Corrosion-Resistant Crystalline-Oxide Coatings on Steel Substrates by An Immersion Anodizing Process

Engineered biomass combustors offer significant reductions in harmful emissions, reduced cooking-times and fuel usage. However, steel components used in the combustors suffer from corrosion degradation due to the aggressive halide salts and water vapor released during biomass fuel combustion. The Department of Energy desires the development of low-cost survivable components that extend the lifetime of critical components that will encourage the adoption of sustainable biomass feedstocks. A unique cyclic high-voltage anodizing process is proposed to convert liquid aluminum-nitrate and zirconium-fluoride electrolytes into crystalline aluminum- oxide and zirconium-oxide coatings on low-cost steel substrates. The thick crystalline oxide has a dense inner zone (nearest the substrate) which prevents corrodent penetration to the steel and encapsulated interior porosity that increases coating toughness/durability. In addition to the enhanced corrosion-resistance and coating toughness, the liquid-immersion process completely envelopes the component, eliminating line-of-sight limitations common for other coating techniques. Thus complex shaped parts, like combustion chambers and exhaust piping, can be coated on the inside surfaces. Furthermore, the process is scalable (power supply and tankage) and only requires 3 stages as opposed to 9 stages for conventional anodizing. In Phase I, crystalline-oxide coatings will be fabricated on steel substrates for coating characterization and testing, which will include adhesion/toughness measurements, coating porosity and microstructure examinations, and exposure to high-temperature (>600C) cookstove environments (continuous and cycling operations). This technology has many applications in several diverse industries, including: 1) pump parts for oil/gas hydraulic-fracturing, 2) erosion- resistant surfaces for air ventilation fan blades in mines, turbine engine blades, aircraft landing gears, 3) abrasion resistance of pipe inside surfaces that carry slurry solutions, 4) abrasion- resistance of recreational vehicle parts, and 5) components of chemical-process combustors. Further development and scaling of this technology in Phase II will enable the cost-effective fabrication of combustor components that offer longer lifetimes and higher durability than current materials used for these components. As a result, cookstoves will be manufactured that last longer and are lower cost than currently offered products. Furthermore, the reduced emissions from these cookstoves will reduce health risks to the operators.