Carbonaceous articles having oxidation prohibitive coatings thereon

A method of protecting a carbonaceous article against oxidation and erosion at elevated temperatures, which comprises:

• (a) applying to the surfaces of said carbonaceous article a first particulate mixture comprising from about 30 to about 75 percent by weight mullite, 0 to about 12 percent by weight silica, from about 15 to about 60 percent by weight aluminum and from about 5 to about 11 percent by weight of a silicon-containing binder material, all percentages being based upon the total weight of the dry ingredients;
• (b) applying to said first particulate mixture a second particulate mixture comprising 0 to about 70 percent by weight mullite, 0 to about 14 percent by weight silica, from about 5 to about 95 percent by weight of a metallic carbide selected from the group consisting of silicon carbide and boron carbide, and from about 5 to about 15 percent by weight of a silicon-containing binder material, all percentages being based upon the total weight of the dry ingredients;
• (c) heating said first particulate mixture either before or after step (b) to a temperature at which silica is reduced by aluminum to produce aluminum oxide and silicon;
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• (d) heating said second particulate mixture either simultaneously with or after step (c) to a temperature at which a rigid, glassy cover layer is formed.

A method of protecting a carbonaceous article against oxidation and erosion at elevated temperatures, which comprises:

• (a) applying to the surfaces of said carbonaceous article a first particulate mixture comprising from about 30 to about 75 percent by weight mullite, 0 to about 12 percent by weight silica, from about 15 to about 60 percent by weight aluminum and from about 5 to about 11 percent by weight of a silicon-containing binder material, all percentages being based upon the total weight of the dry ingredients;
• (b) applying to said first particulate mixture a second particulate mixture comprising from about 25 to about 85 percent by weight borosilicate frit, from about 5 to about 15 percent by weight silicon-containing binder material and 0 to about 65 percent by weight filler material, all percentages being based upon the total weight of the dry ingredients;
• (c) heating said first particulate mixture either before or after step (b) to a temperature at which silica is reduced by aluminum to produce aluminum oxide and silicon;
• and
• (d) heating said second particulate mixture either simultaneously with or after step (c) to a temperature at which a rigid, glassy cover layer is formed.

In the production of steel within an electric arc furnace wherein an electric current is passed through a graphite electrode to establish an electric arc between the tip of said electrode and the molten metal, a process for protecting said electrode against oxidation and erosion at the operating temperatures of said furnace, which comprises:

• (a) applying to said graphite electrode a protective coating comprising a base layer overlying and bonded to said electrode, said base layer being composed of a compliant, low strength, porous, sintered, particulate, ceramic refractory material containing mullite as a major ingredient, said material having a shear strength which is substantially less than the shear strength of the interfacial bond between said base layer and said graphite electrode and a cover layer overlying and bonded to said base layer comprising a rigid, glassy substance which is plastic and flowable at said operating furnace temperatures, said rigid, glassy cover layer having a co-efficient of thermal expansion which is substantially greater than that of said graphite electrode;
• (b) heating said furnace and said coated electrode to said operating furnace temperatures;
• (c) cooling said coated electrode to a temperature which is substantially lower than said operating furnace temperatures whereby said base and cover layers contract at a greater rate than said graphite electrode and produce in said layers a high tensile stress causing microcracks which extend in a direction toward the interface between said base layer and said graphite electrode;
• and
• (d) reheating the coated electrode to said operating furnace temperatures whereby said rigid, glassy cover layer is rendered plastic and flowable and seals off said microcracks prohibiting ingress of oxygen toward said graphite electrode.