Investigation of Two Main Types of Intergranular Corrosion of Aluminium Alloys Depending on Their Structure and Passivation Features

摘要

Intergranular corrosion (IGC) of 3003, 3105, 5456, 2024, 6013 aluminium alloys was studied by corrosion and electrochemistry methods, TEM, X-ray microprobe analysis. It was found that IGC for aluminium alloys may exist in two forms. The first form is crystallographic (CF IGC). It is governed by recrystallization features. High-angle boundaries at a certain recrystallization stage store up a lot of vacancies which coagulate partially. As a result, the boundaries become thermodynamically labile. Boundaries electrode potential is in the pitting (Pt) initiation region and the matrix is in passivity region. The second form of IGC is structure decomposition (SD IGC). It is governed by supersaturated solid solution decomposition features. In this case only a narrow near boundary area, formed near non coherent (NCP) or partly coherent (PCP) second phases, precipitated on grain boundaries, is present in a Pt region. While the decomposition level grows the same areas are formed near NCP or PCP phases inside of grains. It leads to the redistribution of corrosion in a metal volume and to the reduction of corrosion near grain boundaries and in the end to the full transition from SD IGC to Pt.