Magnesium stress-corrosion cracking

N Winzer, A Atrens, W Dietzel, G Song, KU Kainer, Magnesium stress corrosion cracking. Transactions ofNonferrous Metals Society of China, 2007,17, S150-S155 Part A Sp. Iss. 1, Nov. 

Tensile strength diminishes rapidly with increasing temperature above 200°C. The high-magnesium alloys N5, N6 and N8 should not be used above 65°C because higher temperatures make them susceptible to stress corrosion cracking. 

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 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. 

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. 

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...

Stress-corrosion Cracking of Titanium, Magnesium and Aluminium Alloys... 

Stress-corrosion cracking of iron-chromium-nickel wires in boil 1 ing 42% magnesium chloride. Redrawn from Ref 132... 

A.F. Beck and P R. Sperry, The Relationship between Structure and Susceptibility to Stress Corrosion in Aluminum-Magnesium Alloys, Fundamental Aspects of Stress Corrosion Cracking NACE I, R.W. Staehle, A.J. Forty, and D. Van Rooyan, Ed., National Association of Corrosion Engineers, 1969, p 513-529... 

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. 

N. Winter, A. Atrens, W. Dietzel, V. Song, K.U. Kainer, Comparison of the hnearly increasing stress test and the constant extension rate test in the evalnation of transgranular stress corrosion cracking of magnesium. Mater. Sci. Eng. A 472 (2008) 97—106. 

P.B. Srinivasan, C. Blawert, W. Dietzel, K.U. Kainer, Stress corrosion cracking behaviour of a surface-modified magnesium aUoy, Scr. Mater. 59 (2008) 43—46. 

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. 

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]. 

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

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