passivity of metal

2. RASHIDUL ISLAM
20121107007
Department of Applied Chemistry and
Chemical Engineering

3. BEHAVIOR OF
PASSIVATORS
CourseTitle: Nano Materials & Electrometallurgy and Corrosion
Course No: ACCE 402

4. 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.

5. 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 (CrO4
2-), nitrites (NO2
-),
molybdates (MoO4
2-), tungstates (WO4
2-), ferrates (FeO4
2-).
• They act as anodic inhibitors.

6. 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

7. • 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

8. • 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.

9. 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.

10. 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-)

13. 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-passivation/
• https://en.wikipedia.org/wiki/Faraday_paradox_%28electrochemistry%29

14. Have Any Questions……