion have been briefly reviewed and the external agents required according to each theory discussed. It is known that the most essential of these are supplied within the film by the changes in oils and resins, the most obvious being water, carbon dioxide, and acid materials. The fact that a cathodic depolarizing agent (oxygen) is also supplied in very active form as hydrogen peroxide has apparently been overlooked. Basic pigments that are good neutralizing agents are desirable in priming paints; so also is the ability of the pigment t o form reaction products that add to the physical (impervious, etc.) qualities of the film. A metal priming pigment should also have more or less ability to control depolarizing agents such as hydrogen peroxide given off by the vehicle component. It may do this: (1) because its cornbinations with the vehicle (soaps) exist in several stages of oxidation, the lower stages being able to reduce hydrogen peroxide, as in the case of the lead oxides; (2) because the pigment reacts to make a reducing agent such as hydrogen available, as in the case of metallic zinc powder; or (3) because there is maintained in the film a state of alkalinity or other condition under which hydrogen peroxide ii decomposed. The oxidation of the pigment by hydrogen peroxide must not result in the formation of reaction products that promote corrosion.
41
A more careful study of the products resulting from chemical changes in the organic binder is another promising angle from which to attack the problem of formulating still better metal priming paints. ACKXOJVLEDGMENT The author wishes to express his appreciation to R. W. Jamieson and R. L. Eisenhard for conducting the tests given in Tables I1 and 111, and to other members of the research organization of The New Jersey Zinc Company for their criticismq. LITER.4TURE CITED
Baughman and Jamieson, J. Oil &. Fat Ind., 2, 25 (1925). Britton and Evans, Trans. Electrochem. S O C .64, , 51 (1933). Creighton and Fink, “Electrochemistry,” Vol. I , pp. 260, 297 (1924). Cushman, U. S. Dept. Agr., Bull. 30 (1907); Proc. Am. SOC. Testing Materiats, 7 , 211 (1907). Cushman and Gardner, “Corrosion and Preservation of Iron and Steel,” London, The Mining Journal, 1910. Daeves, Trans. Elecfrochem. SOC.,64, 99 (1933). Divers, J. SOC.Chem. I n d . , 24, 1235 (1905). Dunstan, Ibid., 27, 141 (1908). Dunstan, Proc. Chem. SOC.,19 (267), 150-2 (1903); J. SOC. Chem. I n d . , 22, 745 (1903). Electrochem. Soc., “Symposium on Corrosion,” Trans. Electrochem. SOC.,64 (1933). Evans, Chemistrg & Industry, 45, 504-8 (1926). Evans, “Corrosion of Metals,” 1924. Evans and Hoar, Trans. Faradag Soc., 30, 424 (May 5, 1934). Jordan, L. -1.. et al., J . Oil Colour Che7n. Assoc., 16, 3 9 8 4 2 1 (1933). Lewis and Evans, J . Soc. C h e w Ind., 53, 25T (1934). Long, Reineck, and Ball, IXD. ENG.CHEV.,25, 1086 (1933). Mackey and Ingle, J . SOC.Chem. Ind., 36, 317 (1917). Mellor, “Comprehensive Treatise on Inorganic and Theoretical Chemistry,” Vol. X I , p. 279, New York, Longmans, Green, and Co., 1931. Merzbacher, Chem. Cmschau Fette, Oie, Wachse, Harre, 36, 339 (1929) ; Eibner, &Ilexander,“Das Oltrocknen ein kolloider Vorgang aus chemischen Ursachen,” p. 143, Allg. IndustrieVerlag, 1931. Moody, Proc. Chem. SOC.,19 ( 2 6 S ) , 157 (1903). Nelson. “Effect of Adding Zinc Oxide to Iron Oxide Paints.” N . J. Zinc Co. ResearchBull., 1925: Drugs, Oils, Paints, 41, 10-11, 51-3 (1925). Patterson and Hebbs. Trans. Faraday SOC.,25, 204 (1929). Philadelphia Paint & Varnish Production Club, Am. Paint & Varnavh Xfrs.’ Assoc. Circ. 423 (1932). Purdy and Fasig, Paint, Oil,Chem. Rev., 84, 12 (Nov. 17, 1927). Russel. Proc. Rov. Inst. Great Britain. 16-40 (1899-1901). (26j Stieglita, “Qualitative Chemical Analysis,” p: 82 (1921): (27) Stutz, Nelson, and Schmutz, IND.ENQ.CHEY.,17, 1138 (1925). (28) Vernon. Trans. Electrochem. SOC.,64, 31 (1933). (29) Walker, W.H., and Lewis, W. K., J. IND.ENQ.CHEM.,1, 754 (1909). (30) Watts, Trans. Electrochem. SOC.,64, 136 (1933). (31) Whitney, J. A m . Chem. Soc., 25, 394 (1903). RECEIVED September 19, 1934. Presented before the Diviaion of Paint and Varnish Chemiatry a t the 88th Meeting of the American Chemical Society, Cleveland, Ohio, September 10 t o 14, 1934.
-. -Box CONVEYOR IN DULUXENAMEL BUILDING, PHILADELPHIA, PA., OF E. I. -e-
DU
PONTDE NEMOURS & COMPANY
