Optimizing Enamel Adhesion
The way in which enamel bonds to steel can be categorized as mechanical, physical or chemical. A mechanical bond depends on the surface roughness of the steel, which can produce minor improvements in adhesion. A physical bond is typically related to compressive or Van Der Waals forces, which are very weak at best.
Chemical bonding, the bond most often reported in literature, occurs when the oxide dissolves into the glass. Joseph A. Pask relates that good adherence of a glass to a metal surface occurs when both the enamel and steel are saturated with metal oxides at the interface.1 However, if the firing process is so low that the oxide cannot dissolve or so high that it changes the equilibrium of the chemical reaction, the bond will be weak.
During the firing of porcelain enamel, the steel oxidizes. This oxidation typically occurs at the beginning of firing because of the porous glass coating. The porous structure is sealed when the enamel reaches a liquid state, and the oxidation occurs with the transition metals in the glass.
Past work performed by D.B. Clay and R.M. Jamieson showed that iron (Fe) dissolving into a glass is a contributor to good adherence.2 However, if large amounts of iron oxide (FeO) remain at the steel/enamel interface after firing, enamel adherence will decrease. FeO that forms during the firing process must therefore dissolve into the enamel to achieve a good bond. Understanding how and when the chemical bond occurs can be key to optimizing enamel adhesion.
ExperimentsTo help identify how FeO reacts with the enamel at the interfacial area, three glasses of varying amounts of transition metals were prepared. A known good bonding glass used in wet and electrostatic applications was formulated without transition metals and with half the transition metals. The A glass (Enamel A) had no transition metals; the B glass (Enamel B) had the normal amount of transition metals for this particular glass; and the C glass (Enamel C) contained half the metals of the B glass. For this experimental work, the three glasses were milled in a wet mill addition to a specified fineness.
Once the glasses were milled, the three systems were sprayed onto AK Univit cleaned-only, low-carbon steel with a dust coating on the backside. The test panels were dried at 170?F and then fired through a U-type continuous furnace with a 980?F preheat zone.
The optical micrographs of the A glass showed no bond over a four panel range (see Figure 2).
The experimental work by Ralph L. Cook indicates that Fe dissolves as a function of its ionic state.3 Fe does not dissolve as well as FeO, and FeO does not dissolve as well as ferric oxide (Fe2O3). The ionic state is affected by the presence of transition metal oxides, which can be reduced by Fe as it oxidizes to a higher valence.
Achieving Optimum AdhesionChemical bonding of the enamel to the steel is considered the most important contributor to good fired enamel adhesion. The chemical bond is generated when Fe is dissolved into the enamel. Dissolving of the Fe is promoted by a redox (oxidation-reduction) reaction involving transition metals smelted into a glass. Each enamel firing time and temperature below or above the optimum can alter the Fe concentration at the enamel/steel interface and can negatively impact fired enamel adhesion.
The experimental evaluations showed that steel does oxidize based on weight gain, but that the weight gain is less when the steel is enameled. As the oxide is dissolved into the glass, the equilibrium between the enamel and steel must be maintained to ensure good enamel adherence to the steel. Each enamel, with its unique formula and transition metal level, has an optimum firing time and temperature that must be identified to ensure optimum adhesion.