Ceramic Coatings for Nuclear Reactors A Progress Report

Abstract

Ceramic coatings were prepared from materials having low absorption coefficients for thermal neutrons. They were specifically designed for application to typical high‐temperature alloy parts for use in nuclear reactors. Best results were obtained with boron‐free coatings of the frit‐refractory type, in which a high‐barium frit containing small amounts of phosphate, beryllia, lime, zinc oxide, and titania was milled with additions of ceria or mixtures of chromic oxide and ceria. Coating compositions and test data are given.Summary and ConclusionsA total of 85 new frits and more than 200 new coatings were prepared and tested in the development of ceramic coatings for the protection of high‐temperature alloys against oxidation in air at elevated temperatures. The coatings which were designed especially for use in nuclear reactors, where only materials of low absorption coefficients for thermal neutrons can be used, may be suitable for more conventional purposes. The development work is still in progress, but the results to date may be summarized as follows:(1) Boron‐free frits have been developed and tested that have average neutron absorption coefficients in the range 0.15 to 0.50 barn, fritting temperatures of 2600°F. or below, and fiber‐deformation softening temperatures of 1450°F. or above.(2) The best frits developed to date are of the barium silicate type and contain small amounts of BeO, CaO, ZnO, and P2O5. Some of them also contain TiO2.(3) Coatings containing these frits, with admixtures of clay and various refractory oxides, have been developed and tested. The firing temperatures of the most promising of these coatings are in the range 1850° to 2250°F.(4) Of the refractory oxides used as mill additions in the coatings, best results were obtained with Cr2O3 and CeO2 and mixtures of the two.(5) The best ratio of frit to refractory oxide for all frits is 65 to 35 parts by weight.(6) Coatings were applied and adhered well to all the following alloys: types 309, 310, and 321 stainless steel, Inconel, Nichrome V, Stellite 25, and S‐816.(7) The protection given to the various coated alloy specimens varied with the composition of the alloys as well as with the composition of the coatings. Type 321 stainless steel was the most difficult to protect, and Nichrome V and Inconel were the easiest to protect. Type 310 stainless steel was relatively easy to protect.(8) The best evaluation of the degree of oxidation of a coated specimen during long‐time heating in air was obtained by microscopic comparison of metallographic sections of tested and untested coated specimens and determinations of the depth of external oxidation, maximum depth of stringer penetration, and number of stringers per inch of specimen.(9) The better coatings reduced greatly both the depth of stringer penetration and number of stringers per inch on type 310 stainless steel tested at 1900°F. for approximately 150 hours.