It was not corrosion that caused the loss of Boeing 707-321C G-BEBP, it was fatigue cracking plus the failsafe design of the tailplane structure didn't behave in the way predicted. The accident investigation is available online.

ISTR the top spar of the horizontal stab had a known problem with cracking around one fastener. The NDT procedures used at the time to monitor the crack and inspect the centre and lower spars was not able to detect a crack in the centre spar. These cracks caused the stab to fail and fold up against the vertical stab.
Inspection of the worldwide fleet of high time Boeing 707s found several more about to fail. There was a mod. program that involved strengthening the centre spar, stress relief shot peening the upper and lower spars and replacing all the fasteners with Hi-Locks.
I worked on one of these mods at BCAL around 1981. Several weeks of long nights I think.

Dan's did operate holiday charters with the 707-321s, SL and TG.
I Flew on "Spread legs" MAN-GRO in about 1971 on a package holiday, returning on 727 G-BAFZ.

Would it not be more likely that the stab failed downwards, given that during normal 1G flight (even allowing for gusts) the stab is generating a down force?

I see your point about the flight loads on a stab.
This summary just says when Flaps 50 was selected the right horizontal stab separated. I though I'd seen a report that it folded upwards before departing.
https://aviation-safety.net/asndb/329129
Dixi.

I described above how old 707 G-AYSL, Sierra Lima, became universally Spread Legs with its various UK operators, and it even gets a mention as such in Alexander Frater's book "Beyond the Blue Horizon", which all good followers of this thread should have. Flying eastwards from Bangladesh, in 1985, he chances to sit next to a positioning BA Tristar flight engineer, who mentions the aircraft by soubriquet as it having been his first mainstream assignment.

I believe that was the case.

At some point after the Lusaka accident we were not allowed to use speedbrake in flight.
Got quite good at requesting extra track miles!!