Development of Ni-Si-Nb APS Coatings for Sulfuric Acid Resistance
K. A. McNelis, University of Missouri-Rolla, Belle, MO; J. W. Newkirk, D. C. Van Aken, University of Missouri, Rolla, Rolla, MO
Cast alloys based on Ni-Si-Nb were developed at UMR to provide excellent resistance to boiling sulfuric acid in the range of 60 to 80 wt % H2SO4, while maintaining good mechanical properties. One of these alloys (composition in wt% 11.97% Si, 3.49 % Nb, 0.1 %B, balance Ni) was chosen as a candidate for air plasma spray (APS) coating due to its superior performance in immersion testing. Powders for plasma spraying were produced by mechanical milling the cast alloy and APS coatings were deposited on several ferrous substrates, e.g. ductile iron, 1018 carbon steel, 410 and 316 stainless steels. Coatings were evaluated based on microstructure, bond strength, porosity level, and hardness. Corrosion tests were performed in aerated 70% boiling sulfuric acid. Results were very promising and further development is planned for cyclic oxidation and sulfidation testing.
Summary: A highly sulfuric acid resistant Ni-Si-Nb alloy was successfully air plasma sprayed onto a variety of ferrous substrates. Spraying parameters and the use of bond coats were evaluated based on corrosion resistance, microstructure, bond strength, porosity level, and hardness. Immersion corrosion tests were performed in aerated 70% boiling sulfuric acid.