PhD Thesis Colloquium: Mr. Akhand Pratap Singh (10/06/24)

2 minute read

Thesis title:

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

Faculty advisor(s):

Prof. Chandan Srivastava

When?

10th June, 2024 (Monday), 11:00 AM (India Standard Time)

Where

KPA Auditorium, Department of Materials Engineering

Abstract:

Electroplated Sn has been used as a protective coating against corrosion, as an anode material for rechargeable Li-ion battery, decorative finishes, and in electronic packaging. The application of electrodeposited Sn is limited by its electrochemical degradation which reduces reliability and increases the operating cost. In the present work, the corrosion behavior of electrodeposited Sn-based coatings (Sn-Ni, Sn-Zn, and Sn-Bi) has been studied by examining the corrosion products, spatial distribution of phase, solute clustering/partitioning within the matrix, micro-texture, and coating strain. Key observations made in the current work are: (i) optimum incorporation of Ni in Sn coating (6 wt% Ni) produced a highly corrosion-resistant Sn-Ni coating with Ni3Sn4 intermetallic occupying the grain boundaries, low energy (100) orientated Sn matrix, highest fraction of low energy (031) [01̅3] and (01̅1) [011] twin boundaries and lower matrix strain. Contrary, higher Ni addition (13 wt% Ni) degraded the corrosion resistance of the Sn-Ni coating, due to galvanic coupling between the anodic Sn-rich matrix and cathodic intermetallic phase (Ni3Sn4), and evolution of high energy texture characterized by a high fraction of high energy high angle grain boundaries (HAGBs), high matrix strain, and low fraction of low energy twin boundaries, (ii) a non-monotonic variation in the corrosion rate 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 incorporation (14 wt% Bi). Sn clusters in Bi-rich grains strained the Bi grains, thus lowering its effective passiveness and reducing the extent of 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 low energy (101) orientation and highest fraction of low-energy CSL boundaries, (iii) in Sn-14wt%Bi, coatings electrodeposited at different cathodic current densities (5, 12, 25, and 40 mA/cm2), highest corrosion resistance was observed for the coating electrodeposited at the 25 mA/cm2 current density. This was due to low-energy (301) surface texture, evolution of the Sn-1.2at% Bi solid solution phase, and formation of SnO2 oxide passive layer. Lowest corrosion resistance observed for 40 mA/cm2 deposited coating was due to high energy (110) surface texture, formation of Bi-enriched clusters in Sn matrix, and an unstable passive layer consisting of SnO and Sn(OH)2 oxides, (iv) corrosion resistance of Sn-Zn coatings decreased at lower quantities of Zn addition (up to 10 wt%) but increased substantially at higher Zn contents (20-25 wt% Zn). At higher Zn additions, the enhanced corrosion resistance was due to a higher fraction of low energy coincidence site lattice boundaries (CSLs), and preferred low-energy (100) surface texture. On the other hand, a higher corrosion rate for lower Zn addition stemmed from a highly strained matrix and a relatively higher fraction of high-energy, HAGBs, (v) a non-monotonic variation in the corrosion rate of the Sn-23wt% coating with increasing 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 (ZnO and SnO2). Higher energy (110) surface grain orientation, unstable b-Sn matrix due to embedded Zn-enriched clusters, and weak passive layer consisting of Zn0 led to the lowest corrosion resistance for coating electrodeposited at 17 mA/cm2.

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