I hear that it is a significant problem at Iberia with their A340s particularly suffering. Whilst it is not causing corrosion it is causing filters to be blocked and is playing havoc with the wing electrics (fuel contents). So much so that the wiring and tank units have to be replaced on a frequent basis.
I did some work on several BAE 146s that had been used in Columbia. The wings were rotten and needed wing spars, skin and all the pipework replaced.
It is a problem in some parts of the world.