That appears to be oxides that formed at elevated temperatures during heating. There are brush on picking compounds that will remove the discoloration and restore corrosion resistance.
Yes i agree.
The photo on the left is the door which is sealed by a rubber seal and the right photo is the frame of which the door seals against.
This style of corrosion is occurring all around the tank, even after the parts were acided and polished.
We manufacture 100s of tanks a year and this has never happened before
Is it possible the water it was filled with is contaminated?
It could have been some minor water contamination, it could have been insufficient cleaning after fabrication and polishing.
It doesn't take much to get this staining on 304.
I suspect that it wasn't pickled well enough after welding, and then it should have been passivated (cleaned to remove residual iron in the surface) after polishing. That either traces of oxide were still embedded in the surface or that the Cr depeated material from just under the scale had not been removed.
