Graduate Thesis Or Dissertation

Surface Modification to Enhance Corrosion Resistance of Carbon Steels using Additive Manufacturing

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  • Low carbon steels (LCS) due to their high strength-to-cost ratio are one of the most ubiquitous materials used for a wide range of applications, including but not limited to automotive, construction, and transportation. However, the low corrosion resistance of LCS in neutral, acidic, or saline environments limits its utilization and service life. Corrosion resistant alloys, such as stainless steels (316-L and UNS32750 super duplex stainless steels), provide superior corrosion resistance than LCS for such applications, albeit with a significantly higher material cost. The surface modification cladding is one of the viable methods to manufacture a composite with wear and corrosion resistant surface layer on a low-cost substrate with desired mechanical and corrosion properties, as in the case of stainless steel clad on LCS substrate. Various traditional manufacturing techniques such as welding, hot rolling, powder roll bonding, cold/thermal spraying have been used to produce the said clads, however, with limited success. Failure at the clad-substrate interface has been the drawback of the cladded composites produced with the said traditional processes. Recent developments in additive manufacturing (AM) technologies make them an excellent candidate to produce cladded systems with desired properties. Traditionally, the AM techniques have employed to produce three-dimensional (3D) components. However, one of the AM techniques, laser powder bed fusion (LPBF), is a promising method for cladding operations because of its higher resolution and dimensional accuracy than the other AM technologies. Furthermore, the LPBF technique results in lower surface roughness of the components produced with higher material savings per unit volume of print. Therefore, this research makes novel use of the LPBF technique for 2D cladding applications to enhance the corrosion resistance of LCS. This research aims to improve the corrosion resistance of the LCS by cladding it with 316L SS and super duplex stainless steel (SDSS) using the LPBF process. Critical process parameters such as laser power, laser scan speed, hatch spacing, and powder layer thickness were optimized to achieve the best possible metallurgical bonding between the clad and the substrate. Due to high local melt pool temperatures during laser melting, the evaporative losses of the elements resulted in clad layers with lower Cr, Ni content as compared to the feedstock powder. The LPBF process, due to the high cooling rates, is associated with high residual and thermal stresses and as printed parts are characterized by defect density and non-equilibrium microstructures; consequently, additively manufactured clads were subjected to post-printing heat treatment procedures for stress relief and to restore metallurgical, mechanical properties of the as printed clads. The metallurgical and corrosion response of the clads before and after heat treatments were compared. For super duplex stainless steel clads, the as printed (AP) clads showed predominantly δ-ferrite matrix, with allotriomorphic austenite precipitating at the ferrite grain boundaries. Increasing laser scan speeds resulted in decreasing austenite phase fraction, with dominant widmannstatten morphology at higher scan speeds. The heat treatment restored the δ-γ phase balance, thereby increasing the corrosion resistance of the heat-treated (HT) clads as compared to AP clads. Increasing scan speed had a negative impact on the corrosion resistance, and the pitting potential of the AP and HT clads exposed to 3.5 wt. % NaCl aqueous solution. In general, increasing laser scan speeds resulted in decreasing corrosion resistance for the AP and HT clads, as indicated by OCP, EIS, and LPR and CP results. Clads produced at the lowest scan speeds showed comparable corrosion resistance to the as-cast or wrought 316L/SDSS counterparts. Subjecting the cladded composite to tensile stresses (yield and ultimate tensile stress) increased the corrosion rates; however, these stress effects were eliminated by post stressing heat treatments. The SDSS clads showed a superior metallurgical bonding with the LCS substrate did not delaminate even at failure strains at all laser scan speeds. Furthermore, the passivation behavior and the critical chloride thresholds of the additively manufactured super duplex stainless steel clads (SDSS) on carbon steels were also studied in simulated concrete pore solution. The effects of LPBF laser scan speed on the early passivation, full passivation, and critical chloride threshold of the SDSS clads were investigated. Increasing δ-ferrite phase fraction with scan speeds resulted in fast film formation kinetics for early passivation but showed low critical chloride thresholds for the clads. The SDSS clads produced at 100 mm/s, 600 mm/s and 1000 mm/s showed critical chloride threshold values of 4 M, 2.5 M and 1.5 M. The as-cast SDSS alloy did not show any signs of depassivation until 5M Cl- concentration, whereas LCS substrates depassivated at 0.75 M Cl-. Therefore, additively manufactured SDSS clads showed significant enhancement in chloride threshold values over LCS substrates. Finally, the galvanic coupling of the super duplex stainless steel (SDSS) clads on low carbon steel substrate was investigated in 0.1 M NaCl aqueous solution via scanning vibrating electrode technique (SVET). It was found that the clad metallurgy and the microstructural features arising from the LPBF process had a prominent effect on the clad-substrate galvanic coupling. The galvanic coupling exponentially decreased with increasing laser scan speeds. The general corrosion resistance of the clads and the galvanic coupling were found to be proportionally linked with each other; therefore, highly corrosion resistant clads would also give rise to high galvanic coupling. The post-print heat treatment resulted in increased corrosion resistance of the clads, but at the expense of increased galvanic coupling. Since the LPBF process used in this study yielded clads with a very low number of defects and highly dense structures with strong bonding with the substrate, and the clad thickness can be increased on-demand depending on the needs, the galvanic coupling issues that might arise due to possible clad defects or failures are considered to be relatively low. Regardless, more research is needed to improve the galvanic corrosion resistance of clads, especially for cases where cathode-to-anode area ratio could be rather large.
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  • Pending Publication
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  • 2020-06-12 to 2021-01-12



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