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Corrosion of Magnesium, Aluminum, and Steel Automotive Sheet Metals Joined by Steel Self-Pierce Rivets

2015, Master of Science, Ohio State University, Materials Science and Engineering.
The automotive industry is investigating advanced light-weight materials in an effort to increase the fuel efficiency of vehicles. High-strength steels, aluminum and magnesium alloys, and carbon fiber and polymer composites are of interest to automobile manufacturers around the globe. One of the major problems facing the widespread implementation of such materials, especially aluminum and magnesium alloys, is their unique corrosion susceptibility. Not only is the corrosion performance of aluminum and magnesium alloys different from steel that is typically used in automobile manufacturing, but when used in combined, mixed-material systems, galvanic corrosion becomes a significant concern.
The United States Department of Energy, in conjunction with Ford Motor Company, General Motors, and Fiat-Chrysler of America, has established standards that will be enacted to increase the corporate average fuel economy, or fleet-wide average fuel economy, of vehicles to be sold in the United States. This standard is intended to inspire automobile manufacturers to increase the fuel efficiency of vehicles weighing less than 8,500 lbs.
The Materials Technology Subprogram of the Vehicle Technologies Office is responsible for the investigation into new advanced materials that will enable technologies to increase fuel efficiency. The United States Automotive Materials Partnership, LLC. has been established with funding from the Department of Energy in order to design a magnesium-intensive front-end substructure. The first phase of the project resulted in a 44.5% weight reduction and parts-count reduction from 110 to 47.
Now in its third phase, the Partnership has enlisted The Ohio State University in an effort to create a magnesium-intensive front-end demonstration structure, consisting of advanced high-strength steels, aluminum alloys, and magnesium alloys of interest to the original equipment manufacturers participating in the project. The work performed in this thesis has contributed by investigating a case study of an AZ31 cleaning procedure for the MagPASS® conversion coating. Fourier-transform infrared spectroscopy was enlisted to analyze organic compounds on the surface of AZ31 magnesium alloy sheet metal that were interfering with pretreatment uptake. Polymerization of the organic compounds occurred during a warm-forming procedure, causing them to undergo a structural transformation that rendered the cleaning procedure ineffective.
The hydrogen embrittlement of hardened-steel rivets coupled to Mg panels was then studied. Considering their close proximity to magnesium alloys, which generate copious hydrogen during corrosion, it was proposed these rivets (RC 47) could potentially experience degradation of mechanical properties. A unique slow-strain-rate hoop-stress-test was designed in an effort to elucidate changes in mechanical properties of the rivets after hydrogen charging. Unfortunately, the non-ideal geometry of the rivets made hydrogen charging ineffectual, and the hoop-stress-test results were inconclusive.
Finally, changes in lap-shear strength of riveted joints as corrosion dissolved the magnesium surrounding the rivets over time was investigated.. It was found that the riveted joints sustained a significant portion of their lap-shear strength despite significant material loss in the adjacent magnesium. It was also found that the industry-standard Zn/Sn-coating for the hardened steel rivets was superior to aluminum-coatings in mitigating the galvanic corrosion. Student-t statistical analysis was employed to establish the statistical significance of the results.
Gerald Frankel, Dr. (Advisor)
Rudolph Buchheit, Dr. (Committee Member)
126 p.

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Weimer, W. (2015). Corrosion of Magnesium, Aluminum, and Steel Automotive Sheet Metals Joined by Steel Self-Pierce Rivets. (Electronic Thesis or Dissertation). Retrieved from https://etd.ohiolink.edu/

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Weimer, William. "Corrosion of Magnesium, Aluminum, and Steel Automotive Sheet Metals Joined by Steel Self-Pierce Rivets." Electronic Thesis or Dissertation. Ohio State University, 2015. OhioLINK Electronic Theses and Dissertations Center. 11 Dec 2017.

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Weimer, William "Corrosion of Magnesium, Aluminum, and Steel Automotive Sheet Metals Joined by Steel Self-Pierce Rivets." Electronic Thesis or Dissertation. Ohio State University, 2015. https://etd.ohiolink.edu/

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