Adk46

Crevice corrosion. Weird **** happens in a crevice compared to elsewhere, producing differences that lead to electrochemical potentials that make electrons want to move. Losing electrons is the definition of oxidation. The specific weirdness is something about which a corrosion specialist (e.g., my doctoral thesis advisor) is happy to provide many fascinating theories, but can't tell you which one is operating.

All you can do is keep your electrons from wandering about.

More useless info: rubber contains carbon, a common battery electrode. Do not look upon rubber as harmless.

FredR

Things are looking up- good to know you aware of the concept!

Indeed wondering electrons are at the root of the problem but the problem is a bit more complex than that. One only has to look at the prerequisites to understand the potential for such to happen viz:
1. oxygen depletion- the system is closed and oxygen will be depleted. The passivity of the aluminium is thus depleted.
2. Chemical inhibitors exhausted in the crevice- ethylene glycol/water system is quite innocuous at room temperature- the glycol molecule is highly polar [hydroxyl anions] and at temperatures above around 50C it wants to form organic acids such as Oxalic and Carbonic acids to name a couple. This is generally prevented by having an alkaline pH around 9. As I am aware this is what sodium silicate does [as well as inhibition] but as the chemicals are exhausted within the crevice, sooner or later oxidation occurs due to the electrochemical potential difference between the crevice and the main body of coolant and then the acidity rises dropping to about pH2 and off it goes- game set and match eventually.

One would like to think Porsche were well aware of such possibility when they designed these cars but given they could not fathom out how to stop my flex plate clamp migrating 20 years ago and some 30 years ago they went from a bullet proof design to a flawed design for these clamps I would not dismiss any dumb *** possibility with them brilliant though they generally are. My Cayenne has a main drive shaft with an intermediate bearing that fails after 60k miles and costs at least $1k to fix with a franchised dealer. I fixed the flex plate with a $5 bottle of Loctite and on the Cayenne I know a cost free fix for the drive shaft issue!

Adk46

Fred has emulated quite well what my professor might have said, providing two mechanisms to choose from. Along with my carbon theory, we have three! We can blame the alloying additions in the aluminum, and/or the phases they form. And the impurities and/or the "dirt" they create. Or what this stuff does if it dissolves in the stagnant coolant.

Hot corrosion of turbine components is easy to understand compared to aqueous corrosion. You must be familiar with the evils of vanadium sulfate, Fred.

I should clarify that the carbon I'm citing is in the form of carbon black, an addition. One theory of why old wheels leak is related to carbon-aluminum galvanic corrosion at the bead.

FredR

Curt,

As best as I can determine the two points are stages in the crevice corrosion process- not competing options. It is correct to think that if the inhibitors have been exhausted then the coolant [or what remains of it] in and of itself can cause corrosion but then the corrosion would not be limited to the crevice as it would attack all wetted surfaces- just not the case in my motor.

Understanding step 2 is central to understanding the corrosion issues that impact the 928 engine. It also explains why cars can stand for years on end in the same stagnant brew and have no corrosion whatsoever. Simply put, if the coolant does not heat up it does not degrade and corrosion does not take place.

This is how I concluded that the coolant life is not time dependent rather it is most likely run hours dependent. Thus if one measures [as I do] the pH and ensure that it is greater than 7, the coolant should last longer on low usage examples. Chuck a bit more pH buffer in it may be fine. This what the chief chemist of Redline advised me to do many years ago. Use a bottle of water wetter with new coolant and after two years chuck in another bottle and after 4 years in total replace the coolant altogether.

Your bit about rubber baffles me somewhat. As I am concerned rubber is an excellent insulator thus why they use it to insulate electric cables. If it was able to form a galvanic couple I would be worried about going near a cable that was powered up. Maybe there is something I am missing?