The malevolent greaves lasts for 100,000 charges of combat before degrading to nothing - it cannot be repaired. Once equipped, they instantly become untradeable, even if they have not been used in combat.
Losing the malevolent greaves by being killed in the Wilderness will result in a fraction of the original malevolent energy being dropped on death, based on how degraded they were, rounded down. For example, if the greaves had 52% of their charges remaining, 14 malevolent energy would be dropped on death. This can be used to refund most of the cost of the greaves, although the reinforcing plates are not returned.