Corrosion in Eng. Application Lecture 7

8/10/2019 Corrosion in Eng. Application Lecture 7

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Stress corrosion cracking (SCC)

Combined action of applied tensile stress and a corrosive

environment on susceptible material.

Caused by either:

1. Residual internal stress in the metal:

Cold forming

Unequal cooling from high temperature

2. An externally applied stress: Faulty design

Thermal effects (expansion or contraction)


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Requirements for SCC

The following conditions are necessary for SCC:

Figure 7.1 Stress corrosion triangle


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Causes rupture of the film

Occurs in three stages:

Initiation: Slow attack which produces a pit

Slow crack growth plus corrosion enhancespropagation

Rapid failure.

Cause of major industrial costs and safety hazards

Stress corrosion cracking (SCC)


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Characteristics of SCC

Required stress, either residual or applied.

Occurs for stresses below the materials yield stress.

Generally all alloys susceptible to SCC

Cracks initiate and propagate period.

Conditions of cracking are specific to alloy and environments.

Can initiate and crack with little outside evidence of corrosion and no

warning before catastrophic failure. Other forms of corrosion, such as pitting or crevice corrosion can

transition to SCC.

SCC cracks are microscopically brittle in appearance.

Stress corrosion cracks can be both intergranular and transgranular,

depends on alloy, stress level, environments.


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Stress corrosion cracking can propagate either intergranualry or transgranulary

depends on the material properties and the mechanism of crack propagation


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Intergranular SCC Transgranular SCC


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Factors important in SCC:

1- Environmental Factors:

aqueous solutions

Temperature

Electrochemical potential

Time

For example, Brasses crack in NH3

Inconel-600 cracked in pure water at 300C.

Stainless Steels crack in chloride

2- Stress:The greater stress on material, the quicker it will crack.In fabricated components, there are usually:

Residual stresses from cold working, welding, surfacetreatment such as grinding, etc.

Applied stresses from the service, such as hydrostatic,vapour pressure of contents, bending loads, etc.


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Electrochemical dissolution theory

1- the dissolution or oxidation of the metal must be thermodynamically possible.

2- the protective film formed on the metal surface should be thermodynamically

stable.

3- On the basis 1 and 2 it has suggested that a critical potential exists which SCC

occurs.

The electrochemical dissolution mechanism was performed by measuring the

critical potentials for initiations of SCC of 18-8 stainless steels exposed tomagnesium chloride solutions boiling at 130C with and without inhibition

anion addition, Figures 7.2 and 7.3


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7.2 Effect of applied potential on time to failure of stressed moderately cold-

rolled 188 stainless steel in magnesium chloride solution boiling at 130C


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7.3 Effect of applied potential on time to failure of stressed moderately cold-

rolled 188 stainless steel in magnesium chloride solution with sodium

acetate additions, boiling at 130C (2% sodium acetate addition is inhibiting)


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Initiation of SCC and critical potentials

For various metals and solutions, value of

critical potentials, immediately above (or

noble to) SCC initiations.


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Figure 7.4 Effect of applied potential on stress-corrosion cracking of mild

steel in 170 g ammonium carbonate per liter, 70C.


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Figure 7.4 Effect of applied potential on failure times of 0.09% C mild steel at

3 temperatures in 35% sodium hydroxide solution.


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Prevention of SCC

1. Lowering the stress below the threshold value if one exists.

This can be done by:

1. annealing in the case of residual stresses

2. thickening the section, or

3. reducing the load.

Plain carbon steels may be stress-relief annealed at 590

to 650C.

Austenitic stainless steels are frequently stress-relieved at

temperatures ranging from 820 to 930C.

2. Eliminating the critical environmental species by, e.g.

demineralization (salt removal).


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3. Changing the alloy is one possible recourse if neither the

environment nor stress can be changed.

It is common practice to use Inconel (raising the nickel content)when type 304 stainless steel is not satisfactory.

Although carbon steel is less resistant to general corrosion, it is

more resistant to SCC than are the stainless steels.

Thus, under conditions which tend to produce SCC, carbon

steels are often found to be more satisfactory than the

stainless steels.

Heat exchangers used in contact with seawaters are oftenconstructed of ordinary mild steel.


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Chloride Stress Corrosion Cracking (CSCC)

Surface initiated cracks caused by environmental cracking of

300 Series SS and some nickel base alloys under the combined

action of tensile stress, temperature and an aqueous chloride

environment. The presence of dissolved oxygen increases the

probability for cracking.

A ffected Materials

a) All 300 Series SS are highly susceptible.

b) Duplex stainless steels are more resistant.

c) Nickel base alloys are highly resistant, but not immune


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External cracking of Type 304SS instrument tubing under insulation


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Factors affecting CSCC

Temperature : Increasing temperatures increase the susceptibilityto cracking.

Chloride content : Increasing levels of chloride increase the

likelihood of cracking.

pH: SCC usually occurs at pH values above 2. At lower pH values,uniform corrosion generally predominates. SCC tendency

decreases toward the alkaline pH region.

Stress may be applied or residual. Highly stressed or cold worked

components, such as expansion bellows, are highly susceptible to

cracking.

Oxygen dissolved in the water normally accelerates CSCC.


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Alloy composition: Nickel content of the alloy has a

major affect on resistance. The greatest susceptibility

is at a nickel content of 8% to 12%. Alloys with nickel

contents above 35% are highly resistant and alloys

above 45% are nearly immune.

k) Low-nickel stainless steels, such as the duplex(ferrite-austenite) stainless steels, have improved

resistance over the 300 Series SS but are not immune.

Carbon steels, low alloy steels and 400 Series SS are

not susceptible to CSCC


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Morphology

The material usually shows no visible signs of

corrosion.

Characteristic stress corrosion cracks have

many branches and may be visually detectable

Metallography of cracked samples typically

shows branched transgranular cracks

Fracture surfaces often have a brittle

appearance.


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Prevention / Mitigation

Use resistant materials of construction. Alloys with

nickel contents above 35% are highly resistant andalloys above 45% are nearly immune.

Properly applied coatings under insulation.

Avoid designs that allow stagnant regions wherechlorides can concentrate or deposit.

A high temperature stress relief of 300 Series SS

after fabrication may reduce residual stresses.However, consideration should be given to the

possible effects of sensitization that may occur.


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Hydrogen induced failures

Hydrogen atoms may be produced on a metal surface in

an aqueous environment by a corrosion reaction, cathodicprotection, electroplating or acid pickling.

Some of the hydrogen atoms combine to form gaseous

molecular hydrogen on the metal surface and arereleased to the environment. A portion of the atoms are

absorbed by the metal and this entry of hydrogen atoms

into the metal may have some very undesirable effects.

Hydrogen induced cracking (HIC\SOHIC\SCC) and

hydrogen embrittlement are two types of phenomena that

can occur.


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HIC

Hydrogen induced cracks occur when atomic hydrogen diffuseinto the metal and then recombines as hydrogen molecules at

trap sites in the steel matrix such as inclusions and/or regions of

anomalous microstructure.

Cracking that connects adjacent hydrogen induced cracks on

planes in the metal, or near the metal surface is referred to as

stepwise cracking. The linking of the hydrogen induced cracks to

produce stepwise cracking is dependent upon local straining

between cracks filled with accumulated hydrogen under

pressure and the level of dissolved hydrogen in the metal

matrix.


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Hydrogen-induced cracking (HIC)


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Factors Affecting HIC

Steel composition : HIC is usually associated with low strength

ferritic plate steels used in the production of pipelines and

vessels. It is commonly found in steels with high impurity levels.

Environment: The degree of HIC is related to the hydrogen

concentration in the steel, which is dependent on partial

pressure of H2S, pH and temperature.

Stresses: No externally applied stress is needed for the

formation of hydrogen induced cracks.


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Morphology

HIC normally occurs as planar defects aligned in the

rolling direction. Blisters can also form.


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Prevention of HIC

Controlling the metal chemistry to minimise theaffects of inclusions or laminations.The risk of HIC

can be reduced by using HIC resistant steels with

low sulphur content (


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Stress Oriented Hydrogen Induced

Cracking (SOHIC)

SOHIC is similar to HIC but is a potentially moredamaging form of cracking which appears as arrays ofcracks stacked on top of each other.

SOHIC staggered small cracks are formed approximately

perpendicular to the principle stress (applied andresidual) resulting in ladder-like crack array linking(sometimes small) pre-existing HIC cracks.

They usually appear in the base metal adjacent to the

weld heat affected zones where they initiate from HICdamage or other cracks or defects including sulfidestress cracks.


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Stress-oriented hydrogen-induced cracking

(SOHIC)


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Factors Affecting SOHIC

Steel composition and condition: SOHIC can occur in carbon steels,in low sulphur carbon steels, in HIC resistant steels and in ultra lowsulphur advanced steels. SOHIC is relatively uncommonphenomenon usually associated with low strength ferritic pipelineand pressure vessel steels.

Post weld heat treatment, as applied to carbon steels for pressurevessels, reduces residual stresses and hardness differences acrossweld zones, thereby reducing the risk of SOHIC.

Environment: Hydrogen up-take in the steel is required for SOHIC tooccur. The degree of SOHIC is related to the hydrogen concentration

in the steel (which is dependent on partial pressure of H2S, pH andtemperature) and the stress state.

Stresses: External tensile stresses (residual and/or applied) arerequired to induce SOHIC.


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Morphology

SOHIC: Arrays of cracks aligned nearly perpendicular

to the applied stress, which are formed by the link up

of small HIC cracks in the steel. SOHIC is commonly

observed in the base metal adjacent to the HAZ of aweld and is oriented in the through-thickness

direction. SOHIC may also be produced in susceptible

steels at other high stress points such as from the tip

of mechanical cracks and defects and from theinteraction between HIC on different planes in the

steel.


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Sulphide stress corrosioncracking

Cracking of metal under the combined action orresidual and/or applied tensile stress and corrosion inthe presence of water and hydrogen sulphide.

Sulphide stress cracking (SSC) involves hydrogen

embrittlement of the metal by atomic hydrogen that isproduced by the sulphide corrosion process on themetal surface.

The atomic hydrogen can diffuse into the metal and

produce embrittlement, drastically reducing ductilityand deformability and increasing the likelihood ofcracking.


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Schematic showing morphology of sulfide stress cracking in a hard heat affected zone.


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Factors affecting SSC

Metal composition and condition: SSC can occur in carbon steels,low-alloy steels, stainless steels and non-ferrous metals like Ni-

based, Co-based, Al, Cu and Ti alloys.

The tendency for SSC is increased by the presence of hard

microstructures such as untempered or partly temperedtransformation products (martensite), particularly in the heat

affected zones of welds.

pH: Susceptibility to SSC decreases with increasing pH.

Stress: SSC is controlled by the applied stress and by residualstresses from working, forming or welding operations.


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Examples

SSC is most likely found in hard weld and heat

affected zones and in high strength components

including bolts, relief valve springs, compressor

shafts, sleeves and springs.


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Prevention

SSC can generally be prevented by limiting the

hardness of welds and heat affected zones

through preheat, PWHT, weld procedures and

control of carbon equivalents.

PWHT is beneficial in reducing the hardness and

residual stresses that render steel susceptible to

SSC.


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Hydrogen Embrittlement

When electrochemically generated, hydrogen dissolves at the metal surfacethen it migrates to stressed locations.

Hydrogen entry into high strength steels or steels with a hardness can resultin hydrogen embrittlement. A material can fail in a brittle manner at stresses

well below its yield strength.

Hydrogen embrittlement is normally limited to high strength materials due tothe fact that these materials reach tensile strengths high enough to initiatethe failure mechanism.

The susceptibility to hydrogen embrittlement increases with increasingstrength and hardness.

Even though a steel contains hydrogen, no permanent damage occurs unlesssufficient stress is applied to cause the steel to crack.


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Prevention of hydrogen embrittlement

Annealing alloy (softening)

Baking component to remove dissolved hydrogen

Select more resistant alloy

Use inhibitor to minimize corrosion

Change design, avoid sharp corners and eliminate sites

for crevice corrosion

F tti i


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Fretting corrosion

Fretting is a phenomenon of wear which occurs between two

mating surfaces subjected to cycling relative motion of

extremely small amplitude of vibrations.

Fretting appears as pits or grooves surrounded by the corrosion

products.

The deterioration of materials by the conjoin action of fretting

and corrosion is called fretting corrosion.

Fretting is usually accompanied by corrosion in corrosive

environments.

Wear corrosion or friction oxidation are terms that have been

applied to this kind of damage.


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Examples of fretting

Fretting of blade roots of tube blades

Fretting bolted parts, e.g. suspension springs

Fretting of engine components

Fretting damage of riveted joints

Loosing the wheels from axles


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Factors affecting fretting

1-Contact load: wear is a linear function of load and fretting would,therefore increase with increased load.

2- Number of cycles: the degree of fretting increases with number of

cycles.

3- Temperature: The effect of temperature depends on the type of

produced oxide. If a compact oxide is formed can prevents metal to

metal contact, fretting wear is decreased.

4- Relative humidity: The effect of humidity of fretting is opposite site to

the effect of general corrosion where an increase in humidity causes

a decrease in corrosion rate.

Mechanism of fretting corrosion


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Mechanism of fretting corrosion

Adhesion : Contact between two surfaces. The surfaces must be in

close contact with each other. The contact occurs at few sites, calledasperities. The asperities are bonded together at adhesion sites createdby the relative slip of the surfaces.

Oxidation and debris generation: The material removed from the metalsurface due to fretting is called debris. The composition of the debris

differs from one metal another. The debris produced by low carbon steelconsists of mainly ferric oxide.

Crack initiation : Cracks grow in a direction perpendicular to the appliesstress at the fretting area. Some of cracks may not propagate at low

stress. The stage of crack initiation is called fretting fatigue. Crackpropagation at higher stress lead to failure. The crack originates at theboundary of a fretted zone and propagates. During propagation, if acorrosion medium contacts the crack, corrosion fatigue also contributesto the crack propagation.


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Prevention of fretting corrosion

Use low viscosity lubricating oils.

Use gaskets to absorb vibration.

Increase the hardness of the two contactingmetals using shot- peening.

Combination of a soft metal with a hard metal

Design of contacting surface to avoid slip.

Use materials to resist ferreting corrosion

Corrosion in Eng. Application Lecture 7

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