Graduation Year

2020

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Mechanical Engineering

Major Professor

Nathan B. Gallant, Ph.D.

Co-Major Professor

Nathan D. Crane, Ph.D.

Committee Member

Ryan Toomey, Ph.D.

Committee Member

Alberto A. Sagues, Ph.D.

Committee Member

Matthias Batzill, Ph.D.

Keywords

degradation, medical, implant, micromachining, ablation, wetting, picosecond, EIS, electrochemical

Abstract

Magnesium and its alloys are good candidates to manufacture medical implants. They have excellent biocompatibility and because they biodegrade secondary surgical operation to remove the implant could be eliminated. However, in aqueous environments, magnesium alloys rapidly corrode, resulting in premature degradation of the implant along with biologically intolerable hydrogen gas generation. In literature, there are multiple studies focused on creating water repelling hydrophobic magnesium surfaces in order to decrease corrosion rates. Hydrophobic properties can be achieved by creation of a roughness profile on an initially smooth surface combined with a treatment that reduces the free surface energy. In theory, hydrophobic surfaces yield an undisturbed gas layer over the surface which acts as a protective barrier against corrosion in submerged conditions. However, studies in literature have not investigated the mechanism behind increased corrosion protection that comes with hydrophobic surfaces nor the influence of series of processes used to create hydrophobic surfaces on corrosion behavior. In this study, pillar shaped microstructure patterns were fabricated on smooth pure magnesium surfaces by picosecond laser ablation. Some micropatterned samples were further processed by stearic acid modification (SAM). Micropatterned surfaces with SAM had hydrophobic properties with water droplet contact angles higher than 130°, while the micropatterned surfaces without SAM remained hydrophilic. Corrosion properties of all hydrophobic and hydrophilic magnesium surfaces were investigated using electrochemical impedance spectroscopy (EIS) in saline solution. Compared to smooth unmodified surfaces, significantly improved and similar corrosion resistances were observed on both hydrophobic and hydrophilic surfaces. Corrosion rate reduction on micropatterned hydrophilic/phobic surfaces was also verified by prolonged submersion tests in saline solution. Unexpected corrosion inhibition on hydrophilic surfaces was investigated and evidence of local alkalization near microstructures was found. It was concluded that corrosion inhibiting mechanism for hydrophilic surfaces is possibly being caused by local alkalization and the resulting stabilization of Mg(OH)2 layer. This is different than the mechanism behind hydrophobic surfaces’ corrosion resistance, which makes use of gas adhesion at the liquid solid interface as once more verified in this study along with previous studies in literature.

Download

Share

COinS