The coolant type deployed is completely irrelevant to this problem assuming we are talking about an ethylene glycol/water system thus the notion that a specific type or make of coolant can cause this problem is nonsense.

I should politely correct your comment about cars sat in coolant for years on end. Providing there was nothing wrong to start with, coolant can and does sit in these engines for years on end and nothing of any consequence will happen. Folks scratch their heads when they see or hear of this but we have seen such on this list a number of times. When the coolant is at ambient temperature nothing happens. Ethylene glycol is an oil- it is not an agressive fluid- quite the opposite. Water is only a problem when it is oxygenated but even in such condition aluminium and its alloys are impervious to such due to the passivation of the bare metal that is covered with a thin film of aluminium oxide that prevents corrosion. Indeed for the most part the anti corrosion chemicals in the mix never get exhausted or anything vaguely close to it.

On the other hand the buffering chemical package is what gets exhausted eventually and even then it takes a lot of doing to get a coolant into that state. Not sure what they use in modern coolants but historically borax was used to achieve this and thus give the coolant out of the bottle a pH of 9 thus why it has a "slippery" feel to it. What mosts folks do not know is why this is done. Ethylene glycol is thermally unstable at temperatures in excess of 55C. If and when the buffering package gets exhausted and the pH is neutral [pH7] once the coolant gets up to temperature it starts to break down and eventually forms a toxic brew of various organic acids including formic acid, oxalic acid, carbolic acid and whatever else takes its fancy. This then becomes a huge problem for the aluminium alloy because once the pH drops to about pH4 the passive oxide film breaks down completely and then the stuff goes through the alloy like a knife through butter. Despite the potential and popular cooling system mythology- this rarely happens and I have personally never seen a single example of this posted on this list. Thus why all this mythilogy about exhausted coolant is what it is. If it did happen there would be no mistaking it- the entire cooling system would be trashed including the water jacket, the inside passage ways of the heads, the radiator etc etc- strangely enough we have never seen such posted. Bottom line it really takes a lot of effort to get coolant into this state. This is why the state of the coolant can be assessed using test strips for fish tanks- check the pH - if it is OK the coolant is still viable.

Once coolant manages to get into the annulus between the gasket and the head bounded by the areas that are not counter supported on the block side- that is when the fun starts. In the stagnant layer the chemical package and more specifically the buffering chemicals get exhausted and at operating conditions the ethylene glycol starts to break down. This creates a localised acidification process and over time this nibbles away at the Buna N binder material holding the aramid fibres together within the gasket. Once the outer layer of the gasket is breached the gunge can then get between the two sheet layers that are pressed onto a a galvanised barbed steel mesh and then we see this material getting attacked hence the charcteristic staining of the reddish orange ferric oxide [Fe2O3] that is duly formed. This in turn takes up a much greater volume than the steel it consumes and thus the gasket typically swells. The acid gunge also etches the aluminium heads but does not in and of itself create a chronic corrosion. At some stage the acidificaiton process creates a situation wherein the elecro chemical potential difference becomes sufficent to create a flow of electrons. When that happens that is when the crevice corrosion attack is initiated. This is what creates the serious damage when it happens and once that starts it does not stop unless and until the the main body of the coolant is drained. The indisputable fact of this case is that when a pitting attck occurs in a gasketed flanged joint that is a 100% certain indication of a crevice corrosion attack given to is the only corrosion process that can cause such. This i not to be confused with "Pitting Corrosion" that causes similar damage but only does so in open flow channels where the process fluid is circulating freely.

Thus if the system is shutdown and left to stagnate when this process has already started [unknowingly to the owner] the process will continue if the coolant is left in-situ. In such situation serious damage can happen long term. If this process has not started nothing happens. Thus why the trigger condition is permeation of coolant into the annulus. No permeation- no problem.

Now if and when one understands this lot one can then understand why some 928's sit a long time in coolant and never have a problem and some sit a long time in coolant and get trashed. Do you really think "spanner jockey's" are vaguely capable of fathoming this lot out?

40 plus years of experience dealing with similar issues [and way way more] as a world class Engineering and Projects manager in the oil industry with major oil companies - the most corrosive environment known to engineers -I have personally seen and had to deal with the extremes of this business.

I just fall about laughing when I read other opinions about how what I say is nonsense and yet the detractors have absolutely squat to offer other than "it was the coolant".