Graduation Year

2017

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Mechanical Engineering

Major Professor

Wenjun Cai, Ph.D.

Committee Member

Delcie Durham, Ph.D.

Committee Member

Nathan Crane, Ph.D.

Committee Member

Alberto Sagüés, Ph.D.

Committee Member

Shengqian Ma, Ph.D.

Keywords

Aluminum alloy, Magnesium alloy, PVD, EIS, SEM

Abstract

The design of new engineering materials resistant to both wear damage and corrosion degradation becomes increasingly demanding in complex service conditions. Unfortunately, there is typically a tradeoff between wear and corrosion resistance, even for important passive metals such as Al alloys. This is because the presence of precipitates hardens the material but at the same time lead to unfavorable galvanic coupling between the precipitates and the matrix, resulting in accelerated corrosion. This work showed that Al (or Mg) supersaturated solid solution formed using non-equilibrium methods exhibited enhanced corrosion resistance without compromising strength. For Al, alloying with Mn up to ~ 20.at.% simultaneously increased the wear resistance of Al as well as the protectiveness of the passive layer, thus improving the overall tribocorrosion resistance. For Mg, alloying with Y (4.67 wt.%), Zr (0.45 wt%), and Nd (1.79 wt%) in solid solution led to ~ 8 fold increment in corrosion resistance in physiological environment.

Magnetron-sputtered aluminum (Al) and aluminum–manganese (Al-Mn) films with structures ranging from nanocrystalline to amorphous were obtained by tuning the Mn% up to 20.5 at.%. Corrosion behavior of the films was investigated in 0.6 M and 0.01 M NaCl aqueous solutions by potentiodynamic polarization (PD) and electrochemical impedance spectroscopy (EIS). Pitting corrosion was found to be strongly affected by alloy composition. The amorphous Al–20.5 at.% Mn exhibited the best pitting resistance during short term exposure. However, over longer immersion in 0.01 M NaCl up to 108 hrs, nanocrystalline Al–5.2 at.% Mn showed the highest corrosion resistance. The dual-phase Al-11.5 at % Mn alloy was found to have higher nominal corrosion rate compared to its nanocrystalline or amorphous counterparts.

The effects of Mn alloying on the tribocorrosion behavior of magnetron-sputtered Al-Mn thin films with 5.2 at.% and 20.5 at.% Mn were investigated in 0.6 M NaCl aqueous solution. Tribocorrosion resistance of Al-Mn was found to be strongly affected by the alloying composition and applied potential. Higher Mn content increased H/E ratio and promoted the formation of denser and more compact passive film, hence improving tribocorrosion resistance of Al. In particular, alloying with 20.5 at.% Mn led to an increase of the corrosion resistance by ~ 10 times and the hardness ~ 8 times compared to pure Al. The total material loss during tribocorrosion was found to increase with applied potential. When the applied potential was increased from cathodic to anodic, simultaneous contribution of the mechanical and the electrochemical wear leads to accelerated material loss. A galvanic cell model was used to investigate the depassivation-repassivation kinetics during tribocorrosion. It was found that alloying with 5.2 at.% Mn led to more than 10-fold reduction in the current density required to re-passivate similar worn areas compared to pure Al. The origin of wear-corrosion synergy was discussed based on these observations.

Magnesium alloys such as WE43 are considered for biomedical applications including cardiovascular stents and bone implants due to their biocompatibility, good cell adhesion, and mechanical properties close to that of bones. Unfortunately, their high degradation rate and subsequent loss of structural integrity in physiological environments hinders such applications. To improve the corrosion resistance of WE43 magnesium alloy, its microstructure was optimized to prevent micro-galvanic coupling between Mg matrix and precipitates. Chemically homogeneous WE43 with nanoscale surface roughness was obtained by magnetron sputtering with high effective quench rate. The effect of chemical heterogeneity on the corrosion resistance of biodegradable WE43 magnesium alloy was studied by performing corrosion tests in blood bank buffered saline using samples from two metallurgical states, cast and deposited. The microstructure of all samples was investigated by grazing incidence X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The deposited samples, prepared by magnetron sputtering using targets with the same global composition as cast WE43, exhibited chemically homogeneous microstructure without the formation of secondary phases typically observed in the cast alloy. The corrosion behavior was studied by PD and EIS tests. It was found that the deposited alloy showed enhanced corrosion resistance, ~8-fold reduction in corrosion rate compared to the cast alloy, owing to the elimination of micro-galvanic coupling between the Mg matrix and the precipitates. In-situ monitoring of hydrogen bubble evolution during corrosion indicated significantly reduced cathodic reaction kinetics in the deposited alloy. Post-corrosion surface and cross-sectional SEM studies showed that the high corrosion rate in the cast alloy was associated with the formation of severely cracked corrosion products preferably around Zr- and Y-containing precipitates.

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