Defense Date

2008

Document Type

Thesis

Degree Name

Master of Science

Department

Mechanical Engineering

First Advisor

Ramana Pidaparti

Abstract

Corrosion is the destructive result of chemical reaction between a metal or metal alloy and its environment. Airplanes, power generating plants, chemical process and manufacturing plants, concrete structure, and many others which widely uses aluminum alloy and stainless steel alloys are subjected to corrosion. The estimated cost of corrosion damage is in order of 3 to 5 percent of gross national product (GNP). Out of all forms of corrosion, pitting corrosion is most commonly observed in aluminum alloy 2024 and stainless steel type 316. There is a need to study the stress environment around the pits in order to predict the nucleation of the crack. The objective of this thesis is to investigate the correlation between pits and stresses in AA2024 and SS316 alloys under different types of loading. Corrosion experiments were carried out on both alloys samples for a fixed time interval and were imaged on optical and AFM. The optical microscope provided the information on forms of corrosion expected on the surface while the AFM provided the pit profile on the surface. An analysis procedure was developed using CAD and finite element analysis to predict stresses resulting from corrosion pits under different types of loadings. The average corrosion rate of AA2024 is six times higher than that of SS316 in 2 Molar NaCl corrosive environment. Based on the results from the optical microscope, AA2024 usually initiated with localized corrosion along with pitting and localized regions grows in size and soon uniform corrosion is observed. However, the stainless steel SS316 usually initiated with pitting corrosion and soon followed by film forming corrosion. Based on the analysis, it was observed that the stress distribution and levels on the corroded surface varied due to irregularities in the corrosion process. From the stress analysis result of AA2024 under bending, it was observed that there was 80% stress increase during first 30 min of corrosion and then the increase was about 6% from 30min to 60 min and then soon reaches a plateau. Similar results were obtained for both AA2024 and SS316 materials under different type of loadings. Initially, the stress increases sharply as time increases but the amount of stress increase demises as time progress and soon reaches a plateau. There was a sharp increase of Bending and shear loading are induces higher level of stresses compared to tension loading. From these stresses it is possible to estimate the initiation of crack, from which the life can be estimated for failure in the material.

Rights

© The Author

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

December 2008

Included in

Engineering Commons

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