Automobile Finishing Presented before t h e Division of P a i n t a n d Varnish Chemistry a t t h e 100th Meeting of t h e American Chemical Society, Detroit, Mioh.
RESENT-DAY deand decorative effects can be mand for more durable obtained simultaneously if finishes on automobile proper pretreatment and bodies has served to emphapaint finishes are selected. size the need of suitable Active accelerators of chemical treatment prior to rusting are applied to bodies painting. So important is in the soldering fluxes wed this part of any successful in the metal finishing oDerapainting procedure that the tions. It is essentyal-that metal preparation is conthey be completely removed sidered by some as imporin the pretreatment process tant as the composition of prior to painting to avoid the paint itself. Thorough premature failure of the cleaning of metal surfaces finish a t such areas. The is a prerequisite of the appresence of any waterplication of any successful soluble salts or activators V. Nl. DARSEY paint finish. During the such as alkali spots, hand past years the practice of prints, and dirty rag marks Parker Rust-Proof Company, Detroit, Mich. pressure-spray alkali cleanunderneath the surface of paint in combination with ing of bodies in completely moisture will result in blismechanized equipment foltering of the paint coating lowed by chemical treatment a t such areas and premature failure (1). with a phosphate rust-proofing solution containing nitrate The following methods are used for preparing automobile (Bonderizing) has gained wide use. The number of bodies phosbodies for painting: phosphating, phosphoric acid cleaning, phated by this process has increased from 850,000 when first solvent cleaning. introduced in 1936 t o over 2,000,000 bodies in 1940 (Figure 1). Automobile bodies, like most other articles of iron and steel, are painted to preserve them against corrosion and Phosphating Process provide a decorative finish. Like paint, coatings are most Today phosphating is the most widely used of these separate durable when applied over nonreactive surfaces (S), and all methods. The all-steel body made possible the cleaning and pretreatments of easily corrodible metals have as their main phosphating on a similar scale, as used extensively since 1929, objective the formation of a nonreactive coating (4-7) prior to the painting of automotive sheet metal parts such as chemically combined with the base metal, which not only fenders, running boards, and grilles for increasing the adretards corrosion but provides greater adhesion for applied hesion and resultant durability of the paint fmish. Phospaint finishes. Most paint troubles have their beginning phating of automobile bodies is a chemical pretreatment in a t the point of contact between the paint and the metal surwhich the surface of the steel is by chemical reaction converted face @), and any recommended pretreatment should reduce to a protective phosphate coating adapted to inhibiting corthis tendency to change for greater durability of finish. To rosion and increasing the adhesion and durability of applied fulfill this requirement, the base metal must be shielded paint finishes ( 2 ) . By chemical combination of the coating against corrosive agents to prevent corrosion. Protection
P
Bodies must be thoroughly clean before application of paint. Chemical treatment of bodies to provide a phosphate coating for inhibiting corrosion and increasing the adhesion and durability of applied paint has gained wide use. Completely mechanized equipment for cleaning, phosphating, rinsing, and drying of bodies has removed, as far as possible, the human element in securing a satisfactory surface for painting. Frequent testing of finished bodies by an accelerated humidity test serves as a check on all cleaning and chemical pretreatments prior to painting and prevents the output of improperly prepared bodies. Service tests as well as accelerated tests show the advantage of changing the surface of ordinary steel to a more stable phosphated surface, prior to painting, to increase the useful life of the finish.
222
P
February, 1941
*
.,.
INDUSTRIAL AND EkGINEERING CHEMISTRY
with the metal, it has greater adherence than could be expected from any coating which depends upon natural attraction. Before pressurespray alkali cleaning of bodies, rubber dough, chalk marks, and other foreign matter which is not soluble in alkali cleaner are removed from the bodies by suitable solvents. Rust is removed by sanding or with phosphoric acid. The phosphating process consists in spraying the Bonderite solution by means of completely mechanized equipment onto the bodies as they are carried by a conveyor through an enclosed tunnel which includes cleaning, phosphating, rinsing, and drying in one continuous operation. The cleaning is accomplished by forceful spraying of hot alkali cleaner solution onto the bodies as they pass a series of spray nozzles located in spray pipes on the sides and top, inside the tunnel housing, so that every conceivable spot of surface is impinged and scoured by the cleaner to remove all grease, oil, and foreign matter. Following the cleaning procedure, the bodies advance through two stages of spray hot water rinse where the last traces of alkali cleaner solution are removed before phosphating. Each section of cleaning, rinsing, and phosphating has located below it a tank which serves as a reservoir for the respective solutions and, connecting with each tank, a pump with which the solution is sprayed. These tanks are separated by drain spaces to allow each separate solution to drain from the bodies back into its respective tank, where it is recirculated in continuous operation. This method of preparing bodies for painting eliminates so far as possible the human element in obtaining a satisfactory surface for painting. I n the cleaning, phosphating, and rinsing procedure, over 4000 gallons of solution are sprayed onto each body. Such a process ensures the complete removal of all foreign material from the surface as it completely floods every conceivable spot on the body and ensures a chemically clean surface. A solution commonly used for phosphate rust-proofing is prepared by dissolving zinc, or manganese dihydrogen phosphate in water. In order for the phosphate coating to form on metal articles treated with such a solution, free phosphoric acid must be available to dissolve a thin layer of the metal surface and the solution must be saturated with the metal of the phosphate used for preparing the bath or the metal dissolved from the article being treated. A solution of a given free phosphoric acid content will hold in solution a definite amount of either iron, manganese, or zinc, and is commonly referred t o as a balanced solution. Since iron, manganese, or zinc is not soluble in water, it must be held in solution by phosphoric acid. In the coating operation a metal article is treated with the solution; the phosphoric acid of the solution immediately reacts with the metal surface, thus consumes or takes some of the acid out of the solution and causes supersaturation of the solution a t the surface of the article, and zinc or manganese, and iron phosphate precipitate as coating. I n that reaction, zinc and iron phosphates, or manganese and iron phosphates are formed on the article being coated and some iron phosphate dissolvesinto the solution.
223
and free phosphoric acid in solution result from the hydrolysis, and may be represented by the hydrolysis equations:
+ +
+
Mn(HzPOJr (HzO)+MnHP04 HsPO4 3MnHPO4 (HzO) +Mns(P04)~ Hap04
+
When properly cleaned metal articles are treated with the solution resulting from this hydrolysis, a nonmetallic and finely crystalline phosphate coating is formed on the surface. The reaction of the acid phosphates and free phosphoric acid in the bath with the metal being processed may be indicated by the equations:
I n Equation 3 the free phosphoric acid molecules attack the iron articles being treated, and two atoms of hydrogen in the acid are replaced by the iron to form the insoluble secondary phosphate as a coating on the article. Simultaneously the secondary manganese phosphate molecules in Equation 4 react with the iron article, and one atom of hydrogen is replaced by the iron to form normal manganese phosphate coating on the articles. Equation 5 represents the combined reaction which produces on metal objects in the phosphate rust-proofing bath a coating composed chiefly of the normal and secondary phosphates of iron and manganese. The metals present as phosphates in the coating depend entirely on the metals present in the processing bath, and these are generally iron in combination with manganese or zinc. It is essential in these fast processes to eliminate the hydrogen which is formed as indicated and which retards coating action. For this purpose nitrate is used in the solution, and a fast continuous coating operation results. Cleaning
PHOSPHORIC ACIDCLEANERS.The use of phosphoric acid cleaners containing organic solvents provides for the removal
Chemical Reactions Involved in the Phosphating Process
When manganese or zinc dihydrogen phosphate is dissolved in water, acid phosphates
(1) (2)
AUTOMOBILE BODIESENTERING THE BONDERIZING PROCESS
224
INDUSTRIAL AND ENGINEERING CHEMISTRY
of rust, grease, and other foreign matter from the surface of bodies before painting. The application of such cleaning materials is generally by hand, and the human element is a factor in the successful operation of the proc2,000,000 ess. I m m e d i ately following its application, the excess acid is rinsed off with I , 500,000 water and the body is dried before painting. It 1,000,000 is essential that all excess residual acid be removed before 500,000 painting, otherwise its presence underneath the paint will cause premature finish I935 '36 '37 38 '39 '40 failure. I n addiYtAIL5 tion t o providing FIGURE 1. AUTOMOBILE BODIES a clean surface BONDERIZED by this method, the phosphoric acid etches the steel surface and provides increased adhesion for applied paint finishes. SOLVEKT CLEANERS.Application of organic solvents such as naphtha or benzine to the surface of bodies t o remove grease, oil, and other foreign matter is used to a limited extent for preparing automobile bodies for painting. In such a procedure it is necessary t o sand or remove by phosphoric acid any rust present on bodies before painting. In addition to the disadvantage of hand application, no improvement in adhesion of applied paint finishes is obtained over ordinary clean bare steel. ADVANTAGE O F MECKAKICAL CLEANING. A primary advantage of mechanical cleaning and phosphating of bodies prior to painting is to provide a more stable surface and avoid finger prints and hand and rag marks on the surface prior to painting. Inasmuch as the formation of the phosphate coating is dependent upon chemical reaction between the steel body and the rust-proofing solution, the visual evidence that it has formed leaves no doubt as to its cleaning effectiveness. Once the coating has formed on bodies and they are properly rinsed and dried, care is taken that no hands or any other foreign material contact the surface before painting. Experience has shown that such contact marks are sources of weakness in the final paint finish. All operators are required to wear clean canvas gloves for handling the bodies after phosphating and prior to painting.
Vol. 33, No. 2
presence or absence. Advantage is taken of the fact that when in contact with moisture, they form blisters in the paint film a t the contaminated areas. For this purpose a room large enough to accommodate an automobile body is provided where the temperature and humidity are automatically maintained around 100' F. and 100 per cent, respectively. A 24-48 hour period in this test room will reveal the presence of activators and contact marks by blistering in the paint coating a t such areas. The nature and characteristics of the blistered area are generally sufficient evidence to determine the source of the contaminating factor. Once the cause is ascertained, it can be immediately remedied; thus no finished bodies get into service which might fail prematurely and make expensive refinishing charges necessary. Salt Spray Test In addition to the normal outdoor exposure with its wide variation in temperature and humidity, automobile finishes are also subjected to the corrosive action of salt and calcium chloride used for removing ice from the streets in winter and arresting dust on certain roads in other seasons. Figure 2 shows two automobile doors which were subjected to an accelerated salt spray test for 500 hours. The door on the right had been phosphated prior to painting, and the other solvent cleaned only. Confinement of corrosion to the scratched surface (where the paint was purposely removed before placing in test) on the phosphated door reveals the increased protection obtained from paint applied over phosphated surfaces as compared to that afforded by the same paint finish over solvent-cleaned metal without phosphating. Literature Cited Darsey, V. M., Am. SOC.Testing Materials B d . , Jan., 1940 Darsey, V. M., IND. ENG.CHEM.,27, 1142 (1935) Ibid., 30, 1147 (1938). Edwards and Wray, Ibid., 27, 1145 (1935). Kelson, H. A., I b i d . , 27, 1149 (1935). Whitmore, M. R., Ibid., 25, 19 (1933). Winston, Reid, and Gross, Ibid., 27, 1333 (1938). Wray and Van Vorst, Ibid., 25, 842 (1933).
Humidity Test for Bodies Since it is practically impossible to determine by visual inspection the presence of contact marks or activators on the cleaned surface of bodies, frequently a completely finished body is placed in an accelerated humidity test to reveal their
FIGURE 2. EFFECT OF SALTSPRAYON SOLVENT-CLEAKED (left) A X D PHO+ PHATED AUTOMOBILE DOORS (right)
