Corrosion on the ASUS ROG Z690 Formula
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It seems that the VRM block on the ASUS ROG Z690 Formula is corroding due to a reaction between the plated aluminum and other metals found in the water loop.
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Nickel plating is not the best coating for these cases, but if you nickel-plate the aluminum and then use a blowtorch to solder a copper multifin to it, the nickel will go to hell in that area and this is what happens in the following photo:

But with a suitable liquid, it would take a long time, and most importantly, no residue would form, you would just start to have copper with aluminum.
However, if you use tap water or something similar, then the aluminum does form those salts that we see in the following image:
In this last one, a very inappropriate liquid has been used, or there is an element in the circuit that emits something that forms that lump.
I have had a copper block and an aluminum radiator for several years (not recommended), using car coolant, and the only thing I had was the center of the block slightly tinted with aluminum in the center, without any type of residue.
And nickel also doesn't form those curds, it comes off, stops protecting and forms residues.Moreover, in this last photo, the copper doesn't look tinted with aluminum, it looks rather the darkened color that is given when the copper itself forms its protective oxide layer, and this happens after a long time in the open air, or circulating water or another water-based liquid that lost its additives a long time ago.
And in the first image, the level of corrosion of the aluminum is a barbarity, it's as if the copper was completely bathed in aluminum.
That is, we are seeing images of the most extreme cases, in which it seems more likely that extreme chemical corrosion occurred, and the effects of copper-aluminum galvanic corrosion are not as noticeable.As for those blocks for VRM, a clear hint.
Salu2.
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Asus has already offered to refund or replace the affected card