A well-meaning fellow on a VW forum asserted: “The initial problems with the wasserboxer cooling system weren’t because people didn’t change the coolant and it wasn’t due to corrosion of aluminum. It was because the coolant contained phosphate which caused the steel head studs (which go through the water passages in this unique design) to rapidly corrode in preference to the aluminum (i.e. galvanic action of less noble metal) which led to failures of the studs which caused water jacket seal failures.”
Aluminum is one of the least noble metals*, less noble than any grade of steel. So does phosphorus reverse the galvanic series? That’s what would have to happen for this to be true.
Of well over 150 wbx’s I’ve torn down by now, only a handful had corroded head studs. The body of evidence showed these all had clearly received little to no cooling system maintenance, and operated with low fill levels at times, with notably more advanced corrosion of the upper studs than the lowers. Neglecting coolant replacement, or at least additive replenishment, and instead continually adding new water adds new oxygen to the coolant mix. Stud corrosion is not a function of galvanic corrosion, it’s just good old oxidation, aka rust. Stud stretching or failure is NOT a notable contributor to the prevalence of water jacket seal leaks in the wbx. In all of these examples corrosion of aluminum on both sides of the water jacket channel seals and up around the cylinder tops was extreme, and the large exterior crusts of scale and silicate indicated chronic leaking, which probably accounted for the constant additions of new water and operation with low coolant levels, leading to rusting of the studs, which in these cases was probably a late development, rather than something that initiated the leakage.
(Oddly, when I see pictures on the English forum, severely rusted studs are definitely more common, although by no means prevalent, in my reading. Seems to point to unsuitable water supplies, perhaps, but in any case a cavalier approach to cooling system care.)
The removal of aluminum on either or both sides of the channel seals is a process of crevice corrosion, where coolant is initially forced by pressure under any beads of excess sealant that are attached to the inner edge of the channel seal. Once trapped there, it is largely isolated from mixing freely with the body of flowing coolant, and the limited mixing, combined with the high heat, leads to depletion of the corrosion inhibitors in these small trapped volumes, until they become acidic corrosive agents. As metal is precipitated away, more coolant is forced by pressure to occupy the resulting voids, and the depleted coolant gradually undermines the channel seal. I’ve been able to observe this process at all stages of progress, it is without a doubt the mechanism responsible for classic wbx water jacket seal leaks (and boy have we seen some cockamamie theories about that!).
VW addressed this in their TSB’s and in the Bentley procedures for channel seal replacement. If you haven’t noticed, they stress that a continuous bead no wider than 1-2mm of the specified siloxane sealant be applied along the center of the channel seal face, which in my experience is just the right amount when squeezed out to cover the face of the seal without producing beads of excess. They also stress that old coolant must be replaced during this procedure and only their newer phosphate-free product be used.
The first was a method to limit the initiation of crevice corrosion. The second was correcting their own marketing mistake, issuing the vehicles with a phosphated antifreeze while advertising the engine as having a “lifetime coolant” (I’ve seen it claimed here they specified a two-year coolant replacement, but I’ve never seen such an advisory, whereas “lifetime coolant” is in the promotional materials, owner’s manual and official repair guide). Well there is no such thing as a “lifetime” automotive fluid, unless you define that as some finite number of years or miles, but in the absence of such, to laypeople “lifetime” means “forever”, pretty much, so if you’re going to imply to your customers they don’t need to do cooling system maintenance, at least use a factory fill product that plays well with all-aluminum engines and different water supplies.
Crevice corrosion also takes place above the cylinder top o-rings, which do a poor job keeping coolant from being forced by pressure into the space above them, but do a great job of trapping it there once it enters, where it also degrades and eats at the aluminum around the cylinder spigot wall. Once again, the process is hastened by the high temperatures in this area. However, this one doesn’t lead to any failures I’ve ever seen. I’ve seen shadetree rebuilders blame top o-ring failures for fire ring leaks, but I’ve never seen any scrap of evidence linking the two, the fire rings are clamped so tightly there is no void available for coolant to be forced under them to initiate the process. It could conceivably take place over a very very long time, but long before then the engine will desperately need a valve job, at minimum.
*This is actually not a wholly accurate use of the term “noble metal”. Noble metals are a class of metals, such as gold, silver, platinum, etc., which are characterised by their natural resistance to oxidation and other chemical corrosion processes. But the term is commonly used when discussing the galvanic series, where metals are ranked according to their voltage potential in the presence of dissimilar metals and an electrolyte. In that sense, the metals are often called more or less noble, the more noble ones being more resistant to corrosion vs. others lower in the series.