Edge corrosion on steel has a two-fold effect: It is not only aesthetically unpleasant, but also, more importantly, it can affect product life. To address this issue, one needs to consider the best form of material protection. When it comes to coatings, porcelain enamel (i.e., glass coated) over steel substrates performs well for long-term durability and corrosion resistance. In order to understand porcelain enamel’s strengths, one must understand its history, how it is manufactured and what makes it distinctive.
Enameling: A History
Porcelain enameling was believed to take form around the first century A.D. The primary types of enamel were cloisonné and champlevé, depending on the method of fabrication. As time progressed, enamels were applied to gold, then silver, bronze, copper and finally iron. It was on iron that enameling found its most practical use.
Following the application to iron and steel, the next major achievement in porcelain enameling was the application of clay to keep the powdered enamel suspended in water, improving adherence to the metal before firing.
The 1940s through the 1950s was a time of tremendous growth and increased usage as the appliance and hot water tank industries adopted porcelain. Later, its color properties led to the rise of its use in architectural applications, along with in sinks and tables, due to its easy-to-clean and sanitary properties. Porcelain enameling markets include ovens and range tops in the appliance industry; interior hot water heaters; plumbing ware and bolted storage tanks for water; municipal wastewater; manure slurry; and animal feed silos.
Porcelain Enamel Defined
According to the Porcelain Enamel Institute, porcelain enamel is defined as “a substantially vitreous or glassy inorganic coating bonded to metal at a fusion temperature above 800°F.”
The key words here are “vitreous” and “inorganic.” Most are familiar with the terms vitreous or glassy. These materials are encountered in everyday life in many different forms and applications. Porcelain enamel material is noteworthy in that it is designed and formulated to bond or fuse to the metal substrate, much in the same way that glaze is formulated to adhere to a ceramic body. This formula requires ingredients that are not common in glass manufacturing like bottles or windows.
“Inorganic” is less familiar to most, but sets porcelain enamel apart. Most inorganic compounds do not contain carbon and are derived from mineral sources. The raw materials used for porcelain enamels include minerals, rocks, and clays, as well as chemicals manufactured as products or byproducts of the chemical industry. These “of the earth” materials are what contribute to porcelain enamel’s corrosion-resistant, physical, mechanical, electrical, sanitary and color-stable properties.
Another property that enamels exhibit is a lack of coating porosity on the surface. After firing, the enamel forms an impervious moisture barrier. Most, if not all organic coatings have an amount of physical porosity that ultimately can allow water molecules to migrate to the metal substrate.
When it comes to coating performance, adhesion to the substrate is critical. While most organic coatings rely upon cross-linking during the curing process, in which the cured coating attaches itself to the substrate, the porcelain enamel adhesion mechanism is more secure. During high temperature exposure, the enamel/steel interface undergoes a transformation. Iron oxide at the surface of the steel dissolves into the glass, forming a layer where the steel and enamel are intertwined.
For corrosive environments like hot water tanks or liquid storage tanks, the enamel is formulated for each application. The glass frits are selected to promote the necessary adhesion, while at the same time producing a durable surface for the
application—the goal being to prevent corrosion of the base metal substrate.
Below are examples of the properties a porcelain enamel glass must pass in order to be approved for storage tank specifications:
- ISO28706-2:2011 Clause 10. Boiling citric acid (2.5 hours);
- ISO28706-2:2011 Clause 12. Boiling hydrochloric acid – vapor phase (7 days);
- ISO28706-2:2011 Clause 13. Boiling distilled water (48 hours);
- ISO28706-2:2011 Clause 9. Standard detergent solution (24 hours);
- ISO28706-2:2011 Clause 8. Hot sodium hydroxide (24 hours); and
- ISO 6370-2. Abrasion resistance.
Proper selection of the glass frits, clays, refractories, opacifiers, coloring oxides and electrolytes is essential for the specific application and is the responsibility of the supplier and coating manufacturer.
Enamel & Edge Corrosion
One challenge with porcelain enamel is how to uniformly apply and adhere this coating to all surfaces of a metal substrate. Steel edges are the most susceptible to corrosion.
Edge corrosion is not a new phenomenon. Whether the coating is organic-based (i.e., paint/epoxy/urethanes, etc.) or inorganic-based (i.e., porcelain enamel, ceramic) the challenge lies in providing an adequate film of protection on a surface that is not conducive to accepting a coating.
Any thermosetting (organic) or thermoplastic (inorganic) coating must proceed through a melt phase as it heats to the temperature at which it can crosslink or fuse to its substrate. During this phase, the film has an opportunity to flow and level itself, ideally resulting in a smooth and attractive film that is aesthetically appealing and corrosion-resistant. The Porcelain Enamel Institute recommends that when designing parts for enameling, certain radii criteria should be maintained for the enamel to successfully adhere. Depending upon the post-fire enamel thickness, different radii are recommended. Thinner thicknesses can tolerate a tighter radius, while heavier thicknesses (12 mils+) require larger radii.
The Engineered Approach
The engineered approach combines the adhesion and performance of porcelain enamel with properly configured metal substrate geometries. CST has accomplished this by engineering the metal edge and applying the glass in a suitable yet consistent thickness range for maximum corrosion resistance. The result is that the steel panel is completely encapsulated in glass on all six sides.