Biomass is ubiquitous within the Nation and embodies tremendous potential as a renewable energy resource. Virtually all Citizens have access to at least modest quantities of biomass—many counties in the Nation have access to over 100,000 tonnes of local solid biomass resources[1]. One challenge of biomass as a combustion fuel is that, depending on the source, its chemical composition, moisture content, etc. (and thus its combustion behavior) can vary considerably[2]. This is in contrast to fossil fuels such as gasoline, kerosene, diesel, and natural gas which generally have been processed into highly refined hydrocarbon mixtures. Many of these impurities in biomass pose significant corrosive challenges to the types of apparatus (Figure 1) commonly used to carry out the combustion operations, which for cost reasons are often constructed of mild or low-alloy steels. Relevant corrosive species include alkali halides (borne, e.g., by fly ash particulates), mineral/halogen acids and water; as well as various others such as sulfur and nitrogen oxides. While upgrading the combustor components to alloys such as stainless steels or superalloys of nickel, cobalt, etc. would substantially eliminate many corrosion problems; such a change in materials of construction would in general be cost-prohibitive. There is a need to develop corrosion-resistant alloy coatings onto stainless steel base materials with the goal of increasing the functional lifetime of biomass combustion apparatus, while reducing the component cost.

Within this context, the objective of the present work was to develop a cost effective, scalable, and flexible electrodeposition based coating process that can be used to apply cost effective corrosion resistant coatings to low cost stainless steel substrates for enhanced corrosion resistance in sensitive regions of existing and next generation bio-combustors. The manufacturing process involves using electrodeposition for application of binary and/or ternary alloys consisting of [Ni/Co]-Cr-[Mo/Fe] onto a low cost substrate surface, and subsequently accelerated high temperature corrosion evaluation.

Therefore, in this study various electroplated coatings, from [Ni/Co]-Cr-[Mo/Fe] systems, were tested to assess their potential to enhance lifetime performance of low-cost stainless steels exposed to bio-combustor operating conditions. A wide range of electrodeposition processing conditions and electrolytes were explored in order to understand these effects on the deposit composition, structure and high-temperature corrosion resistance properties. Among those evaluated, NiCr coatings with Ni and Cr contents of approximately 60 and 40 wt% exhibited superior corrosion resistance when exposed to an aggressive high temperature corrosive treatment (~700°C, 500 hr, coating surface salted with ~3 mg/cm² every 100 hours). Under this environment, NiCr coating on 304 SS exhibited a 70% lower weight loss than 304 substrate exposed to an identical treatment. This leads to a potential lifetime improvement of up to 3.4 times that of its base material while producing a lower cost alternative to using bulk corrosion resistance materials.

Acknowledgements: The financial support of DOE Contract No. DE-SC0013870 is acknowledged.