CODEPOSITED CHROMIUM AND SILICON DIFFUSION COATINGS FOR FE-BASE ALLOYS VIA PACK CEMENTATION

The simultaneous deposition of Cr and Si into plain carbon, low-alloy, and austenitic steels using a halide-activated pack-cementation proce ss is described. Equilibrium partial pressures of gaseous species have been calculated using the STEPSOL computer program to aid in designin g specific processes for codepositing the desired ratios of Cr and Si into a given alloy. The calculations indicate that NaCl-activated pack s are chr omizing, while NaF-activated packs deposit more Si with less Cr. The use of a ''dual activator'' (e.g., NaF+ NaCl) allows for the deposition of both Cr and Si in the desired amounts. Single-phase ferr itic coatings (150-250 microns thick) with a surface concentration of 20-35 wt.% Cr and 2-4% Si have been grown on AISI 1018, Fe-2.25Cr-1.0M o-0.15C, and Fe-0.5Cr-0.5Mo-0.2C steels using packs containing a 90 wt .% Cr-10Si binary source alloy, a NaF+NaCl activator, and a silica fil ler. Two-phase coatings (approximately 75 microns thick) containing 20 -25 wt.% Cr and 2.0-2.4% Si have been obtained on 304 stainless steel using packs containing a 90 wt.% Cr-10Si binary source alloy, a NaF ac tivator, and an alumina filler. The same pack chemistry allowed the di ffusion of Cr and Si into the austenitic Incoloy 800 alloy without a p hase change. A coated Fe-2.25Cr-1.0Mo-0.15C coupon with a surface conc entration of Fe-34 wt.% Cr-3Si was cyclically oxidized in air at 700 d egrees C for over four months and 47 cycles. The weight gain was very low (<0.2 mg/cm(2)) with no scale spalling detected.; Coated coupons o f AISI 1018 steel, and Fe-0.5Cr-0.5Mo-0.2C steel have shown excellent oxidation-sulfidation resistance in reducing, sulfur-containing atmosp heres at temperatures from 400 to 700 degrees C and in erosion and ero sion-oxidation testing in air at 650 and 850 degrees C.