Ind. Eng. Chem. Res. 2001, 40, 15-20
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Surface Treatments and Coatings for Metals. A General Overview. 2. Coatings: Application Processes, Environmental Conditions during Painting and Drying, and New Tendencies Elisabete Almeida† INETI Surface Treatments and Coatings Laboratory, INETI, Estrada do Pac¸ o do Lumiar, 1649-038, Lisboa, Portugal
Given the diversity and complexity of technologies involved in the surface treatments and coatings sector, it is very difficult to find references in the existing literature that interlink most of them. The work that is presented here is aimed at filling that gap. In view of its length, it has been necessary to divide it into two parts. The first part deals with surface treatments and refers to the preparation of surfaces and the nature of coatings as two of the factors that affect anticorrosive protection by coatings. In this second part the latter are completed by consideration of coating application processes and environmental conditions during application and drying. Finally, a brief review is made of new tendencies in the sector. 1. Introduction Because the quality of surface treatments and coatings for metals is affected by several factors, the author has included their main types in four groups which are each dealt with separately. In this part of the work, to complement the aspects of surface preparation and the nature of coatings already considered in the first part, reference is made to important factors such as application processes and environmental conditions during coating application and drying. Finally, some notes are presented on new tendencies in the “surface treatments and coatings” sector. 2. Other Factors That Affect Anticorrosive Protection by Coatings 2.1. Application Processes. As was noted in paragraph 3 of part 1 of this work,1 application processes also affect the anticorrosive behavior of coatings in general and must be adapted according to the purpose of the coating and its nature. Finally, application processes should be carried out in accordance with the corresponding rules of good practice. 2.1.1. Metallic Coatings. Traditional metallic coatings are normally applied by immersion in a molten metal bath,2 spraying of the molten metal,3 immersion in a chemical bath,4 or cladding,5 as shown in Figure 1. An interesting example is that of zinc coatings on steel, where different characteristics are obtained depending on the application process used, even when the final thickness of the zinc is the same. Thus, if application is by means of a molten zinc bath, the process is known as hot dip galvanizing,2 and the coating obtained is comprised of iron-rich intermetallic alloys next to the substrate but which become increasingly zinc-rich toward the surface (Figure 2). This coating presents good corrosion resistance (depending on its thickness and on the environment) and mechanical strength, but the chemical nature and smoothness of its surface mean † Phone: 351 21 7165141. Fax: 351 21 7160901. E-mail:
[email protected].
Figure 1. Metallic coating application processes.
Figure 2. Diagram of intermetallic alloys in the coating of steel by hot-dip galvanizing.
that it does not constitute a good base for painting, because it needs to be pretreated (see section 3.1.2 of part 11). Coatings obtained by spraying of the molten metal,3 known as metallizing, are obtained from metal in the
10.1021/ie000210k CCC: $20.00 © 2001 American Chemical Society Published on Web 12/02/2000
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Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001
Figure 3. Deposition of molten metal (metallizing).
Figure 5. i ) f(E) curves for the two electrode processes involved in metal plating by galvanic displacement.
Figure 4. Illustrative scheme of nickel electroplating, based on a nickel chloride solution.
form of wire or powder, which is melted in special guns, with the assistance of a power source, allowing the coating to be obtained by the successive anchoring of molten metal particles (Figure 3). In this case there is no chemical interaction between the composition of the substrate and the coating and the coating surface is normally quite rough and therefore must be sealed before painting if it is wished to obtain a coating that is resistant to aggressive media (mixed coating of metallizing + painting; see section 3.2.4 of part 11). The deposition of metals by immersion in a chemical bath can be achieved with or without an electric current (electroless plating5). The characteristics of the final coating will depend on the plating technology used. Electroplating6 consists of the deposition of a metallic coating on an electrically conducting surface which acts as the cathode in an electrolytic cell, whose solution contains ions of the metal to be deposited. This is a relatively complex technology that involves a large number of steps. Figure 4 shows an example of the case of nickel electroplating, based on a nickel chloride solution. Electroless plating consists of a set of different technologies (galvanic displacement, contact, etc.) in which
the electric current is generated internally in the plating cell. Figure 5 shows i ) f(E) curves for the two electrode processes involved in metal plating by galvanic displacement. The idep current is given as equal to the current densities of the anodic and cathodic processes and can be used to calculate the mass M1 deposited by means of Faraday’s law
∆p )
PM1 nF
idept
(1)
in which ∆p is the deposited mass of M1, PM1 is the molecular mass, and t is the immersion time.5 In the case of contact plating, the current is generated by contact of the substrate to be coated with a more active metal that acts as an anode in the galvanic cell. This does not present special advantages over the aforementioned process and is only used in restricted applications such as the internal electrolysis electrogravimetry technique.5 Some of the principles of electroless plating are very old (early 19th century) while others are more recent (1950s and 1960s). This technique is not, however, normally applied in the anticorrosive protection of metals and so will not be especially described here. Other metallic coating processes include cladding,5 which consists of the use of special processes (such as hot rolling or welding) to adhere films of one metal on another that acts as the substrate and which is normally
Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001 17
Figure 6. New coating technologies.
Figure 7. Examples of physical vapor-phase deposition processes.1
cheaper or provides greater mechanical strength but poorer corrosion resistance. 2.1.2. Nonmetallic Inorganic Coatings. Application processes for vitreous enamels include immersion and spraying, both of which can be manual or automated.5 The production line coating of domestic cookers most frequently involves the use of immersion, while baths, shower bases, and sinks are normally coated by spraying. In these technologies the oven used to vitrify the coating is of fundamental importance. It has to fulfill several requirements, and in recent years, heating systems and materials have been altered significantly, with the practical abandonment of the old ovens, which are being replaced by gas burners and by radiant and fiber tubes. For their part, most ceramic materials such as carbides, nitrides, etc., are frequently applied by plasma spraying (barrier coatings7) or by vapor-phase deposition.8 In this last case, if the deposition process is purely
physical, it is referred to as physical vapor deposition (PVD). If the deposition technology involves a chemical reaction with the surface itself, the process is known as chemical vapor deposition (CVD). Figure 7 presents a systematization of the main types of technologies involved in PVD and CVD.9 Vacuum evaporation and cathodic sputtering are both PVD technologies. They are essentially differentiated by the fact that in the former the synthesis of the species to be deposited is achieved by heating (by the Joule effect, by induction, by ion bombardment, or by laser beam; see the example in Figure 7a), while in cathodic sputtering this is achieved by the ejection of atoms from the cathode, by transfer of the amount of movement between ions from the gaseous phase and atoms from the cathode (see the example in Figure 7b). There are different variants of both of these types of PVD techniques9 as well as hybrid techniques, which are often referred to as ion deposition techniques.
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Figure 8. Molten metal spraying techniques.
In the case of CVD, the material to be deposited is obtained by means of a chemical reaction between the cathode material and the reactive medium. The main reactions that can take place in CVD include the following:
(a) Dismutation or Disproportionation T
2AB 79 8 A + AB2 T
(2)
2GeI2(g) S Ge(s) + GeI4(g)
(3)
Example:
(b) Decomposition T
AB(g) 98 A(s) + B(g)
(4)
Example: T
Ni(CO)4(g) 98 Ni(s) + 4CO(g)
(5)
(c) Modification of halogens Example: T′′
TiN:TiCl4(g) + 1/2N2(g) + 2H2O 98 TiN(s) + 4HCl(g) (6) (d) Modification of carbides T
TiC:TiCl4(g) + 4CH4(g) 98 TiC(s) + 4HCl(g) (7) The deposition of ceramic materials is also often achieved by metallizing, by plasma spraying, or by laser, depending on many factors of a technical-economic nature. The technologies of PVD and CVD, LASER, and others are currently referred to as “New Coating Technologies” and can be grouped, as has been noted, according to Figures 69 and 8. Their use is not restricted to the application of nonmetallic inorganic coatings, and in fact one of their common applications is that of special metallic coatings. 2.1.3. Organic Coatings. Organic coatings in liquid form (solutions, dispersions, or emulsions) can be applied by manual spreading (brush, roller, mortar-board, etc.), by spraying (gun or rotary disk), or by immersion (with or without the intervention of an electric current).10,11 Anticorrosive primers must preferentially be applied by manual spreading or cathodic electrodeposition (cataforesis), the latter process normally being
used in industrial production line applications, such as in the case of the automobile industry.12 More recently, they are also applied by autophoresis.13 Finishing coats should be applied by spraying, which may be conventional or airless, with or without the intervention of an electric current (electrostatic or nonelectrostatic).14 In the recent decades, the transfer efficiency (the amount of paint that hits the target to be painted) necessary to reduce to a minimum environmental contamination during liquid paint spraying processes has increased significantly with the development of HVLP (highvolume, low-pressure) special equipment.15,16 Organic coatings in the form of powder (powder paints, plastics, etc.) must be applied in the solid state and, subsequently, by the action of thermal energy (heating) converted to the liquid state (melted) in order to thus be able to wet and adhere to the substrate upon which they are applied. They can be applied in a fluidized bed on preheated substrates or by electrostatic spraying, which is based on the fact that the electrically charged powder particles are deposed on the substrate, which is connected to the earth or oppositely charged. After application the coating will subsequently be heated for the time and duration most suited to its chemical nature. Incorrect heating parameters (curing time and temperature) lead to deficiently polymerized coatings, which consequently provide poorer corrosion resistance, even when the initial selection of the coating material has been correct. When the organic coating is applied in the form of adhesive tapes and sheet linings, special customdesigned equipment will need to be used, such as simple or crossed taping machines. In some cases, the process is assisted by thermal energy in order to achieve the adhesion of the coating to the substrate. In the case of adhesive tapes, which are used in superposed layers of different natures, adhesion is achieved by means of the tapes’ own adhesives. 2.2. Environmental Conditions during Application and Drying. As has been noted throughout this chapter, an anticorrosive coating can only protect the substrate from corrosion if it is sufficiently well adhered. Consequently, particular attention must be paid to environmental conditions during application and, where applicable, during drying and curing.17,18 Only with this special care is it possible to avoid situations in which the substrate surface can become contaminated by impurities, adsorbed layers, moist, etc., which act as weak boundary layers and which would severely prejudice the adhesion of the final coating.
Ind. Eng. Chem. Res., Vol. 40, No. 1, 2001 19 Table 1. VOC Values for Different Paint Technologies27 water-based characteristics
PPa
HSPb
solvent-based
soluble
dilutable
radiation-cured
dry extract/% mol VOC/% mol reduction in VOC emissions/ % compared with solvent-based paints
100 0 to very low 100
75-80 20-25 80-85
35-60 45-65 0
40-50 10-20 70-85
55 2-5 94-97
0 to very low 100
a
Powder paints. b High solid paints.
3. New Tendencies in the Sector “Green” tendencies have undoubtedly influenced the market of thousands of products throughout the world. Though the forecast growth for the anticorrosive products market is similar to the expected gross domestic product growth in the developed countries, it is nevertheless one of the industrial sectors most affected by the referred tendencies because of the nature of most of the products it uses.19 For instance, it is estimated that more than a third of the 3 million tons/year of volatile organic compound (VOC) emissions recorded in Europe are caused by the application of coatings. The products used in this sector also include many toxic, corrosive, carcinogenic, etc., components that will have to be replaced in the short term, as a consequence of emerging legislation. For this reason new products in the field of surface preparation, for degreasing, chemical pickling, phosphating, passivation, and so on, will have to be more environmentally friendly and their effluents quick, easy, and economic to treat. Current tendencies in degreasing and chemical pickling point toward the use of waterbased products containing emulsifiers and acid- or alkaline-active components that replace the traditional mixtures of aliphatic and aromatic solvents, ketones, etc., previously employed. Passivation baths will have to progressively abandon the use of their traditional main component, chromium(VI), and look to alternatives currently subject to intense R&D activity, such as oxidizing baths and chromium-free complexing agents.21 For their part, phosphating baths will have to adapt to new metallic substrates. They will also have to be free of toxic materials and heavy metals and their effluents technically and economically easy to treat.21 Blasting operations with sand will also become a thing of the past, and the tendency will be for descaling areas to be covered and abrasives recycled. With regard to the use of metallic coatings, not only the implementation of the use of traditional galvanized and electrogalvanized steels but also a significant increase in the electroplating of zinc-iron and zincnickel alloys are expected.22 More recently, the use of zinc-cobalt,23 zinc-chromium,24 and zinc-aluminum alloys such as Galfan (zinc alloy with close to 5% aluminum), Galvalume, and others25 is also verified. Profound changes are also taking place in the field of metal spraying application technologies, where there has been a quick transition from flame spraying to the electric arc, and the processes within each of these technologies are becoming increasingly sophisticated, as seen in Figure 7. In the field of organic coatings, solvent-based products are tending to give way to water-based products, high solids products, liquid solvent-free products, and powder products.26 Table 1 summarizes VOC values for some different technologies.27 Toxic pigments are also being
Table 2. Situation of the European Market33 between 1985 and 1995 coating function
millions ($) 1985 1995
optical tribological corrosion thermal barrier decorative
195 196 15 17 2
590 292 240 115 3
annual growth (% year-1) 11 6 28 20 6
Table 3. World Equipment Market33 and Its Growth coating type PVD CVD plasma CVD ionic implantation plasma diffusion plasma spraying
millions of ECUs in 1985
annual growth in 1985-1995 (%/year)
1500 400 22 350
15 25 40 28 18 14
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replaced by more harmless products, and intense R&D work is currently being undertaken in this area.28 With regard to application processes, it is envisaged that airless equipment will remain in use in the traditional sectors, while conventional spraying equipment will be replaced by HVLP equipment.15,16 Meanwhile, airassisted airless equipment, originally developed as hybrid systems, present mixed characteristics halfway between airless spraying and conventional spraying. Another option is electrostatic spraying, whose use will grow significantly in the market sectors of financially healthy companies that are prepared to invest in order to reap the benefits of using this system.14 The coming decade will also see developments in exhaust and effluent treatment systems, to ensure their compliance with new regulations/legislation. The tendency will be toward the installation of gas effluent treatments that fundamentally make use of techniques for the incineration of volatile components, their absorption, and their biological elimination (biofiltering, for instance). Another technology that will undergo a significant increase is the radiation curing of coatings.29 Finally, it should be noted that special surface treatments and coatings, such as those involving the use of lasers,30 plasma, and vapor-phase deposition, either physical (PVD)31 or chemical (CVD),32 in the most diverse versions (see Figure 6), are all ready to invade the market. They offer solutions for all of the different applications of materials, either traditional or new products, particularly in the areas of optics, aesthetics, and behavior at high temperatures but also in relation with anticorrosive protection. Tables 2 and 3 give an indication of their recent evolution in the European market.33 However, it is important to mention that, despite the advantages that they offer, these technologies are normally expensive processes and are not destined for
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the protection of major industrial structures, where the solutions to be adopted will continue to be the new developments of traditional technologies.34,35 4. Conclusions The performance of treated and/or coated metallic surfaces depends on a number of different factors, among which the most important are surface preparation, nature of treatments and coatings, application processes, and environmental conditions during coating application and drying. Thus, in anticorrosive protection, it is fundamental to pay great attention to all of them. The failure of any one of these aspects is sufficient to jeopardize the total quality of the intended anticorrosive protection. Meanwhile, emerging legislation, which is a consequence of the overriding need to protect the Earth’s environment and human health, has obliged the “Surface Treatments and Coatings” sector to make profound changes in the products and processes it employs, though without ceasing to take into account the different factors that affect the anticorrosive protection of metallic surfaces. Literature Cited (1) Almeida, E. Surface Treatments and Coatings for Metals. Ind. Eng. Chem. Res. 2000, submitted for publication. (2) Ambrouse, J. J. Surface 1997, 36 (268), 30. (3) Thermal Spray Technologies. New Ideas and Processes; Houck, D. L., Ed.; ASM International: Materials Park, OH, 1998. (4) Bakket, J.; Meerakker, J. E. J. Appl. Electrochim. 1990, 20, 185. (5) Gentil, V. Corrosa˜ o, 3th ed.; ABDR: Rio de Janeiro, Brazil, 1996; p 235. (6) Dini, J. W. Electrodeposition; Noyes Pub.: Westwood, NJ, 1993. (7) Almeida, E.; Montez, C. Characterization of Thermal Barrier Coatings for Hot Turbo Engine Components. In Protective Coatings and Thin Films, Synthesis, Characterization and Aplications; Pauleau, I., Barna, P. B., Eds.; NATO ASI Series 3; Plenum: New York, 1996; Vol. 21, p 411. (8) Moll, E. Physical Vapor Deposition Techniques II. In Advanced Techniques for Surface Engineering; Euro-Course; Gissler, W., Jehn, H. A., Eds.; Kluwer Acad. Pub.: Dordrecht, The Netherlands, 1992; Vol. 1, pp 1-4. (9) Almeida, E.; Nobre, O. Corros. Prot. Mater. 1991, 10 (1), 33. (10) Cabral, A. M.; Almeida, E.; Marques, C. C. Corros. Prot. Mater. 1989, 8 (1), 12.
(11) Pfeiffer, B.; Schulze, J. W. J. Appl. Electrochem. 1991, 21, 877. (12) Oravitz, J. J. Met. Finish. Guidebook 1999, 240. (13) Thomas, A. Finishing 1996, 20 (10), 28. (14) Gebhard, M. S. J. Coat. Technol. 1994, 66 (830), 27. (15) Mannouch, S. Eur. Surf. Treat. 1993, 13. (16) Deroche, A. G. The Principles of Autobody Repairing and Repainting, 6th ed.; Prentice-Hall: Upper Saddle River, NJ, 1996; p 345. (17) Cronomos, J. J. Prot. Coat. Linings 1999, Mar, 98. (18) Almeida, E.; Piens, M.; Scimar, R. Double Liaison 1980, 298, 12. (19) Almeida, E. New Tendencies of Surface Treatments and Coatings, Plenary Conference, 5th IACP Proceedings, Tenerife, Canarias, Spain, 1995. (20) Almeida, E.; Diamantino, T. C.; Figueiredo, M. O.; Sa´, C. Surf. Coat. Technol. 1998, 106, 8. (21) Goefhmeker, H. Surfaces 1997, May, 74. (22) Elkhatabi, F.; Benballa, M.; Sarret, M.; Muller, C. Electrochem. Acta 1999, 44, 1645. (23) Carpenter, E. O. S.; Farr, J. P. G. Trans. Inst. Met. Finish. 1998, 76 (4), 135. (24) Li, X. Y.; Akiyama, E.; Habazaki, H.; Kawashima, A.; Asami, K.; Hashimoto, K. Corros. Sci. 1997, 39 (8), 1365. (25) Palma, E.; Puente, J. M.; Morcillo, M. Corros. Sci. 1998, 40 (1), 61. (26) Bodnar, E. Eur. Coat. 1991, 1, 206. (27) Almeida, E. New Anticorrosive Painting Technologies at the Beginning of the Twenty-first Century. J. Coat. Technol. 2000, accepted for publication. (28) Almeida, E.; Santos, D.; Uruchurtu, J. Prog. Org. Coat. 1999, 37, 131. (29) Shaobeng, W.; Mattew, T. S.; Soucek, M. D. Prog. Org. Coat. 1999, 36, 89. (30) Frenk, A.; Kurz, W. Laser Surface Treatments. In Advanced Techniques for Surface Engineering; Gissler, W., Jehm, H. A., Eds.; K.A. Publishers: London, 1992. (31) Pauleau, Y. Physical Vapor Deposition Techniques. In Advanced Techniques for Surface Engineering; Gissler, W., Jehm, H. A., Eds.; K.A. Publishers: London, 1992. (32) D’Agostino, R.; Favia, P.; Fracassi, F.; Lamendola, R. Plasma Enhanced Chemical Vapor Deposition. In Advanced Techniques for Surface Enginneering; Gissler, W., Jehm, H. A., Eds.; K.A. Publishers: London, 1992. (33) Hondros, E. D. The Emergence of Interfacial Engineering. In Advanced Techniques for Surface Engineering; Euro-Course; Gissler, W., Jehn, H. A., Eds.; Kluwer Acad. Pub.: Dordrecht, The Netherlands, 1992; Vol. 1, pp 1-4. (34) Zeh, H.; Baumgartl, H. Surf. Coat. Int. 1995, 9, 132. (35) Nakayama, H. J. Coat. Technol. 1998, 70 (887), 63.
Received for review February 18, 2000 Revised manuscript received August 28, 2000 Accepted September 4, 2000 IE000210K
