issue: January 2006 APPLIANCE Magazine
A Comparison of Enameled and Stainless-Steel Surfaces
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by David Fedak and Charles Baldwin, Ferro Corporation
The following paper compares the properties of porcelain enamel and stainless steel used in appliance production. A new stainless-style enamel cover coat is also introduced.
Stainless steel appliances have become very popular in the last several years because of a consumer preference for a bright finish suggesting sophistication and luxury. Furthermore, high-end products have expanded from a niche at the very top end of the market to the masses.
Unfortunately, the increasing demand for stainless steel in all industries has a double impact on the field of porcelain enameling. First, stainless is purchased by the consumer instead of enameled steel. Second, the high percentage of nickel in the stainless increases costs to the enameling industry. In 2005, about two-thirds of all nickel production went into stainless steel and nickel prices were the highest since 1989.
Porcelain (or vitreous) enamel, a glassy coating material suitable for protecting substrates while also decorating them, is a composite material with the best qualities of the metal substrate and the coating. It is generally divided into ground coats and cover coats. Ground coats contain adherence-promoting oxides for good bond to steel and are used in oven cavities, stove grates, hot water tanks, and dual-purpose black enamels. Pyrolytic ground coat is an extremely heat-resistant enamel for self-cleaning ovens, which turns baked-on soils into ash at about 1,000 degrees fahrenheit (427 degrees celcius). Cover coats provide additional chemical, physical or cosmetic properties and require a ground coat as a primer layer. Applications include cooktops, washers, sinks, barbeque grills, and architectural panels. Sometimes considered cover coats, aluminum enamels contain low-temperature glasses to fuse directly onto aluminum. These are most commonly used on cookware.
Stainless steel is defined as alloy steel containing more than 10-percent chromium. A passive chromium oxide layer forms on the surface and provides corrosion resistance. Applications for stainless steel include fasteners, cutlery, flatware, chemical plant equipment, food processing equipment, and kitchen ware.
Stainless steel is classified according to the iron phase present in the alloy. Formed by rapidly quenching austenitic iron from high temperatures, martensite-containing stainless steels, such as alloys 410, 420 or 440C, are hard and corrosion-resistant. These find use in cutlery, turbine blades and other high-temperature applications. Ferritic stainless steels (e.g., 409, 430, 439, and 444) are more corrosion-resistant than the martensitic types, and are thus used in nitric acid service, water storage, food processing, automobile trim, furnaces, and turbines. Austenitic stainless steels, such as alloys 304, 304L, 308, 310, 316, 316L, 317, 321, and 347, are non-magnetic and offer a good combination of corrosion resistance, high- temperature strength and ease of fabrication. This type can be strain-hardened by cold working. The austenitic 304 alloy is widely used in appliances, and there is also some use of ferritic 430. When used on appliances, stainless usually has a brushed finish.
Stainless steel is susceptible to the phenomena of sensitization. During exposure between 1,000 degrees fahrenheit to 1,500 degrees fahrenheit (538 degrees celcius to 816 degrees celcius), complex chromium carbides precipitate at grain boundaries. This can lead to unexpected failure through intergranular corrosion. It is eliminated through heat-treating or the use of extra low-carbon steel (e.g., 304L). Weld spots, particularly on ferritic stainless, require heat treatment to prevent this problem.
Porcelain Enamel Vs. Stainless Steel
A number of tests were used to compare the performance of porcelain enamel to stainless steel in several areas that should relate to a consumer's use of a major appliance. Properties investigated were hardness, abrasion resistance, impact resistance, room- temperature stain resistance, and cleanability.
For porcelain enamel, the five enamels shown in Table 1 were run against 304 brushed stainless steel. The four sheet steel systems were all applied dry electrostatically. The 2C1F white cover coat (Ferro PC46C on PL52) and 2C1F biscuit cover coat (Ferro PC168C) are typically used on cooktops. The pyrolytic ground coat (Ferro PL62D) is used on the interior of self-cleaning ovens and has sufficient heat resistance to withstand many cycles of pyrolytic cleaning. The black dual-purpose ground coat (Ferro PL206) is used as a base coat for 2C1F enamels, but it also has a high enough quality surface to be used as black enamel on range exteriors. Cookware is typically made with stainless steel or enameled aluminum. Therefore, aluminum enamel (Ferro GL4317) was evaluated as another alternative to stainless steel.
Hardness is defined as a measure of a material's resistance to localized plastic deformation such as a scratch. Different test methods are used for readily scratched materials such as paints, malleable metals and hard but brittle ceramics. For organic paints, it is evaluated with ASTM D 33630-00 Standard Test Method for Film Hardness by Pencil Test. For this test, the force required to gouge a coating with a drawing lead of calibrated hardness is assessed. For metals, Knoop, Rockwell or Brinell hardness tests are utilized in which the resistance to deformation with a standard indenter is measured. Ceramics are measured relative to diamond on the Moh's scale. The Knoop, Brinell, Rockwell B, Rockwell C, and Mohs scales are shown side-by-side in Figure 1.
On Figure 1, the point marked SS shows the 88 HRB (Rockwell B) hardness of 304 brushed stainless steel. Point PE shows the hardness of typical porcelain enamel between 5 and 6 on the Mohs scale.
Porcelain was run against stainless on the pencil hardness test. All enamels were off the scale and could not be scratched with the hardest 9H pencil. However, stainless could be scratched with a softer 5H pencil, which correlates with it being softer than enamel on Figure 1.
The abrasion resistance was evaluated using ASTM D 4060-95 Standard Test Method for Abrasion Resistance of Organic Coatings by the Taber Abraser. Panels were run for 2,000 cycles under the hardest CS-17 wheels with a 1-kg load, and the weight loss was measured. Data is shown in Figure 2.
Figure 2 shows the weight loss of stainless steel is similar to enamels, but it does not show the effect of the difference in scratch resistance. Figure 3 shows that the stainless-steel abrasion panel on the left is visually more damaged than the dual-purpose ground coat panel on the right, which barely shows any indication of wear.
Chipping has been a serious issue for the enameling industry. During impact, stainless steel deforms, and possibly dents, but is not seriously compromised. However, the potential for the stainless to be scratched and visually altered permanently is high. For enamels, frit formulations and metal design principles to minimize the chance of chippage are well understood. Furthermore, the porcelain enamel finish readily covers stamping defects and scratches in the metal, which would otherwise be causes of high-cost scrap for stainless steel.
Stain resistance was evaluated for six foodstuffs and three typical household cleaners. The foods were barbeque sauce (Bullseye Original), Heinz ketchup, RealLemon lemon juice, Heinz Worcestershire sauce, Twinings Earl Grey tea, and Heinz vinegar. The cleaners were Easy Off, 409 and (Topco Top Crest) ammonia. Easy Off, containing approximately 5 percent of NaOH, is a very alkaline material with a high pH. Formula 409 contains a mixture of surfactants and a likely smaller amount of NaOH in 2-propanol.
For the foods, soils were placed under watch glasses for 48 hours, while, for the cleaners, the test duration was 24 hours under a watch glass. A value of zero was assigned for no stain, and a one was assigned for a permanent stain. The total number of stains was then added up to generate a ranking for the coating.
Results for food-based soils are shown in Figure 4. All of the porcelains except the aluminum enamel were stained by vinegar. The white cover coat and black dual-purpose ground coat were stained with lemon juice, and ketchup also stained the black ground coat. Comparatively, all six soils left light marks on the 304 stainless steel.
Results for the cleaner testing are shown in Figure 5. The white cover coat and the aluminum enamel were stained with ammonia. Formula 409 left a permanent mark on the black ground coat. The stainless was attacked by both the Formula 409 and ammonia.
To assess the heat resistance, the lab color of panels was measured using a DataColor International Spectraflash SF 450 color machine. "L" measures the brightness, "a" measures the amount of red or green, and "b" measures the amount of yellow or blue. For consistency, the stainless steel panel was always measured with the same orientation to the brushing. Then, the panels were soaked at 750 degrees fahrenheit (399 degrees celcius) for up to 72 hours. Every 24 hours, panels were removed from the furnace, cooled, and the color was measured. The change in color, was calculated using the equation on the right:
Figure 6 shows that the stainless steel changed color, with = 16 after only 24 hours at 750degrees fahrenheit (399 degrees celcius). Visually, this was shown as a sharp shift to more yellow (from b = 4.36 to b = 14.75) and darker. The sheet steel enamels showed little or no change in color readings, and the aluminum enamel shifted slightly brighter and more yellow, probably due to the closeness of the test temperature to the firing temperature of the coating.
The cleanability was assessed by a modified version of the European FAN (Facile Nettoyer, literally Easy-to-Clean) test. First, seven steel rings were glued to the coating surface. Second, grape jam, Egg Beaters(tm), ketchup, salted reconstituted milk, lemon juice, olive oil, and gravy were placed in each ring. Third, the test panel was baked at 450 degrees fahrenheit (232 degrees celcius) for 1 hour. This caused the glue to thermally decompose, allowing the rings to be removed.
Finally, the cleanability of each soil was rated using the system shown in Table 2 with a maximum score of 42 for a completely wipe-clean surface and the minimum of 0 for a material to which all of the burned-on residues strongly adhered.
The cleanability results are shown in Figure 7. Grape jam could not be removed from all of the materials. For stainless, the baked-on egg, olive oil and gravy left stains that were not present on the enamels.
Figure 3. Panels After Abrasion Testing
The comparison testing showed that porcelain enamel is harder to scratch, more stain-resistant, more heat-resistant, and easier to clean than stainless steel. The results show that stainless steel is functionally inferior to porcelain enamel. Since the consumer preference is for the color, porcelain enamels with the appearance of stainless steel have been developed. Using metallic pigments, a low-temperature enamel was invented that fires at about 1,000 degrees fahrenheit (538 degrees celcius) on aluminum or aluminized steel. A high-temperature steel cover coat firing about 1,500 degrees fahrenheit (816 degrees celcius) has also been developed with similar properties to the cover coats tested.
This is an edited version of a paper presented at the Porcelain Enamel Institute (PEI) Tech Forum, held in May 2005 in Nashville, Tennessee, U.S.
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About the Authors
Charles Baldwin holds a master's degree in Materials Science from Case Western Reserve University and joined Ferro as a research engineer in 1998.
David Fedak has been with Ferro Corporation for more than 13 years and is currently a technician working on non-stick, metallic and other new enamel products in Ferro's Porcelain Enamel Division.