RASHIDUL ISLAM20121107007

Department of Applied Chemistry and Chemical Engineering

BEHAVIOR OF PASSIVATORS

Course Title: Nano Materials & Electrometallurgy and Corrosion

Course No: ACCE 402

Passivity•Refers to the loss of chemical reactivity of certain metals and alloys under particular environmental condition.

i.e, certain metals and alloys become essentially inert and act as if they were noble metals such as Platinum and Gold.•The metals which exhibit passivity effect are iron, nickel, silicon, chromium, titanium and alloys of these metals.

•Under limited condition, zinc, cadmium, tin, uranium and thorium also show the passivity effect.

Passivators•Passivators are inorganic oxidizing agents that form a thin protective oxide layer on a metal surface that prevents further oxidation (corrosion).

•They are the solution of chromate (CrO42-), nitrites

(NO2-), molybdates (MoO4

2-), tungstates (WO42-),

ferrates (FeO42-).

•They act as anodic inhibitors.

Behavior of Passivators•A same metal can act as active as well as passive depending on the situation.

•If a metal is immersed in an air free acid solution, it is corroded.

•Increasing the oxidizing power of the solution (by adding oxygen or ferric ion), increases the corrosion rate of the metal.•This is the active state of the metal.

Figure: corrosion rate of a metal as a function of oxidizing power

•If more oxidizing agent (passivator) is added, the corrosion rate shows a sudden decreases.

•This is the passive state of the metal.

Figure: corrosion characteristics of an active-passive metal as a function of oxidizing power

•The passivator is reduced over a large cathodic area of the metal surface that forms a chemically equivalent passive film at small residual anodic areas.

•The small passive areas, in turn, adsorb passivator.•Higher the concentration of passivator, more readily it adsorbs.

•When the passive film is complete, it acts as a barrier between the metal and the electrolyte solution.

Passivation of Iron by HNO3•Fe in concentrated HNO3 → No reaction.•Insulative film is formed on the surface of Fe.•Because concentrated nitric acid is an oxidizing agent so,the potential of the metal is raised to the point that a layer of metastable Fe3O4 forms on the surface and protects it from further corrosion.

Dilute HNO3

•Fe in dil. HNO3 → Rapid corrosion reaction.•Diluted nitric acid is not a strong oxidizing agent and hence does not raise the potential of the metal to the extent that metastable Fe3O4 forms on the surface.

•In this case, the Fe freely corrodes. (Fe0 → Fe2+ + 2e-)

References• Uhlig H.H. and Revie R.W. Corrosion and Corrosion Control.• Fontana M. G. Corrosion Engineering.• https://www.corrosionpedia.com/definition/862/passivity• https://www.corrosionpedia.com/definition/1479/passivator• http://www.besttechnologyinc.com/passivation-systems/what-is-pa

ssivation/• https://en.wikipedia.org/wiki/Faraday_paradox_%

28electrochemistry%29

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