From Zn-Al LDH to ZIF-8@Zn-Al LDH conversion coatings on the surface of AA2024 alloy: inside the process and the effect of the transformation on the protective properties

Abstract

Nowadays, interest in metal organic frameworks (MOFs) as potential materials for corrosion protection of aluminium alloys is increasing. However, application of MOFs in the form of conversion coatings remains limited due to challenging process of MOFs growth directly on Al based surfaces. This obstacle can be overcome by surface pretreatment that promotes further MOF formation. In the current investigation, Zn-Al LDH (layered double hydroxide) grown on the surface of AA2024 aluminium alloy was recrystallised into ZIF-8@Zn-Al LDH coating. In situ synchrotron and ex situ XRD analyses showed that recrystallisation of Zn-Al LDH into ZIF-8 was accompanied by intercalation of 2-methylimidazolate into the LDH gallery under the applied treatment conditions. Such a complex structure of the coating was beneficial for the corrosion protection of AA2024 alloy as the obtained coating contained an increased amount of 2-methylimidazole inhibitive species. Moreover, it was found that the variation of the treatment condition (95–140 °C, 3–24 h) affected the final performance of the ZIF-8@Zn-Al-LDH coating and the coating obtained at 95 °C for 12 h demonstrated the best performance.