Magnesium chloride solution, stress-corrosion

The occurrence of stress-corrosion cracking in the martensitic steels is very sensitive to the magnitude of the applied stress. For instance, a 13% chromium martensitic steel tested in boiling 35% magnesium chloride solution (125.5°C) indicated times to failure that decreased abruptly from more than 25(X)h to less than 0.1 h as the applied stress was increased from 620 MPa to about 650 MPa (Fig. 8.25). However, the effects of stress on time to failure are not always so dramatic. For instance, in the same set of experiments times to failure for a 17Cr-2Ni martensitic steel gradually decreased from more than 800 h to about 8 h as the applied stress was increased from 500 MPa to 800 MPa. 

Fig. 8.30 Effect of nickel content on the susceptibility to stress-corrosion cracking of stainless steel wires containing 18-20% chromium in a magnesium chloride solution boiling at 154°C...

J.I. Dickson, A.J. Russell, D. Tromans, Stress corrosion crack propagation in annealed and cold worked 310 and 316 austenitic stainless steels in boiling (154 °C) aqueous magnesium chloride solution, Can. Metad. Q. 19 (1980) 161-167. 

O.M. Alyousif, R. Nishimura, Stress corrosion cracking and hydrogen embrittlement of sensitized austenitic stainless steels in boiling samrated magnesium chloride solutions, Corros. Sci. 50 (2008) 2353-2359. 

ASTM G36-87 (1987) Standard Practice for Evaluating Stress Corrosion Cracking Resistance of Metals and Alloys in a Boiling Magnesium Chloride Solution. 

Figure 11.35 Time to failure for different austenitic stainless steels subjected to constant-load stress corrosion cracking testing in boiling magnesium chloride solution [15].

Several environments and specimen designs are available for SCC evaluation [74]. ASTM G 36, Practice for Evaluating Stress-Corrosion Cracking Resistance of Metals and Alloys in a Boiling Magnesium Chloride Solution, provides a more severe environment than generally found in service. Resistance in this test depends largely on the nickel content and most austenitic and duplex stainless steels are readily cracked. 

Cracks resulting from stress corrosion cracking (SCC) can occur in stainless steels in solutions containing chloride ions at temperatures above approx. 323 K (50 °C). The cracks are mainly transgranular and they occur when a critical threshold potential is exceeded. Figure 10 shows test results obtained in the boding 42% magnesium chloride solution frequently used in laboratory tests [26]. 

Figure 4.50 Breaking time of iron-nickel-chromium wires in boiling 45% magnesium chloride solutions. (From Copson, H.R. (1959). Phy. Metallurgy of Stress Corrosion Fracture, Interscience Publishers) (88]...

M. Smialowski, J. Kostanski, Creep and stress corrosion cracking of austenitic stainless steel in boiling 35% magnesium chloride solution. Corrosion Sci., 19(12) (1979) 1019-1029. 

In tests lasting for 14 days, Copson found that the susceptibility of steel to stress-corrosion cracking in hot caustic soda solutions increased with increase in nickel content up to at least 8-5%. Alloys containing 28% and more of nickel did not fail in this period. In boiling 42% magnesium chloride the 9% nickel-iron alloy was the most susceptible of those tested to cracking (Table 3.38). Alloys containing 28 and 42% nickel did not fail within 7 days. 

The fracture mode of stress-corrosion cracks in austenitic stainless steels can be transgranular, intergranular or a mixture of both. One of the earliest environments found to cause problems was solutions containing chlorides or other halides and the data due to Copson (Fig. 8.30) is very informative. The test solution for that data was magnesium chloride at 154°C the alloys contained 18-20alloy with a composition of approximately 18Cr-8Ni has the least resistance to cracking in this environment. 

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. 

Transgranular stress corrosion cracks are known [7.49] from i) austenitic steels in acidic chloride solutions, ii) low-strength ferritic steels in acidic media, iii) ferritic steels in phosphate solutions, iv) carbon steel in water saturated with CO2 and CO, v) a-brass in ammonia solutions that do not cause surface films, vi) aluminium alloys in NaCl/K2Cr04 solutions and vii) magnesium alloys in diluted fluoride solutions. For further study of fracture surface appearance, see, e.g. Lees [7.49] and Scully [7.53]. 

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