CORROSION OXIDATION CORROSION PREVENTION AGAINST CORROSION Principles and Prevention of Corrosion D.A. Jones Prentice-Hall, Englewood-Cliffs (1996)

CORROSION

OXIDATION

CORROSION

PREVENTION AGAINST CORROSION

Principles and Prevention of CorrosionD.A. Jones

Prentice-Hall, Englewood-Cliffs (1996)

Attack of Environment on Materials

Metals get oxidized

Polymers react with oxygen and degrade

Ceramic refractories may dissolved in contact with molten materials

Materials may undergo irradiation damage

Oxidation

Oxide is the more stable than the metal (for most metals)

Oxidation rate becomes significant usually only at high temperatures

The nature of the oxide determines the rate of oxidation

Free energy of formation for some metal oxides at 25oC (KJ/mole)

Al2O3 Cr2O3 Ti2O Fe2O3 MgO NiO Cu2O Ag2O Au2O3

1576 1045 853 740 568 217 145 13 +163

For good oxidation resistance the oxide should be adherent to the surface

Adherence of the oxide = f(the volume of the oxide formed : the volume of metal consumed in the oxidation) = f(Pilling-Bedworth ratio)

PB < 1 tensile stresses in oxide film brittle oxide cracks

PB > 1 compressive stresses in oxide film uniformly cover metal surface and is protective

PB >> 1 too much compressive stresses in oxide film oxide cracks

Pilling-Bedworth ratio for some oxides

K2O Na2O MgO Al2O3 NiO Cu2O Cr2O3 Fe2O3

0.41 0.58 0.79 1.38 1.60 1.71 2.03 2.16

If the metal is subjected to alternate heating and cooling cycles the relative thermal expansion of the oxide vs metal determines the stability of the oxide layer

Oxides are prone to thermal spalling and can crack on rapid heating or cooling

If the oxide layer is volatile (e.g. Mo and W at high temperatures) no protection

Progress of oxidation after forming the oxide layer: diffusion controlled activation energy for oxidation is activation energy for diffusion through the oxide layer

Oxide

Metal

Oxygen anions Metal Cations Oxidation occurs at air-oxide interface

Oxidation occurs at metal-oxide interface

• Diffusivity = f(nature of the oxide layer, defect structure of the oxide)• If PB >> 1 and reaction occurs at the M-O interface expansion cannot

be accommodated

Oxidation resistant materials

As oxidation of most metals cannot be avoided the key is to form a protective oxide layer on the surface

The oxide layer should offer a high resistance to the diffusion of the speciescontrolling the oxidation

The electrical conductivity of the oxide is a measure of the diffusivity of theions (a stoichiometric oxide will have a low diffusivity)

Alloying the base metal can improve the oxidation resistance E.g. the oxidation resistance of Fe can be improved by alloying with

Cr, Al, Ni Al, Ti have a protective oxide film and usually do not need any alloying

Schottky and Frenkel defects (defects in thermal equilibrium) assist the diffusion process

If Frenkel defects dominate the cation interstitial of the Frenkel defect carries the diffusion flux

If Schottky defects dominate the cation vacancy carries the diffusion flux

Other defects in ionic crystals impurities and off-stoichiometry Cd2+ in NaCl crystal generates a cation vacancy s diffusivity Non-stoichiometric ZnO Excess Zn2+ diffusivity of Zn2+

Non-stoichiometric FeO cation vacancies diffusivity of Fe2+

Electrical conductivity Diffusivity

Diffusion in Ionic crystals

Frenkel defect

Cation (being smaller get displaced to interstitial voids E.g. AgI, CaF2

Schottky defect

Pair of anion and cation vacancies E.g. Alkali halides

A protective Cr2O3 layer forms on the surface of Fe (Cr2O3) = 0.001 (Fe2O3)

Upto 10 % Cr alloyed steel is used in oil refinery components Cr > 12% stainless steels oxidation resistance upto 1000oC

turbine blades, furnace parts, valves for IC engines Cr > 17% oxidation resistance above 1000oC 18-8 stainless steel (18%Cr, 8%Ni) excellent corrosion resistance Kanthal (24% Cr, 5.5%Al, 2%Co) furnace windings (1300oC)

Alloying of Fe with Cr

Other oxidation resistant alloys

Nichrome (80%Ni, 20%Cr) excellent oxidation resistance Inconel (76%Ni, 16%Cr, 7%Fe)

Corrosion

THE ELECTRODE POTENTIAL When an electrode (e.g. Fe) is immersed in a solvent (e.g. H2O) some metal ions

leave the electrode and –ve charge builds up in the electrode The solvent becomes +ve and the opposing electrical layers lead to a dynamic

equilibrium wherein there is no further (net) dissolution of the electrode The potential developed by the electrode in equilibrium is a property of the

metal of electrode the electrode potential The electrode potential is measured with the electrode in contact with a solution

containing an unit concentration of the ions of the same metal with the standard hydrogen electrode as the counter electrode (whose potential is taken to be zero)

Metalions-ve

+ve

System Potential in V

Noble end Au / Au3+ +1.5

Ag / Ag+ +0.80

Cu / Cu2+ +0.34

H2 / H+ 0.0

Pb / Pb2+ 0.13

Ni / Ni2+ 0.25

Fe / Fe2+ 0.44

Cr / Cr3+ 0.74

Zn / Zn2+ 0.76

Al / Al3+ 1.66

Active end Li / Li+ 3.05

Standard electrode potential of metalsStandard potential at 25oC

Incr

easi

ng p

rope

nsit

y to

dis

solv

e

Galvanic series

Alloys used in service are complex and so are the electrolytes (difficult to define in terms of M+) (the environment provides the electrolyte

Metals and alloys are arranged in a qualitative scale which gives a measure of the tendency to corrode The Galvanic Series

Environment Corrosion rate of mild steel (mm / year)

Dry 0.001

Marine 0.02

Humid with other agents 0.2

Galvanic series in marine water

Noble end Active end

18-8 SSPassive

Ni Cu Sn Brass 18-8 SSActive

MS Al Zn Mg

More reactive

Galvanic Cell

AnodeZn

(0.76)

CathodeCu

(+0.34)

e flow

Zn Zn2+ + 2e

oxidationCu2+ + 2e Cu

Reductionor2H+ + 2e H2

orO2 + 2H2O + 4e 4OH

Zn will corrode at the expense of Cu

How can galvanic cells form?

Anodic/cathodic phases at the microstructural level

Differences in the concentration of the Metal ion

Anodic/cathodic electrodes

Differences in the concentration of oxygen

Difference in the residual stress levels

Different phases (even of the same metal) can form a galvanic couple at the microstructural level (In steel Cementite is noble as compared to Ferrite)

Galvanic cell may be set up due to concentration differences of the metal ion in the electrolyte A concentration cell

Metal ion deficient anodicMetal ion excess cathodic

A concentration cell can form due to differences in oxygen concentrationOxygen deficient region anodicOxygen rich region cathodic

A galvanic cell can form due to different residual stresses in the same metalStressed region more active anodicStress free region cathodic

O2 + 2H2O + 4e 4OH

Polarization

Anodic and Cathodic reactions lead to concentration differences near the electrodes This leads to variation in cathode and anode potentials (towards each other)

Polarization

Current (I) →

Pot

entia

l (V

) →

Vcathode

Vcathode Steady state current

IR drop through the electrolyte

Passivation

Iron dissolves in dilute nitric acid, but not in concentrated nitric acid The concentrated acid oxidizes the surface of iron and produces a thin protective oxide layer (dilute acid is not able to do so)

↑ potential of a metal electrode ↑ in current density (I/A) On current density reaching a critical value fall in current density

(then remains constant) Passivation

Prevention of Corrosion

Basic goal protect the metal avoid localized corrosion

When possible chose a nobler metal Avoid electrical / physical contact between metals with very different electrode

potentials (avoid formation of a galvanic couple) If dissimilar metals are in contact make sure that the anodic metal has a larger

surface area / volume In case of microstructural level galvanic couple, try to use a course

microstructure (where possible) to reduce number of galvanic cells formed Modify the base metal by alloying Protect the surface by various means Modify the fluid in contact with the metal

Remove a cathodic reactant (e.g. water) Add inhibitors which from a protective layer

Cathodic protection Use a sacrificial anode (as a coating or in electrical contact) Use an external DC source in connection with a inert/expendable electrode