i know a lot of people lap their cpus and heatsinks. however, while i was doing a lapping finish, i thought about how copper pennies and the statue of liberty are made from copper...and how i lapped my heatsink and cpu down to their copper innards. eventually, will my heatsink and cpu, now without their nickel finish, rust to green?
Yup. Corrosion will occur if you lap the IHS but it will take 5 years for it to be evident. Something about how the two metal's react to each other. Assuming of course, you have copper against aluminum. In any case, jo and mr. are right, you have nothing to worry about.
I have no idea what material the heatsink and heat spreader is made out of, but your concern is rust/corrosion. Lapping your heatsink/spreader would only affect corrosion if they are plated with a metal that is corrosion resistant. For instance if it was made of Iron, but Nickel plated, it would be bad to lap it.
Nickel has a more stable oxide layer than iron, which is why metals that are corrode more easily are nickel plated. Since we don't see lots of heatsinks and spreaders that are corroded to hell it obvious that they are made out of a metal that is resistant to corrosion.
The more appropriate question to ask would be "Is my heatsink/spreader plated to reduce corrosion?"
If it is then lapping would be bad. If it isn't then you'll only be exposing the same metal and doesn't matter. I've never lapped anything myself, but I would assume it isn't plated for 2 reasons. 1) I've never seen any pictures that showed an underlying metal and 2) If they were to plate the heatsink and/or heat spreader that would decrease the heat transfer coefficient(how efficiently heat transfers from point A to point B). Obviously #2 would be bad, so manufacturers would want to use as few different metals as possible.
I know someone is gonna post mentioning that some heatsinks are copper and aluminum and so why use 2 metals. The bottom line is that the copper core has enough volume to remove heat from the heatspreader, and also has enough surface area to decrease the effect of using 2 metals.
I'm sorry if this post sounds like im trying to tell you how dumb you are for asking. I think your question was very valid and really made me stop and think for a second how bad this would be if you were right. The heat spreader could possibly be made out of 2 metals that look so similar nobody has realized how bad lapping it could be because nobody has lapped their processor long enough ago to see the corrosion rates. I guess we may find out this is a bad idea in a year or 2. AS5 might inhi bit corrosion, but I doubt it seals enough to prevent it completely.
The heat spreader is made of copper and coated with a thin layer of nickel to prevent corrosion. Processors are designed to last around 10 years under any ambient conditions, in any case at stock clock speeds. If there wasn't a nickel coating and the computer was used in a damp environment for a decade or so there would be a high chance of the CPU welding itself to the heatsink. It wouldn't really matter once it was old but it doesn't look good from a marketing point of view.
Removing the nickel layer to expose the copper underneath is said on average to drop CPU temps by around 5 degrees C. If you are an overclocker and replace parts regularly every couple of years or so it might be worth doing as Nickel isn't as thermally conductive as copper. Also the thermal paste should help to prevent corrosion, at least in the short term.
People do run computers over 10 years, my company only stopped using Pentium 4's on socket 468 this year.