Can a polypropylene 55 gallon drum previously filled with chemicals used in the chrome plating process (possibly Nickel Sulfate) ever really be "cleaned out" for re-use? Can polypropylene ever absorb and retain such chemicals, or are these drums practical for this use because they do not?
Polypropylene is hydrocarbon and does not adsorb polar and ionic compounds. So, several treatments with mild hydrochloric acid followed with several treatments with clean water should work. However, I wouldn't store drinking water there without a proven test. For example, leave water there for several weeks and then perform analysis of the water for metal ions. If all concentrations are within safe levels, it should be alright.
Many of the chemicals used in metal plating are extremely poisonous (cyanide), cause severe allergies (Ni), and are carcinogenic (Cr(VI)).
I'd consider it as extremely unprofessional (besides violating many laws and regulations), to re-use anything that has been in contact with these materials for any purpose that requires a 'clean' container, regardless of the theoretical possibility of cleaning everything off.
To use such a drum for food or drinking water would be criminal negligence.
I'd only consider it ok if it was thoroughly cleaned (my myself) and only used by myself as a trashcan in my machine shop outside of my home.
I would even disallow to use it as a trashcan in the machine shop at work. Why risk it?
Even if PP does not absorb ions in principle, there is always the real world:
The guy who does the cleaning gets lazy and thinks you are an idiot requiring him to clean something off he doesn't see and only rinses the drum with water.
The plating bath contained organics that made the ions stick to the drum.
Some stuff precipitated from the plating bath and stuck to the bottom of the drum.
During the cleaning process cleaned and contaminated drums get mixed up.
Some plating solution dried in a crack, or under the rim, or wherever you can't clean in off.
Cleaning is itself a hazardous action that creates a lot of contaminated wastewater.
You don't know where the drums might end up. Of course the risk of using them as a trashcan in the machine shop is minuscule, agreed. But what if a machinist takes them home to his home shop? What if his kids use them to play with? I'd have been delighted as a kid to have a 55-gallon drum as a toy. Maybe the machinist's wife will take them to store food.
There are hundreds of accidents where contaminated containers have been improperly cleaned and re-purposed. Please don't do it, even if it is actually safe.
Even if the drums can be cleaned, imagine anything happening, like some kid being admitted to the ER with a Nickel allergy.
You'd be in a tough position proving that no one got injured from your unprofessionally cleaned drums. The people who used the drums in the first place will have all the paperwork to 'prove' that it wasn't them.
Btw. we had an accident with a metal plating bath that was not properly cleaned and injured a technician who had do do some modifications to it.
It might not hurt anyone, but it's better not to do it.