Was getting a new muffler on one of the cars the other day and whilst there found a brochure relating to the topics of this post.
It was a re-run of an article attributed to "NRMA Tech Topics Bulletin 1/98" and whilst it's long & involved, I'll try to summarise it here as it may answer a lot of questions associated with the problems we get on some of the froggies. These being certain early model BXs having holes eaten through the side of the blocks, Xantias/BX/Pug 307 having heater matrix corrosion problems, CXs having water pumps and thermostat housings eaten away and so on.
Common response is to blame the quality of manufacture of teh original or replacement parts but as I discovered on a couple of occasions on UK boards, often different menufacturers have been used with the same results; why then does this happen?
Again, we often talk about the coolant; it's age, type and consistency and whilst all these factors are attributable to the failures, it's interesting to understand why.
Now before we go too deeply into this, it's worth remembering that these guys were talking predominantly about aluminium radiators but not exclusively. I think that it's important to keep this is context; modern cars = plastic, aluminium, alloys, steel, brass, copper and other synthetic materials in both their body structures as well as their mechanical parts including engines, so as a result, the coolant can be a conduit throughout the system conducting electricity whilst at the same time creating static electricity given the correct set of circumstances. This static electricity then will set off corrosion if allowed to circulate in the system so we have to:
(a) Test to detect if there is any electrical charge in the system.
(b) Try to isolate the source/cause
(c) Rectify and then re-test.

(a) Test to detect if there is any electrical charge in the system.

If the radiator cap is in good condition, attach a probe of a multi meter to the centre (metal) pin on the cap. If this is not possible, a probe needs to be placed into the coolant but touching coolant only - not any part of the radiator. The other probe is then placed onto the negative side of the battery. If the reading is above 50mv, change the coolant!! The reason being that it has been proven that the chemical composition of some coolants does not allow them to dissipate the charged oxygen that has resulted from the stray current.
Mixing of certain coolants has been known to cause formation of hard acids that will strip away the metal with amazing speed. Incorrect dilution particularly if using low quality water can do the same.
It has been also shown that vehicles that have been to the panel shop are very prone to this stray voltage problem due to earth wires being removed, damaged or painted over. Any car that has had any work on it is a potential victim if a wire has inadvertantly been left off, broken or paint or anti-rust preventative allowed to get into the thread of an earth connection anywhere on the car associated with the cooling system.
Don't think that an earth wire fitted to the Aluminium radiator will cure it; it can actually make it worse. (The opposite usually applies to copper cored radiators though.) The internal conduction of electricity is controlled mainly by the coolant; correct (Ethylene glycol) coolant at the correct proportions using a demineralised water and lack of cavitation/aeration reduces this conductivity and percentage of oxydisation.
To test your coolant when you buy;
Shake the bottle & let stand for 5 seconds. If the foam has broken and the fluid returned to normal in that time, chances are it won't foam in the cooling system and risk creating "cavitation corrosion." If the foam remains, find another coolant.
If the problem has been detected, a complete drain out and flush is essential as any traces of the old coolant could contaminate the new. Earth continuity through various parts of the vehicle is essential.


Method of testing.

Have an assistant put as much load as possible on the electrical system whilst you check if stray electric exists. By that we mean, lights, brakes, air/con everything electrical turned on, but do it systematically, one at a time while you watch the multi-meter so if there's a sudden jump in the reading, the source is easier traced. (ie) if say when the reversing or brake lights are activated and the reading doubled, the earth in the region of the tail light assembly would be a likely suspect.

When completed doing the coolant change and reckecking, cleaning and/or replacing dodgy earthes, simply revert to (a) and do a complete retest. Do it again after a period of a few weeks or a few thousand klms and this should prove if the fault has been detected and rectified.

I hope this might help a few who I know have had ongoing corrosion & cooling problems.
I still have the article at hand so if there's anything I've missed, just ask & I'll see if I can find it from this Tech bulletin & answer if possible.

Alan S