Effect of Sr Addition to a Modified AA3003 on Microstructural and Corrosion Properties

Abstract

Sr is known to transform the morphology of the eutectic silicon phase as well as the Fe-rich β phase in Al-Si alloys, improving their mechanical properties. However, little is known about the effect Sr has on the (local) corrosion properties of aluminium alloys. This study investigates the effect of Sr addition to a modified AA3003 heat exchanger material on the morphology of the different phases present, especially the Fe-rich phases, as well as on the (local) corrosion properties of this material. This work reports the formation of a Sr-rich phase, which slightly increases the macrohardness of the material. The Fe-rich phases are not shown to be refined/influenced by the addition of Sr Potentiodynamic polarization experiments showed an increase in pitting potential by increasing the amount of Sr in the material up to 0.4 wt.%. Nevertheless, the analysis of the corrosion morphology revealed that the Sr-containing particles did not contribute to the corrosion process despite their cathodic behaviour compared to the Al matrix as measured by Scanning Kelvin Probe Force Microscopy. This behaviour was attributed to the thicker oxide layer found on the Srrich particles.

Methods—On the Application of Ambient Scanning Kelvin Probe Force Microscopy to Understand Micro-Galvanic Corrosion Phenomena: Interpretation and Challenges

“…An example can be seen in Fig. 4, taken from the work by Verkens et al 53 Al 2 SrSi 2 inclusions in Sr-containing AA3003 appeared to be electrochemically inactive in NaCl solution (see Fig. 4a), despite their relatively high potential with respect to the surrounding alloy matrix (∼160 mV higher than the matrix, as measured by SKPFM), as seen in Fig.…”

“…Adapted with permission from. 53 (b) Topography and CPD map of a Sr-containing particle, which was found cathodic with respect to the aluminium matrix from SKPFM measurements prior immersion. The histogram of the CPD map reveals that the potential of the Sr-rich phase is about 160 mV higher than the potential of the aluminium phase.…”

“…(a) Scanning electron micrograph of Sr-containing AA3003 aluminium alloy after 5 d immersion in 0.1 M NaCl solution.Galvanic corrosion was only initiated around Fe-containing intermetallics, while Srcontaining particles were found to be electrochemically inactive. Adapted with permission from 53. (b) Topography and CPD map of a Sr-containing particle, which was found cathodic with respect to the aluminium matrix from SKPFM measurements prior immersion.…”