Kai-Hung Lau

Corrosion-resistant metallic coatings for applications in highly aggressive environments

Surface modification to improve the corrosion resistance of low-cost alloys is an economically attractive alternative to the use of expensive corrosion-resistant alloys. A low-grade steel surface modified by a suitable metallic diffusion coating can provide excellent corrosion resistance similar to that obtained with an expensive super alloy. We have used the fluidized bed reactor chemical vapor deposition (FBR-CVD) technique for the preparation of diffusion coatings containing Cr, Ni, Si and Ti on carbon and low-grade stainless steels. Several formulations of diffusion coatings on 409 SS have shown corrosion resistance similar to that of alloys AL6XN® or AL29-4C® in chemical heat pump applications. In this paper, we describe the preparation, characterization, and performance evaluation of a number of corrosion-resistant metallic diffusion coatings.

Corrosion-resistant metallic coatings on low carbon steel

Abstract Corrosion resistant coatings of various metals and alloys such as Si, Ti, Ni, and TiNi were formed on steel rebars by fluidized bed chemical vapor deposition (FBR-CVD), paint-and-heat or FBR-plasma spray techniques. The paint-and-heat metallization and FBR-plasma spray are powder coating techniques that can be applied easily and economically on new components as well as on existing steel structures, such as bridges. These metallic powder coatings provide non-sacrificial, superior corrosion protection for a long time. TiNi (70:30 wt. %) coatings on steel rebars provided a 20 fold increase in corrosion resistance over uncoated steel. These metallic coatings can be used to prevent corrosion in many industrial applications.

Diffusion Coatings for Corrosion Resistant Components in Coal Gasification Systems

Heat-exchangers, particle filters, turbines, and other components in integrated coal gasification combined cycle system must withstand the highly sulfiding conditions of the high temperature coal gas over an extended period of time. The performance of components degrades significantly with time unless expensive high alloy materials are used. Deposition of a suitable coating on a low-cost alloy may improve its resistance to such sulfidation attack, and decrease capital and operating costs. The alloys used in the gasifier service include austenitic and ferritic stainless steels, nickel-chromium-iron alloys, and expensive nickel-cobalt alloys. The primary activity this period was preparation and presentation of the findings on this project at the Twenty-Third annual Pittsburgh Coal Conference. Dr. Malhotra attended this conference and presented a paper. A copy of his presentation constitutes this quarterly report.

CVD coatings from the Al-B-Si system on carbon

Abstract Borides of aluminum and silicon have previously shown superior resistance to oxidation at high temperature and other useful properties. In this paper, carbon substrates were coated using chemical vapor deposition (CVD) from the Al-B-Si-H-Cl-Ar system in a cold-wall reactor. B-Si compounds such as SiB4 ± x and SiB6 were deposited as thick coatings by varying the B/Si molar ratio in the gas phase and the deposition temperature. A transition from nodular to faceted grain growth was observed between 1150 °C and 1250 °C. Al-B coatings were also deposited, but consisted of a boron-rich phase with faceted grains irrespective of the B/Al ratios or deposition temperatures used in this investigation. Although aluminum acted as a growth modifier to improve the quality and increase the size of grains, Al-B and B-Si compounds could not be deposited simultaneously from the gas phase and no ternary compounds from the Al-B-Si system were observed.

DIFFUSION COATINGS FOR CORROSION RESISTANT COMPONENTS IN COAL GASIFICATION SYSTEMS

Heat-exchangers, particle filters, turbines, and other components in integrated coal gasification combined cycle system must withstand the highly sulfiding conditions of the high temperature coal gas over an extended period of time. The performance of components degrades significantly with time unless expensive high alloy materials are used. Deposition of a suitable coating on a low cost alloy may improve is resistance to such sulfidation attack and decrease capital and operating costs. The alloys used in the gasifier service include austenitic and ferritic stainless steels, nickel-chromium-iron alloys, and expensive nickel-cobalt alloys. During this reporting period we conducted two exposure tests with coated and uncoated coupons. The first one was aborted after a short period, because of a leak in the pressure regulator of a CO/CO{sub 2}/H{sub 2} gas mixture gas cylinder that was used to prepare the simulated coal gas stream. Nevertheless, this run was very instructive as it showed that during the brief exposure when the concentration of H{sub 2}S increased to 8.6%, even specialty alloys such as HR160 and I800 were badly corroded, yet the sample of a SS405-steel that was coated with Ti/Ta showed no signs of corrosion. After replacing the pressure regulator, a second run was conducted with a fresh set of coated and uncoated samples. The Ti/Ta-coated on to SS405 steel from the earlier runs was also exposed in this test. The run proceeded smoothly, and at the end of test the uncoated steels were badly damaged, some evidence of corrosion was found on coupons of HR160 and I800 alloys and the Cr-coated steels, but again, the Ti/Ta-coated sample appeared unaffected.