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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21 NACE INTERNATIONAL: VOL. 53, NO. 12 MATERIALS PERFORMANCE DECEMBER 2014 Information on corrosion control and prevention —K.R. Larsen were seen after 4.5 h. Inhibiting behavior up to one day in the presence of Ca 2+ was conf irmed with mass gain, mass loss, and H 2 evolution measurements. Additionally, the pH did not exceed a value of 10 during immersion, indicating that it was not pos- sible under the experimental conditions for the system to reach the passivating pH of 12, and the inf luence of the pH value on the corrosion rate was found to be limited in the pH region of 8 to 11. "During immersion tests within the presence of calcium, a grey-white, volu- minous, and open-porous deposition layer formed on the surface of magnesium parts, and it covered the entire surface after 12 days—with a thickness greater than a tenth of a millimeter," notes Grabowski. "An electrically conductive connec- tion of magnesium with more noble met- als such as aluminum results in a rapidly increased corrosion rate, which was effec- tively reduced by the presence of calcium by a factor of two," he says. This inhibiting effect was seen within a short immersion time of less than a day without the inf luence of galvanic cou- pling. "Under long-immersion conditions of more than a day, the presence of cal- cium caused a slightly detrimental effect on the corrosion behavior of magnesium alloys," he adds. Another key discovery was that the deposition layer that covered the surface after several days of immersion within the presence of calcium mainly consisted of calcium carbonate (CaCO 3 ), which was similar to hard-water lime scale. "These layers aren't easily soluble in water and are diff icult to remove. In other words, the deposits exhibit good adhesion to the magnesium substrate," explains Grabowski. "The CaCO 3 layer appears during corrosion as a by-product, but this layer doesn't change the fact that we need corrosion protection systems directly on our magnesium surfaces, such as coat- ings. If there are defects in the coating, it's good to know that we have insoluble CaCO 3 deposition layers on the surface in the region of the defect." The group's work provides a deeper un- derstanding of the corrosion mechanisms of magnesium alloys under simulated am- bient conditions. "This is a first step to- ward tailoring corrosion protection sys- tems to ensure that no visible or invisible defects caused by corrosion occur during a car's product life cycle," says Grabowki. "Our goal is to enable lightweight magne- sium technology and to make automotive mobility more sustainable." Te CORROSION paper can be viewed at Reference 1 M . G r a b o w s k i , D. B l ü c h e r, M . K o r t e , S. Virtanen, "Influence of Ca 2+ in Deicing Salt on the Corrosion Behavior of AM50 Magne- sium Alloy," Corrosion 70, 10 (2014): pp. 1,008- 1,023.

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