Chapter 17 Part 2

Corrosion and Degradation of Materials

Electrochemical Cells

• Electrochemical cells can be set up under different situations and can lead to the corrosion of a metal

• Grain/Grain Boundary– Generally the grain boundary is a region of disarray, and has higher

energy, and atoms can be “pulled out” of the solid more easily• This phenomenon is used in metallography, when polished metal

surfaces are etched with acids to reveal grain boundaries– Sometimes, segregation of solutes to grain boundaries may make them

more “noble” than the surrounding grain, resulting in corrosion in regions adjacent to grain boundaries

• When certain stainless steels are cooled slowly, chromium precipitates as chromium carbide along grain boundaries, robbing the surrounding grain of the protective chromium oxide. The grain boundary is cathodic compared to surrounding grain

Electrochemical Cells

• In multiphase materials, one phase may be anodic with respect to another– Corrosion rates are higher in multiphase materials.– For example

• In pearlitic gray cast iron, graphite is more noble than pearlite, leading to corrosion of pearlitic regions

• Martensite (single phase) is more corrosion resistant than austenite that has been slow cooled to form pearlite (two phase)

• Tempered martensite corrodes more easily than martensite– Low temperature tempering results in finer Fe3C particles and

more corrosion sites

– Higher temperature tempering results in coarser Fe3C particles and fewer corrosion sites

Corrosion Kinetics

• Up to this point, we have dealt with the thermodynamics of corrosion, i.e. which combinations of conditions results in anodic and cathodic regions under equilibrium

• Corrosion does not occur under equilibrium conditions

• Of interest is the corrosion kinetics, i.e. the rate at which a metal corrodes

• For each atom of a metal that participates in the oxidation reaction, n electrons need to get transported away

• The weight of a metal that is lost due to corrosion is given by Faraday’s law

• M Mn+ + ne-

w = weight loss during corrosion (or weight gain during electroplating)

I = current in amps = iA

i = current density

A = area of corroding surface

t = time in seconds

M = atomic mass g/mole

n = number of electrons involved in the corrosion reaction

F = Faraday’s constant

= 96,500 C/mol

nF

ItMw

Corrosion Penetration Rate (CPR)

• If a metal exposed to a corrosive environment, dissolves uniformly, the thickness removed can be calculated

• If a sample with a surface area of A (in2) is exposed for t (hours) and a weight loss of w (mg) is measured then

• CPR is the rate of thickness loss

• CPR<20 mil/yr or about 0.5 mm/yr is acceptable

)/(534

)/(6.87

)/(103652454.2

10

)/(54.2

10

)(54.2

10

)(54.2)(

10

)/(

10

2

3

2

3

2

3

222

3

3

3

yearmiltA

w

yearmmtA

w

yearmmtA

w

hrcmtA

wlossthicknessofRate

cmA

wlossThickness

cmAinAareaExposed

w s Volume los

cmg Density

w (g) (mg) wloss Weight -

Polarization• In an electrochemical half cell the

metal atoms are in a state of equilibrium with its ions in solution– There is an equilibrium

exchange current density i0

associated with the transfer of electrons at the standard emf potential E0 (or V0) of the half cell

• There is an i0 and E0 associated with the anodic and cathodic reactions

• However, potential differences cannot be maintained in a conductive metal, such as Zn

• There is a displacement of the electrode potentials and currents from points A and B to C

• This displacement of electrode potentials is called polarization

Point A: -0.763V, 10-7A/cm2

Point B: 0V, 10-10A/cm2

Point C: ~-0.5V, 10-4A/cm2

icorr = 10-4A/cm2 is used in CPR calculations

Activation and Concentration Polarization

• Activation polarization: In a multistep electrochemical reaction the rate is controlled by the slowest step.

• Concentration polarization: Corrosion reaction may result in

a build up or depletion of the ions or atoms that are required for a corrosion reaction.

Passivation

• Passivation is the loss of chemical reactivity in presence of a environmental condition.– The formation of surface layer of reaction products that inhibit

further reaction

• Oxide film theory: A passive film of reaction products acts as a diffusion barrier.

• Adsorption theory: Passive metals are covered by chemisorbed films of oxygen.

• Examples: Stainless steel, nickel alloys, titanium and aluminum alloys passivate in certain environments

Polarization Curve for Passivation• Initially, the potential of the metal increases with current density, i.e. the

metal undergoes active corrosion• When potential reaches Epp the primary passive potential, current density

decreases, i.e., the corrosion rate also decreases • In order to make the metal active again, there may need to be an

externally applied potential

Galvanic Series

• The Standard emf series gives the relative oxidation or reduction behavior under standard conditions.

• The Galvanic series ranks materials on the basis of corrosion behavior in sea water

Types of Corrosion

• Though the basic corrosion mechanism is the same, i.e. the oxidation of a metal due to transfer of electrons, conditions under which the process occurs can be vastly different, and has lead to the classification of corrosion into the following types:– Uniform– Galvanic– Crevice– Pitting– Inter-granular– Selective leaching– Erosion/cavitation– Stress corrosion– Fretting

Types of Corrosion

• Uniform or general corrosion– Most common in terms of

weight loss and destruction, but also the easiest to control

• Galvanic or two metal corrosion– When dissimilar metals are

in contact– Very sensitive to the

relative areas of the anodic and cathodic regions

– If the anodic region is small, then corrosion can be rapid and deep

– Tin coating of steel in “tin” cans• If the coating is scratched, the

underlying steel rusts quickly– Zinc coating of steel by

galvanizing• If the coating is scratched, the

galvanic cell results in corrosion of the coating, but due to large surface area of Zn, the CPR of the Zn coating is relatively low

Types of Corrosion

Crevice Corrosion• Localized electrochemical

corrosion in crevices and under shielded surfaces where stagnant solutions can exist.

• Occurs under valve gaskets, rivets and bolts in alloy systems like steel, titanium and copper alloys.

Anode: M M+ + e-Cathode:O2 + 2H2O + 4e- 4OH-

• As the solution is stagnant, oxygen is used up and not replaced.

• Chloride ions migrate to the crevice to balance positive charge and form metal hydroxide and free acid that causes corrosion

Rivet holes

Types of Corrosion

• Pitting– Initiates at non-uniformities in

composition– Growth of pit involves

dissolution of metal in pit maintaining high acidity at the bottom.

– Anodic reaction at the bottom and cathodic reaction at the metal surface.

– At the bottom of the pit

MCl + H2O M(OH) + H+ + Cl-

– The presence of H+ at the

bottom of the pit pulls more M into solution

– Some metals (stainless steel) have better resistance than others (titanium).

Types of Corrosion

• Intergranular corrosion– Localized corrosion at and/or adjacent to highly reactive grain

boundaries resulting in disintegration.– When stainless steels are heated to or cooled through sensitizing

temperature range (500-800C) chromium carbide precipitate along grain boundaries.

– When exposed to corrosive environment, the region next to grain boundaries become anodic and corrode.

Types of Corrosion

Stress Corrosion Cracking (SCC)• Cracking caused by combined

effect of tensile stress and corrosive environment

• Stress might be residual and applied

• Only certain combination of alloy and environment causes SCC

• Crack initiates at pit or other discontinuity

• Crack propagates perpendicular to stress

• Crack growth stops if either stress or corrosive environment is removed.

Types of Corrosion

Erosion corrosion• Acceleration in rate of corrosion

due to relative motion between corrosive fluid and surface.

• Pits, grooves, valleys appear on surface in direction of flow.

• Corrosion is due to abrasive action and removal of protective film.

Cavitation damage• Caused by collapse of air bubbles

or vapor filled cavities in a liquid near metal surface.

• Rapidly collapsing air bubbles produce very high pressure (60,000 PSI) and damage the surface.

• Occurs at metal surface when high velocity flow and pressure are present.

Types of Corrosion

Selective leaching:• Selective removal of one element of alloy by corrosion• Example: Dezincification or selective removal of zinc

from copper and brasses • Weakens the alloy as single metal might not have same

strength as the alloy

Fretting corrosion• Occurs at interface between materials under load

subjected to vibration and sliding• Metal fragments get oxidized and act as abrasives

between the surfaces

Oxidation of Metals

• Metals oxidize when exposed to air– Oxidation partial reaction: M M2+ + 2e-

– Reduction partial reaction: ½ O2 + 2e- O2-

• Oxidation starts by lateral expansion of discrete oxide nuclei.• The oxide layer is the medium through which:

– Metal ion diffuse to the surface.– O2- ions diffuse to oxide-metal interface– Electrons diffuse to oxide gas interface

• The oxide therefore serves as both the electrolyte for the electrochemical process as well as the electrically conductive medium for the flow of electrons

• If the oxide is electrically non-conductive or does not allow the diffusion of ions, then a protective oxide layer can form

Oxidation of Metals

• Whether an oxide films form depends on following factors:

– Pilling-Bedworth Ratio

• PB ratio should be close to 1

– Good adherence

– High melting point of the film

– Low oxide partial pressure

• Should not evaporate

– Coefficient of expansion equal to that of metal

– High temperature plasticity

– Low conductivity and diffusion coefficients of metal ions and oxygen.

OM

MO

MM

OO

A

A

A

A

consumedmetalofVolume

producedoxideofVolumeratioPB

/

/

WhereAO = molecular wt. of oxideO = density of oxideAM = molecular wt. of metalM = density of metal

Oxidation of Metals

Oxidation of Metals

• Oxidation rate is expressed as weight gained per unit area.

• Parabolic oxidation behaviorW2 = K1t+K2

– K1 = Parabolic rate constant– K2 = constant– Ion diffusion is controlling the step

(Eg – Fe, Cu, Co)• Linear oxidation behavior

W = K3t – W=weight gained – per unit area– K3 = linear rate constant.– t = time– Porous or flakey oxide layer

allows continuous supply of oxygen to the metal surface. Ex. Na, K, Ta

• Logarithmic oxidation behaviorW = K4 Log(K5t + K6)

– K4, K5, K6 = constants– Ex. Al, Cu, Fe (at slightly elevated

temperature)

Linear: Catastrophic oxidationParabolic or logarithmic: a protective oxide may form

Corrosion Control

Corrosion Control – Material Selection

• Metallic Metals:– Use proper metal for particular environment– For reducing conditions, use nickel and copper alloys– For oxidizing conditions, use chromium based alloys

• Nonmetallic Metals:• Limit use of polymers in presence of strong inorganic

acids• Ceramics have better corrosion resistance but are

brittle

Coatings

• Metallic Coatings: Used to protect metal by separating from corrosive environment and serving as anode.– Coating applied through electroplating, roll bonding or

hot dipping– Might have several layers

• Inorganic coatings: Coating with steel and glass.– Steel is coated with porcelain and lined with glass.

• Organic coatings: Organic polymers (paints and varnishes) are used for coatings.• Serve as barrier but should be applied carefully.

Design

• General design rules:– Provide allowance for corrosion in thickness.– Weld rather than rivet to avoid crevice corrosion.– Avoid dissimilar metals that can cause galvanic corrosion.– Avoid excessive stress and stress concentration.– Avoid sharp bends in pipes to prevent erosion corrosion.– Design tanks and containers for easy draining.– Design so that parts can be easily replaced.– Design heating systems so that hot spots do not occur.

Alteration Environment

• Lower the temperature reduces reaction rate.• Decrease velocity of fluids reduces erosion corrosion.• Removing oxygen from liquids reduces corrosion.• Reducing ion concentration decreases corrosion rate.• Adding inhibitors which are retarding catalysts reduce

corrosion

Cathodic Protection

• Electrons are supplied to the metal structure to be protected.Example: Fe in acid

Fe Fe2+ + 2e-

2H+ + 2e- H2

• Corrosion of Fe will be prevented if electrons are supplied to steel structure.• Electrons can be supplied by:

– External DC supply• Externally impressed anodic currents form protective passive films

on metal and alloy surfaces.– Galvanic coupling with more anodic metal

• A more easily corroded material is used as a sacrificial anode

Degradation of Polymers

• Polymers degrade by a physico-chemical process rather than an electrochemical process

• Different forms of degradation include– Swelling and Dissolution:

• A diffusion of liquid or solute into the polymer• Small solvent molecules enter into the space

between the macromolecules of the polymer causing chains to be forced apart.

• Reduced intermolecular secondary bonding• Increased ductility and lower strength• Polymers can dissolve in solvents, especially when

the solvent has a similar chemical structure– Example: Hydrocarbon polymers in gasoline

Degradation of Polymers

Degradation of Polymers

• Bond Rupture or Scission by exposure to Radiation

– , electron beam, X-rays, UV can all cause bonds to break, i.e. an electron that makes a covalent bond may be removed

– Subsequently, the bond may result in the breaking of the carbon chain (scission) or result in cross-linking, changing the structure of the polymer

– Exposure to radiation can sometimes be used in constructive processes, such as Rapid Prototyping, in which cross-linking is enhanced by exposure to UV radiation

• Bond Rupture by exposure to chemicals

– Oxygen O2, Ozone O3,and other substances can cause chain scission, especially if there are double bonds, such as those in un-vulcanized or partially vulcanized rubber

Degradation of Polymers

Degradation of Polymers

• Thermal Effects– High temperature breaks covalent bonds

• C-F bond in PTFE (Teflon) is stronger than the C-H bond in most polymers and the C-Cl bond in polymers like PVC

• Teflon use temperature is higher than other thermoplastics

• Weathering– A general term referring to the degradation of

polymers due to exposure to outdoor conditions that may include a combination of several of the above effects