Cathodic Electro Deposition
Electrodeposition Coating Process for Automobile Bodies* By Yoshiaki 0YABU,** Nobuo FURUNO,*** Yoji HIRASAWA*** and Hiroshi OMORI **** Synopsis The earlier electrodeposition processwas mainlypursuitedfor the quality control in terms of mass production. However, the coating efficiency was picked up to be a serial problemfrom a standpoint of the harmony between natural circumstancesand human’s living-life conditions and the recovery usage of the paint was earnestlysearched. This trend led to the development in various technicalfields.
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The second impetus on further improvementof electrodeposition paint was due to oil crisis in 1973. The new technology relating to save-energy was developed. In thepaint industries, the further improvedanti-corrosive performance was strongly demanded. This demand also involvedthe developmentof new materials for automobile bodies and even the design of bodiesstructure. In this report, the introduction of electrodeposition process into automobile industries in Japan is hystrically reviewedand the electrodeposition mechanismand the mechanism which deposited materials from the coating film are briefly explained.
As the demandfor anti-corrosiveperformance is increased, the coating system in which the anodic oxidation and the dissolutionfrom the object metal can not occur, was aimed and recentlyit was rapidly accepted as the superior system. On the other hand, there is an oppositeopinionfor this trend on the basis of the metal analysis detectedin the deposited film. The characteristics of cationic electrodeposition,in particular, the superior corrosion resistance with regard to the metal dissolution in the wholeelectrodeposition processis explained.
In Chapter III, throwingpower which is characteristic for electrodeposition processis described. It is referred that throwingpower is not necessarily determined by the paint performance. Even the electrodeposition coating system, the areas into which the paint liquid does not intrude can not be coated. And it is pointed out that the design of bodiesstructure is inevitablynecessaryas well as the improvement paint performances. of In Chapter IV, the role of the main component,vehicleresin, is introduced. The deviceswhich disperse resin into solvent` water’, and factors which determine the polarity of electrodeposition explained.
And the are typical resin structure is shown. The paint performanceis greatly effected by the vehicleresin, but it is also deeplyrelated how thepaint is produced and how it is coated. Therefore, it is inferred that thepaint performances are relativelyestimated. In Chapter V, the whole system of electrodepositioncoating process is given. The so-called ` closed-loopsystem’ which is remarkably characteristic in electrodeposition process, and which is suitably fitted for mass production of automobile bodies is explained.
Besides, the pH control technologyof paint bath and the possibility on its future development are referred. Thefurther improvement the recycleusage technology f paint of o is pointed out and the possibility to replyfor the social needssuch as saveenergyand lesspolution is explained. In Chapter VI, thepresent status of electrodeposition processand various technicalproblemsare explained. The summary of this review and authors’ views for future development are given in thefinal chapter. I. Introduction The development of the electrodeposition coating echnology in automobile bodies industry commenced around 1964 in Japan. In those days, dip coating of water-borne paint was applied to automobile parts after metal pretreatment, and it was confirmed when voltage was applied, the vehicle was deposited at the electrode. This was the beginning of electrodeposition coating. However, the essential point of this technology was not properly comprehended, while the unpublished work was progressing. Before 1963, the small products to which latex emulsion was mainly applied had been produced industrially.
In Great Britain, the first major industrial use of electrodeposition coating system was in 1963 when the Pressed Steel Company commenced priming petrol tanks at their Oxford Works. In U. S. A. , the patent on electrodeposition system for automobile bodies and parts such as wheels was published in 1963 from the Ford Motor Company. And when patents emerged, all the automobile industries aimed at them. As the research was going on, the coating technology was proved to attract us with peculiar interests . such as high corrosion resistance of coating and the superior adaptability to mass production.
The paint industries in Japan made a great effort to develop this coating technology, while they introduced it from the developed countries which it was attained in a higher level. The Japanese industries, in particular, automobile industries were in rapid progress, and they invested in the installation for the new coating system. Owing to this days trend, the electrodeposition was satisfactorily accepted by most of automobile industries. And in a short period of time, Japan got ahead of Europe and then U. S. A. n its distribution. In the beginning of electrodeposition paint which was mainly composed of natural oils, the film performance as a primer itself was inferior to that of solvent-type polyester primer modified with epoxy resin by spray coating, but the electrodeposition coating system was not followed in terms of throwing power and the coating efficiency. Around 1971, natural circumstances grew worse in proportion to the rapid progress of industrial products and people became sensitive to the recovery of the nature in Japan.
The electrodeposition coating system does not emit an organic solvent in the air and in this point of view it fitted the days’ demand. How- * ** *** **** Received April 11, 1983. © 1983 ISIJ Director of Research Department, Nippon Paint Co. , Ltd. , Minami-shinagawa, Shinagawa-ku, Tokyo 140. Technical Center, Nippon Paint Co. , Ltd. , Ikeda-nakamachi, Neyagawa 572. Industrial & Marine Coatings Division, Nippon Paint Co. , Ltd. , Fukushima, Fukushima-ku, Osaka 553. (994) Review Transactions ISIJ, Vol. 23, 1983 (995) ver, the great mass of water was utilized for rinsing after electrodeposition and it caused the problem of waste treatment. In response to this problem, the treatment of rinsed water was studied, leading to the development of the closed system. In 1969, PPG (the Pittsburgh Plate Glass Company, U. S. A. ) published the patent on ultrafiltration closed-loop system for electrodeposition coating. The waste treatment of rinsed water after electrodeposition was remarkably improved and coating efficiency reached 100 %.
This progressed the improvement of the quality and the preparation of large size of ultrafilter and facilitated the industrialization with use of it. Since the oil crisis in 1973, the textile industries were chased up closely by the developing countries which manufactured products cheaply and they were not allowed to keep the technology for mass production. They were demanded to develop the new technological product being provided specific properties by utilizing their fine chemical technology. It can be said that this trend in the textile industries accelerated the development of ultrafilter.
In 1975, it happened that an automobile engine was dropped off from the body by severe corrosion of supporting panels which was caused by the salt dispersed on the road in Canada. This led to the big compensation and with a help of its impetus the corrosion protection of automobile bodies were taken as the serial problem which had been prone to be neglected and the design of automobile bodies were begun to study in terms of corrosion protection. Around that time, in order to decrease the weight of automobile bodies, inorganic and organic materials were started to be investigated in esponse to the days’ demand of saving labor and energy and cationic electrodeposition coating attracted remarkable attention. PPG’s patented cathodic electrodeposition system was introduced in 1972. Its first commercial use was for applying the primer on household appliances to gain the advantages of increased detergent resistance and outstanding corrosion resistance. The corrosion resistance of cationic electrodeposition coating was far superior to that of anionic one and the former was substituted for the latter in a few years in Japan. In those days, there had been reported only several papers on the steel surface.
In the industrial metal pretreatment line which the phosphating bath was controlled in the proper treatment values, even if the satisfied phosphate coating was not obtained due to the variation of steel surface, the phosphating bath was changed to fit the varied steel surface. Around the time cationic electrodeposition coating was examined to apply to the industrial line, there had been technological competition to protect automobile bodies against corrosion either by the improvement of primer paint or by the development of precoated steel plated with Zn, Ni, or Al.
However, the cationic electrodeposition paint of PPG was proved to be far ahead of others and most of the automobile industries in the world accepted it. Consequently, the anti-corrosive performance of the precoated steel for automobile bodies came to be evaluated by applying electrodeposition coating to it. Even the steel industries were involved in the world trend from mass production to the development of the functional materials. The steel industries commenced earnestly the development of precoated metal possessing the corrosion resistance, while producing steel by continuous annealing pickling line system.
With the development of precoated metal, papers on the analysis and its interpretation on steel surface have begun to appear. (The development of the instrument of chemical analysis has given a big help to them. ) And the technology of the phosphate coating has progressed in combination with the technical reports on steel surface. At present, the electrodeposition system are mainly from PPG’s patent. The distribution of primers for automobile bodies in each region is shown in Table 1. At present it is impossible to decide with technical investigations what sort of steels are suitable for automobile bodies.
Though one side and both side galvanized steels have been utilized for automobile bodies, the development of corrosion protection would progress as a total technology while giving answers how to select the precoated steel and how to use it. The authors inferred that the essential technology for electrodeposition coating system could be throwing power and the control of deposition bath paint, and carried out the analysis of throwing power and the deposition mechanism in 1969. In the references published in the 1960’s, electrodeposition coating is described as ` electrophoretic coating’.
The authors reported that the film forming reaction was due to water electrolysis, water removal by electroosmosis, aggregation of colloidal particles and their melting by Joules heat. In this review, the technology of the electrodeposition coating system is discussed on the basis of authors’ work and views referring to many papers and patents. II. Mechanism of Electrodeposition Coating 1. Signj/icance Electrodeposition of Coating Processof AutomobileBodies As structural materials for automobile bodies, steels have been widely utilized as they are supplied cheaply and in large quantities besides their strength.
The main purposes of the coating are to protect the substrate from the corrosive attack and to provide it with a good appearance. Table 1. Distribution of primers in each region. (investigated in 1982, autumn) (%) (996) Transactions ISIT, Vol. 23, 1983 The method of applying primers using the electrodeposition coating process has become quite common in the automobile industry, since it was introduced during the 1960’s because of its inherent processing advantages. The electrodeposition coating method fits well into the coating process in the automobile industry either as an in-line processing method or as an off-line system.
The coating time is short, 2 to 3 min full immersion, for complete bodies or parts , and it coats the exterior, interior, and recessed areas in a single dip application. One technician normally operates the processing system. Bodies or parts to be coated are fully immersed in the electrodeposition bath. The electrodeposition paint formulated proper materials provides a uniform coating over all electrically conductive surfaces in the given coating time. The paint is water-borne. There are also small amounts of solvent present, approximately 5 % or less.
This composition and the process efficiency are very compatible with goals to minimize air pollution. When the electrodeposition coating process is used with a ` closed-loop ‘ system, it results in high utilization of the paint. Properly designed systems have a paint utilization efficiency approaching 100 %. Most systems are designed to achieve this high efficiency. There are numerous other characteristics of the electrodeposition process, such as non-flammability. Various reactions relating to electrodeposition in connection with the object are reviewed in this chapter.
Mechanismof Electrodeposition In anionic electrodeposition developed in the beginning, it was assumed that the deposition was due to the following reaction, where charged water-soluble or water-dispersible macroion R~ was changed to R at the anode. R—;R+e Resin particles stabilized by macroion is called hydrophobic colloid. The deposition mechanism was assumed to be electrophoretic deposition of hydrophobic colloidal particles and its coating process was called ` electrophoretic coating process ‘.
But, it was failed to explain characteristics of electrodeposition such as superior throwing power and high productivity by the above electrode reaction with an electron transfer. Many reports including the authors’ research’ were published and it was clarified that the deposition mechanism was due to the coagulation of hydrophobic colloidal particles by H+ generated by water electrolysis. This result greatly contributed to the development of the cationic electrodeposition coating process. And by the industrial utilization of anionic and cationic electrodeposition, the above mechanism based on electrolysis was widely accepted.
In electrodeposition, charged water-soluble or water-dispersible macroions are attracted by the electrode of the opposite polarity where they undergo electrochemical reactions and are deposited. If the 2. electrodepositing vehicle is represented by RX, where R~ is the macroaaion, then R+ will be deposited at the cathode (cathodic electrodeposition). Conversely, with macroanion R-, deposition will take place at the anode (anodic electrodeposition). The anodic electrodeposition relies on carboxyl groups and the cathodic on amine groups.
Thus electrodeposition resins are designed and prepared like other coating polymers but are required to carry ion-forming groups, e. g. , -NH2 or -COOH . These resins are utilized in electrodeposition as indicated in Table 2. When the voltage is applied, the main reactions at the electrode are electrolysis of water and the deposition of macroion. 1) Electrolysis of water Anode 2H20 –> 4H++02+4e 40H- –p 02+2H20+4e Cathode 4H2O+4e —~ 4OW+2H2 4W+4e -p 2H2 2) Polymer deposition Anionic electrodeposition RCOO-+H+ –~ RCOOH Cationic electrodeposition R3NH++OH- — f R3N+H2O As shown in Fig. , anionic electrodeposition and cationic electrodeposition are similarly symmetry. The mechanism of electrodeposition and others would be substantially considered to be the same phenomena. 3~ Table 2. Electrodepositable tion. 2> resins and mode of deposi- Fig. 1. Comparison tionic one. of anionic electrode position with ca- . Transactions ISIJ, Vol. 23, 1983 (997) 3. Mechanismof Film Formation When the constant potential difference (100 V) was applied, the logarithm of the current was linear to time as shown in Fig. 2.
This relationship is called the law of logarithm in the film formation, and indicates that the current decreases due to insulation of the electrode. When the electrodeposition period was longer than 6 sec, this relationship deviated from the linear shape and the square of the reciprocal of the current density became linear to time as shown in Fig. 3. This relationship is called the law of parabola in the film formation, and demonstrates that the mechanism of electroconduction of the electrodeposited film is ionic, controlling the rate of the film formation.
From these results the mechanism of the electrodeposition film formation is discussed using the model shown in Fig. 4. When the applied potential difference is low, the melting process of the electrodeposited particles cannot occur. In this case, the particles rinsed off from the electrode by washing with tap water after electrodeposition. As the applied potential difference is increased, the melting process occurs faster. Materials formed by this melting process get increased the electric resistance. But it is not allowed to increase beyond a certain value.
The film conduction depends on ionic species in the pin holes which are built up by releasing the bubbles. These bubbles and pin holes are visible although they are very small. The colloidal particles precipitate in the pin holes as shown in Fig. 4, the growing point of the film is no longer adjacent to the electrode but within a diffusion layer a finite distance away from the electrode. When the applied potential difference is too high, the random growing ruptures the uniformity of the film. The film rupture voltage is believed to depend on the rheological properties of the film and the rate of electrolysis. . E ffect of Metal Dissolution When the potential difference is applied in an electrolyte solution, the following metal dissolution may occur at the anode. M° –> Mn++ne The redox potential of the metal affects the possibility of the above reaction, and as the potential becomes higher, the reaction possibility becomes larger. But this does not imply the continuous reaction progress. When anionic species such as C1 are presented too much in the electrolyte, the dissolution reaction occurs easily and Cr04-, S04- and C03- disturb the above reaction.
In anionic electrodeposition, the object is anodized and the following polymer deposition occurs; that is nRC00-+Mn+ –p (RC00),z-M Fig. 2. The time relation during between the early the logarithm at of current a constant and period applied potential of 100 V. Fig . 3. The relation between the square that of reciprocal between the curpa- -),andand rent (time rameter p and time (- Fig . 4. A model of the electrodeposited film. In other words, at the anode the dissolution reaction is decreased by the polymer deposition.
These reactions would not occur evenly over the automobile bodies, because of their complex-shaped structure. Since the potential difference is not the same in all the locations of automobile bodies, the electrodeposition coating process provides an uneven coating. This would deteriorate the performance of coating such as corrosion resistance and appearance. The dissolved amounts of the metal during reactions differ largely in electrodeposition bath compositions. And they are affected by the surface treatment of the steel.
The adaptability of the newly developed metal materials should be strictly examined. In cationic electrodeposition the object is the cathode, and the above mentioned anodic dissolution reaction does not occur at the object. This point was aimed at and the cationic electrodeposition coating process has been developed which gives an excellent corrosion resistance. Anderson4; analyzed the metal in cationic electrodeposition films and characterized the peculiar relations of cationic electrodeposition against anionic one (Fig. 5).
He presumed that the following reaction would occur at the cathode. Review (998) Transactions ISIJ, Vol. 23, 1983 M°+40H- [MO2]n-+2H2O+(4-n)e Since then, his data have been cited by many researchers with a little discussion. Being based on the results electrodeposition film weight increases with an increase of the deposition time. The authors can not agree with the following approach. The metal detected in electrodeposition film is assumed to be due to the electrode reaction by a flow of current. Metallic substrate dissolves chemically without current flow.
For example, corrosion with hydrogen evolution is illustrated as follows. 2Fe+3H20 Zn+H2O –p + Fe203+3H2 ZnO+H2 ~’ T Fig. 5. Changes cold-rolled in iron steel content with of electrodeposited deposition time. films on 1′ 2A1+3H2O –pA1203+3H2 These reactions do not participate in electron transfer and they are not related to current flow. These corrosion reactions differ in electrodeposition bath compositions, electrodeposition process conditions, metal materials and their surface conditions. Therefore, the analysis of corrosion reactions are quite difficult.
However, the practical difficulties arising from the above mentioned factors have been clarified. In cationic electrodeposition, the working electrode is the anode and anodic dissolution causes a contamination of electrodeposition bath paint. Care must be taken in cationic electrodeposition being different from anionic electrodeposition. In order to prevent the contamination by dissolved ion, the working electrode is settled inside diaphragm which ion-exchange membrane is preferably used. III. Throwing Power _l. Signficanceof Throwing Power in ProtectiveCoating In coating applications or complex-shaped automobile bodies, throwing power, the ability to extend paint films into highly recessed areas, as well as the corrosion resistance of coating are greatly demanded. These two specifications have been parallely taken as the index for the improvement and development of the electrodeposition coating process. Recently, the materials such as the cold-rolled steel, galvanized steel and other steels have been widely utilized for automobile bodies, and the estimation for two specifications (corrosion resistance of coating and throwing power) become complexed and varied.
In order to improve the protective performance of automobile bodies, they should be examined particularly in the throwing power. In this chapter, we review in throwing power. 2. Throwing Power Test In case of the design for corrosion protection of automobile bodies, there are two following purposes to estimate throwing power. 1) To design the structure of automobile bodies and materials’ adaptability 2) To design electrodeposition paint and deposition coating process Review Fig. 6. Types of throwing cells (dia grammatic).
For the first purpose it is recommended to make a model on the characteristics of the bodies structure. The modeling of the bodies structure generally arises from the know-how on the design for corrosion protection of the automobile industry. There have been proposed the following devices for measuring throwing power of the electrodeposition paint since it was developed. The most simple device is a comparative test. Many possible methods were described briefly by Tawn and Berry,5) and are illustrated diagrammatically in Fig. 6. This diagram enables one to appreciate the meaning of throwing power. 1) Bar test (Fig. 6(a)) In this type of test, either or both of two functions can be measured : the length of the anode coated under given conditions, and the gradient of film thickness down the anode. (2) Multiple plate test (Fig. 6(b)) In this form of test, the efficiency, that is the weight or film thickness of coating on the inside plates, can be compared with that of coating on the outside plates. (3) Wedge test (Fig. 6(c)) In this test, it is only possible to estimate the percentage of the total area not coated with the paint.
Mathematical expression of the result is difficult, due to difficulties of measuring areas accurately and defining the boundary between coated and uncoated areas. (4) Tube test (Fig. 6(d)) Several methods of this type exist, one of which is illustrated in Fig. 6(d), but in essence this is similar to the bar test, except for the restriction applied to the electrical bath by the non-conductive tube itself. (5) Cell test (Fig. 6(e)) Particular conditions may be applied to the anode in the cathode compartment, and the deposition on the anode in the separate compartment may be determined separately.
Another purpose to estimate throwing power is to detect quantitatively the inherent ability of the used paint. In these several measuring methods, the tube test (Fig. 6) has been recognized to be mostly accepted method. The size of this test tube is defined fully by automobile industries. For examples, Ford Cell and GM Cell are popularly accepted in U. S. A. 3. Analysis of Throwing Power The electrodeposition paint industries have analyzed the mechanism of throwing power as corresponding to the different size of each automobile industry.
Oyabu and Furuno6-8~ showed that throwing power could be related to the geometry of the test cell and coating properties by the following equation: Th2 = -. E . ,c …………………… (1) a LJ the cross-sectional area of the cell the perimeter of the cell the bath conductivity the applied voltage the current density at the coated surface of the tube at the end of the deposition experiment The equipotential surfaces at the start of the electrodeposition process are shown at the left, and those in progress at the right of Fig. . The dense distribution of equipotential surfaces along to the anode corresponds to the electrodeposited film. In this area the current density is approximately uniform. To verify the relationships between a and Th, and between L and Th in the above equations, three kinds of experiments were carried out. A steel panel was inserted into a glass tube. If the thickness of the panel is expressed by d and the width by l(d