The Role of Hydrogen in the Stress Corrosion Failure of High Strength Al-Zn-Mg Alloys.
Annual technical rept.,
IMPERIAL COLL OF SCIENCE AND TECHNOLOGY LONDON (ENGLAND)
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Two alloys, A1 6 Zn 3 Mg 1.7 Cu and A1 6 Zn 3 Mg 0.14 Cr artificially aged at 120 C for times up to 400 hours have been tested at room temperature in tension in two aqueous environments, distilled water and 3.5 sodium chloride solution at strain rates of 10 to the minus 4th powersecond, 10 to the minus 5th powersecond, and 10 to the minus 6th powersecond. The results have been compared with previous data on fast and slow strain rate pre-exposure tests. Also conventional dead load tests were carried out in laboratory air and distilled water using the copper containing alloy. Both the slow strain rate technique and the dead load test are closely related and the observations are consistent with the hydrogen model for embrittlement via reaction of the metal with the testing environment during straining or loading. The hydrogen adsorbed into the grain boundaries and the grain matrix causes grain boundary decohesion and cleavage. The ability to trap this hydrogen as innocuous hydrogen bubbles improves the embrittlement resistance. A peak embrittlement resistance is found to be around 150-250 hours of ageing at 120 C for the copper quaternary alloy. The embrittlement resistance mechanism and kinetics are not fully understood and may be complicated by the dissolution processes which accompany hydrogen generation. In this context the observation of grain boundary oxide films on the fracture surfaces of a small number of the dead load samples may be of importance.
- Properties of Metals and Alloys