Stress corrosion cracking in alumnium alloy AFNOR 7020-T6 water tank adaptor for liquid propulsion system

A torroidal shaped tank in the liquid propulsion system provides water for initial run of the turbo pump and for cooling various components. This is made of high strength aluminium alloy Al–4.5Zn–1.5Mg designated as AFNOR 7020. Pressurisation adaptors are initially shrink fitted to the openings provided in the water tank and later TIG welded circumferentially to configure the tank assembly. During one of the functional checks, a crack was noticed on the weld joint of the adaptor. Detailed metallurgical analysis indicated that the failure was due to stress corrosion cracking (SCC).This paper brings out the metallurgical analysis carried out which confirms the SCC.