Handbook Corrosion (1992) WW Part 5 doc

Table 10 Some standard galvanized fasteners Description ASTM specification Grade or type Carbon steel bolts A 307 Grade A, B High-strength bolts A 325 Type 1 Transmission tower bolts A 394 Type 0, 1 Quenched-and-tempered alloy steel bolts A 354 Grade BC Galvanized coatings in bearing-type connections, which develop shear resistance by allowing the bolts to bear on the plates, are not detrimental to performance and have along history of use for example, in electrical utility transmission line towers. In friction-type connections, all loads in the plane of the joint are transferred by the friction developed between the connected surfaces. The load that can be transmitted is determined from the clamping force applied to the bolts and the coefficients of friction of the faying surfaces. Clean, galvanized mating surfaces have a coefficient of friction that is slightly lower than that of as-rolled steel. The coefficient of friction of galvanized surfaces can be made equal to that of uncoated steel surfaces by wire brushing or light grit blasting (Ref 40, 41). Neither treatment should be severe enough to produce breaks or discontinuities in the galvanized coating. The fatigue behavior of galvanized steel connections equals that of uncoated steel connections, regardless of whether the galvanized surfaces were wire brushed or gritblasted after galvanizing (Ref 40, 41, 42). Welding. Zinc-coated steel can be satisfactorily welded by all common welding methods, but attention must be given to the possible generation of zinc fumes. Adequate ventilation, operator respiration units, or a fume-extracting welding unit should be used to avoid potential harmful effects. Extensive tensile, bend, radiographic, and fatigue tests show that the properties of sound metal inert gas or metallic are welds on galvanized steel are equivalent to those on uncoated steel (Ref 35). Penetration of molten zinc into the weld metal is the primary factor in cracking of galvanized steel weldments. The crack begins at the root of the weld and may or may not extend to the surface. Recommendations to avoid fillet weld cracking include (Ref 35): • Treat the base metal to reduce the amount of available zinc by, for example, beveling the standing plate in a tee joint at an angle of 15 to 45° • Remove zinc from both faying surfaces by burning with an oxygen fuel gas torch or by shotblasting • Provide a parallel gap of 1.6 mm ( 1 16 in.) between the weld elements • Choose consumables that will give a low silicon content weld, for example, manganese silicate flux and 2% low manganese/low silicon electrodes for submerged arc welding. For CO 2 welding, low- silicon filler wire gives freedom from zinc penetration cr acking, but causes a small amount of porosity. For shielded arc welds, use a low-silicon electrode and a rutile covering Weld porosity can occur because of the volatilization of zinc during welding. Porosity can be reduced by making adequate provision for the escape of gases evolved during welding. Normal porosity does not reduce the static tensile strength below that specified for satisfactory low-carbon steel weld metal, but it will affect the fatigue strength of a fillet weld. Weld Damage Repair. Galvanized materials damaged by field welding can be touched in with an organic or inorganic zinc-rich paint. Organic paint does not require a high degree of surface preparation and dries quickly. The paint must have a zinc dust content of 95% and should be applied in several coats to a thickness of three times the galvanized thickness to provide equivalent protection. Low-melting zinc-cadmium or zinc-tin-lead alloy rods can also be used for repair. The rod is heated to about 330 °C (625 °F) with an oxyacetylene torch, and the melted alloy is rubbed and spread over the damaged area. This gives equal protection, but is more expensive than paint touch up. The damaged area can also be zinc metallized by thermal spraying, but the equipment requirements may prevent convenient field use. Additional information on repair techniques is given in Ref 43. Painting Galvanized Steel Galvanized coatings, when used without further treatment, offer the most economical corrosion protection for steel in many environments. The galvanized coating makes an excellent base on which to develop a paint system. Painting of galvanized steel is desirable for aesthetics, as camouflage, as warning or identification markings, to prevent bimetallic corrosion, or when the anticipated environment is particularly severe (see the article "Organic Coatings and Linings" in this Volume for supplementary information on painting for corrosion protection). In corrosive atmospheres, a duplex system of galvanized steel top coated with paint has several advantages that make it an excellent system for corrosion prevention: • The life of the galvanized coating is extended by the paint coating • The sacrificial and barrier properties of the zinc coating are used if a break occurs in the paint film • Undercutting of damaged paint coatings, a major cause of failure of paints on steel, does not occur with a zinc substrate (Fig. 13) • Surface preparation of a weathered zinc surface for maintenance painting is easier than that for rusted steel. Fig. 13 Illustration of the mechanism of corrosion for painted steel (a) and painted galvanized steel (b). (a) A void in the paint results in rusting of the steel, which undercuts the paint coating and results in further coating degradation . (b) A void in the coating of a painted galvanized steel is sealed with zinc corrosion products; this avoids the undercutting seen in (a) and prevents further deterioration of the painted coating. Synergistic Effects of Galvanized and Painted Systems. The galvanized coating prevents rusting of steel by acting as a barrier against the environment and by sacrificially corroding to provide cathodic protection. Painting the galvanized coating extends the service life of the underlying zinc because the barrier property of the paint delays the reaction of zinc with the environment. If a crack or other void occurs in the paint and exposes the galvanized coating, the zinc corrosion products formed tend to fill and seal the void; this delays further reaction. When painted steel is exposed to the environment, rust forms at the steel/paint interface. Because rust occupies a volume several times that of the steel, the expansion resulting from rusting leads to rupture of the steel/paint bond. Further, rust is porous; it accumulates moisture and other reactants, and this increases the rate of attack on the steel. The result is undercutting, flaking, and blistering of the paint film, leading to failure of the paint coating (Fig. 13). Zinc corrosion products occupy a volume only slightly greater (20 to 25%) than zinc; this reduces the expansive forces and conditions that lead to paint failure. A coating system consisting of painted galvanized steel provides a protective service life up to 1.5× that predicted by adding the expected lifetimes of the paint and the galvanized coating in a severe atmosphere (Ref 44). This is demonstrated in Table 11. The synergistic improvement is even greater for mild environments (Ref 45, 46). Table 11 Synergistic protective effect of galvanized steel/paint systems Galvanized steel Paint Galvanized plus paint Thickness Thickness Thickness Type of atmosphere μm mils Service life (a) , years μm mils Service life (a) , years μm mils Service life (a) , years 50 2 10 100 4 3 150 6 19 75 3 14 150 6 5 225 9 29 100 4 19 100 4 3 200 8 33 Heavy industrial 100 4 19 150 6 5 250 10 36 50 2 19 100 4 4 150 6 34 75 3 29 150 6 6 225 9 52 100 4 39 100 4 4 200 8 64 Metropolitan (urban) 100 4 39 150 6 6 250 10 67 50 2 20 100 4 4 150 6 36 100 4 40 100 4 4 200 8 66 Marine 100 4 40 150 6 6 250 10 69 Source: Ref 45 (a) Service life is defined as time to about 5% red rust. Paint adhesion is the primary concern in painting galvanized steel. The surface of the zinc is nonporous and does not allow mechanical adhesion of the paint. Surface contaminants, such as oils, waxes, or postgalvanizing treatments, also effect adhesion. A fresh zinc surface is reactive to certain paint ingredients, such as fatty acids; this can produce zinc soaps and disrupt the zinc-paint bond. Galvanized coatings can be successfully painted immediately after galvanizing or after extended weathering. The deliberate use of weathering is not recommended, because weathering may not be uniform, the time required is long (6 to 12 months), hygroscopic impurities can form that may be difficult to remove, and there is exposure to atmospheric pollutants. Chemical etchants, such as acids or copper sulfate, should not be used. The action of these chemicals is difficult to control, surface preparation may be nonuniform, and the galvanized coating could be damaged if allowed to remain in extended contact with the chemicals. Long-term adhesion will suffer with this type of treatment, although initial adhesion may be obtained. Mechanical roughening of the zinc surface through the use of a light blast can provide a good surface for painting. However, careful control of the blast pressure and flow rate must be exercised to avoid excessive removal of the galvanized coating. Initial adhesion of the paint can be achieved through the use of a pretreatment primer to provide an adequate base for further coating. Long-term adhesion is obtained by the selection of top coat that is compatible with the primer and galvanized steel. Pretreatment Methods. As with all painting operations, the surface to be painted must be free of contaminants that could affect the adhesion of the paint or the appearance of the painted article. Non-oily substances can be removed by brushing or scrubbing, then rinsing with water. Oily materials should be removed with a solvent, such as naphtha, turpentine, or mineral spirits, followed by a final wiping with clean cloths and clean solvent, to avoid spreading oil films on the surface. Two-component wash primers meeting Steel Structures Painting Council (SSPC) specification SSPC-Paint 27 are ideal pretreatments. The primer is applied by spraying in thin coasts according to the recommendations of the manufacturer. The freshly prepared primer should be applied to a dry film thickness of 7.6 to 13 μm (0.3 to 0.5 mils). Finish Coating. The primed material should be finish coated as soon as possible after priming treatment. Wash primers are moisture sensitive, and the vehicle may gel under high-humidity conditions and lose adhesion. The primer usually dries in 15 to 30 min, and it is dry enough to recoat after 30 to 60 min. Almost all paints will adhere to the wash primer. Direct Application Systems. A more convenient alternative to natural weathering or pretreatment of the galvanized surface is the use of a paint system directly compatible with the surface. Alternatives are discussed below. Zinc Dust-Zinc Oxide Paints. Federal specification TT-P-641G describes three types of zinc dust-zinc oxide paints. All three contain the same pigmentation of 4 parts zinc dust to 1 part zinc oxide; the only variation is in the paint base material. Types 1 and 2 (linseed oil and alkyd resin bases, respectively) are recommended for general use, and type 3 (phenolic resin base) is especially formulated for severe moisture exposure or underwater service. All are useful as primers for adherence and are satisfactory as finish coats. If color is required, the top coat can be pigmented. Other compatible top coats may be used, but those with very strong solvent systems should be avoided, particularly if used before proper aging of the base film. Portland Cement in Oil Paints. These paints are compatible with either fresh or weathered galvanized coatings. Although they tend to be brittle, adherence is excellent. They are not as versatile as the zinc dust-zinc oxide paints and are usually limited to applications in which a high gloss is not required and an oil base is suitable. They are available in a wide choice of colors. Other Direct Application Systems. Newer paint systems that have been successfully used on direct application to galvanized steel include epoxy resin based paints, chlorinated rubber based paints, vinyl copolymer based paints, coal tar epoxy paints, and acrylic latex emulsion paints. Economics of Hot Dip Galvanizing Corrosion control results in a negative cash flow for the owner of any facility. In selecting a system for corrosion control, the economic consequences of the selection should be determined. Initial cost should not be the determining factor in selecting a system. Instead, the desired service life of the project should be reasonably estimated, and the life cycle cost of several systems should be evaluated to determine the most economical system for the particular project. A number of models have been developed for the economic evaluation of corrosion protection systems (Ref 47, 48, 49). The projected structure life, inflation and discount (cost of capital) rates, number of years in the maintenance cycle, estimated costs for future maintenance, and original system costs must be determined based on discounted cash flow techniques. In the private sector, tax and investment incentives must also be considered. Table 12 illustrates a simple analysis that does not take tax or investment incentives into account. Although the initial galvanizing cost is set at 25% above the paint cost, this is often not the case; in reality, the cost of galvanized steel is frequently lower than that of painted steel. More information on the use of engineering economy is available in the article "Corrosion Economic Calculations" in this Volume. Table 12 Discounted cash flow analysis of galvanized versus painted steel The time value of money is an important consideration when analyzing the economics of different coatings. This example assumes an inflation rate of 4%, a discount rate of 10%, a rep aint cycle of 10 years, and an expected service life of 50 years. No tax or investment considerations are made. A repaint cost of 75% of the original cost is assumed, and the galvanizing cost is 25% greater than the paint cost. Galvanizing Paint Year Original cost NPW (a) Original cost NPW (a) 0 $1.25 $1.25 $1.00 $1.00 10 . . . . . . . . . $0.43 20 . . . . . . . . . $0.24 30 . . . . . . . . . $0.14 40 . . . . . . . . . $0.08 Total lifetime costs . . . $1.25 . . . $1.89 (a) NPW, net present worth of inflated future maintenance costs Selected Applications of Hot Dip Galvanized Steel Hot dip galvanized coatings are found in a wide variety of applications requiring long-term maintenance-free corrosion protection. The examples in this section will demonstrate the scope of galvanized steel use. Bridges. The first all hot dip galvanized bridge in the United States was erected in 1966 at Stearns Bayou, Ottawa County, MI. The bridge is 128 m (420 ft) long and has a 9.1-m (30-ft) wide roadway with a 1.5-m (5-ft) walkway on each side. All the components of this bridge were hot dip galvanized. To avoid possible effects from road salting, telescoping splash plates in the joints were installed to divert deck drainage away from the beams. When inspected in 1986, the average coating thickness on stringers and diaphragms was 112 μm (4.4 mils), which is enough to last another 60 to 100 years. The bearing pads, the most deteriorated component of the bridge, showed a coating thickness of 90 μm (3.5 mils). The article "Corrosion in Structures" contains detailed information on the use of hot dip galvanized steel structural members. Pulp and paper mills contain a number of areas that, in the absence of corrosion protection, would rapidly deteriorate because of exposure to various chemicals. A paper mill in the northwest United States used galvanized structural steel pipe supports, ladders, cages, and miscellaneous steel items in the stock tank, black liquor, lime kiln, and paper machine areas. Galvanized steel was used during the original construction 19 years ago and for subsequent expansions. All of the galvanized steel is located outside in a humid environment subject to salt spray. When inspected in 1985, coating thicknesses ranged from 90 to 160 μm (3.5 to 6.5 mils). More information on materials of construction for pulp and paper mills is available in the article "Corrosion in the Pulp and Paper Industry" in this Volume. Recreation. The Gettysburg Observation Tower overlooks the historic Gettysburg battle-ground in Gettysburg, PA. The structure was erected in 1974 with galvanized pipes and rolled-steel sections. An inspection in 1984 revealed coating thicknesses of 100 to 150 μm (4 to 6 mils); this is above the specification for newly galvanized material. Service life can be expected to be 100 years or more, assuming the environment does not change appreciably. Utility Industry. A galvanized substation located near Knoxville, TN, and owned by the Tennessee Valley Authority was constructed in 1936 to handle 100,800 kW of electricity produced by a nearby dam. Substation structures are of a bolted lattice construction. The minimum coating thicknesses measured in a recent inspection were 70 μm (2.75 mils) on lattice angles and 50 μm (2.5 mils) on bolt heads. Based on estimations of the original coating weight, this substation will last another 30 years before maintenance coating will be required. Other Applications. Hot dip galvanized steel coated by the batch process is used in oil refineries and petrochemical industries and for miscellaneous highway uses, such as guard rail, light and sign standards, and fencing. The coating has found extensive use in water and wastewater treatment plants, both in atmospheric and immersion service. Such applications of electrical utility transmission towers, microwave transmission stations, pole line hardware, cooling towers and cooling tube bundles, nuts, bolts, and various fasteners all involve extensive use of galvanizing. Galvanized reinforcement bars, ties, and lintels for concrete and masonry reinforcement and support provide long-term corrosion protection in vital areas that are not normally visible. The use of galvanized materials in concrete reinforcement and masonry applications is ideal, because the normal pH of these materials before setting is about 12 to 12.5, which corresponds to the pH range in which corrosion of zinc is at a minimum. Galvanizing Specifications Galvanized coatings produced by the hot dip (batch) process are covered by numerous specifications. In addition to those already discussed in the text, the following are pertinent specifications under the authority of ASTM: • A 123 "Standard Specification for Zinc (Hot-Galvanized) Coatings on Products Fabricated from Rolled, Pressed, and Forged Steel Shapes, Plates, Bars, and Strip" • A 153 "Standard Specification for Zinc Coating (Hot Dip) on Iron and Steel Hardware" • A 384 "Standard Recommended Practice for Safeguarding Against Warpage and Distortion During Hot- Dip Galvanizing of Steel Assemblies" • E 376 "Standard Recommended Practice for Measuring Coating Thickness by Magnetic-Field or Eddy- Current (Electromagnetic) Test Methods" References 1. G.J. Slunder and W.K. Boyd, Zinc: Its Corrosion Resistance, 2nd ed., International Lead Zinc Research Organization, 1983 2. R.J. Krepski, The Influence of Bath Alloy Additions in Hot Dip Galvanizing A Review, St. Joe Minerals Corporation, 1980 3. Report of Subcommittee XIV on Inspection of Black and Galvanized Sheets, Committee A-5, in ASTM Proceedings, American Society for Testing and Materials, 1952, p 113 4. F.M. Reinhart, in Twenty-Year Atmospheric Corrosion Investigation of Zinc- Coated and Uncoated Wire and Wire Products, STP 290, American Society for Testing and Materials. 1961 5. L.P. Devillers and P. Niessen, The Mechanism of Intergranular Corrosion of Dilute Zinc- Aluminum Alloys in Hot Water, Corros. Sci., Vol 16, 1976, p 243-252 6. S.E. Hadden, Effect of Annealing on the Resistance of Galvanized Steel to Atmospheric Corrosion, J. Iron Steel Inst., Vol 171, 1952, p 121-127 7. H.S. Campbell et al., Effect of Heat Treatment on the Protective Properties of Zinc Coatings on Steel, J. Iron Steel Inst., Vol 203, 1965, p 248-251 8. J.J. Friel, Atmospheric Corrosion Products on Al, Zn, and Al-Zn Metallic Coatings, Corrosion, Vol 42, 1986, p 422-426 9. G. Schikorr, Corrosion Behavior of Zinc, Vol 1, English ed., American Zinc Institute and Zinc Development Association, 1965, p 72 10. L. Kenwort hy and M.D. Smith, Corrosion of Galvanized Coatings and Zinc by Waters Containing Free Carbon Dioxide, J. Iron Steel Inst., Vol 70, 1944, p 463-489 11. "Method for Estimating the Service Life of Metal Culverts," Test Method 643-B, California Department of Public Works, 1963 12. M. Romanoff, "Underground Corrosion," NBS 579.227. National Bureau of Standards, 1957 13. R.M. Burns and W.W. Bradley, Protective Coatings for Metals, 3rd ed., Reinhold, 1967, p 165 14. S.G. Denner et al., Hot Dip Aluminizing of Steel Strip, Iron Steel Int., June 1975, p 241-252 15. G. Eggeler et al., On the Influence of Silicon on the Growth of the Alloy Layer During Hot Dip Aluminizing, J. Mater. Sci., Vol 21, 1986, p 3348-3350 16. H.F. Graff, Aluminized Steel, in Encyclopedia of Materials Science and Engineering, Pergamon Press, 1986, p 138-141 17. V.I. Kelley, in Atmospheric Corrosion Investigation of Aluminum-Coated, Zinc- Coated, and Copper Bearing Steel Wire and Wire Products (A 12 Year Report), STP 585, American Society f or Testing and Materials, 1975 18. H.P. Godard et al., The Corrosion of Light Metals, John Wiley & Sons, 1967, p 11 19. L. Allegra et al., Resistance of Galvanized, Aluminum-Coated, and 55% Al- Zn Coated Sheet Steel to Atmospheric Corrosion Involving Standing Water, in Atmospheric Corrosion, W.H. Ailor, Ed., John Wiley & Sons, 1982, p 595-606 20. G.E. Morris and L. Bednar, Comprehensive Evaluation of Aluminized Steel Type 2 Pipe Field Performance, in Transportation Research Record 1001, National Research Council, Transportation Research Board, 1984, p 49-60 21. J.C. Zoccola et al., Atmospheric Corrosion Behavior of Aluminum- Zinc Alloy Coated Steel, in Atmospheric Factors Affecting the Corrosion of Engineering Metals, STP 646, American Society for Testing and Materials, 1978, p 165-184 22. H.E. Townsend and J.C. Zoccola, Atmospheric Corrosion Resistance of 55% Al- Zn Coated Sheet Steel: 13-Year Test Results, Mater. Perform., Vol 18, 1979, p 13-20 23. H.E. Townsend and A.R. Borzillo, Twenty-Year Atmospheric Corrosion Tests of Hot Dip Coated Sheet Steel, Mater. Perform., to be published 24. J.H. Payer, Electrochemical Methods for Coatings Study and Evaluation, in Electrochemical Techniques for Corrosion, R. Baboian, Ed., National Association of Corrosion Engineers, 1977 25. J.B. Horton et al., Corrosion Characteristics of Zinc, Aluminum, and Al- Zn Alloy Coatings on Steel, in Proceedings of the Sixth International Congress on Metallic Corrosion (Sydney, Australia), 1975 26. S.A. Kriner, unpublished research, 1985 27. A.J. Stavros, Galvalume Corrugated Steel Pipe: A Performance Summary, in Transportation Research Record 1001, National Research Council, Transportation Research Board, 1984, p 69-76 28. D. Horstmann, Reaction Between Liquid Zinc and Silicon-Free and Silicon- Containing Steels, in Proceedings of the Seminar on Galvanizing of Silicon-Containing Steels, International Lead- Zinc Research Organization, 1975, p 94 29. R.W. Sandelin, "Effects of Microstructure on the Galvanizing Characteristics of Steel ," Paper presented at the Annual Meeting, American Hot Dip Galvanizers Association, Sept 1964 30. Galvanizing Characteristics of Structural Steels and Their Weldments, International Lead Zinc Research Organization, 1975 31. "Standard Recommended Practice for Safeguarding Against Embrittlement of Hot- Dip Galvanized Structural Steel Products and Procedure for Detecting Embrittlement, A 143, Annual Book of ASTM Standards, American Society for Testing and Materials 32. The Design of Products To Be Hot Dip Galvanized After Fabrication, American Hot Dip Galvanizers Association, 1985 33. Recommended Details of Galvanized Structures, American Hot Dip Galvanizers Association, 1983 34. "Standard Recommended Practice for Providing High-Quality Zinc Coatings (Hot-Dip)," A 385, Annual Book of ASTM Standards, American Society for Testing and Materials 35. Welding Zinc Coated Steels, American Welding Society, 1973 36. R.M. Burns and W.W. Bradley, Protective Coatings for Metals, 2nd ed., Reinhold, 1955, p 128 37. S.K . Coburn, C.P. Larrabee, H.H. Lawson, and G.B. Ellis, Corrosiveness at Various Atmospheric Test Sites as Measured by Specimens of Steel and Zinc, in Metal Corrosion in the Atmosphere, STP 435, American Society for Testing and Materials, 1968, p 371-372 38. C.J. Sunder and W.K. Boyd, Zinc: Its Corrosion Resistance, 2nd ed., International Lead Zinc Research Organization, 1983, p 113-150 39. "Code of Practice for Protective Coating Iron and Steel Structures Against Corrosion," BS 493, British Standards Institution, 1977, p 23 40. P.C. Birkemoe, W.D. Crouch, and W.H. Munse, "Design Criteria for Joining Galvanized Structurals," Annual Report, ZM-96, International Lead Zinc Research Organization, April 1969 41. R.A. Sanderson, "Fatigue Behaviour of High Streng th Bolted Galvanized Joints," M.A. thesis, University of Toronto, 1968 42. D.J.L. Kennedy, High Strength Bolted Galvanized Joints, J. Struct. Div., Vol 98 (No. St12), Dec 1972, p 2723-2738 43. "Standard Practice for Repair of Damaged Hot-Dip Galvanized Coatings," A 780, Annual Book of ASTM Standards, American Society for Testing and Materials 44. J.F.H. van Eijnsbergen, Twenty Years of Duplex Systems, Metallwissenschaft Technik, Vol 29 (No. 6), June 1975 45. J.F.H. van Eijnsbergen, Supplement (to Twenty Years of Duplex Systems), Thermisch Verzinken, Vol 8, 1979 46. D.S. Carr, "Performance of Painted Galvanized Steel," Paper presented at the Semi- Annual Meeting, Houston, TX, American Hot Dip Galvanizers Association, Sept 1982 47. B.R. Appleman, Economics of Corrosion Protection by Coatings, J. Protec. Coatings Linings, Vol 2 (No. 3), March 1985 48. T.J. Kinstler, "Probability Functions in Corrosion Economics or a Corrosion Engineer Goes to Monte Carlo," Paper presented at Corrosion/82, National Association of Corrosion Engineers, March 1982 49. "Recommended Practice: Direct Calculation of Economic Appraisals of Corrosion Control Measures," NACE RP-02-72, National Association of Corrosion Engineers Porcelain Enamels Introduction PORCELAIN ENAMELS are glass coatings that are applied primarily to fabricated sheet steel, cast iron, or aluminum parts to improve appearance and to protect the metal surface. Porcelain enamels are distinguished from other ceramic coatings by their predominantly vitreous nature and the types of applications for which they are used, and they are distinguished from paint by their inorganic composition and the fusion of the coating matrix to the substrate metal. Porcelain enamels of all compositions are matured at 425 °C (800 °F) or above. Because they offer only barrier protection to the metal substrate, porcelain enamel coatings must be free from defects and coating discontinuities to provide optimum protection. The most common applications of porcelain enamels include major appliances, water heater tanks, sanitary ware, and cookware. Porcelain enamels are also used in a wide variety of applications ranging from chemical-processing vessels, heat exchangers, agricultural storage tanks, piping and pump components, and barbeque grills to architectural panels, signing, specially executed murals, and microcircuitry components. Table 1 lists some additional applications for porcelain enamels. Normally, porcelain enamels are selected for products or components where there is a need for one or more of the special service requirements that porcelain enamel can provide, such as chemical resistance, corrosion protection, weather resistance, specific mechanical or electrical properties, appearance or color needs, cleanability, or thermal shock capability. Table 1 Some applications for porcelain enamels Industrial products Chemical reactors Commercial heat exchangers Food-processing vessels Induction heating coils Ion gun parts Jet engine components Microcircuitry boards Mufflers Transformer cases General products Camping equipment [...]... enamel Water-resistant enamel SiO2 33.74 36.34 56 .44 48.00 B2O3 20.16 19.41 14.90 12.82 Na2O 16.74 14.99 16 .59 18.48 K2O 0.90 1.47 0 .51 Li2O 0.89 0.72 1.14 CaO 8.48 4.08 3.06 2.90 BaO 9.24 8 .59 ZnO 2.29 Al2O3 4.11 3.69 0.27 ZrO2 2.29 8 .52 TiO2 3.10 3.46 CuO 0.39 MnO2 1.43 1.49 1.12 0 .52 NiO 1. 25 1.14 0.03 1.21 Co3O4 0 .59 1.00 1.24 0.81 P2O5 1.04 0.20 F2 2.32 2.33 1.63 1.94 Weather... in either continuous or batch-type furnaces at temperatures of about 4 25 °C (800 °F) for steel parts and 52 5 to 55 0 °C (980 to 1020 °F) for aluminum More information on the porcelain-enameling process is available in the article "Porcelain Enameling" in Surface Engineering, Volume 5 of ASM Handbook, formerly 9th Edition Metals Handbook Process Variables The thickness of the applied layer of porcelain... substrate metal and the service requirements of the part On aluminum, porcelain enamel is applied to produce a fired enamel thickness ranging from about 65 to 1 25 μm (2 .5 to 5 mils) A tolerance of ±13 μm (±0 .5 mil) is required in order to maintain uniform opacity for a white enamel coating 1 15 μm (4 .5 mils) thick On sheet steel, a ground coat about 50 to 100 μm (2 to 4 mils) thick is used to promote... wet-process coatings range in thickness from 255 to 6 35 μm (10 to 25 mils) Coating thicknesses for hot-water tanks normally range from 150 to 230 μm (6 to 9 mils), with 150 μm (6 mils) a generally accepted minimum thickness Heat-exchanger surfaces, depending on end use, are sometimes double coated for added durability The thickness of the porcelain enamel on a large part of simple configuration can be closely... Bathtubs To 49 To 120 5- 9 Water, cleansers Chemical ware To 100 To 212 12 Alkaline solutions To 100 To 212 1-2 All acids except hydrofluoric 1 75- 230 350 - 450 1-2 Concentrated sulfuric acid, nitric acid, and hydrochloric acid Home laundry equipment To 71 To 160 11 Water; detergents; bleach Range exteriors 21-66 70- 150 2-10 Food acids; cleaners Range oven liners, conventional 66-3 15 150 -600 2-10 Food acids;... 65 to 90 μm (2 .5 to 3 .5 mils) are desirable Wet electrostatic spraying of porcelain enamel is used to reduce losses in material by charging the porcelain enamel slip during atomization to a potential of 100,000 to 120,000 V The electrostatically charged droplets are attracted to the grounded parts being sprayed A well-operated electrostatic unit can deposit up to 85% of the sprayed material on the part. .. required, and allows the parts to be handled more easily Parts are either air dried or dried with radiant or convection dryers Convection drying consists of gradual heating of the parts to 120 °C ( 250 °F); cycle times range from 2 to 5 min The coating is still wet during the initial stages of drying, so drying must be accomplished in an atmosphere free of dirt, scale, or dust The parts are then fired in... by a mechanical spraying system that is adapted to the part For example, mechanically applied porcelain enamel on curved silo panels measuring 2 × 3 m (6 × 9 ft) can be maintained within ±13 μm (±0 .5 mil); however, when application is by hand spraying, the variation in enamel thickness is 50 μm (±2 mils) An enamel thickness of 65 to 180 μm (2 .5 to 7 mils) is desirable for aluminum architectural panels... aluminum architectural panels When white or lightcolored enamel is used, however, the enamel thickness ranges above 75 μm (3 mils) in order to produce acceptable opacity Two coats with a total thickness of about 1 25 μm (5 mils) result in more uniform opacity than one coat that is 1 25 μm (5 mils) thick Additional details are available in Ref 1 Firing Time and Temperature Firing of porcelain enamel involves... liners for household refrigerators are fired at 8 05 °C (1480 °F) for 2 1 min or at 790 °C (1 450 °F) for 4 min In all cases, there is a minimum practical 2 temperature for the attainment of complete fusion, acceptable adherence, and desired appearance Most ground-coat enamels for high-production steel parts exhibit acceptable properties over a firing range of 55 °C (100 °F) at an optimum firing time However, . years 50 2 10 100 4 3 150 6 19 75 3 14 150 6 5 2 25 9 29 100 4 19 100 4 3 200 8 33 Heavy industrial 100 4 19 150 6 5 250 10 36 50 2 19 100 4 4 150 6. 4 4 150 6 34 75 3 29 150 6 6 2 25 9 52 100 4 39 100 4 4 200 8 64 Metropolitan (urban) 100 4 39 150 6 6 250 10 67 50 2 20 100 4 4 150 6 36 100 4. Aluminum-Coated, and 55 % Al- Zn Coated Sheet Steel to Atmospheric Corrosion Involving Standing Water, in Atmospheric Corrosion, W.H. Ailor, Ed., John Wiley & Sons, 1982, p 59 5-606 20. G.E.