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The reagent keeps colour on good passivated stainless steel. A black spot will appear when the surface has not been passivated correctly. Various qualities of passivation will give graduations of grey shades, ranging from light (acceptable) to dark (unacceptable).
Also refer this link ::: Ferroxyl Test Kit for free IRON passivation ( http://rxmarine.com/Ferroxyl-Test-Kit-for-free-IRON )
Passivity Test Kit for PASSIVATION ( http://rxmarine.com/Passive-Film-Testing )