Turbulent fluid flow at high Reynolds numbers presents significant degradation risks to active-passive metals due to enhanced localized degradation phenomena. A multidisciplinary experiment was proposed to study the relationship between hydrodynamics in fully-developed pipe flow and both the growth and performance characteristics of passive films.
Preliminary work was performed to set up (i) an environmental chamber for the experiment, (ii) design a custom wall shear stress sensor and constant temperature anemometer traverse system to monitor hydrodynamic conditions in-situ, (iii) monitor in-situ degradation through an array of ultrasonic thickness transducers, and (iv) acquire data and control the environment via a LabVIEW routine. A validation experiment was conducted on a 1220 mm long experimental section of 45.7 mm inner diameter AA2024-T3 tubing in simulated seawater. Extensive degradation was observed in-situ and confirmed with ex-situ techniques after sequential exposure to fully-developed turbulent flow at an expected wall shear stress of 10 Pa for 180ks (Reynolds number of 122,000) and then at 40 Pa for 630ks (Reynolds number of 262,000). No typical erosion-corrosion hydrological features were observed, however significant pitting and intergranular corrosion were observed with corrosion product caps covering 47% of the total ultrasonic transducers' measurement area. Passive film and pit growth were recorded via ultrasonic thickness measurements with an observed simultaneous decrease in dissolved oxygen content. The validation experiment successfully demonstrated the capability of the designed and constructed sensors for the proposed experiment. Numerous areas of suggested development and research were identified to ensure accuracy and improve interpretation of future experiments. / Master of Science / The durability and degradation resistance of aluminum and its alloys are of significant interest due to their widespread applications across several industries in which they experience a wide variety of environmental conditions. These materials are attractive for many reasons, most notably due to their low density, cost, and the ability to tailor their material properties for specific applications through selective alloying, heat treatments, and/or cold working.
Aluminum displays active-passive behavior and naturally develops a protective oxide that can be improved through anodization, known as a passive film. The oxide provides a physical, chemical, and electrical boundary between the substrate and its environment that resists degradation, but the oxide is still vulnerable to highly aggressive environments due to tribocorrosion. Specifically, aggressive turbulent fluid flow at high Reynolds numbers presents significant degradation risks to aluminum alloys and other active-passive metals.
An original multidisciplinary experimental program was proposed and designed to study the relationship between hydrodynamics in fully-developed turbulent pipe flow and both the growth and performance characteristics of passive films. The study was motivated by the lack of and disagreement between reported hydrodynamic data that lead to passive film failure. It is believed that the characterization of this relationship may yield suggested procedures for improving the corrosion resistance of both passive films and artificial coatings.
In order to perform the proposed experiment, several preliminary tasks were conducted. An environmental chamber was modified and characterized to ensure the desired hydrodynamic conditions could to achieved and sustained. The environmental chamber, known as the Virginia Tech High Turbulence Corrosion Loop is single direction, variable Reynolds number environment where a material is exposed to fully-development turbulent pipe flow. A custom wall shear stress sensor and a constant temperature anemometer traverse system were designed to measure the hydrodynamic parameters of wall shear stress and degrees of turbulence during experimental operation. An array of fifteen ultrasonic thickness transducers provided in-situ degradation measurements of experimental materials. Degradation measurements and environmental conditions were conducted and monitored remotely and frequently via a custom software routine.
A validation experiment was conducted on a 1220 mm long experimental section of 45.7 mm inner diameter AA2024-T3 tubing in simulated seawater to verify the capability of performing the proposed experiment with the constructed equipment and provide a baseline for expected degradation. Extensive degradation was observed in-situ and confirmed after with ex-situ techniques after sequential exposure to fully-developed turbulent flow conditions at an expected iv wall shear stress of 10 Pa for 180ks (Reynolds number of 122,000) and then at 40 Pa for 630ks (Reynolds number of 262,000). No typical erosion-corrosion hydrological features were observed, however significant pitting and intergranular corrosion were observed with corrosion product caps covering 47% of the total ultrasonic transducers’ measurement area. Passive film and pit growth were recorded via ultrasonic thickness measurements with an observed simultaneous decrease in dissolved oxygen content.
The validation experiment successfully demonstrated the capability of the designed and constructed sensors for the proposed experiment. Numerous areas of suggested development and research were identified to ensure accuracy and improve interpretation of future experiments.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/83770 |
| Date | 25 June 2018 |
| Creators | Todoroff, Peter Kent |
| Contributors | Materials Science and Engineering, Hendricks, Robert Wayne, Schetz, Joseph A., Trueman, Elissa, Corcoran, Sean G. |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Thesis |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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