Effect of Microstructural Evolution on the Electrochemical Degradation Behaviour of Electrodeposited Tin-Based Coatings

Abstract

The electrodeposited/electroplated Sn has been used as a protective coating to enhance corrosion resistance, anode material for rechargeable Li-ion battery as an alternative to graphite, decorative finishes, and electronic industries. The application of electrodeposited Sn in various industries has a long history and however, the electrochemical corrosion degradation of Sn based coatings led to reduced longevity, poor performance, and hence increased operational cost. In the view of improving the corrosion resistance of pristine Sn coating, one proven technique is to develop Sn alloy coatings. In the present work, the corrosion behavior of Sn based coatings are studied by examining corrosion product, spatial distribution of phase, solute clustering/partitioning within the matrix, microtexture and strain of the matrix. The striking observation of the current work are: (i) an optimum addition of Ni in Sn coating (6 wt.% Ni) produces a highly corrosion resistant Sn-Ni coating with Ni3Sn4 intermetallic occupying the grain boundary, low energy (100) orientated Sn matrix, highest fraction of low energy (031) [01 ̅3] and (01 ̅1) [011] twin boundaries and lower matrix strain. Contrary, higher Ni addition (13 wt.% Ni) degrades the corrosion resistance of the Sn-Ni coating, mainly due to the higher galvanic coupling between the anodic Sn-rich matrix phase and cathodic intermetallic phase (Ni3Sn4), and high energy texture which is characterized by high fraction of high energy HAGBs, higher matrix strain, and least fraction of low energies twin boundaries. (ii) the corrosion resistance properties of Sn-Zn coatings decrease with lower quantities of Zn addition (up to 10 wt%) but increase substantially at higher Zn contents (20-25 wt% Zn). This anomaly remains unresolved. At higher Zn additions, the enhanced corrosion resistance of Sn-Zn coatings was due to the evolution of a higher fraction of low energy, coincidence site lattice boundaries (CSLs), and preferred low-energy (100) surface texture of the coatings. On the other hand, a higher corrosion rate for lower Zn addition is due to a highly strained matrix and a relatively higher fraction of high-energy, high-angle grain boundaries. (iii) A non-monotonic variation in the corrosion rate of the Sn-23wt.% coating with increase in the deposition current (7, 12, 17, and 22 mA/cm2) was noticed. The coating deposited at 12 mA/cm2 exhibited the highest corrosion resistance due to relatively lower energy surface texture, higher fraction of lower energy grain boundaries, lesser coating strain and evolution of protective oxide film with stabler oxides. Apart from illustrating the microstructure-corrosion property correlation in Sn-Zn coating, this is the first report on the imaging of atomic clusters and establishment of its role in determining the corrosion behaviour of electrodeposited Sn-Zn coatings. (iv) A non-monotonic variation in the corrosion behavior of the Sn-Bi coatings with Bi addition (0-17 wt% Bi) was noted. A highly corrosion-resistant Sn-Bi coating was produced at an optimum Bi content (14 wt.% Bi). Nanoclusters of Sn atoms were observed (by atom probe tomography-based analysis) in Bi-rich grains, causing strain within Bi grains, thus lowering its effective passiveness, and reducing the micro-galvanic coupling between Sn-rich (anodic) and Bi-rich (cathodic) grains. The increase in the corrosion resistance for optimum Bi content was also due to the evolution of surface texture (of the matrix Sn phase) corresponding to orientation near (301) plane, which has very low surface energy and the highest fraction of low-energy CSL boundaries. (v) Sn-14wt%Bi coatings electrodeposited at different cathodic current densities (5, 12, 25, and 40 mA/cm2). The highest corrosion resistance was observed for the coating electrodeposited at the 25 mA/cm2 current density. This was due to low energy (301) grain orientation at the surface, the evolution of the Sn-1.2at% Bi solid solution phase, and the formation of the SnO2 oxide passive layer. The lowest corrosion resistance was observed for 40 mA/cm2 deposited coating. This was due to high energy (110) surface texture, Bi-enriched nanoclusters in the Sn matrix, and an unstable passive layer consisting of SnO and Sn(OH)2 oxides.