What is the difference between porosity and corrosion?

Russ

I've been browsing auctions for Hard Times tokens lately and come across quite a few that say the surfaces have porosity. How can I tell whether it's something that is part of the planchet naturally, or corrosion that happened later? And, are these prone to porosity, or is a seller who's calling it that trying to camouflage what it is really corrosion?

Russ, NCNE

braddick

Porosity usually is damage that has eaten at the coin while corrosion is the same damage, but also has signs of the damage 'above' the coin's surface.
Almost as if you pictured rust (corrosion) and then the item when the rust is removed (porosity).

Porosity can also indicate water damage while corrosion is usually from a source (such as soil) that is still evident.

cladking

Porosity can also be caused by rusted dies.

Libertad

Corrosion is the metal's intrinsic nature, trying to achieve a lower state of energy level.

Moisture is the heavy foot on the gas pedal of hydrogen-driven oxidation and sealing out the moisture (or acid, or alkali, or oxide-combined impurities) is the key to slowing down the process.

Ask anybody in the South who collects coppers, worse by the seashore.

Here's a helpful link to understanding the processes.

Maybe the old-timers were on target when they shellac'd or laquered the copper coins in their care?