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 CCl 4 or storing in desiccator for 24 h. During the immersion in CCl 4 , 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 CCl 4 . Considering that CCl 4 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.

Effect of Air Storage on Stress Corrosion Cracking of ZK60 Alloy Induced by Preliminary Immersion in NaCl-Based Corrosion Solution

“…The low-temperature peak indicated as #1 in Figure 12 b is attributed to hydrogen, which freely evolves from the CP layer at room temperature. This is evidenced by the absence of this peak on the thermal desorption spectra corresponding to the pre-exposed specimens subjected to air storage at 24 °C for 24 h. Moreover, the bubbles of hydrogen gas emanating from the pre-exposed specimen can be seen by the naked eye when this specimen is submerged in an inert liquid such as CCl 4 [ 26 ]. This hydrogen can be associated with the molecular or atomic hydrogen liberated from the corrosion product layer as well as with hydrogen, which is produced in situ by the corrosion reaction [ 26 ].…”

“…This is evidenced by the absence of this peak on the thermal desorption spectra corresponding to the pre-exposed specimens subjected to air storage at 24 °C for 24 h. Moreover, the bubbles of hydrogen gas emanating from the pre-exposed specimen can be seen by the naked eye when this specimen is submerged in an inert liquid such as CCl 4 [ 26 ]. This hydrogen can be associated with the molecular or atomic hydrogen liberated from the corrosion product layer as well as with hydrogen, which is produced in situ by the corrosion reaction [ 26 ]. Since the pre-exposed specimens do suffer from appreciable embrittlement after 24 h of air storage at 24 °C but do not exhibit peak #1 in the thermal desorption spectra, the hydrogen associated with this peak is not sufficient to be the sole reason for PESCC.…”

“…Hydrogen can be produced during pre-exposure of the specimen as well as during the subsequent air storage if the corrosion solution sealed within the corrosion product layer interacts with the bare metal of the specimen. Indeed, it was shown recently that a large portion of hydrogen gas evolved from the pre-exposed specimen during its 24 h storage in CCl 4 [ 26 ]. The volume of this hydrogen summed with that of hydrogen extracted from the same specimen during subsequent gas analysis was twice as high as the volume of hydrogen extracted from the counterpart specimen subjected to the gas analysis right after pre-exposure.…”

“…Along with time after extraction of the specimen from the corrosion solution, the weakly bonded part of this hydrogen desorbs from the corrosion product layer even at room temperature. This is witnessed by the low-temperature desorption peak in Figure 12 b as well as by the naked-eye-visible hydrogen bubbles released from the specimen immersed in CCl 4 right after pre-exposure [ 26 ]. The remaining part of hydrogen sitting within the corrosion product layer is bonded more strongly and, therefore, desorbs only at higher temperatures and longer times.…”

“…Thus, it was concluded that there must be an aggressive medium retaining inside the corrosion product layer on the side surface to form the corrosion products on the fracture surface. Later on, the presence of the corrosion solution within the corrosion product layer was additionally evidenced by the evolution of extra hydrogen generated by the corrosion reaction inside the specimens after pre-exposure to corrosive media [ 26 ].…”