Already outdated missile warhead, mostly phased out of service but still used by some navies less advanced in terms of technlogy. It uses very powerful acid equivalent (one that works - and does it greatly - in space and can melt through modern composites in seconds. Each of corrosive warheads is in fact a container filled with a lot of pressured acid, that is dispersed in moment of detonation. It was always considered a last defensive measure against overpowering missile salvoes - when used en masse it could create a short term barrier of acid, thick and wide enough to stop a lot of enemy missiles by melting through them. Using them was always a last ditch attempt, done when everything else failed. They were mostly phased out due to progress on the field of both acid-resistant materials and evasive maneuvers that allowed missiles to circumvent the acid wall as long everything wasn't counted perfectly and detonated when they were no longer able to do that before hitting it. It can still be used, but not on a scale that makes actually producing them a viable idea.