Corrosion is an equipment mechanic.
Betweenlands axes, pickaxes, shovels, most swords (except the Shockwave Sword), the Weedwood Bow, Syrmorite Shears, and the Sickle are corrodable. Corrosion is a value [0,255] stored as "Decay" in item stack NBT as an integer tag. There are 6 visible texture variants and 6 tooltip hints to the stage of corrosion. Take note that corrosion does not affect non-Betweenlands items, and only occurs in the Betweenlands (so if a Betweenlands tool is taken to the Overworld it will not corrode at all).
The more an item has corroded the greater the effectiveness and attack damage are reduced. For completely corroded items their effectiveness and attack damage are reduced by 50%. The maximum power of a weedwood bow is reduced by 15%. Items dropped from plants using Syrmorite Shears and the Sickle become chance drops, with the chance decreasing the more the tool is corroded.
Only items currently in a players' inventory have the chance to corrode. The probability of a corrodable item corroding at all within one second is 4.88%, if the player is in water it is 13.98%, if the player is currently using or swinging the item the previously stated probabilities are multiplied by 1.025, the former probability becoming 5% and the latter 14.33%. The higher a players' corrosion level the closer the probability is to being halved.
Corroded items can be 'purified' using a Purifier, which resets the corrosion value back to zero. This will not affect the durability of the item. Corrodable items can also be coated with Scabyst, keeping the item's corrosion level at the level it was when coated until broken but making it unable to be purified again. Items do not corrode if they are inside a Weedwood Chest, but will resume corroding upon being taken out.