While I am not familiar with the specific material used on the Volvo engine, I can tell you that there are certain materials which are know to suffer from "stress corrosion cracking". I learned that when I was working as a structural engineer on the Lunar Module (1968) at Grumman Aerospace Corp in Long Island , NY. We had LM 3 in the clean room (no possible corrosion there), yet a part installed in the Aft Equipment Bay was found cracked. I got all excited and phoned my boss (on a Sunday). He calmed me down and explained it was a new phenomenon just discovered in certain metals. If when they are fabricated, or installed and the part is under a constant tension stress, then a small crack will develop and the part will eventually crack (fail) with no other load on it. Aluminum 7075-T651 and other types of alloys that are very susceptible to stress corrosion cracking. So, it could be that the Volvo part was casted (made) with a residual tension stress in it (possibly during cooling), so it just took a long time (not loading) to crack it. For more information go to the Internet and type "stress corrosion cracking" and you may get a mouth full of information on the phenomena. We in the aerospace industry have learned what to avoid it in terms of material and alloys, but also in preventing "clamp-up" which leaves a tension residual on certain parts when they are attached on a not smooth surface.
It was time to retire that part.
