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SCC is the brittle cracking of a metal due to the result of combinedeffects from localized corrosion and a tensile stress.

Stress Corrosion Cracking



• Can be Intergranular (Al-Cu, Cu base alloys) transgranular (Mg alloy or18/8SS) or mixed mode of cracking.

• Residual stresses due to cold working, welding etc.,

• Compressive stress does not cause

• Threshold stress is required

• Most of areas unaffected

• Specific to certain environmentsBrass - SCC in solutions with ammonia.Steel - SCC in caustic (high pH), amine solutions.Stainless steels and aluminium alloys - SCC in solutions containing chlorides.Ti-alloys - SCC in nitric acid or methanol.

Characteristic



Mechanism



Control(Material + Environment+ Tensile Stress)

1. Lower stress level below the threshold value (annealing,thickening the section)2. Eliminate critical species3. Apply cathodic protection4. Add inhibitor (Phosphate in boilers)

Mechanism

Electrochemical theory

Fissures at weak points of oxide film favour anodicdissolution & initiation of SCCStress sorption theory

Adsorption of ions on metal weakens metal atom bond

Mechanism



Failure of wall of a hydrofluoricalkylisation plant due tohydrogen blistering

Formation of cavities in thesteel due to hydrogen blistering

Brittle mechanical failure caused by penetration and diffusion of atomic hydrogen

into the crystal structure of an alloy.

(e.g) boiler tubes, plating, crude oil pipe lines picklingSteels in oil and gas industry

Hydrogen Embrittlement



The corrosion reactions in presence of hydrogen sulphides –

Anodic reaction: Fe → Fe2+ + 2e –

Dissociation reaction:H2S→ H+ + HS-

HS-→ H+ + S-

Cathodic reaction :

2H+ + 2e – → 2H → 2H2 (gas)

Corrosion process

H+ + e – → H H2O + e –

→ H+ + OH –

• H-bearing environment (heat treatment, wielding or other manufacturingprocess)

• Decomposition of water vapour or steam on hot surfaces.

Source of Hydrogen



Hydrogen Induced Cracking (HIC) – The corrosion damage is in theform of blisters and / or internal cracks in absence of STRESS

Sulfide stress Cracking (SSC) – Presence of STRESS

Posions - Presence of S2- and As3+ delay the recombination of H atoms.

P, Sb, Se, Te and Cyanide are other poisons.

Fe (Surface) + H2S (gas)→ FeS (Surface) + 2H (solution)



• Weakening of the metallic bond strength by the dissolve H.

• Diffusion of atomic H into metal and forms molecular H2 in voids/defectsforms blisters. This build up high pressure causes rupture.

• Diffusion of atomic H into metal and reacts with alloying elements to

form brittle hydrides

Prevention of HE

• Modification of environments

• Use of materials resistant to HE (High strength materials with low inclusion/voids)

Mechanism



• Large smooth area and a small corroded area

• Specific to high strength materials

• Transgranular failure

• Endurance limit is decreased by corrosive agent

Brittle failure of an alloy caused by fluctuating stress in a corrosive environment.

(Different from SCC)

(e.g) Sucker rods & drilling rods in oil wells rail vehicle springs, motor shaft working incorrosive environment

Characteristics

Fatigue occurs when a material is subjected to repeated loading and unloading.

Corrosion Fatigue



• Deep pits are initially formed

• Cracks initiates & propagates across the metal

• The frquency of cyclic stress is important. Lower frequency leads togreater crack propagation per cycle.

• Lowering the general corrosion rate will delay or prevent CF.

• Addition of inhibitors

• Barrier coating (Coating Zn, Cr, Ni, Cu & nitride)

• Reduce cyclic stresses by shot peening

Prevention

Mechanism



Also known as Selective Leaching or Selective dissolution or Parting

Removal of one element from an alloy leaving an altered residual structure.

Dezincification:

Selective removal of Zn from Brass with its yellow colour and its colourchanges to red (copper colour)

Dealloying



Selective dissolution of iron from Grey cast iron (E.g. Water pipes buried in soil)

Graphitization

Graphite flakes are cathodic to iron, and corrosion is localized to iron which startsleaching (becomes a porous mass) and leaves a rich residue of graphite flakes.



Characteristics

• Loss of mechanical strength without change in shape

• It leaves a porous structure• White cast iron does not undergo since C is present as Fe3C but leavesbehind a network of graphite flakes

Mechanism

C is present as graphite formPotential difference exists between graphite and ironLocal cell promotes corrosion of iron.

Prevention

Coal – tar epoxy coating on metal to prevent graphitizationTackling at desgin stageCupro-nickel or addition of 1% Sn to 70-30 brass reduces.Addition of 2% Al to Brass also prevents



Loaded metal surfaces under relative motions in the presence of corrosiveenvironment. e.g) Ball bearing, electrical switch gear

• Discolour & deep pit formation

• Relative motion small as 10-8 cm

Mechanism

• Wear – Oxidation – Rupture causes metal removal and oxidation• Oxidation wear – oxide layer ruptured and oxide debris formed• Exposed metal is further oxidized

PreventionLubricate with low viscosityIncrease hardnessUse gaskets to absorb vibrationRoughen the surface

Fretting Corrosion



MechanismRepeated formation of bubbles act as hammer and remove metal.

PreventionMetal must be high Corr. Resistant.Coating with neopreneUse dense high strength tensile material.

• Corrosion caused by impact of air bubbles in a medium (e.g) Marinepropellers, boiler tubes

• Pitting type of appearance and surface roughening

Cavitation Damage



Erosion-Corrosion



Erosion-Corrosion

Corrosion

A

B

C

Laminar

flow

Cavitationerosion

Turbulentflow

Erosion

A = Erosion - corrosionB = Cavitation erosion – corrosion

C = Turbulent flow corrosion



0

y

xd

notional profile in the absence offriction at the pipe walls

laminar flow (parabolic profile)turbulent flow

Fully developed velocity profiles in a circular section pipe

Types of Flow



entry zone unstable flow turbulence

laminar flow

inlet

inlet

(a)

(b)

Types of Flow



v (∞)

laminar layer

intermediatelayer

turbulent layer

laminar sub - layer

y

x0x CRIT

Hydrodynamic (prandl) boundary layers near the surface of a flat plate formed by a uniform

fluid velocity such that turbulent flow can develop



entry length Fully developed laminar flow

boundary layers boundary layers merge andbecome fully developed

Development of a hydrodynamic boundary layer for solution flowing through a tube



0 1 2 3 4

C o r r o s i o n r a t e m m / y e a r

0.0

0.2

0.4

0.6

0.8

Tidal 7.6 m/s 30.5 m/s

0.05

0.5

0.75

5 m/s

Max. recommendedin-service velocity

Critical velocity is not well defined for NAB. In seawater service conditions a maximum

flow rate of 5 m/s is normally recommended

Influence of fluid velocity on NAB

corrosion performance



Corrosion rate vs Velocity



Protective oxide film on NAB

• prevents corrosion• approx. 800 nm• complex layers of CuO, Cu2O and

Al2O3

• Level of protective film damage isdependent on particle impact angle

• Mechanical removal and/or ruptureenables charge transfer at varyingrates

• ‘Recovery’ aspects then become

important (system dependent)

Protective film integrity



For a ductile material, the

erosion mechanisms are (1)+ (2) microcutting, (3) plasticdeformation depending onthe angle of impingement.

Ductile erosion mechanisms



Cracksystems

• Fracture toughness plays an important role.

• Can depend on direction (perpendicular/transverse to coating / substrateinterface).

e.g. oxides at splat boundaries

Brittle erosion mechanism



T = E + C + S

or

S = T - ( E + C )

whereT is material loss under erosion-corrosion

E is material loss by pure mechanical erosion processesC is solids free flow corrosionS is synergy, the difference between erosion-corrosion (T) and the summationof erosion (E) and corrosion (C).

Synergistic effects can be:

– Negative (equivalent to extra safety factor)

– Positive (additional safety factor required)

Synergy



Zero Resistance AmmeterMicrobial Induced CorrosionImmediately after immersion, a metal surface

undergoes a sequence of biological and

chemical changes that lead to the formation of abiofilm which is causative for corrosion .



• highly complex structures containingvoids, microbial clusters or layers.

• diffusion in biofilms is dependent onflow conditions and structure.

A biofilm isheterogeneous innature and thedistribution ofmicroorganisms is

not uniform:

Slime, extracellular polymeric substances(EPS) - enzymes, proteins, nutrients, trappedinorganic material – possible modification ofoxygen reduction mechanisms

Microbial Induced Corrosion



Sulphate Reducing Bacteria



Copper alloys are more resistant to biofouling than most metals due to the toxicity ofthe released copper-ions.

When coupled to other metals the release of copper-ions can be greatly reduced.

The ennoblement of passive metals, such as titanium and stainless steels is well-documented:

• This ennoblement has been related to the formation of aerobic biofilms, althoughthe mechanism governing the processes is still a subject of much debate.

• Biofilms on titanium have been reported to catalyse the cathodic reduction ofoxygen – thus increasing the overall cathodic efficiency.

Biofilms and galvanic corrosion



References

• S.N.Banerjee, “An Introduction to Corrosion Science and Corrosion Inhibition”,

Oxonian Press P.Ltd., New Delhi, 1985.

• Zaki Ahmad, “Principles of Corrosion Engineering & Corrosion Control”, ButterworthHeinemann, 2006.

• M.G.Fontana & N.D. Greene, “Corrosion Engineering”, McGraw Hill, New York ,1978.

• L.L.Shrier “Corrosion”, Vol. I & II, Butterworth Heinemann, 1994.

• H.H.Uhlig and R.W.Revie, “Corrosion and Corrosion Control”, A Wiley – InterScience Publication John Wiley & Sons, New York, 3rd Edition, 1985

and etc.

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