Microstructure and texture engineering strategies to enhance the corrosion resistance of tin based coatings
Coating corrosion-prone material with metal/alloy and passivated oxide layers is the primary protection method in the ever-evolving corrosion protection area. To improve the corrosion resistance of the coatings, the coating matrix is often blended to form a composite with nanomaterials like nanofibers, nanosheets, and nanoparticles. Sn and its alloys have been widely used as a corrosion-resistant material for coating applications in the electronic and canning industries due to their protective and non-toxic nature. This thesis discusses various strategies employed to enhance the corrosion protection efficiency of the Sn-based electrodeposited coatings. The first part of the thesis concerns with the investigation of the micro-texture changes in the Sn and SnCu alloy coatings with the change in the electrodeposition parameters (deposition temperature, current density, and alloy composition) and its implication on corrosion resistance. Increasing the electrodeposition current density from 5 mA.cm−2 to 80 mA.cm−2 changed the predominant crystallographic orientation from (100) to (110) while changing the electrodeposition temperature from 15˚C to 80˚C changed the dominant crystallographic orientation from (100) to (001). Electrodeposition at 70˚C and 20 mA.cm−2 led to the highest corrosion protection efficiency in the Sn electrodeposits, which also exhibited the highest fraction of low energy (031)[01 ̅3] twin boundaries. The second part of the thesis work discusses the morphological, microstructural, and macrotexture changes and their implications over the corrosion protection ability of Sn and Sn-based alloy coatings when graphene oxide (GO) is incorporated into the coating matrix as a secondary phase. Four systems were studied: (i) Sn electrodeposit, (ii) Sn-Cu electrodeposit, (iii) Sn-Bi electrodeposit, and (iv) Sn-Co electrodeposit. In the Sn-GO system, the corrosion rate first decreased to a certain GO volume fraction range and increased continuously. The Sn coating with high GO volume fractions exhibited an even higher corrosion rate than the pristine Sn coating. Thus, an optimum GO volume fraction for maximum corrosion resistance is established for the Sn-GO coating system. A high GO volume fraction led to high corrosion rates in Sn-GO composite coatings because of the galvanic coupling phenomenon between the anodic Sn matrix and the cathodic GO sheets. SEM analysis revealed that incorporation of GO led to uniform and compact coating morphology in the SnCu-GO composite coatings. XRD analysis revealed a shift in preferred growth texture low index orientations like (020) and (220) in the Sn-rich phase. Morphological improvements and the impervious nature of the dispersed GO sheets towards the corrosive Cl− ion medium improved the corrosion resistance of the SnCu-GO coating matrix. In the SnBi-GO system, the incorporation of GO affected the purity of the Sn-rich and Bi-rich phases. Uniformly dispersed GO in the SnBi-GO coating matrix led to the enrichment of the Sn-rich phase and subsequently increased the corrosion resistance of the SnBi-GO coatings. High GO volume fractions led to GO agglomeration within the coating which led to the formation of micro-cracks and pinholes in the coating matrix and thus decreased the corrosion resistance. In the SnCo-GO system, the higher absorption affinity of Sn compared to that of Co led to an increase in Sn content with an increase in GO volume fraction. TEM analysis revealed that GO incorporation led to a transition towards a layered microstructure where the electrochemically inert Co3Sn2 phase shielded the Co2Sn phase and thus increased the corrosion resistance in the SnCo-GO coatings. In the later part of the thesis, Cu was increasingly incorporated into the electrodeposited Sn coatings (0 at. % Cu to 33.0 at.% Cu). For the SnCu electrodeposits, the SnCu coating with ~21.2 at.% Cu exhibited the lowest corrosion rate while the SnCu coating with ~9.9 at.% Cu exhibited the highest corrosion rate. It was observed that the incorporation of Cu into the Sn electrodeposits changed the predominant growth texture from (001) to (100). The corrosion rate dependence on the distribution of the Cu6Sn5 phase within the coating matrix was also explored. The corrosion mechanism for all the systems was modeled using an equivalent electrical circuit.