But im thinking the weight/drone needed to carry such a device is going to be impractical (and expensive) and most importantly long delays to deploy.
White phos seems to burn at near enough 3000 degrees C, near on double the melting point of steel and self ignites when in contact with the air. A good strike around the turret / barrel connection would do enough damage that even if the tank is still driveable, the main barrel would be unusable / too risky to fire a round down due to potential warping / blowout. For the price of a Phantom 4 drone £1k and a pickle jar filled with white phosphorous you can have an army little suicide drones that can be rapidly acquired and deployed![]()
It would depend on the thickness (thus thermal sink) of the energy the white phosphorus / thermite is generating. If the energy the metal can conduct/heatsink away is larger than the WP then it's just going to burn on the top and warm the tank up a bit. I assume the size of payload would be the weight of a grenade so quite difficult to keep a lump and be accurate with it.