Materials Performance

DEC 2014

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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20 DECEMBER 2014 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 53, NO. 12 MATERIAL MATTERS Study aids understanding of deicing salt infuence on magnesium alloy corrosion Magnesium alloys are used to fabricate automobile wheels. T o gain an understanding of the corro- sion mechanisms involved with magnesium alloy materials exposed to deicing salts, a team of researchers with the University of Erlangen-Nuremberg (Erlangen, Germany), SINTEF Materials and Chemistry (Trondheim, Norway), and Audi AG immersed samples in electroly tes that simulated conditions found on salt-treated roadways and accel- erated the corrosion process to study the results. Magnesium alloys are among the lightest-weight construction materials available today. Compared to aluminum alloy materials commonly used in auto- mobiles, magnesium alloys can reduce the weight of a component by about one- third. The problem with using magne- sium alloys is corrosion—these alloys have the lowest corrosion resistance of all the construction metals. Establishing a means to overcome the corrosion chal- lenges and take advantage of the benef its of using lightweight magnesium alloys would be extremely benef icial to the auto- motive industry. In a paper published in the journal CORROSION, 1 Michael Grabowski, a Ph.D. candidate at the Uni- versity of Erlangen-Nuremberg, and his colleagues describe their investigation of the corrosion mechanisms of magnesium alloys and how deicing salt inf luenced the corrosion rate. The chemical components in deicing salt cause an increase in corrosivity by increasing the electrical conductivity of the electroly te and destabilizing protec- tive surface layers of the alloy. The group's work focused on identif ying the impact of deicing salt components such as sodium chloride (NaCl), magnesium chloride (MgCl 2 ), and calcium chloride (CaCl 2 ) on the corrosion behavior of magnesium alloys. "Of particular interest was CaCl 2 , because no technical data about its inf lu- ence had been published," says Grabowski. CaCl 2 is commonly used to lower the freezing point of deicing salt. To study the inf luence of the presence and absence of the deicing salt ingredient Ca 2+ on the corrosion behavior of die-cast magne- sium alloy A M50 (Mg-5A l-0.3Mn), the researchers used electroly tes that were close to the deicing salt composition, but one included Ca 2+ and one did not. The goal of the work was to determine the dif- ferences in the corrosion mechanism when exposed to deicing salt with and without the addition of Ca 2+ . To explore how to best protect the magnesium alloys, the researchers simu- lated the corrosive environments com- monly found on the roadways by dissolv- ing 3 w t% of the deicing salt ingredients in water for several days, and then con- ducting immersion experiments. They also used several electrochemical charac- terization methods to help accelerate the corrosion process so that years' worth of corrosion would happen in a few minutes. Electrochemical investigations, H 2 evolu- tion, and weight-loss characterization were used during the experiment and evaluation of the samples. The experiments carried out in the laboratory revealed that the presence of calcium may have the capability to signif- icantly decrease the corrosion rate of magnesium caused by galvanic coupling. Under polarizing conditions, the result- ing current densities were signif icantly lower in the presence of Ca 2+ . Electro- chemical impedance spectroscopy (EIS) measurements taken up to 4.5 h showed polarization resistance (R P ) increasing with time; however, a breakdown of R P values in the presence of Ca 2+ , combined with the appearance of an inductive loop,

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