Hi Nigel

The mechanism for stress corrosion cracking is two fold. Firstly the material must be under a tensile stress, either a residual stress or an applied stress. Neither is significant to the failure of the material. SCC does not occur when the stress is from compression, so shot peening an object will prevent SCC.

The corrosion aspect is not completely understood. When certain materials are in the presence of certain mediums, a passive film is formed, but it appears that in SCC that within the crack that this film fail within the crack region (I suspect it breaks down at the crack tip, where the stress is exposing fresh material).

The cracks themselves can be intergranular or transgranular. However the particular form that occurs, is dependent upon the specific alloy - environment combination. Final failure of the component is by ductile overload usually.

I not aware of which environments cause problems for 7075 aluminum alloy, however aluminum it typically susceptible to seawater, NaCl solutions, NaCl-H₂O₂ solutions, water vapour and would you believe, air.

I hope this helps

Nigel,

I realise that this might be a bit late, but I would suggest reading Stress-Corrosion Cracking materials performance and evaluation by Russel H Jones, he has a chapter dedicated to the SCC of aluminium alloys, the 7075 alloy is used frequently as an example throughout.  If you are interested in the potential differences created I would suggest looking at the Pourbaix diagrams.

I agree with AlanD, with the various environments he has suggested it is the halide ions which are damaging, in particular Cl-, thus most lab tests use Cl.

Hope this is of some use