Hello everyone. I'm looking for an answer regarding odd behavior of chemical process I'm generally familiar with. When restoring metal parts I often use weak solution of phosphoric acid, which reacts with oxides of iron, leaving steel surfaces gray, mat and slightly dusted. Such surface is passivized and ready to be painted over.
Recently I tried to use this method to clean inner surface of a tank, to prepare it for an epoxid paint. Immediately after the application the surface looked as expected, consistently silvery gray, shiny, with the characteristic smell of steel. What was my surprise some 24h later, after curing and drying. The whole surface seems to be smooth and painted-like with red-brownish glossy paint! The first idea was of some major impurity coming into the process but repeated procedures with a different bottles of acid resulted in the same outcome. Any guesses what is happening there? What is that surface, and how stable is that? I'm not much into the risk of painting over that without a knowledge of what happened, for one of reasons is the epoxid paint is nearly impossible to be completely removed even when not having a proper bond with an underlying material. I'm attaching pictures before and after the process, unfortunately I didn't make one freshly after applying the acid.
Before first try:(https://i.postimg.cc/ZBbfGH6w/20200416-175615536-i-OS.jpg) (https://postimg.cc/ZBbfGH6w)The result: (https://i.postimg.cc/Rqj7fGq2/20200513-160847551-i-OS.jpg) (https://postimg.cc/Rqj7fGq2)