Immersion magnesium alloys

In considering the corrosion of magnesium and its alloys it is important to examine the methods available for assessing corrosion tendencies and particularly those known as accelerated tests. Tests carried out by immersion in salt water or by spraying specimens regularly with sea-water are worthless as a means of determining the resistance of magnesium alloys under any other than the particular test conditions. Extrapolation to less corrosive conditions is not valid and even the assessment of the value of protective measures by such means is hardly possible. The reason is to be found in the fact that corrosion behaviour is directly related to the formation of insoluble... 

It may be felt that the initiation of a stress-corrosion test involves no more than bringing the environment into contact with the specimen in which a stress is generated, but the order in which these steps are carried out may influence the results obtained, as may certain other actions at the start of the test. Thus, in outdoor exposure tests the time of the year at which the test is initiated can have a marked effect upon the time to failure as can the orientation of the specimen, i.e. according to whether the tension surface in bend specimens is horizontal upwards or downwards or at some other angle. But even in laboratory tests, the time at which the stress is applied in relation to the time at which the specimen is exposed to the environment may influence results. Figure 8.100 shows the effects of exposure for 3 h at the applied stress before the solution was introduced to the cell, upon the failure of a magnesium alloy immersed in a chromate-chloride solution. Clearly such prior creep extends the lifetime of specimens and raises the threshold stress very considerably and since other metals are known to be strain-rate sensitive in their cracking response, it is likely that the type of result apparent in Fig. 8.100 is more widely applicable. 

Other investigations indicate that half-tide conditions give results similar to fully immersed conditions, and that in sea-water the 99-5% aluminium coating is preferred to aluminium-zinc or aluminium-magnesium alloy coatings. 

Example 17.1 Plots for Mg Alloys Consider the global impedance response for an as-cast magnesium alloy AZ91 presented in Figure 17.11 for the AZ91 alloy at the corrosion potential after different immersion times in 0.1 M NaCl. What quantitative information can be obtained without considering a detailed process model such as discussed in Example 10.3 ... 

In aqueous exposures, only strong alkalis, concentrated hydrofluoric acid, and chromic acid are considered not to induce SCC in magnesium alloys [5]. In other exposures from deionized water to a wide variety of aqueous s t solutions SCC failures have been observed in full, partial, or alternate immersion [32]. The most severe acceleration of failures has been found to occur with NaCl + K2Cr04 solutions, which have been used in many laboratory studies of magnesium SCC however, the results have been found to correlate poorly with service experience [28,31,33]. 

To evaluate magnesium alloys both immersion and salt spray tests are often used. Table 7.6 shows corrosion rates obtained after different types of accelerated corrosion tests taken from the literature (52, 62, 63). The corrosion rates show a wide scatter. For some of the accelerated corrosion tests such as the immersion in NaCl solution the corrosion rates are extremely high. 

The combination of immersion, hydrogen collection and weight loss measurement is an easy corrosion evaluation method, particularly for magnesium alloys. The method, first established and used by Song et al. [5] to estimate and monitor the corrosion rate of magnesium in a NaCl solution, has been widely adopted as a common corrosion rate measurement technique for magnesium alloys in various aqueous solutions. The reliability of the method has been theoretically and experimentally demonstrated [24,25] and the details will not be repeated here. 

As a final example in this section, we consider the work of Karavai et al. who used Mg and pH-selective microelectrodes in combination with SVET to study the corrosion inhibition in microdefects on sol-gel-coated AZ31 magnesium alloy immersed in chloride media [89]. The inhibitors investigated were 1,2,4-triazole, F, and Ce ". Both types of ion-selective microelectrodes were... 

Continuous immersion test for 7000 series (aluminum-zinc-magnesium) alloys (Ref 36) aqueous solution containing 3% NaCl, 0.5% hydrogen peroxide (30%), 100 m/L 1 N sodium hydroxide, and 20 m/L acetic acid (100%) pH 4.0... 

Other Baths. Other forms of 2inc plating are also in use. Immersion 2inc deposits are used as a preparatory step in electroless plating or electroplating of aluminum (146), magnesium (147), and beryUium (148) alloys. Formulations vary with the appHcation typical baths are Hsted in the references cited. 

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