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This paper presents the progress of an ongoing vehicle micro corrosion environment study. The goal of the study is to develop an improved method for estimating vehicle corrosion based on the Total Vehicle Accelerated Corrosion Test at the Arizona Proving Ground (APG). Although the APG test greatly accelerates vehicle corrosion compared to the field, the “acceleration factor” varies considerably from site-to-site around the vehicle. This method accounts for the difference in corrosivity of various local corrosion environments from site-to-site at APG and in the field. Correlations of vehicle microenvironments with the macroenvironment (weather) and the occurrence of various environmental conditions at microenvironments are essential to the study. A comparison of results from APG versus field measurements generated using a cold rolled steel based corrosion sensor is presented.

Bare magnesium is not compatible with the zinc phosphate process commonly used in automotive manufacturing today. If a magnesium component is assembled onto the vehicle prior to the phosphate process, it must be coated beforehand to prevent magnesium dissolution in the phosphate bath. In addition, for a component that must function in very corrosive environments, galvanic corrosion problems also need to be addressed because the component is most likely to be connected with a more noble metal such as steel fasteners or to another component made of steel or aluminum. In this paper, several coatings were evaluated for their effectiveness in stopping magnesium dissolution during the phosphate process and preventing galvanic corrosion. The results show that a porous anodized coating or a very thin chemical conversion coating does not completely stop magnesium dissolution during the phosphate process.

Automotive corrosion is a complex issue since a vehicle is comprised of many materials and different locations on the vehicle experience different corrosion environments. As a result, multiple corrosion mechanisms are encountered. Hence, development of an accelerated corrosion test for automobiles that correlates well to real world corrosion situations is a challenging task. Most corrosion tests currently used in the automotive industry were designed for corrosion of steel. With an increasing use of aluminum and magnesium alloys, galvanic corrosion becomes a critical issue. Applying corrosion tests designed for steel to evaluate galvanic corrosion of lightweight alloys could lead to erroneous conclusions since the acceleration factors for the two corrosion mechanisms may be very different.

General corrosion protection of sheet materials such as steel used in automobile construction has reached a high level of performance, due primarily to the incorporation of mill-applied treatments such as electrogalvanizing, galvannealing and other coil-coating processes developed over the last half century. While such treatments have greatly extended the corrosion resistance of steel and its various body constructs, attention is now focused on aspects of the manufacturing process wherein these intended protections are compromised by such features as weldments, joins, cut edges and extreme metal deformations such as hems. A novel metal deposition process, based on high-velocity impact fusion of solid metal particles, has been used to extend the corrosion resistance of base steel and pre-galvanized sheet, by selectively placing highly controlled depositions of zinc and other sacrificial materials in close proximity to critical manufacturing details.

Two passenger cars have been instrumented for a comparative study of vehicle micro environments (data gathered from onboard vehicle sensors) and the macro environment (data gathered from a Detroit metro area weather station). Environmental sensors were installed at more than thirty corrosion prone sites on each of the two vehicles to measure temperature and relative humidity of the air and temperature and time-of-wetness of the surface. The weather station data include temperature, relative humidity, and daily rainfall. Data collected over a one year period are analyzed and the results are presented. The results indicate that the micro environment (or “environmental corrosion load”) varies considerably from site to site around the vehicles. These vehicles will continue to gather data for another year. They will then be sent through several cycles of a total vehicle corrosion test at an automotive proving ground. The micro environment response during the test will also be recorded.