INDUSTRIAL AND ENGINEERING CHEMISTRY
1152
applied over almost noncorrodible metals. (Additional exposure of these specimens may alter this conclusion.) It is believed that water penetration of the paint film is the main factor of all paint blistering. Assuming that a paint film was absolutely impervious to moisture and applied over clean metal (surface free of any active substance), it would not blister. Contrary to popular opinion, corrosion is a result of paint blistering and not a cause. Carefully cleaned glass specimens were painted and exposed in the humidity test; they were found to exhibit the same degree of blistering as steel specimens prepared in the same manner. Sandblasting or etching the glass with hydrofluoric acid so as to roughen its surface failed to prevent blistering when it was painted and tested as described. In general, the adhesion of the paint film to its supporting base influenced the size of the blisters but was found not to affect their number appreciably. Increased adhesion of paint films to their base is conducive to preventing blisters from increasing in size and thereby to extending its useful life. Expressed by Wray and Van Vorst (IO): “The permeability of the paint film to moisture is a fundamental property of practical importance.
VOL. 30, NO. 10
Moisture is necessary for the common types of corrosion; the exclusion of moisture by the paint film is an important function of a metal protective paint, and an understanding of the mechanism of moisture penetration and its measurement has become essential in modern paint technology.”
Literature Cited Darsey, V. M., IND. ENG.CHEM.,27, 1142 (1935). Edwards and Wray, Ibid., 27, 1145 (1935). Evans, U.R., Trans. Am. EZectrochem. SOC.,55, 243 (1929). McKay and Worthington, “Corrosion Resistance of Metals and Alloys,’’ A. C. S. Monograph 71, New York, Reinhold Publishing Corp., 1936. Nelson, H. A.,IND. ENG.CHIOM., 27, 1149 (1935). Speller, “Corrosion Causes and Prevention,” New York, McGraw-Hill Book Co., 1935. Tanner and Lodeesen, U. 5. Patent 1,911,726(1933). Whitmore, M.R.,IND.ENG.CHEM.,25, 19 (1933). Winston, Reid, and Gross, Ibid., 27, 1333 (1935). Wray and Van Vorst, Ibid., 25, 842 (1933). RECEIVEDApril 2, 1938. Presented before the Division of Paint and Varnish Chemistry a t the 93rd Meeting of the American Chemical Society, Chapel Hill, N. C., April 12 t o 15, 1937.
Pretreatment of Metal F. N. SPELLER National Tube Company, Pittsburgh, Pa.
0
F ALL factors that influence the protection obtained
from paint, those associated with the metal-surface condition are probably the most important, even more so than the composition of the paint itself. Since more than half of the iron and steel in use is painted, even if only for appearance, we will discuss the subject with particular reference to ferrous metals, although the same principles apply to the protection of nonferrous metals. Consideration of the subject will be mainly from the viewpoint of the user of paints, with the hope of encouraging the development of better and more economical methods of preparing metals for painting, and their more general use in everyday practice on steel used in larger structures. One of the purposes of painting is to form a physical barrier between the metal and its environment to prevent corrosion. However, all paints are more or less permeable to moisture, and, where it finds access, damage to the metal and the inner paint film follows. Therefore, both chemical and physical protection of the metal is required. The former may be controlled by corrosion inhibitors, such as zinc chromate, now commonly used in the priming coat. Wherever moisture is present, this treatment passivates the metal to some extent.
Adhesion of Paints The Underground Corrosion Conference held in Washington, D. C., November 15 to 17, 1937, included a useful and timely contribution on the principles of “Adherence of Organic Coatings to Metals’’ by Schuh (16). At this time, therefore, it is necessary only to refer briefly to the important factors that affect the adherence of paints. Every one knows that it is impracticable to make paint adhere properly when there is moisture or loose foreign matter on the metal sur-
face. All such unbonded matter should be removed completely. However, there are other factors that are not so generally recognized in preparing the surface for painting. The bond between a paint film and the metal may vary greatly, depending upon several factors. The film should be directly in contact with the metal or so near (i. e., within several molecular layers) that it is within the close range of attractive forces a t the interface between these materials. Furthermore, reactions sometimes take place between the metal and paint, which in some cases weaken the bond. Specific adhesion between paint and different materials cannot yet be measured. But for any one metal, all that is required is a reliable test for relative adhesion. In the following discussion of the principal factors that influence adhesion, it will be noticed that these are often interrelated. METALSURFACEFILMS.On all the common metals, surface films form by reaction of the metal with its environm e n t f o r example, the oxides, phosphates, or chromate combinations. I n fact, oxide films form so rapidly in the atmosphere that it is doubtful whether paints are ever applied to a perfectly clean metal. Surface films that are tightly bonded, stable, and absorptive usually afford a better surface for painting than the metal itself. Evans (4) showed an oxide film to be blended into the metal surface structure. The bond of a surface film to metal, the cohesive strength of the film, and the adhesion between paint and surface hare all of fundamental importance in protection by paints. Phosphated surfaces generally give good paint adherence. A number of processes based on this principle are now available for treating a wide variety of zinc and steel surfaces. The choice is based on service requirements and cost of application. Metallic phosphate combinations, as applied by the Parkerizing or Electrogranodine processes, are examples of effective but relatively expensive methods of making a better bond between paint and metal and of protecting it to some extent from corrosion. The influence of other less expensive chemical treatments on adhesion will be referred to later.
OCTOBER, 1938
1153
INDUSTRIAL AND ENGINEERING CHEMISTRY
ROUGHNESS.Another important factor that materially improves adhesion is roughness, which increases the actual surface area. For instance, a metal surface that has been roughened by steel-grit blasting may have well over fifteen times the area of a smooth surface (3, 16); even a very fine abrasive increases the surface area several times over a smoothly polished surface. The contour of the surface depends mainly on the particle size of the grit. Excessive roughness may invite corrosion, because of the thinner paint film that covers the high spots. Metal spray adheres mechanically to a sand-blasted metal surface but not to one that is smooth-machined. The surface area is also increased when a nonreactive stable film, such as iron phosphate, is formed on the warm metal surface (see the preceding .paragraphs). This method of providing an enlarged inert surface for the reception of paint seems preferable in some ways t o increasing the metallic surface, which is subject to corrosion. The weathering of hot-galvanized steel prior to painting is a well-known illustration of the beneficial effect of chemical roughening and metal surface films in improving adherence of paints.
Influence of Mill Scale on Durability of Paints There has been much controversy on the question of whether or not mill scale should be completely removed before painting. An oxide layer builds up on the metal surface as it cools after the last pass of the rolling mill, and its composition and structure vary with the temperature a t which the metal leaves the rolls. Therefore, there is considerable difference between mill scales, and their composition and adhesion cannot always be controlled in finishing steel products. About twenty years ago, a process was introduced by the National Tube Company to produce a uniform thin scale on pipe. The temperature was lowered to about 1650' F. before the finishing pass was entered in order to crack off the relatively thick welding scale and form a new thin scale of more uniform thickness which is comparatively easy to remove by pickling. Russian iron, blued sheets, and many of the colored metal finishes are examples of oxide films that provide considerable protection in some environments. However, the properties of mill scale on the larger steel products are not always under such control.
Surfaces for Painting - DRYSURFACE. Where the relative humidity is above 70, there is always the chance that an invisible adsorbed layer of moisture may be present on the metal, particularly when the metal temperature is below that of the air (18). Warming the metal sufficiently assures a dry surface and also improves the strength of bond to the coating. METALCOMPOSITION.The composition of the metal is of some importance with respect to the properties of the surface film formed in air. For example, on account of the more adherent film on copper steel, p:int holds better and lasts longer on this metal than on plain steel under most atmospheric conditions (17). CHEMICAL ACTION. Chemical action between the metal and the decomposition products of the paint vehicle may loosen the bond. For example, the adhesion of most oil paints to a new hot-galvanized surface weakens after a short time so that the paint tends to peel. This is thought to be due t o the formation of zinc formate on the interface between the paint and the zinc (23). If the galvanized surface is previously roughened by chemical or physical means, or if a considerable amount of zinc dust is added to the paint, adhesion is much improved. The bond of asphalt coating applied hot over an asphaltic primer may be weakened in time by separation of an oil which fluxes the undercoat when the latter is very different in type or consistency from the outer coat. Therefore, it is not wise to conclude that adhesion of new coating combinations is good until so proved by a prolonged test under service conditions. Changes in strength and ductility of the paint film also tend to destroy adhesion. Shrinkage, particularly in thick films, may overcome the adhesive bond. The composition, permeability to moisture, and aging properties of paints may have a marked effect on the initial adhesion or permanence of adhesion. When present in amounts over 25 per cent of the total pigment in the primer, inhibitors such as zinc chromate retard or prevent metal corrosion under paint that otherwise would destroy the bond between paint and metal.
I
During the past two or three years, the corrosion committee of the British Iron and Steel Institute, and other investigators in England, have taken a rather definite stand in favor of removal of all mill scale (7). However, it is sometimes difficult and expensive to descale large fabricated steel units completely. It is therefore important to consider the matter carefully and determine what, if any, advantage is obtained by the additional expense involved. The bond of mill scale to the metal varies considerably on the same piece of steel. In some cases the strength of this bond amounts to several thousand pounds per square inch, before weathering, and the paint adheres firmly to dry mill scale surfaces just as it does to the clean metal. It cannot be denied that many successful painting jobs have been done where only the loose scale and other foreign matter have been removed. On the other hand, where moisture is present the scale is cathodic to the metal by about 0.25 volt, and corrosion tends to proceed underneath the scale, which eventually loosens. If this happens after the steel is painted, failure of the coating is likely to be hastened. Later in this paper metal surface treatment will be discussed that may be applied to a clean metal or to one that is partly covered with a tight scale t o prevent or retard corrosion under paint. The best and most economical practice of surface treatment will therefore be determined by consideration of all of the factors involved, including cost and service conditions. Where a highly finished painting job is essential, as on automobile bodies, there is little question that all scale should be removed unless it is very thin and adherent and a corrosion inhibitor is applied. On the other hand, when the thicker bituminous or Portland cement coatings are used as for protection of pipe underground, it does not pay to spend much, if anything, on removing tight scale. The National Bureau of Standards' tests (10) of bare pipe in corrosive soils show no material difference between the pitting of pipe carrying a standard mill scale and the same pipe with very little or no scale. General conclusions cannot
INDUSTRIAL AND ENGINEERING CHEMISTRY
1154
A.
As-rolled 6nirB. seratcli-biurhed
B.
Pickled, dipped in oliromate idiihitina F.. and rsrlied wrth
solution at 180-190”
C.
VOL. 30, NO. 10
Pickled trehted with PiLouDiLoria w i d ohrodate inhibitor. not washed
tap water
OIL LACPUEK AB’TER PnETREATmNTS NOTED Fronir~1, STEEL PAR EL^ GIVENTwo C o a ~ sOP LINSEED-TUVG Asom, A N D ExPosEn 53 WEEKSos m h - ROOFI N P ~ ~ ~ ~ s n rSsMr-runnsTaIA1, .~o~’s ATMOSPHERE
be drawn from these tests because metal underground is subjected to local differences of potential due to variations in the soil and mater in contact uith the pipe, vhich are often in pxcess of the potential generated hetween mill scale and metal.
Methods of Cleaning the Metal Surface There are various ways of cleaning the sorface before i t is paint.ed. Where desirable, most of the mill scale can be removed or loosened by exposure to a corrosive atmosphere for several months. The percentage of scale removed depends mainly upon the corrosiveness of the air and the period of exposure. For instance, a few purchasers of pipe for gas and oil pipe lines order the material shipped vithout any coating heeausc they find that. after being weathered in transit, thr pipe is easier to clcsn Iry sand, steel-grit blasting, or other oieaiis. Sandblasting is one of the most effective means for renioving mill scale. Under some conditions, however, it i s ohjeet.ionahle on account of tiie dust probleni, the cost, and the difficultyof application. The air used in the blast should be fairly dry; otherwise, when it expands, moisture may be deposited on the clean metal. Ko matt.er how the surface is cleaned, in corrosive air it should receive a rust-inhibitivc priming paint immediately after cleaning. A method of hydraulic sandblasting is being developed that may have some advantages particularly in preventing dust irr Foundries. Mechanical impact appliances have proved effective for cleaning steel, particnlarly after the mill scale has been loosened by weathering. Cleaning machines of this type have been devised that remove mill wale and rust from the pipe, and also roughen the surface. Power-driven xire brushes in the hands of reliable operators can he used most effectively in reeonditioning old striictures that require repainting. Pickling in hot dilute acid and rinsing in hot water areoften less expensive and more satisfactory than send or steel-grit hlasting. One of the latest methods is to pickle the metal free from scale in 5 per cent sulfuric acid at 160’ F. (6). After being rinsed in hot water, i t is immersed in a 2 per cent free phosphoric acid bath containing approximately 0.5 per cent of iron in solution at 185” F. for about 3 or 4 minutes. On removal from tlie phosphoric acid bath the pipe is allowed to dry without washing and is painted while warm. Phosphoric acid gives an iron phosphate inhibitive finish well adapted to hold paint, as pointed out hefore. Where practicahle, this
treatment is favorable t,o good painting. Even scrubbing with a water-alcohol solution of phosphoric a.oid is reported to be useful in preparing the surface for painting. If there is any suspicion that a film of moisture is present on the surface of a metal, i t may be wiped with a clean gas flame immediately before the paint is applied. (Wet metal should not be dried in this way.)
Application of Inhibitors to Metal Surfaces I t is well known that the electrochemical process of corrosion is retarded or prevented under certain conditions when the electrolyte contains a certain amount of an inhibitor, such as sodium chromate. This is apparently due to the polarization of anodic areas by t.he formation of self-healing films (5, f4,19). The potential of a nietd to water is t1ia.t of the natural metal surface lilrn and is materially affected by the composition of applied paint coatings. Time-potential curves indicate that the cliemical protective effect is just as important as physical protection of paints on metals (8). The use of a slightly soluble zinc chromate in the priming coat of paints is now generally recognized as good practice. Red and hlue lead are mild inhibitors of corrosion but are much improved when used with more than 25 per cent of zinc chromate. The general use of red lead as priming paint on the as-rolled surface of iron or steel probahly accounts for the comparatively little trouble experienced in painting over tightly attached mill scale. Apparently the metal is rendered more or less passive by a sniall amount of inhibitor dissolved by moisture which finds access through defects in the paint. Addition of the sodium oliromates to water greatly retards or stops the galvanic action between mill scale and steel. The aniount required to do this depends upon the composition of tlie water. Rased on qualitative tests in Pittsburgh city water, only a trace of corrosion is found on steel carrying mill scale when the dichromato concentration is about 800 p. p. in. I n the ahsence of mill scale, about 300 p. p. in. of dichromate will he sufficient to prevent corrosion under these conditions. The normal clrrorrratc (pH 8.5) is still more effective. It would seem to he only one stop further to apply a chromate inhibitor to the metal before painting. The phosphated fihn formed by reaction between iron and phosphoric acid acts as a milder inhibitor and serves a useful purpose in improving adherence of paint. Combination of these materials has proved useful in preventing or retarding corrosion in industrial water.
SNDUSTRIAL AND ENGSNEEKSh-G CHEMISTRY
OCTOBER, 1938
1155
Within the past few years, a number of water-inhibitive solutions have appeared on the market for pretreatment of metal surfaces on a large scale, prior to painting, to improve adhesion and retard corrosioii under the paint. These contain essentially orthophosphoric acid or chromic acid. At. least one contains both phosphoric acid and chromium salts with other proprietary oil-dispersing ingredients. The effect of these inhibitors depends upon their proper application and somewhat upon conditions of exposure. For instance, in indust.ria1 air and in the salt-spray chamber, B marked improvement, 'ims fouiri in the rpsistanrr of a paint 5 o t inhibited
Inliibiied
Hnrie lead chromate priniei
Zinc chromato-iron "Tide m i n i
/: t
The adherent fims that these inhibitors form on t.he metal increase the surface available for contact with the paint and a t the %me time have apassivatingeffect that may be quite usefid in neutralizing local potentials such as those doe to mill scale or rust. These inhibiting materials have not been appiied in practice on a large scale long enough to develop filly all necesaary precantions in application and determine the degree of protection that can he depended upon under various service conditions. However, much the same principle Iras been employed for some time in various forms (in hot soliitions) for the preparation of highly finished steel for painting, suoh as for automobile body parts. Chromate inhibitors would probably not be effective in an atmosphere that was decidedly acid, since acidity dissolves the fim and increases corrosion. The addition of a basic pigment or zinc dust should he h e l p ful in the priming coat, under such conditions. The amount of moisture that can penetrate a good paint fdm is quite limited, so that a very small amount of soluble cbr+ mate should give the concentration necessary to passivate the metal. The solnIility of zinc chromate in water is about 800 p. p. In.
Summary and Conclusion It is evident that metal corrosion and the paint problem are intimately related. To obtain the best protection of iron and steel by paint, proper preparation of the surface is of fundamental importance. This includes, a t least, removal of all foreign matter not tightly bonded to the metal or
1156
IXDUSTRIAL AND ENGINEERING CHEMISTRY
VOL. 30, NO. 10
(Left) FIGURE WEATBERED,
3. STEEL StiRF.4CE &-ELII SCRATCH-BRUSKED,
AND
PAINTED WITH A PHENOLIC ParmEn CON'I'AININO I'AININO 50 PER CENTZrm CLIROMRTE CXROMRTE AND h P a E l r O t I C FINlSVINO FINlBVINO IX I Y LORRIN, OHIO,
COAT;
IXMERSED IXXERSED FOR
HARBOR WATER ABOUT8 MONTHI~
(Below) FIaunE 4. STEELSUnrACE DEny PICSLING AXD PAINTED VITH A BAKELITE Pxrma AND FINISBINO COAT; IMMERSED IT LORAIK,OHIO, HARBOR !VATER FOR A m v r 8 MONTES SCALED
Suiiaer inhibited yith phosphoric ncideiiromate aulution before psintiog
unsuitable as a base for paint. Various methods for removing mill scale under different conditions of service have been discussed. The surface should be actually dry, preferably warm, and free from reactive suhstances when paint is applied. Water solutions of inhibitors applicable a t Suriaoe inhibited with Bakelite primer and Surlaee not inhibited before painting noma1 or higher temperatures are being definishing Cost beiore psinting veloped that react with the metal to form inhibitive films. The rate of this reaction d e creases with the temperature so that precautionsmust be taken to ensure either the completion of the reaction or the removal Bibliography of all soluble material that has not reacted before the surface (1) Am. 600. for Testing Materials, Symposium on Paint and Paint is dried prior to painting,^ properly this Materials, 1935. in certain hss been found to improve paint adhesion: ( 2 ) urn^, R. M., snd ~ ~u. E., ~rTnns. ~i ~~ ~see., ~ ~69, , t it tends to polsrize the iron mill-scale couple, and to reduce 169-83 (1936). (8) D m w . V. &I., IND.E m . CHEM.,27, 11424 (1935). local differencesin potential. The advantage of an efficient (4) Evans, U. R., "Metallic Corrosion Psssivity and Proteetion," inhibitive pigrient in $he p r i l ~ n g is no,v generally retPP. 5P60, London, Edward Amold'and Co., 1937. ognized. The Combination of an inhibitor properly applied (5) Ibid., Dp. 2g8-g. to the metal and in the primkg paint tends to reduce iiurther io) Pootzrer, H. R., J . Oil Coiour Cfimi. ~ s s a c . ,20.34 (&fa,., 1'33ij. (7) Iron and Steel Inst. (London), 4th Rout. Corrosion Committce. the chance of destnlction of the bond between paint and metill by corrosion. However, the practical value of these low-cost ( 8 ) Jordan, PP. 13Land . A,,3 9 4sot. 2 (1930); CILm,, 5th Ind., Rept., 56, 301-71 pp. 305(1g37), and 351 (1938). water inhibitors to fabricated or structural steel has not been (9) xsppler. F., z. vrr.deut.mp., 80, 183-5 (1936). fully demonstrated by long-time teste in service. The rs(10) Logan, K. H., J. Research Natr. Bur. Sucndords, 16,453 (1936). (11) Nelson, E. A., IND. ENQ.Cam.. 27, 3&41 (1935). markable developments in paints and painting technic in r+ cent years have been accomplihed gradually by testing ard Reu,, No, 9, 12-10 (1854j. experience. The successful appiication of watersoluble in(14)Roetheli, B. E., a d cox. G. I,., IND. ENR. caEar., 23, 10% hibitors to the metal on a large scale will probably have to go (1931). (15) Sohuh, A. E., Proc. Am. Soc. Teslip iw&riars,38, Pt. I1 (1938). through a similar process of development. Where better and (I6) Simonds. B. R.1 Iron Age, 139.635-&640.642.644; 140.3743. more expensive are to be applied, it still more k,,. 52 (1937). portant that all necessssy precautions be taken in the prepti(17) speiler, F. N., "corrosion. and prevention,"znd cd., ration of the surface.' PP. 115-16, New York. McGraw-Hill Book Co.. 1935. (35) rbid.. P. 148. ¢ tests and experience indicate that, as a rule, when carried out with d l necessary precautions, the most desirable ~ ~ ~37, No, ~ (Dee,, ~ 1933), m L , preparation of steel surfacea for painting (particularly for un(21) Taylor. G. o., paint Manut,, 6, ,9 (1936), derwater service) is complete descaling, followedby phosphat( 2 2 ) Vernon. PI. H. J., Mdar rnd. (London), 46, 2 3 ~ 1 2. 7 % ~ . 307-8 (1935). ing or similar chemical treatment. J ,
iiii
ik:j
After this paper w.s written, the Ameriesn Sooiety for Testing Materials est w B ne* subcommittee (XXIX of Committee W - l ) a" Preparation of Iron and Steel for Painting This subeummitteo should be a good medium for testing all isrtorr having LO do with prepamtion of the metal 6urfeDe8 for minting.
~~~~~~5)&4r.di.
&$z$,
(23) Wing. H. 3 . . 1x0. ENG.CBEM.,28, 2 4 2 3 (Feb., 1936).
L
R ~ c e r v e o4 ~ i i l12, 1938. Presented befois the Division of Paint and Varnish Chemistry at the 95th .Meeting of the Ameriosn Chemical soriets. DeIlss. Teras. i p i i l I S Lo 22, 1938.
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