All ETDs from UAB

Advisory Committee Chair

Manoj K Mahapatra

Advisory Committee Members

Shane A Catledge

Robin D Foley

Haibin Ning

Ruigang Wang

Document Type

Dissertation

Date of Award

2020

Degree Name by School

Doctor of Philosophy (PhD) School of Engineering

Abstract

The solid oxide fuel cell (SOFC) electrochemically converts chemical energy into electrical energy. Interconnects are used to provide electrical connection to various individual cells in a stack for drawing useful electrical energy in the kW-MW range. By lowering the operation temperature to 800oC, the chromia-forming ferritic alloy interconnects become applicable. Their corrosion resistant chromia layer at the surface can form gaseous chromium oxides that lower the SOFC output by the cathode poisoning phenomena. Application of protective coatings can mitigate oxidation and form a resistant layer over the chromia. In this study, the electroless and electrolytic deposition methods were used to obtain uniform deposition despite substrate geometry. The nickel coating was selected based on its satisfactory electrical conductivity and coefficient of thermal expansion as NiO as well as its status as a model material for electrolytic and electroless deposition. While the electrolytic deposition forms a pure, crystalline nickel coating, the electroless forms an amorphous nickel-phosphorus coating. As both coatings have been used commercially for corrosion protection, it is believed that their protection ability can be improved by pre-oxidation and pre-reduction modifications. Modifications for improved oxidation resistance have been studied in literature reports. However, changes in the oxide scale development were not reported. The oxidation behavior of the coated and modified coated ferritic alloys in the SOFC anode and cathode environments need to be studied. This research focuses on investigating the high temperature oxidation behavior differences of the nickel and nickel-phosphorus coatings with respect to coating modification at 800oC. X-ray diffractometry (XRD), scanning electron microscopy (SEM), and energy dispersive x-ray spectroscopy (EDS) were used for the coating and morphology study. The coating parameters were optimized to form a uniform ~5µm thick coating. NaCl, 1-propanol, or ethanol can be added to the electrolyte bath to deposit a Ni coating with cone/pyramid morphology. SDS can be added to deposit a nickel coating with a clustered morphology and pyramidal particles. But the additives especially NaCl were found to lower the adhesive strength of the Ni coating on the AISI 430. This study also considered the thermal decomposition of Ni salts. Thermogravimetric analysis (TGA) and XRD showed the ligands bonded to Ni were removed at ~800oC for all to Ni salts. The uncoated, Ni-coated, and Ni-P-coated AISI 430 were studied in SOFC environmental conditions for their oxidation behavior. Ar-3%H2 and Ar-3%H2O atmospheres simulated the SOFC anode environment while air-3%H2O simulated the SOFC cathode environment. XRD, SEM, and EDS were used to identify the oxide scale behavior. The oxidation rates of the samples increased from Ar-3%H2 < Ar-3%H2-3%H2O < air-3%H2O. The oxide layers became more uniform and homogeneous as the atmosphere became more oxidizing. The Ni-coated AISI 430 showed the lowest weight change of the three for all experiments indicating that Ni-coated AISI 430 is effective at oxidation resistance. The Ni-coated AISI 430 formed an Ni-Fe intermetallic outermost layer in reducing atmospheres and NiO outermost layer in oxidizing atmospheres. The Ni-P coated AISI 430 formed Fe3(PO4)2 outermost layer in Ar-3%H2 and FexOy outermost layer in humid and oxidizing atmospheres. In all samples, a chromia layer formed between the AISI 430 and outer layer. The study investigated the pre-oxidation of AISI 430 (Ni-coated P-AISI 430, P-sulfamate) and pre-reduction of coated AISI 430 (R-sulfamate and R-electroless) modifications for their changes on oxidation behavior in an SOFC cathode simulated environment. The modifications improve the Ni-coated and Ni-P coated AISI 430 samples oxidation resistance. The R-sulfamate showed the overall lowest oxidation rate. The P-sulfamate formed an NiO outermost layer with remaining Ni as an intermediate layer. The initial chromia layer formed from pre-oxidation was shown to mitigate elemental diffusion between the Ni coating and the AISI 430. The R-sulfamate formed an FexOy outermost layer. The initial chromia layer and Fe0.5Ni0.5 outermost layer formed from pre-reduction mitigated elemental diffusion. Oxidation studies were performed at high temperatures in different atmospheres.

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