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Passivity, passive state
In passivized state, ordinary metals behave similarly to precious metals. For example, iron dissolves in a dilute nitric acid solution, but does not in a concentrated solution. After the iron is inserted into a concentrated nitric acid solution and is then immersed into a sulfuric acid -copper solution, the precipitation of iron does not take place by substitution with copper. The reason for this is that a thin oxide layer has formed on the metal, completely covering it. As a rule, this so-called passivation takes place immediately if the solution into which the metal is dipped contains a sufficient amount of oxidant. In this case, we speak about self-passivation. Stainless steel alloys are often self-passivizing. Therefore, it should be ensured that the passivizing layer is removed prior to soldering.