I saw the blue color on the steel surface. some people call that "blue rust" but I think it is second oxide scale. How the second oxide scale occur on steel? How can I avoid it?
XPS will give you the electronic state of oxide scale in surface and coupling with other characterization techniques you may able to find the reason for its formation.
if you have it formed on cold rolled steel, it might be only a layer of about 40nm. This can be obtained by injecting water vapor ( in a controlled way) at the end of batch annealing. On continuous annealing process, it can happen too , by accident, during cooling.For both cases the contact steel-oxygen needs to be limited in time, and occur in a specific temperature range. If you have it formed on hot rolled steel, it's mostly due to temperature/time evolution of the scale formed on roll out table, during coiling and cooling. in certain conditions you get dismutation of wustite into magnetite and Fe (4FeO gives Fe3O4+Fe)
You haven't said what you have done to have the blue scale appear but guessing it is a heating process you could do the treatment in a vacuum or a gas combination with a low oxygen partial pressure.
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