Corrosion Effects of Magnesium Chloride and Sodium Chloride on Automobile Components

The project consisted of two phases. In the Phase I study, the relative corrosiveness of the two salts [magnesium chloride (MgCl2) and sodium chloride (NaCl)] were examined by SAE J2334 test and ASTM B117 test. In the Phase II study, SAE J2334 test and NACE TM-01-69 test (as modified by the Pacific North States) were applied. Representative metals examined in the project included stainless steel 410 and 304L, aluminum 2024 and 5086, coated automobile body sheets, copper wires, and mild steels. Experimental results of SAE J2334 test indicated that MgCl2 was more corrosive than NaCl to the bare metals tested. However, the experimental results of ASTM B117 test showed opposite conclusions. Because of the conflicting conclusions, further tests were conducted using NACE TM-01-69 (as modified by the Pacific Northwest States). Again, opposite conclusions were obtained from SAE J2334 and NACE TM-01-69 tests. In order to investigate the causes responsible for the inconsistency, the experimental conditions of both SAE J2334 and NACE TM-01-69 tests were modified and various modified modes of the two tests were conducted. It was found that the inconsistency in the test results was not a result of different chemical concentrations of chloride solution, different immersion times, testing periods, or testing temperatures. The inconsistency was attributed to the different moisture conditions and different properties of the two salts under high humidity environment. There are three basic moisture conditions in the three testing methods used in the project: dry, wet (saturated moisture), and dip (immersion). Since the MgCl2 solution has higher viscosity and stronger hydraphilicity than the NaCl solution, it is much easier for the MgCl2 solution to stick and crystallize on the surface of the metals under the dry condition, and then become solution on the metal surface under the wet condition. This dry-wet effect is responsible for the different corrosion behaviors of MgCl2 under different testing conditions. Therefore, depending on service conditions experienced by automobile components, MgCl2 is more corrosive than NaCl under humid environments, and NaCl is more corrosive under immersion and arid environments. This conclusion was obtained based on the experiments with the deicing salts used in the state of Colorado. Study findings resulted in specification changes for deicing chemicals used by the Colorado Department of Transportation (CDOT). The new specification on corrosiveness requires MgCl2 used by CDOT to be no more corrosive than NaCl on aluminum and stainless steel as tested by the NACE TM-01-69 method.