Evidence for the presence of corrosive solution within corrosion products film in magnesium alloy ZK60

Abstract

It has been recently suggested that it is the corrosion solution sealed within the corrosion products layer that can be the primary cause of pre-exposure stress corrosion cracking (PESCC) of Mg alloys. In the present study, we attempt to find additional evidence for the presence of the retained corrosive media in the corrosion products layer deposited on the surface of the ZK60 alloy. The samples of this alloy were pre-exposed to the corrosive solution for 1.5 h and then subjected to the thermal desorption analysis (TDA). The specimens were tested either immediately after pre-exposure or after the post-exposure immersion in CCl4 or storing in desiccator for 24 h. During the immersion in CCl4, the volume of hydrogen desorbing from the samples was assessed. The main finding of the present study lies in that the corrosion reaction remains within ZK60 samples even after their extraction from the corrosive solution. This is evidenced by the extensive hydrogen desorption which continued and not ceased even after 24 hours after pre-exposure. It is established that the largest part of this hydrogen cannot be detected by short TDA of the sample right after pre-exposure to corrosive media, thus it should be produced in-situ during the immersion in CCl4. Considering that CCl4 is inert with respect to Mg, it is concluded that the corrosion reaction producing hydrogen can be caused only by aggressive media retained within the corrosion products layer.