On the matter of oxidation, I can't see how it would occur much with distilled water, but then -- what do I know? And if it occurs on a heatsink or water-block base/plate, how would that adversely affect cooling? I'm only guessing "not much."
I could imagine, even so, if the interior surface of a water-block had been plated, and if the plating would begin to flake off, you could really have some troubles. So . . . no nickel-plating for me -- when I "get there."
the parts which are exposed to air oxidizes making the block look really ugly.
It turns brownish, instead of its original copper brilliance. I would say copper tarnish.
You cant really reverse this without using some form of clear coat polish, which is worse then nickel plating.
The nickel plating is also a very thin layer. Its electrostatic plating, and if done correctly, it doesnt really flake off unless ur running an injector through the block.
If your running an injector, then your running steel, which naturally oxidizes in water leaving a black layer which people often confuse for corrosion.
It doesnt impact the cooling potential on the block to any noticeable degree.
The nickel chromium plate is also very resistant to tarnish.
So it stays shiney for a long time.
And not all of us are straight performance junkies..
We do watercooling as Art in a sense.
This is why "bling" can triple the cost of your H2O gear in an instant.
I WILL stick by my own story, though. I measured an improvement of as much as 2C per surface when the nickel was removed. Others who do "lapping" noted that it was only the flatness of the surface area, but I was sure I had that covered in my little experiments.
Thats probably cuz the heat sink was bowed..
Remember my thread on bowed heat sinks, and how poor they were with the imporper mounts required for bowing?
By lapping you most likely made the sink flat which offered a better contact surface with the IHS.
Example:
Pass1:
Notice how the copper is showing in the center, which indicates a bow.
Pass 2:
Shows how much of a bow the Heat sink had.
This type of bow requires a tremendous amount of mounting pressure which is very difficult to do on a heat sink of that size due to Torque.
On a waterblock, its fairly easy to get the required mounting pressure... on a Tower heat sink... no so much due to the torque presented on the top portion unless the board is mounted horizontally.
This is why i <3 horizontal mounted board cases like ITX, and the Coolermaster one.