07-Mechanical Aspects of Corrosion

7/28/2019 07-Mechanical Aspects of Corrosion

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MECHANICAL ASPECTS OF CORROSION

Dr. Ramazan Kahraman

Chemical Engineering DepartmentKing Fahd University of Petroleum & MineralsDhahran, Saudi Arabia

Reading Material: Chapter 8 inPrinciples and Prevention of Corrosion, Denny J ones, Prentice-Hall, 1996.


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Mechanical Aspects of Corrosion

z Static stress

stress-corrosion cracking (SCC) hydrogen embrittlement or hydrogen induced cracking (HIC) liquid metal embrittlement (liquid metals can permeate down

grain boundaries and cause intergranular cracking, e.g.,mercury on brass and aluminium alloys, liquid zinc onstainless steel)

z Dynamic stress corrosion fatigue cracking (SCC) fretting corrosion

z Stress per se does not affect corrosion processes much

z Plastic strain can have a large effect: increased dislocation density

rupture of passive films


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Stress-Corrosion Cracking (SCC)z Cracking of a metal under the combined effects of a

static stress and a specif ic chemical environment(SCC occurs in specific environments only)

z Cause of major industrial costs and safety hazards


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Stress Corrosion Cracking Systems

z Brass and ammonia (often in local atmospheres).

z Austenitic stainless steels and chloride solutions(70 oC).

z Carbon steels in caustic, carbonate/ bicarbonate,nitrate and phosphate.

z High strength aluminium alloys in water or water vapour.


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Mechanism of Stress-Corrosion Cracking

z Several possible mechanisms, still notfully understood

z Mechanisms Anodic dissolution Film-induced cleavage

Bond weakening through damagingadsorption


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Anodic Dissolution

1 The walls and tip of the crack are passive2 The passive film at the crack tip is ruptured by the plastic

strain, and active corrosion occurs3 The crack tip repassivates4 Go back to 1


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Film-Induced Cleavage

1 The walls and tip of the crack are covered by a brittle film(either an oxide film or a de-alloyed layer)

2 The film at the crack tip is ruptured by the plastic strain3 The brittle crack continues into the metal4 The crack is blunted by plastic strain


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Damaging Adsorption

It was proposed that the cohesive bonds between surface metal atoms areweakened through adsorption of damaging components of the environment. Thesurface energy of the metal is said to be reduced, increasing the tendency of themetal to form a crack under tensile stress.

But only those specific adsorbates are effective which successfully reduce theattractive force of adjoining metal atoms for each other located at the extremeroot of a notch subject to high tensile stress and experiencing some plasticdeformation.

It appears likely that damaging adsorption of the kind described occurs onmobile defect sites generated continuously at the yielding crack tip. Hence, thekinds of anions that adsorb and their subsequent effect on metal properties differfrom the effects observed in adsorption measurements on the unstressed lattice.

A critical potential for SCC is defined as that value above which damaging ionsadsorb on defect sites and below which they desorb.


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Stress-Corrosion TestingConstant Load Testing or Constant Strain/Extension Rate Testing

S t r e s s

log(Time to Failure)

Threshold stress

z Then assess by fracture surface change in elongation or reduction in area

time to failure


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Control of Stress-Corrosion Cracking

z Remove stress (often difficult, especially for residual stresses)z Avoid the necessary environmentz Apply electrochemical protection where possible such as cathodic

protection or inhibitors

Note that cathodic protection prevents SCC but accelerate HICz Coatings are often ineffective or impractical because they do not

withstand the aggressive chemical and/or physical environmentsassociated with SCC

z Use a different materialz Live with it


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Hydrogen Embrittlement

1 Hydrogen produced by the cathodic reaction or comesfrom environment

2 Hydrogen diffuses to region of tri-axial tensile stressahead of the crack

3 Hydrogen causes brittle fracture4 Crack blunts by plastic deformation as it runs out of

hydrogen

HH

H

HH

HH

H


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Hydrogen Embrittlement (Cont.)

z sources of hydrogen welding electroplating

contact with gaseous hydrogen corrosion, especially in the presence of sulphides

z higher strength materials are moresusceptible to hydrogen embrittlement


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Mechanism of Hydrogen Embrittlement

z

Development of internal pressure by release of interstitial atomic hydrogen as molecular hydrogenat voids or other favored sites under extremepressure (as much as 300 atm).

z Another proposed mechanism is that hydrogendiffuses to and adsorbs at imperfections at the tip of the crack, reducing surface energy of metal atoms,

lowering their cohesive forces, producing micro-cracks ahead of the main crack, and allowing thecrack to extend under a tensile stress (stress-sorption cracking).


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Hydrogen Embrittlement of Steel

Hydrogen Induced Cracking (HIC)z Internal cracking of lower strength steels

(e.g. pipeline steels) due to high pressurehydrogen collecting at inclusions.

Hydrogen Crackingz Internal cracking of steels at higher

temperatures due to reaction of dissolvedhydrogen with carbon to form methane.


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Control of Hydrogen Embrittlement

z Remove or decrease stress (often difficult, especially for residualstresses)

z Remove the hydrogen sourcez Apply electrochemical protection where possible such as inhibitors,

reduction of oxidizers (such as dissolved oxygen), or an increasein pH to reduce corrosion rate and consequent degree of hydrogenproduction on the surface.

z Avoid cathodic protectionz

Use inert coatings to exclude hydrogen-bearing environmentz Use a different materialz Live with it


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Corrosion Fatigue Cracking (CFC)

z Metal fatigue results in crack propagation due to a cyclic stress

z Corrosion makes both crack initiation and propagation easier

z Fatigue strength of any metal is the stress at or above whichfailure occurs within a stated number of cycles.


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Corrosion Fatigue - S-N Curve

Air

Corrosion

log (cycles to failure, N f)

S t r e s s

A m p

l i t u d e

Fatigue/Endurance Limit


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Control of Corrosion Fatigue

z Remove fatigue or reduce stressz Avoid the corrosion environmentz Apply electrochemical protection including inhibitors, cathodic

protection (when hydrogen induced cracking is not a danger),reduction of oxidizers (such as dissolved oxygen), or increase inpH.

z Barrier coatings to exclude the corrosive solution from the alloysurface and sacrificial zinc (galvanized) coatings to cathodicallyprotect steel at breaks in the coating might also prevent CFC.

z Use a different materialz Live with it


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Characteristics of Environmentally Induced Cracking

[Principles and Prevention of Corrosion, Denny J ones, Prentice-Hall, 1996]


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Fretting Corrosion

z Rubbing of two metals removes oxide filmand allows oxidation

z The oxide may also act as an abrasive

z Prevention of relative motion and allowinglarger relative movement may prevent theproblem


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Home Exercise Problems

z

Prb. 1 of Chapter 8in Principles and Prevention of Corrosion, Denny J ones, Prentice-Hall, 1996.


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References

z Principles and Prevention of Corrosion, Denny J ones, Prentice-Hall, 1996.z Web Site of Dr. R. A. (Bob) Cottis.

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07-Mechanical Aspects of Corrosion