Porcelain enamel coating for high-carbon steel

Details Porcelain enamel coating for high-carbon steel Porcelain enamel coating for high-carbon steel

1999-06-17

2001-01-23

Turner, Archene (Department: 1775)

Stock material or miscellaneous articles

Composite

Of metal

C427S372200, C427S375000, C427S376100, C427S376200, C427S402000, C427S419200, C427S419300, C427S419400, C428S212000, C428S428000, C428S469000, C428S697000, C428S699000, C428S701000, C428S702000

Reexamination Certificate

active

06177201

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to porcelain enamel coatings used to coat sheet steel, and more particularly to multi-layered porcelain enamel coatings used to coat hot-rolled, high-carbon sheet steel substrates.
BACKGROUND PRIOR ART
In the past, it has been problematic to coat high-carbon, non-enamel grade steel with porcelain enamel coatings due to defects that occur during firing of such coatings. This is especially problematic when attempting to coat the steel using a single firing. Such defects are often caused by what is known as the primary boiling effect which can be observed during firing of porcelain enamel coatings. The primary boiling effect is caused by gaseous products of hydration and combustion of the carbon in the high-carbon steel sheet. These gases consist of carbon monoxide, carbon dioxide, hydrocarbons and traces of nitrogen. The boiling gases cause defects in the finished enamel coating, such as blisters, bubbles, voids, pinholes, boil-outs, copperheads and other defects. Such defects adversely affect the strength, weather resistance, corrosion resistance and appearance of the coating.
Additionally, such defects on the surface of the enamel coating may cause spalling due to freezing and thawing action of water which enters into the defects at the enamel surface. This initial spall can then open up more bubbles or pores and through subsequent freeze/thaw cycles produces more spalling.
It is believed that during the firing of the glass directly to the steel at elevated temperatures, water or moisture from a number of sources reacts with the steel to form hydrogen gas. During the high-temperature firing process of the porcelain enamel, the hydrogen gas formed by this reaction penetrates into the steel. After firing, and as the enamel coated steel is cooled, the hydrogen, being less soluble in the cooler steel, is discharged from the steel and builds up a pressure beneath the solidified enamel coating. This is especially problematic in cases where all of the surfaces of the steel substrate are coated with the enamel. The result is that the coating pops off in small flecks known as “fish scales” due to the pressure of the trapped gas. Defects which are caused by the trapped hydrogen are known as hydrogen defects.
U.S. Pat. No. 2,940,865 issued to Sullivan discloses the use of a thin layer of nickelous oxide coated on the steel substrate prior to enameling with a single layer of enamel. The stated function of the nickelous oxide layer is to reduce hydrogen defects and increase the adherence of a single thick layer of porcelain enamel coating to a steel base. The '865 patent discloses it is believed that the thin coating of nickelous oxide reacts with the water during the firing process away from the steel surface. Atomic hydrogen that is produced by the reaction, rather than penetrating into the steel, will combine to form molecular hydrogen which will not penetrate into the steel. It is also believed that the nickelous oxide layer is porous and is believed to resist the progress of the atomic hydrogen from penetrating into the steel so that the atomic hydrogen will combine into molecular hydrogen rather than penetrate into the steel.
However, it has been found that, with the layer of nickelous oxide as disclosed in the '865 patent, boiling defects, especially pinhole defects, may still occur in the single layer of porcelain enamel coating. Discontinuities, such as pinholes, in the porcelain enamel coating can expose the steel substrate, thereby subjecting it to corrosion. Accordingly, if discontinuity defects are minimized, then coating performance improves.
It would be desirable to provide for a porcelain enamel coating that would allow for the coating of higher carbon steel and reduce the defects in a porcelain enamel coating that are caused by the boiling of gases and trapped hydrogen pressure upon the porcelain enamel coating, as well as provide for desirable coating characteristics. It would also be desirable to provide such a coating which could be prepared in a single firing.
SUMMARY OF THE INVENTION
The current invention provides a multi-layered, functionally-gradient porcelain enamel coating which can be used to coat steel, including higher carbon, non-enamel-grade steel, that controls hydrogen defects and boiling defects in the finished coating. This invention permits coating all surfaces of a high-carbon steel sheet with minimal pinhole defects. The coating of the invention may also be prepared in a single firing.
One object and feature of the invention is to provide a multi-layer porcelain enamel coating including a ground coat having nickelous oxide mixed substantially uniformly therein to provide for resistance to hydrogen defects.
Another object and feature of the invention is to provide a porcelain enamel coating for coating higher carbon steel, the coating having increased resistance to defects caused by boiling of gases during the firing process.
Another object and feature of the invention is the use of a nickelous oxide layer underneath a multi-layered, functionally-gradient porcelain enamel coating having a soft ground coat and a hard cover coat.
Another object and feature of the invention is to provide a multi-layered, functionally-gradient porcelain enamel coating having a highly viscous cover coat such that during firing, the cover coat allows for smaller and fewer bubbles within the cover coat and thereby reduces the number of defects in the cover coat that may cause spalling due to freeze-thaw cycles. This permits use of the coating in applications such as water towers which may be subjected to freezing conditions.
One embodiment of the invention provides a multi-layered porcelain enamel coating composition for coating a steel substrate. The coating composition includes a ground coat layer of porcelain enamel for coating directly onto the steel, the ground coat layer including a soft ground coat frit and nickelous oxide separate from the frit dispersed substantially uniformly throughout the ground coat. The coating also includes a cover coat layer of porcelain enamel for coating over the ground coat layer, the cover coat layer including a hard frit.
In another embodiment of the invention, the coating composition is substantially the same as discussed above, but the nickelous oxide, rather than being mixed with the ground coat layer, is coated directly onto the steel substrate. The ground coat is then coated over the nickelous oxide layer, and the cover coat is coated over the ground coat. The ground coat and cover coat are fired together.
In other embodiments, a third layer, which is more dense than the cover layer, may be used to provide for further spall resistance by reducing the size and number of bubbles and other defects at the surface of the multi-layered enamel coating.


REFERENCES:
patent: 2755210 (1956-07-01), Sutphen et al.
patent: 2940865 (1960-06-01), Sullivan
patent: 3011906 (1961-12-01), Davis et al.
patent: 3765931 (1973-10-01), Kyri et al.
patent: 3956536 (1976-05-01), Schoenemann et al.
patent: 4012239 (1977-03-01), Brun et al.
patent: 4064311 (1977-12-01), McLean et al.
patent: 4250215 (1981-02-01), Mayer
patent: 4460630 (1984-07-01), Nishino et al.
patent: 5266357 (1993-11-01), Preuss et al.
patent: 5296415 (1994-03-01), Podesta
patent: 5516586 (1996-05-01), Singer et al.
patent: 5547768 (1996-08-01), Topolski et al.
TI Vitreous Enamels Ltd and University of Leeds, “Vitreous Enamelling, A Guide to Modern Enamelling Practice”, pp. 20-27, 36, 50-59—published sufficiently before filing date such that the month is not an issue.

Affiliated with

Also associated with

Rating

Porcelain enamel coating for high-carbon steel does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Porcelain enamel coating for high-carbon steel, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Porcelain enamel coating for high-carbon steel will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2481438