Understanding how and when the iron oxide chemical bond occurs can be key to 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.
To 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.
Figure 1. Bond vs. temperature for the sample glasses.
Each panel was bond tested and evaluated, with zero indicating no adherence and 10 indicating the best adherence (see Figure 1). The A glass, with no transition metals, had no adherence over the entire firing range. The B glass, with full metals, started to bond at a lower temperature of 1390?F, compared to the C glass, which started to bond at 1420?F. At 1670?F, the B glass bond dropped off drastically while the C glass bond held.
Figure 2. The A glass shows no bond over a four panel range
Optical and scanning electron microscopy tests were run on selected samples to determine the relationship between the bond and the iron at the enamel/steel interface.
The optical micrographs of the A glass showed no bond over a four panel range (see Figure 2).
Figure 3. The B glass has an optimum firing temperature of 1500¿F; at 1480¿F there is excellent bond, and the bond begins to weaken at 1670¿F.
Enamel B had an optimum firing temperature of 1500?F (see Figure 3). The micrograph at 1360?F did not show any bond. At 1390?F, the bond was beginning, and at 1480?F there was excellent bond. The bond began to weaken at 1670?F, which supports Figure 1.
Figure 4. The C glass shows no bond at 1390¿F, a good bond at 1420¿F, and it maintains an excellent bond through 1670¿F.
Enamel C showed no bond at 1390?F (see Figure 4), where the B glass was just starting to exhibit bond. At 1670?F, the B glass adherence was getting weaker while the C glass still had excellent adherence.
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.
Figure 5. The weight gain of the steel during firing.
Using an electronic scale, weight gain analysis after firing was performed on raw steel and on the same type of steel with enamel B applied (see Figure 5). The 1390?F raw steel panel and the enameled steel were fired together, and the 1480 and 1670?F panels were fired the same way. The raw steel panels showed a much higher weight increase compared to the enameled panels. This may indicate that the oxide being formed on the enameled panels was being dissolved into the glass, reducing the weight. The enameled panel at 1670?F showed a higher weight gain. This can be referred back to Figure 3, which shows a weaker bond at this temperature because of an excess of some form of Fe present in the enamel.
Figure 6. At the 1480¿F firing, Enamel A has no bond and does not show any reaction at the enamel/steel interface.
Scanning electron maps of 30-degree cross-sections were run on glasses A and B to determine the relationship between the bond and the presence of Fe. Enamel A at the 1480?F firing had no bond and did not show any reaction taking place at the enamel/steel interface (see Figure 6). The map also showed that the surface of the steel was rough, but a rough surface alone cannot create an enamel bond on this type of steel.
Figure 7. The Si map - the light area indicates high Si in the glass, while there is no Si evident in the steel.
The silicon (Si) map also showed no reactions (see Figure 7).
Figure 8. The Fe map. The light area indicates high Fe in the steel, but there is no Fe in the glass.
Note on the Fe map (Figure 8) that no Fe was present in the glass.
Figure 9. Enamel B at 1390¿F. Iron is migrating into the glass and being dissolved.
The scanning electron map of the B enamel at 1390?F, where the bond was beginning, showed a chemical reaction occurring—iron migrating into the glass and being dissolved (see Figure 9). This bond could also be considered mechanical because the chemical reactions taking place at the surface of the steel were creating a rougher surface.
Figure 10. The Si map - the dark area is near the interface where the Si is diluted by the Fe in the glass.
The Si map showed areas low in Si at the interface (see Figure 10),
Figure 11. The Fe map. The light gray area shows where Fe has dissolved into the glass at the low Si sites.
where the Fe map (Figure 11) showed Fe at the low Si sites.
Figure 12. Enamel B at 1480¿F shows excellent bond due to higher chemical reactions occurring at the interface.
The 1480?F map of Enamel B showed excellent bond due to higher chemical reactions occurring at the interface (see Figure 12).
Figure 13. The Si map - the gray area near the interface shows the lower Si levels.
Si levels appeared to be low at the interface on the Si map (see Figure 13),
Figure 14. The Fe map. An Fe concentration is apparent at the low Si areas.
while the Fe map showed an Fe concentration at the low Si areas (see Figure 14).
Figure 15. The Enamel B bond begins to deteriorate due to a large concentration of Fe at the interface.
At 1670?F, the Enamel B bond was getting weaker. The scanning electron map showed a much higher concentration of Fe at the interface area (see Figure 15). An area low in Fe appeared to be directly above the steel surface.
Figure 16. The Si map - the lighter area indicates higher Si levels just above metal.
The Si map showed a low Si level, but the area directly above the steel was high in Si (see Figure 16).
Figure 17. The Fe map just above the interface. The dark area indicates low Fe levels.
The iron map confirmed that the area was low in Fe (see Figure 17). It was believed that the equilibrium had been changed between the enamel and steel, and no additional Fe could be dissolved into the enamel, creating the weaker bond.
Figure 18. The Fe weight percent and the distance the Fe travels into the enamel increases in correlation with firing time.
It has been well documented by D. Ritchie and the late D. White, Ph.D., that when the firing time is increased, the Fe weight percent and the distance the Fe travels into the enamel also increases (see Figure 18).4
Higher temperatures would also increase the mobility of Fe in the glass.
Figure 19. Enamel B at 1390¿F.
Optical micrographs on 30-degree cross-sections of Enamel B were taken on the panels at 1390, 1480 and 1670?F intervals. The 1390?F micrograph showed a hazy iron area at the interface (see Figure 19). At this temperature, the adherence was beginning.
Figure 20. Enamel B at 1480¿F.
At 1480?F, the hazy Fe area had expanded further into the enamel (see Figure 20). There was excellent bond at this temperature.
Figure 21. Enamel B at 1670¿F.
At the 1670?F, the Fe layer had expanded much further into the enamel (see Figure 21). The large amount of iron oxide migrating into the enamel could not be dissolved into the enamel, so adherence was impacted negatively. The area low in Fe at the interface, shown on the scanning electron map, could not be seen in the optical view.
Achieving Optimum Adhesion
Chemical 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.
For More Information
For more information about enamel adhesion, contact Ferro Corp., Porcelain Enamel Division, 4150 E. 56th St., P.O. Box 6550, Cleveland, OH 44101; (216) 641-8580; fax (216) 641-2049; or visit www.ferro.com.