I worked as a materials and process engineer for Douglas Aircraft Company and Boeing. My degree is in Chemistry.
There is a process to etch/remove selected areas of aluminum materials called Chemical Milling.
@HankRearden replicated this process by accident. The purple power began to slowly dissolve any aluminum present (freezer liner and pistons). The chemical dissolution of the aluminum produced hydrogen gas (H2) and heat. The heating element raised the temperature of the solution which sped up the reaction of the purple power with the aluminum. The heat from the chemical reaction kept being added to the heat produced by the heating element and in turn sped up the reaction even more which produced more heat. Meanwhile, the sped up reaction continued to produce hydrogen gas which had no where to escape since the freezer lid was either locked or was heavy enough to allow an increasing H2 pressure above the liquid until the pressure caused the walls of the freezer to bulge until the lid blew open. The chemical reaction and the increasing heat used up all of the water in the purple power. The whitish-gray sludge or powder remaining behind was a combination of different oxides of aluminum (aluminum hydroxide and sodium aluminate).
It is fortunate that there was no short in the heating element as any electrical spark would have ignited the hydrogen gas and the entire freezer would have exploded like a bomb. If the lid was not latched then any spark would have caused the exploding gas to push the lid open and thus venting the burning gas to the outdoor air.
Our chemical milling and plating tanks were made out of 316 CRES (Corrosion REsistent Steel) Steel or 304 Stainless Steel because those materials are not susceptible to chemical attack. Aluminum is very susceptible to chemical attack by acids and bases (caustic) which can be found in many cleaning agents such as purple power (probably caustic).
Our heating elements were connected to rheostats to allow for temperature control which was monitored by thermometers. We also monitored the water content of the solutions and added water to maintain the proper chemical balance.
Cast iron is also susceptible to chemical attack by strong acids and strong bases. Test the cleaning agent on a section of the cast iron to determine if any chemical attack will occur.
Most, if not all, 55 gallon drums are lined with a thin layer of a polymer that is chemically resistant to what the drum will contain. Test the cleaning agent on the inside of the drum to determine its compatibility. Some cleaning agents may remove the liner and then allow chemical attack or rust formation.
I hope this helps everyone who is contemplating making a home built cleaning tank.