CORROSION

OXIDATION

CORROSION

PREVENTION AGAINST CORROSION

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

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

UNIVERSALITY OF CORROSION

• Not only metals, but non-metals like plastics, rubber, ceramics are also subject to environmental degradation

• Even living tissues in the human body are prone to environmental damage by free radicals-Oxidative stress- leading to degenerative diseases like cancer, cardio-vascular disease and diabetes.

CORROSION DAMAGE

• Disfiguration or loss of appearance• Loss of material• Maintenance cost• Extractive metallurgy in reverse- Loss of

precious minerals, power, water and man-power

• Loss in reliability & safety• Plant shutdown, contamination of product etc

COST OF CORROSION

• Annual loss due to corrosion is estimated to be 3 to 5 % of GNP, about Rs.700000 crores

• Direct & Indirect losses• Direct loss: Material cost, maintenance cost, over-

design, use of costly material• Indirect losses: Plant shutdown & loss of production,

contamination of products, loss of valuable products due to leakage etc, liability in accidents

WHY DO METALS CORRODE?

• Any spontaneous reaction in the universe is associated with a lowering in the free energy of the system. i.e. a negative free energy change

• All metals except the noble metals have free energies greater than their compounds. So they tend to become their compounds through the process of corrosion

ELECTROCHEMICAL NATURE

• All metallic corrosion are electrochemical reactions i.e. metal is converted to its compound with a transfer of electrons

• The overall reaction may be split into oxidation (anodic) and reduction (cathodic) partial reactions

• Next slide shows the electrochemical reactions in the corrosion of Zn in hydrochloric acid

ELECTROCHEMICAL REACTIONS IN CORROSION

DISSOLUTION OF ZN METAL IN HYDROCHLORIC ACID,

222 HZnClHClZn -------------------- -(1) Written in ionic form as,

22 222 HClZnClHZn ----------------------(2)

The net reaction being,

222 HZnHZn ------------------------- (3)

Equation (3) is the summation of two partial reactions,

eZnZn 2*2 -----------------------------------------(4) and 222 HeH ------------------------------------------(5)

Equation (4) is the oxidation / anodic reaction and Equation (5) is the reduction / cathodic reaction

ELECTROCHEMICAL THEORY

• The anodic & cathodic reactions occur simultaneously at different parts of the metal.

• The electrode potentials of the two reactions converge to the corrosion potential by polarization

PASSIVATION

• Many metals like Cr, Ti, Al, Ni and Fe exhibit a reduction in their corrosion rate above certain critical potential. Formation of a protective, thin oxide film.

• Passivation is the reason for the excellent corrosion resistance of Al and S.S.

FORMS OF CORROSION

• Corrosion may be classified in different ways

• Wet / Aqueous corrosion & Dry Corrosion

• Room Temperature/ High Temperature Corrosion

CORROSION

WET CORROSION DRY CORROSION

CORROSION

ROOM TEMPERATURECORROSION

HIGH TEMPERATURECORROSION

WET & DRY CORROSION

• Wet / aqueous corrosion is the major form of corrosion which occurs at or near room temperature and in the presence of water

• Dry / gaseous corrosion is significant mainly at high temperatures

WET / AQUEOUS CORROSION

Based on the appearance of the corroded metal, wet corrosion may be classified as

• Uniform or General• Galvanic or Two-metal• Crevice• Pitting• Dealloying• Intergranular• Velocity-assisted• Environment-assisted cracking

UNIFORM CORROSION

• Corrosion over the entire exposed surface at a uniform rate. e.g.. Atmospheric corrosion.

• Maximum metal loss by this form.

• Not dangerous, rate can be measured in the laboratory.

GALVANIC CORROSION• When two dissimilar metals

are joined together and exposed, the more active of the two metals corrode faster and the nobler metal is protected. This excess corrosion is due to the galvanic current generated at the junction

• Fig. Al sheets covering underground Cu cables

CREVICE CORROSION

• Intensive localized corrosion within crevices & shielded areas on metal surfaces

• Small volumes of stagnant corrosive caused by holes, gaskets, surface deposits, lap joints

PITTING

• A form of extremely localized attack causing holes in the metal

• Most destructive form• Autocatalytic nature• Difficult to detect and

measure• Mechanism

DEALLOYING

• Alloys exposed to corrosives experience selective leaching out of the more active constituent. e.g. Dezincification of brass.

• Loss of structural stability and mechanical strength

INTERGRANULAR CORROSION

• The grain boundaries in metals are more active than the grains because of segregation of impurities and depletion of protective elements. So preferential attack along grain boundaries occurs. e.g. weld decay in stainless steels

VELOCITY ASSISTED CORROSION

• Fast moving corrosives cause

• a) Erosion-Corrosion, • b) Impingement attack ,

and • c) Cavitation damage in

metals

CAVITATION DAMAGE

• Cavitation is a special case of Erosion-corrosion.

• In high velocity systems, local pressure reductions create water vapour bubbles which get attached to the metal surface and burst at increased pressure, causing metal damage

ENVIRONMENT ASSISTED CRACKING

• When a metal is subjected to a tensile stress and a corrosive medium, it may experience Environment Assisted Cracking. Four types:

• Stress Corrosion Cracking• Hydrogen Embrittlement• Liquid Metal Embrittlement• Corrosion Fatigue

STRESS CORROSION CRACKING

• Static tensile stress and specific environments produce cracking

• Examples:• 1) Stainless steels in hot

chloride• 2) Ti alloys in nitrogen

tetroxide• 3) Brass in ammonia

HYDROGEN EMBRITTLEMENT

• High strength materials stressed in presence of hydrogen crack at reduced stress levels.

• Hydrogen may be dissolved in the metal or present as a gas outside.

• Only ppm levels of H needed

LIQUID METAL EMBRITTLEMENT

• Certain metals like Al and stainless steels undergo brittle failure when stressed in contact with liquid metals like Hg, Zn, Sn, Pb Cd etc.

• Molten metal atoms penetrate the grain boundaries and fracture the metal

• Fig. Shows brittle IG fracture in Al alloy by Pb

CORROSION FATIGUE

• Synergistic action of corrosion & cyclic stress. Both crack nucleation and propagation are accelerated by corrodent

• Effect on S-N diagram• Increased crack

propagation

AirAir

CorrosionCorrosion

log (cycles to failure, N f)

Stre

ss A

mplit

ude

Log (Stress Intensity Factor Range, K

log

(Cra

ck G

row

th R

ate,

da/

dN

)

PREVENTION OF CORROSION

• The huge annual loss due to corrosion is a national waste and should be minimized

• Materials already exist which, if properly used, can eliminate 80 % of corrosion loss

• Proper understanding of the basics of corrosion and incorporation in the initial design of metallic structures is essential

METHODS

• Material selection• Improvements in material• Design of structures• Alteration of environment• Cathodic & Anodic protection• Coatings

MATERIAL SELECTION

• Most important method – select the appropriate metal or alloy .

• “Natural” metal-corrosive combinations like• S. S.- Nitric acid, Ni & Ni alloys- Caustic• Monel- HF, Hastelloys- Hot HCl• Pb- Dil. Sulphuric acid, Sn- Distilled water• Al- Atmosphere, Ti- hot oxidizers• Ta- Ultimate resistance

IMPROVEMENTS OF MATERIALS

• Purification of metals- Al , Zr• Alloying with metals for:• Making more noble, e.g. Pt in Ti• Passivating, e.g. Cr in steel• Inhibiting, e.g. As & Sb in brass• Scavenging, e.g. Ti & Nb in S.S• Improving other properties

DESIGN OF STRUCTURES

• Avoid sharp corners• Complete draining of vessels• No water retention• Avoid sudden changes in section• Avoid contact between dissimilar metals• Weld rather than rivet• Easy replacement of vulnerable parts• Avoid excessive mechanical stress

ALTERATION OF ENVIRONMENT

• Lower temperature and velocity• Remove oxygen/oxidizers• Change concentration• Add Inhibitors

– Adsorption type, e.g. Organic amines, azoles– H evolution poisons, e.g. As & Sb– Scavengers, e.g. Sodium sulfite & hydrazine– Oxidizers, e.g. Chromates, nitrates, ferric salts

CATHODIC & ANODIC PROTECTION

• Cathodic protection: Make the structure more cathodic by– Use of sacrificial anodes– Impressed currents

Used extensively to protect marine structures, underground pipelines, water heaters and reinforcement bars in concrete

• Anodic protection: Make passivating metal structures more anodic by impressed potential. e.g. 316 s.s. pipe in sulfuric acid plants

COATINGS• Most popular method of corrosion protection• Coatings are of various types:

– Metallic– Inorganic like glass, porcelain and concrete– Organic, paints, varnishes and lacquers

• Many methods of coating:– Electrodeposition– Flame spraying– Cladding– Hot dipping– Diffusion– Vapour deposition– Ion implantation– Laser glazing

Surface Treatment (Coatings)• Organic paints• Chromating and phosphating:

– The Process - chromating and phosphating are surface-coating processes that enhance the corrosion resistance of metals. Both involve soaking the component in a heated bath based on chromic or phosphoric acids. The acid reacts with the surface, dissolving some of the surface metal and depositing a thin protective layer of complex chromium or phosphorous compounds

• Anodizing (aluminum, titanium)– The Process - Aluminum is a reactive metal, yet in everyday objects it does not corrode or discolor. That is because of

a thin oxide film - Al2O3 - that forms spontaneously on its surface, and this film, though invisible, is highly protective. The film can be thickened and its structure controlled by the process of anodizing. The process is electrolytic; the electrolyte, typically, is dilute (15%) sulfuric acid. Anodizing is most generally applied to aluminum, but magnesium, titanium, zirconium and zinc can all be treated in this way. The oxide formed by anodizing is hard, abrasion resistant and resists corrosion well. The film-surface is micro-porous, allowing it to absorb dyes, giving metallic reflectivity with an attractive gold, viridian, azure or rose-colored sheen; and it can be patterned. The process is cheap, an imparts both corrosion and wear resistance to the surface.

Surface Treatment (Coatings)• Electro-plating

– The Process -Metal coating process wherein a thin metallic coat is deposited on the workpiece by means of an ionized electrolytic solution. The workpiece (cathode) and the metalizing source material (anode) are submerged in the solution where a direct electrical current causes the metallic ions to migrate from the source material to the workpiece. The workpiece and source metal are suspended in the ionized electrolytic solution by insulated rods. Thorough surface cleaning precedes the plating operation. Plating is carried out for many reasons: corrosion resistance, improved appearance, wear resistance, higher electrical conductivity, better electrical contact, greater surface smoothness and better light reflectance.

Bluing

– Bluing is a passivation process in which steel is partially protected against rust, and is named after the blue-black appearance of the resulting protective finish. True gun bluing is an electrochemical conversion coating resulting from an oxidizing chemical reaction with iron on the surface selectively forming magnetite (Fe3O4), the black oxide of iron, which occupies the same volume as normal iron. Done for bolts called “blackening”

Hot-dip Coating (i.e. galvanizing)– Hot dipping is a process for coating a metal, mainly ferrous metals,

with low melting point metals usually zinc and its alloys. The component is first degreased in a caustic bath, then pickled (to remove rust and scale) in a sulfuric acid bath, immersed (dipped) in the liquid metal and, after lifting out, it is cooled in a cold air stream. The molten metal alloys with the surface of the component, forming a continuous thin coating. When the coating is zinc and the component is steel, the process is known as galvanizing.

– The process is very versatile and can be applied to components of any shape, and sizes up to 30 m x 2 m x 4 m. The cost is comparable with that of painting, but the protection offered by galvanizing is much greater, because if the coating is scratched it is the zinc not the underlying steel that corrodes ("galvanic protection"). Properly galvanized steel will survive outdoors for 30-40 years without further treatment.

CONCLUSION

• Corrosion is a natural degenerative process affecting metals, nonmetals and even biological systems like the human body

• Corrosion of engineering materials lead to significant losses

• An understanding of the basic principles of corrosion and their application in the design and maintenance of engineering systems result in reducing losses considerably