phosphoric de rusting tank cleaning

[mikegolding]

I have a couple of containers ( large bucket and blue plastic barrel ) that have been used with a milkstone remover solution for de rusting steel that are coated with a hard white / grey scale like buildup that I want to clean . I assume this is iron phosphate but chemistry is not my strong point so maybe not . I tried caustic soda previously but it turned into what looked like rust and was no easier to remove . Anyone have any preferably chemical suggestions ?
Thanks Mike.

 

[Ross365]

Citric acid would be one safe and cheap option; there a good numebr of threads on here discussing it.

 

[mikegolding]

Citric acid / washing soda mixture was what I was going to try next , but I want to get the old residue out of the containers first in case it has a detrimental effect .
Thanks Mike.