US 2697670 A
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Dec. 21, 1954 A GAUDENZ] ETAL 2,697,670
CERAMIC COATED CHROMIUM STEEL Filed July 20, 1955 INV ENTORS ATTORNEY5 UnitedStates Patent O CERAMIC COATED CHROMIUM STEEL Arthur Gaudenzi, Wettingen, and Hannes Giger, Baden, Switzerland, assignors to Aktiengesellschaft Brown, Boveri & Cie., Baden, Switzerland Application July 20, 1953, Serial No. 369,188
Claims priority, application Switzerland `luly 28, 1952 11 Claims. (Cl. 117-70) This invention relates to ceramic-coated chromiumcontaining metallic objects and more particularly to structural parts such as turbine blades which are subject to thermal and mechanical stresses and to the corrosive action of hot gases containing corrosive materials such as vanadium compounds.
It is well known to coat thermally stressed metallic structural elements such as the exhaust pipes of internal combustion engines used on aircraft and the blades of gas turbines with protective layers of ceramic material. Such layers serve not only as heat insulation but also to protect the coated surface against corrosion and erosion by hot gases.
The use of fuel oil for the operation of gas turbines has given rise to the problem of protecting the turbine parts, particularly the blades against the attack of the corrosive slag formed when the oil is burned and particularly slag containing vanadium compounds. Vanadium compounds, when encountered under such conditions, prevent the formation of a protective oxide layer on steels and destroy existing oxide layers so that a progressive oxidation and erosion of steel structural parts such as combustion chamber walls and gas turbine blades which are contacted with combustion gases formed from vanadium containing fuel oils occurs.
It is known that magnesium oxide is strongly resistant to the attack of corrosive slags such as the vanadium containing slag derived from fuel oil. Chromium metal also provides a strongly resistant coating. But such protective coatings must adhere strongly to the parts to be protected and this is a problem in the case of the highly heat resistant chromium containing steels.
An object of the present invention therefore is to provide a strongly adherent and corrosion and erosion resistant coating for chromium containing steel objects and particularly a coating which will withstand the action of vanadium containing slags such as those produced by the combustion of fuel oil.
We have solved this problem in the following manner. First we apply a coating of a chromium oxide containing enamel. Such a coating is not particularly resistant to erosion but it has the merit of strong adhesion to the chromium steel surface and the further merit of strong adhesion to a superposed coating or coatings which are resistant to erosion. We then apply successive coatings of magnesia containing enamels, each successive coating having a larger content of magnesia until the desired thickness of coating and the desired magnesia content is attained. And finally we may apply a layer of chromium metal which is strongly adherent to the magnesia-containing layer. The resulting composite protective coating either with or without the chromium layer has a high heat conductivity and good stability under fluctuating temperature.
By chromium containing steel we mean alloys of chromium with iron containing about 14-30% of chromium and not more than about 0.5% of impurities such as carbon, silicon, manganese, sulfur, phosphorus etc.
The initial chromium oxide enamel layer should have a heat expansion which is less than that of the chromium steel. For example for use on a chromium steel containing to 30% of chromium and having a coefficient of expansion a=9.5-l2 l06 the enamel may have a coeicient of expansion a=8l0 l05. Thus after the enamel has been burned onto the steel surface and the product is cooled the steel shrinks more than the enamel and the latter is put under pressure and is much less likely to break than if it were put under tension. The difference should not however be too large as in the case of chromium-nickel steel (18% Cr and 8% Ni) which has a coecient of expansion a=18 l06. The chromium oxide enamel is burned onto the surface of the steel and the superposed layers of magnesia containing enamel are each burned onto the preceding layer.
The invention is illustrated in the accompanying drawings in which:
Fig. l is a diagrammatic section of a protective coating built up entirely of ceramic materials and Fig. 2 is a diagrammatic section of a coating comprising a metallic chromium top layer.
The coating of Fig. l is formed as follows: A coating layer 2 of chromium oxide enamel composition is applied to the surface of the body 1 of chromium steel having a chromium content of about 21%. This coating should have a heat expansion which is somewhat less than that of the body 1. A suitable chromium oxide enamel composition is the following, the parts being by weight. 4 parts of borax, 28 parts of feldspar, 23 parts of quartz, 5 parts of iiuorite, 15 parts of cryolite, 5 parts of soda and 4 parts of chromium oxide. The mixture is melted, cooled, pulverized, mixed with water sufcient to form a liquid coating, applied to the steel surface in any suitable manner as by brushing, spraying or dipping, dried and burned or baked at 850 C. to 900 C. for a few minutes. After cooling a layer 3 of the same enamel composition to which 10% to 20% by weight, based on the weight of the solid components, of magnesium oxide powder has been added is applied, dried and burned in the same way as the layer 2. Then layer 4 is applied, dried and burned,
said layer consisting of the original enamel composition to which 25 to 35% by Weight of magnesium oxide powder has been added. And finally layer o is applied dr1ed and A burned, said layer consisting ofthe original enamel composition with the addition of 40-60% by weight of magnesium oxide.
The layers of high magnesium oxide content provide excellent protection of the steel surface against vanadium containing slag. The layers adhere well to each other and the base layer 2 adheres well to the steel so that the entire multi-layer coating does not peel or crack even when subjected to sharp temperature changes. Obviously any desired number of layers may be applied and the magnesium oxide content may be increased to the desired extent by increasing the additions of magnesium oxide to the composition of the rst layer. The coating generally is built up to a thickness of at least l mm. and the thickness of the layers and the number thereof may be varied accordingly. It is desirable to increase the magnesia content of the layers by relatively small increments as illustrated but this is not essential as will appear from the following example.
Referring to Fig. 2, a layer 2 of the chromium oxide enamel composition described in connection with Fig. 1 is first applied to the chromium steel base 1 and baked. Then a layer 3 of said enamel to which preferably 40 to 60% by weight of magnesium oxide has been added is applied and baked. In effect layers 3 and 4 of Fig. l are omitted. Then the chromium layer 6 is applied by sprinkling metallic chromium in `powdered form onto the layer 3 while it is at a bright red heat. This results in the chromium being incorporated into the surface of and bonded to the ceramic layer. The resulting coating is particularly advantageous for use in turbine blades.
The above described coating formed by sprinkling chromium powder upon the hot ceramic layer of ceramic material may be reinforced by electroplating it with chromium. The chromium covering layer may be polished leaving the sharp edges in the case of turbine blades, intact. Due to the heat insulating action of the ceramic layer the temperature attained by the metal base of a turbine blade provided with a ceramic coating will be somewhat lower than an unprotected blade subjected to the same gas temperature.
The ceramic protective coating described above practically eliminates the attack of vanadium containing slags such as are derived from fuel oil. This greatly lengthens the life of combustion chamber walls and other structural parts which are exposed to such slags.
The invention is not limited to the particular enamel 'compositions vdescribed above. The base for vthe chromium oxide containing enamel may be any good enamel suitable for use on unalloyed steel. Many such enamels are well -known. Examples of such enamels Aare the following, the parts being :by weight.
Parts Parts (a) Borax 43.8 Sodium nitrate 2.0 Feldspar 27.7 Potassium oxide 0.3 Quartz 27.8 Manganese oxide 0.4 Fluorspar 2.9 Nickel-oxide 1.0 ASoda 8.8 Clay 11.6 (b') Borax 24.2 Cryolite 17.0 Feldspar .28.5 Potassium nitrate 5.0 Quartz v28.4 oda 5.7 4l-iluorspar `5. Kaolin 2.1
The chromium oxide content ofthe enamel may vary within the range from 212%.
1. An article comprising a metallic object formed of an alloy consisting essentially of iron and from about 14% to about 30% of chromium, anda ceramic'coating on said object said coating comprising a first layer in contact with said object of ceramic enamel substantially free of magnesium oxide and containing from about 2% to `about 12% of chromium oxide land at least one layer .of ceramic enamel superposed upon said -first layer, said superposed layer containing at least about of magnesium oxide.
2. An article as defined in claim 1 in which the superposed layer of ceramic enamel consists of a mixture of ctlhe composition of the first layer with magnesium oxi e.
3. .An article as defined in claim l in which the first layer contains about 4 parts of chromium oxide to 100 parts of other solids.
-4. An-article as defined in claim l in which the first layer is the product formed byheating to about850900 C. a mixture of finely divided borax, -feldspan quartz, fiuorite, cryolite, soda and chromium oxide.
5. An article as defined in claim l in which the first 'layer has a coefficient of expansion less than that of the metallic object. l
6. An article as defined in claim 1 in which the coating has a top layer of metallic chromium.
7. An article as defined in claim 1 in which the first layer is burned onto the surface of the metallic object and the coating comprises a plurality of superposed ceramic layers, each successive superposed layer containing more magnesium oxide than the layer below it and being burned onto the layer below it, the topmost of said ceramic layers containing at least about 30% by weight of magnesium oxide.
8. An article as defined in claim 7 comprising a layer of metallic 4chromium superposed upon the topmost ceramic layer.
9. An article as defined in claim 8 in which said chromium layer comprises finely divided chromium fused -intothe surface of the ceramic layer.
10. An article as defined in claim 8 in which said lchromium `layer is an `electro deposit vof fchromium.
111. An article comprising a metallic object yformed'of an alloy consisting essentially of iron and from about 10% to about 30% chromium, and a ceramic coating on said object said coating comprising a first layer in contact with said object of ceramic enamel substantially free of magnesium oxide and containing from about 2% to about 12% -ofchromium oxideand at least-one layer vof ceramic enamel superposed upon said first layer, said superposed 'layer containing at least about 10% of Amagnesium oxide.
ReferencesCited in the file of this patent Moore et al., National Advisory Committee for Aeronautics, Technical Note 2380, pp. 1-14 (1951). (Copy in Science Library.)
Harrison, American Society for testing Materials, Symposium on Corrosion of Materials at Elevated Temperatures, Special Technical Publication No. 108 (1950), pp. 114-121. (Copy in Science library.)