types of corrosion.ppt

8/11/2019 types of corrosion.ppt

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Types of Corrosion

Dr. Hamdy A. Kandil


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Metallic corrosion


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Forms of corrosion

Uniform corrosionPittingCrevice corrosionGalvanic corrosion

Erosion corrosionCavitationFretting corrosion


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Forms of corrosion

Intergranular corrosionExfoliationDealloying (selective leaching)Environmental cracking Stress

Corrosion Cracking (SCC)Corrosion fatigueHydrogen embrittlement


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Uniform corrosionUniform corrosion is characterized by corrosive attackproceeding evenly over the entire surface area, or alarge fraction of the total area. On the basis of tonnagewasted, this is the most important form of corrosion.


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PittingPitting corrosion is a localized form of corrosion by whichcavities or "holes" are produced in the material.Pitting is considered to be more dangerous than uniformcorrosion damage because it is more difficult to detect,predict and design against.


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A Pitdeep pit (anode) general attack(small pits)

An aluminum

A92519 specimenexposed to a 3,5%NaCl solution forseven days. Thewidth of the pictureis approximately 1mm.

intact(cathode)


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Causes of pitting

Localized chemical or mechanical damage tothe protective oxide film.

Low dissolved oxygen concentrations and highconcentrations of chloride (as in seawater)Localized damage to, or poor application of, aprotective coatingThe presence of non-uniformities in the metalstructure of the component, e.g. inclusions.


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Passive metals pitPitting corrosion on a stainless steel barexposed to an alkaline solution loadedwith chlorides.


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Morphology of pitting

narrow/deeppits

elliptical pits

shallow pits

mesa attack


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Morphology of pittingSubsurface pits

Undercutting pits

vertical grain attack

Horizontal grainattack


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A pitting-related accidentThe sewer explosion that killed 215 people in Guadalajara,Mexico, in 1992. Besides the fatalities, the series of blastsdamaged 1,600 buildings and injured 1,500 people.Damage costs were estimated at 75 million U.S. dollars.


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Crevice corrosionCrevice corrosion is a localized form of corrosion usuallyassociated with stagnant solutions in shielded areassuch as those formed under gaskets, washers, insulationmaterial, fastener heads, surface deposits, disbondedcoatings, threads, lap joints, clamps, etc.


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Example of crevice corrosionCrevicescorrosion of aS30400stainless steel

washer after 30days in 0.5FeCl3 + 0.05 MNaCl solution


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Another example of crevice

corrosion

Stainless steel bolt used in seawater after 5 years ofexposure.


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Initiation of crevice corrosionCrevice corrosion is initiated by changes in localchemistry within the crevice:

Depletion of inhibitor in the crevice Depletion of oxygen in the crevice A shift to acid conditions in the crevice Build-up of aggressive ion species (e.g. chloride) in

the crevice

Popped rivets found on anexternal section of amaritime patrol aircraftfuselage.


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Analysis of a crevice-corrosionrelated problem (continued)

Underside of panel where severe corrosion was found


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Analysis of a crevice-corrosionrelated problem (continued)

Close-up picture showing the severity of corrosion


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Advanced crevice corrosionpillowing

Notice how the rivet heads appear to be lower than thesurrounding skin surface.


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Stage 0 of crevice corrosionConsider the case of apassive metal corrodingAssume that the maincathodic species isdissolved oxygenCrevice shown is notdrawn to scale (it is muchnarrower and longer in

reality)

Initially, the dissolved oxygen content in the crevice isequal to the one outside. The corrosion rate is uniform.


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Stage 1 of crevice corrosionDue to the difficult accesscaused by the crevicegeometry, oxygen is rapidlydepleted in the

crevice by uniform corrosion. The corrosion reactions nowseparate, i.e. metaldissolves more in the crevice(anode) and oxygen isreduced more on the outsidemetal surface (cathode).

The large cathodic surface (Sc) vs. anodic surface (Sa) ratio (Sc/Sa)leads to an acceleration of the anodic (corrosion) reaction in the crevice.


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Stage 2 of crevice corrosionThe metal ions producedby the anodic corrosionreaction form corrosionproducts (hydrolyze)

giving off protons (acid) and.The pH in a crevice canreach very acidic values,sometimes equivalent topure acids.

The acidification of the local environment produces aserious increase in the corrosion rate of most metals.The corrosion products seal even further the crevice

environment and the problem gets worse.


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Galvanic corrosionThe driving force for corrosion is a potential differencebetween the different materials. In a bimetallic couple,the less noble material will become the anode of thiscorrosion cell and tend to corrode more compared with

the uncoupled condition. The more noble material will actas the cathode in the corrosion cell and corrode less.


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Example of galvanic corrosionStainless screw in contact with a cadmium plated steelwasher


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Nobility and galvanic corrosion

The relative nobility of a material can bepredicted by measuring its corrosion potential.

The well known galvanic series lists the relativenobility of certain materials in a givenenvironment (e.g. seawater).

A small anode/cathode area ratio is highlyundesirable. In this case, the galvanic current isconcentrated onto a small anodic area leading toa very high corrosion rate.


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Galvanic series in seawaterPlatinumGoldGraphiteTitaniumSilver

Chlorimet 3Hastelloy C18-8 Mo stainless steel (passive)18-8 stainless steel (passive)

Chromium steel >11 % Cr (passive)Inconel (passive)Nickel (passive)


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Galvanic series in seawater

(continued)Silver solderMonelBronzesCopperBrassesChlorimet 2Hastelloy BInconel (active)Nickel (active)TinLead


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Galvanic series in seawater

(continued)Lead-tin solders18-8 Mo stainless steel (active)18-8 stainless steel (active)

Ni-resistChromium steel >11 % Cr (active)Cast ironSteel or iron

2024 aluminumCadmiumCommercially pure aluminiumZincMagnesium and its alloys


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Erosion corrosionErosion corrosion is an acceleration in the rate ofcorrosion attack in metal due to the relative

motion of a corrosive fluid and a metal surface.


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Example of erosion corrosionpump rotor destroyed by erosion corrosion

Aggravating factors: high flow velocity

flow disturbances presence of solids multiphase flow


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Example of erosion corrosionInside of the stainless steel pump column spools pumpingseawater serving in the Arabian gulf.


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CavitationCavitation occurs when a fluid's operational pressuredrops below the vapor pressure causing bubbles to formand then increases causing them to violently collapse.


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Cavitation usually happens At the suction side of a pump, especially ifoperating near the net positive suctionhead

At the discharge of a valve or regulator,especially when operating in a nearclosedposition

At flow expansions in other processesincurring sudden pressure drops andincreases


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FrettingFretting corrosion is damage at contact surfacesrubbing against each other (under load and inrepeated relative surface motion, as induced forexample by vibration).


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Intergranular corrosionIntergranular corrosion is localized attack along the

grain boundaries, or immediately adjacent tograin boundaries, while the bulk of the grainsremain largely unaffected.


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Causes of intergranular corrosion

This form of corrosion is usually associatedwith chemical segregation effects

(impurities have a tendency to be enrichedat grain boundaries) or specific phasesprecipitated on the grain boundaries.

Such precipitation can produce zones ofreduced corrosion resistance in theimmediate vicinity.


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Example of intergranular corrosion

Sensitization of stainless steels :Chromium-rich grain boundary precipitates

lead to a local depletion of Cr immediatelyadjacent to these precipitates, leavingthese areas vulnerable to corrosive attackin certain electrolytes.Reheating a welded component duringmulti-pass welding is a common cause ofthis problem.


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ExfoliationExfoliation is a form of intergranular corrosion associated

with high strength aluminum alloys that have beenextruded or otherwise worked heavily, with amicrostructure of elongated, flattened grains.

failed aircraft componentmade of 7075-T6 aluminum(picture width = 400 m)


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Anisotropic grain structure in wrought aluminum alloys


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Example of exfoliationexfoliation of an aircraft component


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Dealloying (selective leaching)Dealloying or selective leaching is a selectiveremoval of one or more elements from an alloyby the corrosion processes.

Examples:dezincification of unstabilized brassgraphitization of gray cast iron


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Environmental cracking

Stresses that cause environmental crackingarise from:

Residual cold workWeldingGrinding

Thermal treatmentService conditions To be effective the stresses must be tensile.


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Types of environmental cracking

Stress Corrosion Cracking (SCC) Sulfide stress cracking

Chloride induced SCC Caustic induced SCC

Hydrogen embrittlementCorrosion fatigue


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Cracks

The cracks form and propagate approximately atright angles to the direction of the tensile

stresses at stress levels much lower than thoserequired to fracture the material in the absenceof the corrosive environment.

As cracking penetrates further into the material,

it eventually reduces the supporting crosssection of the material to the point of structuralfailure from overload.


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Stress Corrosion Cracking

Stress corrosion cracking (SCC) is thecracking induced from the combinedinfluence of tensile stress and a corrosive

environment.

intergranular SCC of anInconel heat exchanger tube


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Stress Corrosion Cracking

Usually, most of the surface remains unattacked,with fine cracks penetrating into the metal.In the microstructure, these cracks can have anintergranular or a transgranular morphology.Macroscopically, SCC fractures have a brittleappearance.

SCC is classified as a catastrophic form ofcorrosion, as the detection of such fine crackscan be very difficult and the failure not easilypredicted.


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Example of SCC

SCC in a 316 stainlesssteel chemicalprocessing piping

system containingchlorides.Chloride stress corrosioncracking in austeniticstainless steel ischaracterized by themulti-branched "lightningbolt transgranular crackpattern.


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A catastrophic case history On October 4, 1992 an EL AL

Boeing 747 freighter crashed in Amsterdam, killing all fourpeople on board and over 50people on the ground.The cause of the crash was thenumber 3 and 4 engines

separated from the wing,causing a loss of control.The reason for the number 3engine separation was abreakage of the fuse pinweakened by a crack.

The pin was designed to breakwhen an engine seizes in flight,producing a large amount oftorque.


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Hydrogen embrittlement

Also called: hydrogen induced crackingInvolves the ingress of hydrogen into the

metal causing: Reduced ductility and load-bearing capacity, Subsequent cracking and Catastrophic brittle failures at stresses below

the yield stress of susceptible materialsMost vulnerable are high-strength steels,titanium alloys and aluminum alloys


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Sources of hydrogen

In the metal making processIn meatl processing (phosphating, pickling)From weldingIn storage or containment of hydrogen gas

As a by-product of general corrosionFrom cathodic protectionIn electroplating


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Hydrogen embrittlement of

stainless steelHydrogen (atoms) present at the surface entersthe steelHydrogen diffuses along the grain boundariesand combines with the carbon, which is alloyedwith the iron, to form methane gasMethane gas is not mobile and collects in smallvoids along the grain boundaries and reducesductilityThe gas can build up enormous pressures thatcan initiate cracks


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Corrosion FatigueCorrosion-fatigue is the result ofthe combined action of analternating stress and acorrosive environment.The fatigue process is thought

to cause rupture of theprotective passive film, uponwhich corrosion is accelerated.The introduction of a corrosiveenvironment often eliminates

the normal fatigue limit of aferrous alloy, thereby creating afinite life regardless of stresslevel.

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