Hydrogen-induced pore formation in Ni–P-plated Al–Zn–Mg alloys revealed by synchrotron X-ray computed tomography and hydrogen detection

The internal microstructures of Ni–P-plated Al–Zn–Mg-based alloys were investigated using synchrotron X-ray computed tomography, together with various analytical methods for hydrogen detection. We clarified that hydrogen-induced pores were generated and segregated near the top and bottom surfaces of a rolled sheet of a commercial-purity Al–Zn–Mg-based alloy with Ni–P plating. In contrast, such segregation of hydrogen micropores was not observed in the high-purity-grade Al–Zn–Mg alloys with Ni–P plating. This result suggests that hydrogen enters the Ni–P plating layer during the plating operation and generates pores in the surface region owing to surface inclusions. Additionally, the hydrogen-induced defects deteriorated the tensile properties such as tensile strength and elongation of the commercial-purity Al–Zn–Mg-based alloys containing higher levels of impurities such as Fe and Si with Ni–P plating.