Degree Type

Thesis

Date of Award

2018

Degree Name

Master of Science

Department

Mechanical Engineering

Major

Mechanical Engineering

First Advisor

Pranav Shrotriya

Abstract

High strength aluminum alloys of 2XXX and 5XXX series are widely utilized in recent years for marine, transportation and structural applications, due to low cost, high strength–to-weight ratio as well as good corrosion resistance. However, the aluminum alloys are susceptible to Intergranular Corrosion (IGC) or Intergranular attack (IGA), a form of metal degradation along the selective grain boundary element in a corrosive environment. For instance, Al-Cu-Mg alloy, AA2024-T3 is vulnerable to IGC with the dissolution of S phase, Al2CuMg particles that precipitate along the grain boundaries. Similarly, Al-Mg alloy, AA5083 is susceptible to IGC with the precipitation of β phase, Mg2Al3 particles along the grain boundaries of the alloy when exposed to long term elevated temperature (60 ˚C ~ 180 ˚C).

In the present work, we utilized in-situ stress measurement during IGC of 2024 and 5083 to determine the mechanism relating corrosion induced stress fields and grain boundary dissolution. Phase shifting curvature interferometer, a method proven to be effectively detects electrochemical reaction induced curvature changes which is utilized to monitor the sample curvature during IGC. AA2024-T3 and sensitized AA5083-H116 samples were cut and polished then mounted in electrochemical cell with the front surface exposed to solution (aqueous 1 M NaCl for AA2024-T3, aqueous 3.5 wt% NaCl for sensitized AA5083-H116) and the mirror polished back surface utilized for in situ stress measurements. The electrochemical tests were performed with a three-electrode setup. Anodic potentiodynamic polarization was performed at a rate of 0.167 mV/s starting from -0.9 V to -0.3 V followed by a 30-minute open circuit potential (OCP). Subsequently, the samples were held at 0.1 V above the OCP value (-0.535 V for AA2024 samples and -0.63 V for sensitized AA5083) to monitor stress development during anodic dissolution. Corrosion surface morphology was examined with Scanning Electron Microscopy (SEM) and Energy-Dispersive Spectrometer (EDS).

Experimental results show that anodic dissolution of both 2024 and 5083 are associated with development of compressive stresses in the sample surfaces. The compressive stresses development is at low magnitude during open circuit exposure and stepping to the higher potential results in large dissolution current and rapid development of compressive stresses. Microstructural and compositional characterization of the sample surfaces show evidence of grain boundary dissolution and formation of pits. The grain boundaries show higher concentration of oxygen indication formation of oxides during the anodic dissolution. The observation of compressive stress generation during anodic dissolution and presence oxides at grain boundaries indicates that dissolution of intermetallic particles segregated at the grain boundary results in formation of oxides. The volume increase associated with oxide formation acts as wedges in the grain boundary resulting in compressive stresses on the sample surface. The oxide formation induced wedging stresses may contribute to intergranular stress corrosion cracking of these alloys.

Cuprous Oxide or Copper (I) Oxide (Cu2O), a p-type semiconductor is a promising candidate for various applications such as photocatalyst for solar driven water splitting of and H2 generation, electrode for lithium ion batteries and p-type semiconductor in heterojunction with n-type ZnO for photovoltaic applications. The Cu2O has a direct bandgap of 2.17eV that is able to absorb major portion of the visible light spectrum which favours the PEC hydrogen production. According to Shockley-Queisser limit graph, Cu2O with a direct band gap of 2.0–2.2 eV, which has an approximate maximum efficiency of 18%. Moreover, Cu2O is low cost, abundantly available in nature and easily produced via electrodeposition and sputtering where the production methods are simple, economical and scalable. A new phase shifting curvature interferometry, a technique used for high-resolution in-situ stress measurement of electrochemical reactions. The curvature changes that could be resolved by curvature interferometry system is as small as 1 x 10-4 m-1. It was demonstrated that small curvature change rates of 2.5 x 10-8 m-1s-1 could also be reliably measured, indicating the applicability of the system to measure bulk samples. In the present work, the phase shifting curvature interferometry is utilized to measure the deflection and stress development on the Indium Tin Oxide-Au (ITO-Au) sample during electrodeposition of Cu2O. Experimental results show that electrodeposition of Cu2O are associated with the compressive stress development in the sample surface. The Au-Cu2O interface layer exerted high stress on the sample due to mismatch of lattice between the Au and Cu2O while the remaining Cu2O deposited layers are in stress free zone.

Copyright Owner

Brendan Shin Hau Yeah

Language

en

File Format

application/pdf

File Size

67 pages

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