Magnesium alloys testing

The magnesium alloys tested became more sensitive to small notches and increased in tensile and yield strength as the temperature was lowered. 

Some tests indicate that magnesium alloys are resistant to loam sod. However, in the presence of chlorides, corrosive attack may be serious particularly if galvanic couples are present as a result of coupling to iron stmctures. 

Standard Test Methods For Chemical Analysis of Magnesium and Magnesium Alloys, ASTM E-35 to 88, American Society for Testing and Materials, Philadelphia, Pa., 1992. 

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. 

Magnesium-air air cells with NaCl-electrolyte were developed and investigated. The current-voltage and the discharge characteristics of the cells with were studied. Air gas-diffusion electrodes suitable for operation in NaCl-electrolytes were designed. Various carbon-based catalysts for the electrochemical reduction were tested in these air electrodes. Magnesium alloys suitable for use as anodes in Mg-air cells were found. 

Slow strain test. The strain rate chosen frequently for the tests, based on several studies, indicates important susceptibility to cracking at about 2 x 10 6 s 1 for steels, aluminum and magnesium alloys. However, the tests refer to open-circuit conditions and the strain rate sensitivity of cracking is dependent upon potential as well as solution composition. Where necessary the potential of the specimens can be controlled using a potentiostat during slow-strain-rate tensile testing.171 The reduction of area is a simple and appropriate way to quantify the susceptibility to SCC. 

Corrosion Resistance of Aluminum and Magnesium Alloys Understanding. Performance, and Testing Edward Ghali... 

The effect of plasma electrolytic oxidation (PEO) treatment on the SCC of surface-modified magnesium alloys was studied [167]. PEO coating offered improved corrosion resistance. However, the barrier film did not improve the SCC resistance in ASTM D 1384 test solution. The SCC of PEO-coated specimens was attributed to the development of micro cracks in the coating, leading to substrate cracking under SSRT test conditions [167]. 

B. A. Shaw, Corrosion resistance of magnesium alloys, in ASM Handbook, Vol. 13A, Corrosion Fundamentals, Testing, and Protection, ASM International, Materials Park, OH, 2003, p. 693. 

Table I. Results of Tensile Tests of Welded Aluminum-Magnesium Alloy Plate at Low Temperatures... 

Tensile tests of notched specimens from plain and welded plate have been made to measure the notch sensitivity of aluminum-magnesium alloys at low temperatures. For the plain plate, specimens with four different notches, as shown in Fig. 2b were tested at room temperature and at -320 F. For the welded plate, Type I specimens from across the weld with the deep sharp notch machined in the center of the weld itself were tested at the same two temperatures. The results were compared with those obtained on standard 1/2 in. diameter tensile specimens (ASTM E8-57T). 

The obvious effect of specimen geometry on strength indicates that data obtained from tests of notched specimens should be interpreted with caution. Considerably different ratios were obtained with the various notches and no one notch is wholly suitable for the evaluation of notch sensitivity under conditions other than those in this test. However, the data are sufficient to indicate that plain and welded aluminum-magnesium alloy plate is relatively insensitive to notches, at least to temperatures as low as -320 F. 

Table V- Average Results of Tear Tests of Some Aluminum-Magnesium Alloys... 

J. Sawkill and D. James, "Tear Tests on Aluminum-Magnesium Alloy Plate, British Welding Journal (November, 1958). 

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