Occurs all the timeOccurs all the time

Attacks many structures in a plant Attacks many structures in a plant

Shortens useful life of plant equipmentShortens useful life of plant equipment

Increases maintenance requirementsIncreases maintenance requirements

Creates safety and environmental problemsCreates safety and environmental problems

Increases production downtimeIncreases production downtime

Costs you money!!!Costs you money!!!

Chapter 3. Corrosion of MetalsChapter 3. Corrosion of Metals

Corrosion Corrosion caused leakage caused leakage which triggered which triggered

the fire that the fire that destroyed this destroyed this

Nypro Reactor Nypro Reactor

Corrosion causes catastrophic Corrosion causes catastrophic failures !!failures !!Corrosion causes catastrophic Corrosion causes catastrophic failures !!failures !!

Chevron 2001Chevron 2001Chevron 2001Chevron 2001

The leak caused by corrosion at this elbow started the fire that destroyed this refinery

PhillipsPhillips -- Pasadena – 1989Pasadena – 1989: : Ethylene Ethylene rreactor eactor exploexplosionsionPhillipsPhillips -- Pasadena – 1989Pasadena – 1989: : Ethylene Ethylene rreactor eactor exploexplosionsion

TYPES OF CORROSIONTYPES OF CORROSION   There are many different ways of looking at real caseThere are many different ways of looking at real casess of metallic corrosion: of metallic corrosion:

• • low-temperature corrosion and high-temperature corrosionlow-temperature corrosion and high-temperature corrosion • • dry corrosion and wet corrosion:dry corrosion and wet corrosion:

atmosphere, industrial gasesatmosphere, industrial gasesaqueous solutions, liquid chemicalsaqueous solutions, liquid chemicals

• • chemical corrosion and electrochemical corrosionchemical corrosion and electrochemical corrosion • • types of metals: steels, aluminium alloys, ceramics, etc.types of metals: steels, aluminium alloys, ceramics, etc. • • types of environment: sulphuric acid, alkalis, marine, etc.types of environment: sulphuric acid, alkalis, marine, etc.

  One classification regards industrial metallic corrosion in 10 categories:One classification regards industrial metallic corrosion in 10 categories:

• • uniform attackuniform attack• • galvanic corrosiongalvanic corrosion• • crevice corrosioncrevice corrosion• • pittingpitting• • intergranular corrosionintergranular corrosion• • selective leachingselective leaching• • stress-corrosion cracking and corrosion fatiguestress-corrosion cracking and corrosion fatigue• • hydrogen damagehydrogen damage• • oxidationoxidation• • high temperature corrosionhigh temperature corrosion

UNIFORMUNIFORM CORROSION CORROSION

uniform removal of metal over the entire surfaceuniform removal of metal over the entire surface

•• it is the it is the most common type of corrosionmost common type of corrosion; it is; it is most metal-consuming most metal-consuming

•• all metals are attacked by uniform corrosionall metals are attacked by uniform corrosion

•• it could be it could be either chemical or electrochemicaleither chemical or electrochemical

•• steady corrosion over the entire surface exposed to the corroding mediasteady corrosion over the entire surface exposed to the corroding media

•• least objective in engineering design: easy testing, easy inspection, easy least objective in engineering design: easy testing, easy inspection, easy prediction of failureprediction of failure

  

Uniform corrosion is dealt with most effectively byUniform corrosion is dealt with most effectively by

•• proper selection of materialsproper selection of materials

•• application of protective coatingsapplication of protective coatings

•• addition of inhibitorsaddition of inhibitors

•• cathodic protectioncathodic protection

Question: do you know how the rate of uniform corrosion is determined? This is an important parameter for both the design and maintenance

Coating holidays Coating holidays cause localised cause localised corrosioncorrosion

corrosion products corrosion products of the unprotected of the unprotected re-bars expend in re-bars expend in

volume and cause volume and cause cracking to the cracking to the

concreteconcrete

GALVANICGALVANIC CORROSION CORROSION   

The corrosion of one metal caused by another in an electrochemical The corrosion of one metal caused by another in an electrochemical process driven by the potential difference between the two metals.process driven by the potential difference between the two metals.

In this process, the corrosion in one metal is accelerated (the anode) while in In this process, the corrosion in one metal is accelerated (the anode) while in the other suppressed (the cathode)the other suppressed (the cathode)

  Three Essential Conditions• potential difference• presence of an electrolyte• electrical connection between the two electrodes Factors Influencing Galvanic Corrosion•• potential difference: potential difference:

electromotive force (emf) of pure elementselectromotive force (emf) of pure elementsgalvanic series of alloysgalvanic series of alloys

•• environment: electrolyte conductivity, temperature, etc.environment: electrolyte conductivity, temperature, etc.be aware that under certain environment conditions a galvanic coupling may be aware that under certain environment conditions a galvanic coupling may reveres their cell potential difference: reveres their cell potential difference: galvanised steel in hot water systems galvanised steel in hot water systems

•• cathode-to-anode area ratiocathode-to-anode area ratio•• distance effectdistance effect

Galvanic series, the Galvanic series, the driving force of driving force of

galvanic corrosiongalvanic corrosion

Restoration of the Statue of Liberty in Restoration of the Statue of Liberty in 1986, due to galvanic corrosion damage1986, due to galvanic corrosion damage

  PreventionPrevention Techniques Techniques

•• selecting metals of similar electrode potentials to minimise the driving force selecting metals of similar electrode potentials to minimise the driving force of the processof the process

• • protection against moisture condensation to eliminate the chance of protection against moisture condensation to eliminate the chance of forming an electrolyteforming an electrolyte

• • insulation between dissimilar metals to avoid electrical connectioninsulation between dissimilar metals to avoid electrical connection

• • coating for electrical insulation or isolation of metal from electrolytecoating for electrical insulation or isolation of metal from electrolyte

• • installing a third metal which is anodic to both metalsinstalling a third metal which is anodic to both metals

•• designing for easy replacement of the anode metal or thicker section for designing for easy replacement of the anode metal or thicker section for longer service lifelonger service life

  

Beneficial Applications of Galvanic CorrosionBeneficial Applications of Galvanic Corrosion

• • Cathodic protection, sacrificial anode protectionCathodic protection, sacrificial anode protection

Galvanised steels:Galvanised steels: ZnZn coating is coating is anodic to steel, anodic to steel, act as a act as a sacrificial metalsacrificial metal

• • Cleaning silverCleaning silver

blackened surfaces:blackened surfaces: silver sulphidesilver sulphide

rubbing with an abrasive?rubbing with an abrasive? bad for silver platesbad for silver plates

Ag in Al pan: soda solution: Ag in Al pan: soda solution:

cathodic reaction reduces silver sulphide to Agcathodic reaction reduces silver sulphide to Ag

CREVICE CORROSIONCREVICE CORROSION

Crevice Crevice corrosioncorrosion is a localised attack occurring within crevices or is a localised attack occurring within crevices or other shielded other shielded areas where a small volume of stagnated solution areas where a small volume of stagnated solution presentspresents

MechanismIn a seawater environment: H2O, O2, Na+ and Cl-

Metal oxidises in balance with the reduction of oxygen: 

M M+ + e- (the anodic reaction)O2 + 2H2O + 4e- 4OH- (the cathodic reaction)M+ + OH- MOH

Under stagnant conditions, the concentration of M+ increases due to a decreasing concentration of O2. The positively charged crevice attracts negatively charged ions: Cl- and OH-. Being a smaller ion, Cl- travels faster than OH- into the crevice. 

The increased concentration of Cl- in the crevice promotes production of M +, encourages M+ + OH- MOH, and more hydrolysis of H2O, leading to

increased [H+] concentration.

Increased concentrations of [H+] and [Cl-] accelerate the corrosion crevice corrosion is self-catalytic.

Crevice corrosion on the face Crevice corrosion on the face of a flange caused by of a flange caused by

absorbent gasket, also known absorbent gasket, also known as gasket corrosionas gasket corrosion

Crevice corrosion of a boltCrevice corrosion of a bolt

Crevice corrosion is auto-catalyticCrevice corrosion is auto-catalytic

Characteristics• crevices of the width of 25 - 100 m are most effective• concentration of Cl- in a crevice is found to be 3-10 times higher than that in surrounding areas in a case of dilute neutral NaCl solution: pH value drops from 7 to 2-3.• crevice corrosion is characterised by an initiation period with a very slow start and an ever-increasing corrosion rate.• the oxygen reduction reaction provides cathodic protection in surrounding areas, making the attack inside the crevice very difficult to inspect.• it occurs in many mediums and is most intense in Cl- solutions.• metals and alloys having corrosion-resistant oxide films or passive layers are susceptible to crevice corrosion:

films destroyed by high concentration of Cl- and H+

stainless steels and Al alloys are typical examples Typical Conditions• holes on surfaces• gaps underneath bolt head or between lapping parts• porous mediums: gaskets, wood, fabrics, sand• deposition of dirt or corrosion product• water droplets, waterlineExample: an 18-8 stainless steel tank for a saline solution was safe in a dyeing plant; but when a stainless steel bolt accidentally fell to the bottom of the tank, rapid attack developed under the bolt after a brief period, causing leakage.

Minimising Crevice Corrosion• use welded butt joints instead of riveted or bolted joints• seal crevices: continuous welding/soldering• regular and thorough cleaning or complete draining to remove deposits or avoid stagnation• filter or screen flow to remove solids in suspension• use non-absorbent gaskets, such as Teflon, whenever possible.• use crevice corrosion resistant alloys Maximum crevice corrosion resistance is achieved in alloys of

• a narrow active-passive transition• a small critical current density• an extended passive region

 Titanium and high nickel alloys are examples of such materials. Type 430 stainless steel has a large critical current density, a wide active-passive transition and a limited passive region. It is extremely susceptible to crevice corrosion. Stainless steels as a family are very poor in resisting crevice corrosion.

Minimising Crevice Corrosion – tutorial questions

Due to the similar mechanism many other forms of corrosion are also considered crevice corrosion. These include:

Deposition corrosionWaterline corrosionInlet corrosionGasket corrosionDroplet corrosionDifferential aeration corrosion

What are these forms of corrosion?

Why are they considered similar to crevice corrosion?

How is crevice corrosion resistance (or tendency) evaluated for materials?

What about pitting resistance?

Most corrosion is found in the splash zone, where wet-dry

conditions alternate

PITTINGPITTINGPitting is a highly localised form of corrosion. It is characterised by

pits or holes of various sizes:

• small diameters and depth-to-diameter ratio of >>1• often in clusters• fail because of perforation with small weight loss • most destructive• very difficult to detect• difficult to evaluate by laboratory tests• develop and grow in the direction of gravity• undercut surface as they grow

Corrosion pitsCorrosion pits

Factors Affecting Pitting• Solutions

solutions containing chloride or chlorine-containing ions: sea water hypochlorites (HClO3) have strong pitting tendencies; oxidising metal ions with chlorides are extremely aggressive pitters:

cupric (CuCl2) and ferric (FeCl3) chlorides• Flow

Pitting is associated with stagnant conditions. Increasing flow velocity decreases pitting attack.• Alloys

As a class, stainless steels are more susceptible to pitting corrosion than are any other group of metals or alloys.

Solution-quenched austenitic SS exhibit better pitting resistance.Cold working increases pitting attack of 18-8 steels, preferentially on edges.Surface finish affects pitting resistance. Polished surfaces are more resistant than etched or ground surfaces.Cr, Ni, Mo and N as alloying elements increase pitting resistance of SS.

 Type 316 SS is more resistant to pitting than type 304 due to the addition of 2%Mo. Type 304 is considered unsuitable for applications in seawater whereas type 316 is sometimes recommended. Ti has excellent resistance to pitting, owing to its protective film being inert to Cl- and H+.

The MechanismThe process of pitting corrosion consists of two stages: the initiation and the growth. Initiation:Pitting starts with an initiation period of very slow corrosion rate. Pitting selectively initiates at areas of surface irregularities chemical, microstructural, physical.• a surface scratch or other mechanically induced break• en emerging dislocation or slip step• a compositional heterogeneity such as an inclusion, segregate or precipitateThe initiation of pitting is very fragile and young pits are unstable.  Growth:Following the initiation, a pit grows at an ever-increasing rate with an identical mechanism to crevice corrosion. It is an autocatalytic process. The same mechanism implies that alloys that show pitting attach are also susceptible to crevice corrosion.

EROSION CORROSIONEROSION CORROSION

Erosion corrosion is a result of the combined effect of chemical attack and mechanical abrasion.

 The Attack:The damage appears as groves, waves or holes, following the direction of the flow.

• Typical conditions:submarine propellersinterior of slurry pumpsexterior parts of high speed boats and shipshigh flow rate pipelines

• Special locations:elbows and junctions, extra angular accelerationsudden reduction of pipe diameter, high velocitysudden increase of pipe diameter, turbulencevalves, high velocity + turbulence

• Alloys: Most alloys are susceptible to erosion corrosion, particularly those that have low hardness and rely on protective surface films for corrosion resistance, such as Al, Pb and Cu alloys, and stainless steels.

Solid erosion corrosion of Solid erosion corrosion of impellers in slurry mediaimpellers in slurry media

Liquid impingement and Liquid impingement and impingement erosionimpingement erosion

Erosion corrosion at Erosion corrosion at pipeline elbowpipeline elbow

Factors Affecting Erosion Corrosion

• MediumMany mediums can cause erosion corrosion. These include gases, aqueous

solutions, organic systems, and liquid metals. Solid particles in suspension in fluid are most destructive by destroying surface films. • Velocity

Increasing velocity generally increases erosion corrosion rate. There usually exists a critical velocity beyond which the rate of corrosion is suddenly increased. 

- laminar flow moving at a velocity removes metal ions from metal surface and break local equilibrium balance, encouraging further dissolution of metal 

- low flow velocity helps avoid stagnant conditions, replenish oxygen and bring inhibitors to metal surface, leading to a decrease in corrosion rate

 - corrosion tests under static or slow motion conditions often do not represent the real situation.

• TurbulenceTurbulence provides a greater agitation of the fluid and greater mechanical impact to the surface of the metal. Instantaneous high pressure pulses associated with the

formation and explosion of microbubbles cause most damage to metal surfaces. • Impingement

Impingement create a local environment of very high velocity, very strong turbulence, and very high pressure pulses and thus is very destructive in causing erosion corrosion.

 • Cavitation

Cavitation damage is a special form of erosion corrosion, commonly observed on components moving at very high velocities through fluid. It is caused by the formation and

collapse of liquid vapour bubbles, which may create local pressure pulses as high as 400 MPa, causing local plastic deformation and destruction of surface films to the metal. 

Typical cases of cavitation damage:

- leading edge of the wing of supersonic airplanes caused by rain droplets- leading edge of propellers of sea going vessels- body of high speed boats- liner on the coolant side of vehicle engines caused by vibration

Cavitation damagesCavitation damages

Minimising Erosion Corrosion

• MaterialsSolid solution hardening is effective in improving resistance to erosion corrosion. Solution hardening is more effective than other hardening methods in improving corrosion resistance is due to the fact that other methods tend to produce heterogeneous microstructure or cause mechanical instability to surfaces. 

- High silicon iron is an improvement to cast irons and it is widely used in severe erosion corrosion conditions, such as slurry carrying pipelines.- Alloying with noble metals to be inherently more resistant to corrosion

80%Ni-20%Cr alloy ferritic stainless steels (80%Fe-20%Cr)

- Alloying to form more stable and impermeable surface films316 stainless steel to improve 304 aluminium brass to improve Cu-Zn brass

- Stainless steel is considered to have the greatest resistance towards cavitation

Shape memory alloys are excellent in resisting cavitation, due to their ability to deform non-destructively during impact.

• DesignErosion corrosion is closely related to the structure of a system and the flow pattern of the liquid; thus, many erosion corrosion situations may be avoided or minimised by proper

design. Increasing tube diameter to reduce flow velocity and ensure laminar flow- Increasing tube diameter to reduce flow velocity and ensure laminar flow- Using streamline bends and expanded junction section to minimise impingement effect- Lining with second metal at high risk locations (galvanic corrosion !!!)- Easy to replace- Protruding pipe ends at inlet and out let, delivering turbulence away from the vessel wall into the middle of liquid.- Very smooth surface to minimise the chance of vapour nucleation as against cavitation 

• EnvironmentSettling and filtering to remove solids in suspension are helpful. Inhibitors may also be

added to the liquid. Decreasing temperature always reduces the rate of corrosion. • Surfacing

Some surface coatings are effective to prevent other forms of corrosion, but may not have satisfactory mechanical properties to stand against erosion corrosion, particularly when a heavily suspended slurry solution is involved. Hard facings, welded overlays and

replaceable inserts are widely used.

proper design is the most effective way of preventing erosion corrosionproper design is the most effective way of preventing erosion corrosion

INTERGRANULAR CORROSIONINTERGRANULAR CORROSION

Intergranular corrosion is a localised corrosion that occurs preferentially along grain boundaries inside a metal.

• Grain Boundary AttackGrain boundary regions are generally subjected to a higher degree of corrosion, because of the relatively higher free energy state, chemical segregation and

impurity concentration. However, in most cases grain boundary corrosion does not impose a serious problem to the performance of a structural component. • Sensitisation of Austenitic Stainless Steels

Austenitic stainless steels tend to form carbide precipitates (Cr23C6) along grain boundaries at 400-850 °C, causing a local depletion of Cr. The sensitised regions are anodic to the grains and are attacked preferentially. A common cause of sensitisation is welding, known as weld decay.  Common heat treatments of austenitic stainless steels:

(i) stress relieving at 350-450 °C to avoid sensitisation(ii) solution annealing at 1000-1100 °C followed by quenching to eliminate the

effect of sensitisation; more problems with cast austenitic stainless steels

The problem of weld decay may be avoided by using:- L grades: 304L, 317L, 316L: <0.03%C , standard 18-8 steels: 0.06-0.15%C -  Stabilisers: strong carbide forming elements: Nb, Ta (type 347), Ti (type 321)- Electric arc welding instead of flame welding: more rapid heating cycle,

narrower heat affected zone, lower tendency to form carbides in HAZ- Laser cutting instead of oxy-flame cutting

 • Other Alloys Susceptible to Sensitisation

Some high strength Al alloys are also susceptible to sensitisation. For example, the strengthening precipitates CuAl2 in Al-Cu alloys cause local depletion of Cu, reducing their corrosion resistance.

Acceperated grain boundary corrosion due to Cr depletion caused by formation of Cr carbides

Heat affected zones provide a condition for SS sensitisation

Fusion cutting is another case

SELECTIVE LEACHINGSELECTIVE LEACHING

Selective leaching, also known as dealloying and parting, is the selective preferential removal of one elemental spices in an alloy system.

dezincification of yellow brassdealuminification of aluminium bronzegraphitisation of grey irondechrominification

 • Dezincification

zinc dissolves in pure water by a hydrolytic reaction.Better resistance to dezincification:

red brass: 85-15%Znalloying: "inhibitors", such as Sn, As and P

 Al bronze and Si bronze are attacked by selective leaching

Al, Si and Zn are anodic to Cu 

• GraphitisationGraphitisation is the selective removal of iron from the surface of grey cast iron due to a galvanic reaction between the graphite and iron. It is the most costly damage large-diameter underground water mains.

 - Graphite is cathodic to iron, forming excellent galvanic cells- Moist soil under the ground and aqueous solution it carries inside provide the environment for selective leaching- Earth movement may cause failure as a result of reduced strength- Use of nodular or malleable iron is effective in avoiding graphitisation

Dezincified yellow brass Dezincified yellow brass showing red Cu colourshowing red Cu colour

STRESS CORROSION CRACKINGSTRESS CORROSION CRACKING The AttackStress corrosion cracking (SCC) is a cracking failure of materials caused by the combined action of tensile stresses and corrosive environments. SCC occurs to many different materials, including plastics, Al alloys, Cu alloys, Mg alloys, carbon steels, stainless steels, Ti  It occurs only with specific material-environment combinations:

stainless steels in 50-60 °C chloride-containing solutionscarbon steels in caustic solutionsAl alloys in chloride solutionsCu alloys, particularly brasses, in ammonia atmosphere

 The environments in which SCC occurs sometimes are not highly corrosive to the metals in question. When SCC happens the metal may be virtually unattacked over most of its surfaces. The stress that induces SCC is also often very much lower than the failure stress of the metal. It is the combination of the two that causes the attack. 

Morphology of cracksWedging effect of corrosion productsIntergranular and transgranular cracks

Factors Affecting Stress Corrosion CrackingFactors Affecting Stress Corrosion Cracking

• Stress:Stress:residual stresses: welding, cold working, heat treatment and castingresidual stresses: welding, cold working, heat treatment and castingappliedapplied stresses: gravitation, mechanical assembling stresses, temperature stresses: gravitation, mechanical assembling stresses, temperature

variation, etc.variation, etc.a critical stress seems to exist for SCC for each metal-environment combinationa critical stress seems to exist for SCC for each metal-environment combination

  • TimeTime

Materials fail by SCC in brittle fracture manners - corrosion is responsible for Materials fail by SCC in brittle fracture manners - corrosion is responsible for nucleation of cracks and failure occurs by mechanical cracking.nucleation of cracks and failure occurs by mechanical cracking.

short time SCC testsshort time SCC testsdecreasing stress and temperature increases failure timedecreasing stress and temperature increases failure time

  • Metallurgical FactorsMetallurgical Factors

Generally speaking, pure metals have lower tendency towards stress corrosion Generally speaking, pure metals have lower tendency towards stress corrosion cracking than alloys. Single phase structure better than multiphase structures. cracking than alloys. Single phase structure better than multiphase structures. Segregation of precipitates raises the susceptibility to stress corrosion cracking. Segregation of precipitates raises the susceptibility to stress corrosion cracking. However, a soft phase inclusion, such as ferrite domains in austenite stainless However, a soft phase inclusion, such as ferrite domains in austenite stainless steel matrix, may relax the stress concentration at crack tips and slow down their steel matrix, may relax the stress concentration at crack tips and slow down their propagation.propagation.

• EnvironmentEnvironmentNo general rules to what environments cause SCCNo general rules to what environments cause SCCSpecific metal - environment combinationsSpecific metal - environment combinationsRefer to the list of established data for known Refer to the list of established data for known

combinationscombinationsConduct new tests for combinationsConduct new tests for combinations

Cracks may propagate in Cracks may propagate in intergranular or intergranular or

transgranular manner.transgranular manner.

Chloride SCC. SCC always occurs at the tensile stress site

The Process of Stress Corrosion Crackingunclear mechanism - complex interplay of metal, environment, stress states and interface properties

 • Initiation

Corrosion is the primary reason for crack initiation - cracks are observed to start at the base of a pit.

A tensile stress assists breaking up protective films and enhance the elastic energy of surface atoms.

• Propagation In the intermediate stages, stress corrosion cracking proceeds by the conjoint action of

stress (concentration) and corrosion at crack tips. Cracks have been observed to propagate in discontinuous steps, emitting acoustic waves when jumping. The contribution of corrosion to crack propagation is evident in experiments when acoustic waves are stopped at the application of cathodic protection and resumed after the removal of the cathodic protection. 

preferential attack at crack tips:local plastic deformation, increased defect density, emerging slip steps, decreased resistance to chemical attack

 • Cracking

The final failure caused by unstable propagation of cracks is basically a mechanical process. However, the presence of corrosive chemicals and water molecules at the crack tips lowers the critical stress for cracking.

Summary

Stress corrosion cracking is an electrochemical-mechanical process. It initiates at small but chemically active areas such as a stress induced film rupture, a high energy grain boundary, and chemical segregation sites.

 Stress corrosion cracks propagate in discontinuous steps. The slow motion of corrosion builds up stress concentration at a crack tip until a critical moment

when the crack propagate to a new location and stopped when the stress concentration is relaxed. 

Stress corrosion cracks may follow intergranular and transgranular paths. In materials having complex slip systems or high stacking-fault energies, cracks propagate along intergranular paths. 

The role of corrodant in the mechanism of stress corrosion cracking is the least understood.

PreventionAvoid dangerous metal-environment combinations by using a different metal. Carbon steels sometimes are used in preference to stainless steels due to their higher resistance to stress corrosion cracking. Inconel is used to replace type 304 SS for the same reason.

 Eliminate the tensile stress component whenever possible. More than often a tensile stress is from residual sources; thus stress relieving heat treatment is commonly recommended. Furthermore, creating a compressive surface residual stress condition is highly effective in suppressing stress corrosion cracking. A compressive surface stress state may be achieved by various means including thermal treatment, blasting, ion implantation.

 Reduce the corrosivity of environment, such as to change the pH of a fluid, to eliminate oxygen or chloride from a solution. Inhibitation, cathodic protection and design modification may also render the environment less harmful.

 Cathodic protection is effective. It may be applied either with an external power supply or consumable anodes. Precautions must be taken when applying cathodic protection to guard for hydrogen embrittlement.

Corrosion FatigueThe presence of a corrodant, or the action of corrosion, tends to reduce the fatigue life, or decreases the fatigue limit, of a metal.

 Little is known about corrosion fatigue beyond the knowledge of stress corrosion cracking.

 Corrosion fatigue is characterised by transgranular cracks that do not show much branching. The final cracking is largely a mechanical process.

 • Factors Affecting Corrosion Fatigue

Fatigue life in the case of pure mechanical loading is determined by the number of cycles; the effect of cycling frequency is negligible. In the case of corrosion fatigue, however, stress-cycle frequency has a strong influence on the fatigue life of a metal. Corrosion fatigue is most pronounced at low stress frequencies. Low frequencies allow a better contact of corrodant to the metal at crack tips.

 • Prevention

In addition to those applied to stress corrosion cracking, vibration should clearly be avoided to prevent corrosion fatigue, by, for instance, proper designs.

Fretting corrosion is a special Fretting corrosion is a special form of corrosion fatigueform of corrosion fatigue

HYDROGEN DAMAGEHYDROGEN DAMAGEHydrogen in environment is damaging to metalsDamage is associated with hydrogen absorptionForms of damage: embrittlement, blistering & decarburisationSources of atomic hydrogen:

corrosion processapplication of cathodic protectionweldingelectrolysis & electroplating

 • Hydrogen BlisteringHydrogen blistering is caused by the formation of micro-bubbles of high pressure hydrogen gas inside the metal The most damaging fact is that the equilibrium pressure between H2 and H is very high - several 100,000 atm, sufficient to rupture any known engineering material.  Hydrogen blistering is most prevalent in petroleum industry. It occurs in storage tanks and in refining processes.

• Hydrogen EmbrittlementHydrogen Embrittlement

Hydrogen embrittlement is the brittle cracking of metals caused by Hydrogen embrittlement is the brittle cracking of metals caused by hydrogen absorption. The actual embrittling mechanism is not clear. hydrogen absorption. The actual embrittling mechanism is not clear.   Ti alloys: formation of brittle hydrides Ti alloys: formation of brittle hydrides Irons & steels: interaction of H & crack tipsIrons & steels: interaction of H & crack tipsDifferent from SCC: cathodic current suppresses SCC but encourages Different from SCC: cathodic current suppresses SCC but encourages hydrogen embrittlementhydrogen embrittlement  Hydrogen embrittlement is a more serious problem in high strength Hydrogen embrittlement is a more serious problem in high strength materials:materials:

- HSLA steels - HSLA steels - high strength grades of carbon steels- high strength grades of carbon steels- Ti alloys - Ti alloys - Cu alloys- Cu alloys

• PreventionPrevention

• "Clean" steels: "Clean" steels: rimmed steels: high % microvoidsrimmed steels: high % microvoidskilled steels: voids-free structurekilled steels: voids-free structure

• Coating & lining: Coating & lining: impervious to hydrogen penetrationimpervious to hydrogen penetrationelectroplating, cladding with ASS or nickelelectroplating, cladding with ASS or nickelporous materials: brick liningporous materials: brick lining

• Resistant alloys:Resistant alloys:Ni-containing steels & Ni alloysNi-containing steels & Ni alloyslow diffusion rate of H in Nilow diffusion rate of H in Ni

• Baking: Baking: absorption of H in metals is reversibleabsorption of H in metals is reversiblebaking at 100-150°C removes dissolved H in steelsbaking at 100-150°C removes dissolved H in steels

• Process control:Process control:pickling, plating & weldingpickling, plating & weldingLow-H or H-free welding techniques Low-H or H-free welding techniques

• Decarburisation & Hydrogen AttackDecarburisation & Hydrogen Attack

Removal of C in C-containing alloys at high T:Removal of C in C-containing alloys at high T:[C] + 4[H] = CH[C] + 4[H] = CH44

CHCH44 formed in microvoids exerts a high pressure to the matrix of formed in microvoids exerts a high pressure to the matrix of steels, steels,

causing embrittlcausing embrittlementement – – known as known as hydrogen attackhydrogen attackHydrogen attack occurs inHydrogen attack occurs in

- petrochemical plants- petrochemical plants- oil refineries- oil refineries- processing lines for ammonia & methanol synthesis- processing lines for ammonia & methanol synthesis- conventional power stations- conventional power stations

• Environmental EffectEnvironmental Effect

Hydrogen attack occurs in carbon steels at 250 - 600 °C with critical Hydrogen attack occurs in carbon steels at 250 - 600 °C with critical hydrogen partial pressures of 1 -75 MPa. Increasing temperature hydrogen partial pressures of 1 -75 MPa. Increasing temperature decreases the critical hydrogen partial pressure required for the attack. At decreases the critical hydrogen partial pressure required for the attack. At temperatures below 200°C, carbon steels are not attacked by hydrogen gas temperatures below 200°C, carbon steels are not attacked by hydrogen gas even under relatively high pressures.even under relatively high pressures.  

• Metallurgical FactorsMetallurgical Factors

The sensitivity of carbon steels to hydrogen attack increases with The sensitivity of carbon steels to hydrogen attack increases with carbon carbon contentcontent: :

- commercial pure iron (0.004%C) resisted attack at 540°C & 4.8 MPa - commercial pure iron (0.004%C) resisted attack at 540°C & 4.8 MPa for 500 hrs for 500 hrs

- steel (0.009%C) was attacked under the same conditions- steel (0.009%C) was attacked under the same conditions  Alloying with carbide-forming elementsAlloying with carbide-forming elements ( (Ti, V, Cr, Mo and WTi, V, Cr, Mo and W)) improves improves resistance to hydrogen attack. resistance to hydrogen attack. TThese elements stabilise hese elements stabilise CC in the matrix, in the matrix, mostly by forming complex alloy carbides, such as (Fe,M)mostly by forming complex alloy carbides, such as (Fe,M)33C or MC or M77CC33..

ProcessingProcessing- Cold worked steels embrittle- Cold worked steels embrittle easily easily in high in high-T-T, high, high-P -P hydrogenhydrogen- Surface stresses accelerate - Surface stresses accelerate HH absorption absorption andand cause hydrogen attack cause hydrogen attack- HAZ in a weldment is more susceptible to hydrogen attack- HAZ in a weldment is more susceptible to hydrogen attack- Quench - Quench andand tempered steels are more resistant than normalised ones tempered steels are more resistant than normalised ones - - Spheroidising improves resistance to hydrogen attackSpheroidising improves resistance to hydrogen attack

OXIDATIONOXIDATIONReaction between a metal and O2 at the absence of water.

Dry oxidation is only a process of appreciable rate at elevated temperatures for most metals

Pilling-Bedworth RatioIt is suggested that oxidation resistance of a metal depends on the properties of the metal oxide on the surface, as determined by the Pilling-Bedworth Ratio: 

PB ratio =  PB ratio ~ 1 gives good oxidation resistanceRequirements on protective film:

- good adherence, coherent to base metal- good mechanical properties: strength, ductility and toughness, no

rupture under applied or thermal stresses- high melting temperature- similar thermal expansion coefficient to metal- low diffusion coefficient for oxygen and metal ions

( )( )

volume oxidevolume metal

Electrochemistry of OxidationElectrochemistry of Oxidation

Oxidation process of metals in gaseous oxygen is effectively an Oxidation process of metals in gaseous oxygen is effectively an electrochemical process rather than a simple chemical reaction, in analogy electrochemical process rather than a simple chemical reaction, in analogy to aqueous galvanic processto aqueous galvanic process, for example:, for example:

Cu + OCu + O22 = CuO = CuO

Anodic reaction at metal-scale interface: Anodic reaction at metal-scale interface: Cu = CuCu = Cu2+2+ + 2e + 2e--

Cathodic reaction at scale-gas interface:Cathodic reaction at scale-gas interface:OO22 + 2 + 2e-e- = O = O2-2-

Most metal oxides conduct electrons and ions to some extentMost metal oxides conduct electrons and ions to some extent

Morphology of OxidesMorphology of OxidesDiffusion of either metal ions or oxygen ions through the oxide controls the Diffusion of either metal ions or oxygen ions through the oxide controls the oxidation rateoxidation rate  Fe, Ni, Cu, Cr, Co: oxides grow preferentially at the scale-gas interface by Fe, Ni, Cu, Cr, Co: oxides grow preferentially at the scale-gas interface by outwards cation diffusion certain protection of the scaleoutwards cation diffusion certain protection of the scale  Ti, Ta, Zr, Hf: oxides grow inwards at the metal-scale interface, non-Ti, Ta, Zr, Hf: oxides grow inwards at the metal-scale interface, non-protective due to scale fracturingprotective due to scale fracturing  Oxidation RateOxidation RateControlling factor:Controlling factor: conductivity of oxide conductivity of oxide

Anion deficient oxides: Anion deficient oxides: n-type oxidesn-type oxidesCation deficient oxides:Cation deficient oxides: p-type oxidesp-type oxides

  Alloying with higher valence metals reduces anion vacancy concentration of Alloying with higher valence metals reduces anion vacancy concentration of n-type oxides & reduces the rate of diffusion-controlled oxidationn-type oxides & reduces the rate of diffusion-controlled oxidation  Alloying with lower valence metals reduces cation vacancy concentration of Alloying with lower valence metals reduces cation vacancy concentration of p-type oxides & reduces the rate of diffusion-controlled oxidationp-type oxides & reduces the rate of diffusion-controlled oxidation

Oxidation KineticsOxidation KineticsDifferent metals show different oxidation kinetic behaviourDifferent metals show different oxidation kinetic behaviour

  - Linear:- Linear: W = ktW = kt

metals having non-protective surface filmsmetals having non-protective surface filmsNa, K Na, K PB ratio ~0.5PB ratio ~0.5Ta, Nb Ta, Nb PB ratio ~2.5PB ratio ~2.5

- Parabolic:- Parabolic: WW22 = kt + C = kt + Cmetals having protective surface filmsmetals having protective surface films

Fe, Co, Cu, NiFe, Co, Cu, Ni

- Logarithmic:- Logarithmic: W = klog(Ct + A)W = klog(Ct + A)empirical observationempirical observationthin oxide layers at low temperaturesthin oxide layers at low temperatures

Al, Cu, FeAl, Cu, Fe

- Catastrophic:- Catastrophic:oxidation with continuously increasing rateoxidation with continuously increasing rateignition and self-sustained combustion of metals ignition and self-sustained combustion of metals

Mg, Al (powder), ZnMg, Al (powder), Zn

Oxidation Resistance of Fe-Ni-Cr Oxidation Resistance of Fe-Ni-Cr AAlloyslloys

Fe, Ni and Co exhibit only moderate oxidation resistance. Alloying with Cr, Fe, Ni and Co exhibit only moderate oxidation resistance. Alloying with Cr, Si and Al enhances their resistance. Fe-Ni-Cr alloys are the most commonly Si and Al enhances their resistance. Fe-Ni-Cr alloys are the most commonly used alloys for general purpose high temperature oxidation resistance used alloys for general purpose high temperature oxidation resistance applications, largely due to their relatively low costs, moderate oxidation applications, largely due to their relatively low costs, moderate oxidation resistance, and good mechanical properties resistance, and good mechanical properties