Hi. One of my hobbies is working on cars and, as you know, phosphoric acid is commonly used on car sheet metal to treat rust and to protect and etch the metal in panel repairs before painting. I have been reading a lot of car specialized forums and that has been very usefull but I thought it would be a good idea to ask here about some doubts I have. As I am not a chemical engineer (I am an electronics engineer) I would appreciate to hear the opinion of the chemical experts about this.
I know, from experience and from what I have read, that when you treat a rusty sheet metal with phosphoric acid you get zones of black-dark grey-bluish color and zones of white-yellowish color. These white-yellowish zones are generally covered in a powdery stuff.
What I would like to know if I am right that the white-yellowish stuff is iron phosphate III and that the black-dark grey-bluish stuff is iron phosphate II or if any of those could be something else or if there is something else apart from iron phosphate there.