A wrong type of grease used when fitting press fit bolts to the booms caused corrosion, thus weakening the lower starboard boom at the outboard bolt hole
That is from the aforementioned Viking accident report.
The Valiants suffered corrosion at Station O which is where the wing spars were mounted onto the fuselage truss. They had one rear spar fracture in flight but the other spar held the wing together. The crew were lucky as there was quite a split on the wing's skin.