Copper does not react with water, but it slowly reacts with atmospheric oxygen forming a layer of brown-black copper oxide. In contrast to the oxidation of iron by humid air, this oxide layer stops further bulk corrosion. A green layer of verdigris (copper carbonate) can often be seen on old copper constructions. Water cooling equipment usually tarnishes when exposed to sulfides, which react with it to form various copper sulfides.
Tarnished copper looks discouraging but has no measurable impact on performance - the oxidation layer is too thin to have any effect on thermal conductivity.
Photo above illustrate oxidized water block copper base (source: overclock.net)
It is nearly impossible to avoid the naturally occurring copper tarnishing (oxidation). However, it's recommended to use nitrite or latex gloves during installation as human skin oil is slightly acidic. Those of you who don't prefer the looks of tarnished copper are advised to use nickel-plated copper water blocks. The use of quality market-proven biological growth- and corrosion-inhibiting coolants is always strongly recommended for any liquid cooling system. For best results, EK recommends the use of EK-CryoFuel Coolants.