It depends on how the spell really works. 

From your description I get two possible situations:

1. Target is frozen to 1K instantly AND covered with thick ice at the same temperature. 

2. Target is not frozen, but instantly covered with thick ice at 1K.

The scenarios are quite different.

In the first case the target is with most probability destroyed (barring some counter magic): at 1K the crew would die instantly, all electronics will be shattered by thermal stresses and even the component materials (even most metals) would become so brittle they will shatter easily (maybe also from the sheer weight of the tank). Moreover most chemical reactions will effectively stop.

In the second case, it depends heavily on the thermal shielding of the tank and the total mass of the ice produced. The 1K ice will begin drawing thermal energy from the environment (outer side) and from the tank (inner side), rising its temperature and lowering the tank temperature in the process. This process continues until the tank, the ice and the external environment reach thermal equilibrium. It's quite a difficult calculation to perform without knowing the actual construction of the tank, but in the end, it depends on the relative thermal capacities of the mass of ice and of the tank. 

If there is enough ice to bring down the temperature of the tank to dangerous level (for the materials and/or crew), then it would be an effective spell.

Keep in mind that the freezing effect will cool down the tank from outside, so before the thermal equilibrium is reached, the outer tank parts will be cooled faster, so the first parts affected are the outer joints (e.g the joints between tracks elements, for example), which could become stuck due to thermal shrinkage, and maybe break if the tank was moving.