Inhibiting stress corrosion cracking by removing corrosion products from the Mg-Zn-Zr alloy pre-exposed to corrosion solutions

On the Corrosion Fatigue of Magnesium Alloys Aimed at Biomedical Applications: New Insights from the Influence of Testing Frequency and Surface Modification of the Alloy ZK60

“…Nevertheless, one can notice that the flat, fibrous fracture surface topology bears a lot of similarity to that reported for magnesium alloys fractured in physiological solutions under stress corrosion cracking (SCC) conditions (e.g., AZ91D [103,104] or AZ31 and ZK60, as shown by some of the present authors in dedicated investigations [28,105,106]). Figure 13 illustrates some of the observed features, which are typical of SCC in many Mg alloys (including the same ZK60 [28,[105][106][107][108]) and are not seen in the specimens conventionally fatigued in air-there are visible signatures of intergranular brittle fracture (marked by red arrows) and multiple secondary cracks-and the characteristic fluted relief (marked by white arrows), which is indicative of SCC-induced plasticity ahead of the crack tip (flutes represent specific elongated dimples formed when voids are nucleated along slip-band intersections [109]). Of course, these features, which are sporadically observed on the fracture surface of the specimens fatigued in corrosive media, do not represent the predominant fracture mechanism, but rather, they indicate the plurality of the mechanisms involved in the fracture processes.…”

“…Ca, Y, and Zr are by far the most popular additions to Mg-Zn systems when seeking a balanced combination of mechanical and biofunctional properties [11][12][13][14][15][16][17][18][19][20][21][22][23]. The commercial alloy ZK60 has been proven versatile enough for processing by various techniques, from conventional extrusion and rolling to a wide range of severe plastic deformation techniques, resulting in substantial microstructure refinement and the enhancement of tensile [24], fatigue [25] and corrosion properties [26][27][28]. One of the factors impeding the broader uptake of Mg alloys is their poor corrosion resistance, resulting in a faster-than-desired degradation rate in chlorine-containing solutions, including human body fluids [29][30][31].…”

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

“…However, this interpretation of the origin of PESCC has been challenged by the present authors recently on the basis of several new experimental findings reported in [11][12][13]. Specifically, it was found that neither the loss in ductility nor the appreciable concentration of diffusible hydrogen is observed in Mg alloys pre-exposed to the corrosive solution if the corrosion products layer is removed from the specimens' surface prior to the subsequent tensile testing in air [12,13]. These findings imply that PESCC should be attributed merely to the corrosion products layer.…”

“…The cylindrical samples of 6 mm diameter and 30 mm length were machined from the hot-extruded bar of the commercial alloy ZK60, which is known to be strongly susceptible to PESCC. The chemical composition, microstructure and mechanical properties of this alloy as well as its susceptibility to SCC and PESCC at various conditions, were documented in detail in our previous reports [11][12][13][14][15][16]. The samples were immersed in the aqueous corrosive solution of 4 % NaCl + 4 % K 2 Cr 2 O 7 at the open-circuit potential and ambient temperature of 24°C.…”

“…Since the corrosive solution does not interact directly with the metal during mechanical testing, and, consequently, the effects associated with AD can be excluded, PESCC is commonly attributed to hydrogen being absorbed by the metal during pre-exposure to corrosive media [6 -10]. However, this interpretation of the origin of PESCC has been challenged by the present authors recently on the basis of several new experimental findings reported in [11][12][13]. Specifically, it was found that neither the loss in ductility nor the appreciable concentration of diffusible hydrogen is observed in Mg alloys pre-exposed to the corrosive solution if the corrosion products layer is removed from the specimens' surface prior to the subsequent tensile testing in air [12,13].…”