Collect the security override from Freeze's suit to disable the Freeze Gun | Main story Batman: AC Guide

Freeze's suit can be found inside the Armory which you have already visited just a while ago. Take the route back to the Torture Chamber and note that a major part of the ice has already melted by now #1. As a result you have to jump down onto the platform #2 and use the Grapnel Gun to swim north, making use of the grapple points.

Once again use the ladder and follow the passage into the Armory. After reaching it, stay on the upper level and explore the north-west part of the room #1. A cutscene will start after you reach the capsule inside which the suit is. Note that Batman has gained the Disruptor #2. For now, the only thing you can use this device for is disrupting the Freeze Gun stolen by Penguin, though it will also gain other functions later on.