corrosion mechanism - 2

8/8/2019 Corrosion Mechanism - 2

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CORROSION MECHANISM

Yves GUNALTUN


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CORROSION MECHANISMS

CAUSES OF EXTERNAL CORROSION

MAIN PARAMETERS

CORROSON CONTROL BY CATHODIC PROTECTION

INTRODUCTION TO CATHODIC PROTECTIONINTRODUCTION TO CATHODIC PROTECTION


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CORROSION MECHANISM


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BEHAVIOUR OF METALS IN THE NATUREBEHAVIOUR OF METALS IN THE NATURE

In nature, elements are present in their most stable form(s)

Some elements are in metallic form, for example gold.

Most of elements are in "combined" form (chemical compounds like

mineral salts)

Iron, which is the main construction material, is present mainly as iron

oxide(s) and iron sulphur.


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IRON OXIDEIRON SULPHIDE

IRON (METAL)

EXTRACTIVEMETALLURGY ( G > 0)

CORROSION ( G < 0)

IT IS IMPOSSIBLE TO STOP CORROSION.

BUT

IT CAN BE DELAYED OR ITS RATE CAN BE REDUCED.

STABILITY OF ENGINEERING MATERIALSSTABILITY OF ENGINEERING MATERIALS

Different metals and environments have different energy levels

associated with them.

Any reaction can occur only if it is thermodynamically possible.

The change in energy must always be from the higher energy

state to the lower one.


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CORROSION IS AN ELECTROCHEMICAL PHENOMENON

However, in addition, pure chemical reactions or entirelyphysical process can also be occurring.

CORROSION:

DAMAGE OF ENGINEERING MATERIALS BY A CORROSIVE AGENT.

But not every process necessarily leads to damage.

Damage is said to occur when the function is impaired.

DEFINITIONDEFINITION

DAMAGE CAN BE CLASSIFIED AS UNIFORM, LOCALISED(SHALLOW PIT, PITTING, CREVICE CORROSION, SELECTIVEDISSOLUTION) AND CORROSION CRACKING.

But metal loss is not necessarily the unique cause of failure.

Hydrogen embrittlement is the penetration of hydrogen intometal matrix under a cathodic process.


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Liquid water Wetting of the metal

CAUSES OF CORROSION IN OIL AND GAS PRODUCTIONCAUSES OF CORROSION IN OIL AND GAS PRODUCTION

CO2 H2S Organic acidsO2 Cl2Corrosive agent

1 day 1 month 1 year 10 years...Time

Flow effects SandSalts ConstraintsTemperatureProduction parameters

Bacteria Conditions of use Bad design /constructionHuman impact

Heterogeneities In the electrolyte On the metal surface In the metal matrix

Steel

Water

Favourable / accelerating parameters

Liquid water


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Corrosion takes place if

CORROSION - CONDITIONSCORROSION - CONDITIONS

free water is in contact with metal

corrosive agent is dissolved in the water phase. Corrosive agent

can be O2, CO2, H2S, acids (H+), Cl2.

there is heterogeneity in the corrosive medium or on the metal

surface resulting in potential difference between two locations

on the metal surface. Some other parameters can modify the corrosivity of the effluent

Corrosion resistance of the material

Time

Environmental / operating conditions

External causes


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CONSTRUCTIONCONSTRUCTION

Bad construction practices can be more dangerous than any corrosive agent.



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EXTERNAL CORROSION

By external environment:

atmosphere,

Oxygen

sea waterOxygen, bacteria

soils,

Geological cells

Oxygen, bacteria

INTERNAL CORROSION

By the produced effluents:

CO2, H2S, organic acids,

Solids

By the injected effluents

Oxygen, bacteria By the fluids used for

utilities


INTERNAL CORROSION

By the produced effluents:

CO2, H2S, organic acids,

Solids

By the injected effluents

Oxygen, bacteria, chlorine By the fluids used for

utilities

IN OIL AND GAS PRODUCTION THE TEMPERATURE OF THE PRODUCED

FLUID IS GENERALLY BELOW 250C.


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HETEROGENEITY IN THE CORROSIVE MEDIUMHETEROGENEITY IN THE CORROSIVE MEDIUM

Heterogeneity in the corrosive medium is generally initiated by the corrosion

reaction. Example: depletion of oxygen in the cavities.

AIR ARGONIIRON

Na+

Cl -Cl -

Na+

STEEL

WATERTEFLON

The bubbling of a gas like Nitrogen

or Argon in water allows the

removal of the dissolved oxygen.

Bubbling of air in water allows

saturation of water in oxygen

(increase of oxygen concentration

in water).

HETEROGENEITY IN THE CORROSIVE MEDIUM


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STEEL

WATERTEFLON


Oxygen concentration 1 ppm 0.1 ppm

Potential - 650 mV - 680 mV

Oxygen concentration 10 0.1 ppm

Potential - 610 mV - 680 mV

Ea - Ec = 30 m

Ea - Ec = 70 mV

REFERENCE ELECTRODEREFERENCE ELECTRODE


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POROUS PLUG

CuSO4

Cu ++

Cu

SO4-

-

E=Eo + (RT/nF) * Ln (Me++)

REFERENCE ELECTRODEREFERENCE ELECTRODE

When an electrical current flows through the metal /electrolyte phase

boundary, a potential difference arises at the interface. The potential difference between metal and electrolyte can not be directly

measured.

This potential difference is measured against a reference electrode.

POTENTIAL MEASUREMENTPOTENTIAL MEASUREMENT


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POTENTIAL MEASUREMENTPOTENTIAL MEASUREMENT

REFERENCEELECTRODE

V

IRON

Fe / Fe ++

ELECTROLYTE

Cu ++

SO4--

Cu / Cu ++



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STEEL

WATERTEFLON

CATHODIC SURFACE

ANODIC SURFACE

1/2 O2 + H2O + 2e 2OH-

Fe + 3H2O Fe (OH) 3 + 3 H+ + 3e pH

pH


A chemical reaction may change locally the corrosivity around

the corroding surface.

THERMODYNAMICTHERMODYNAMIC


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THERMODYNAMICTHERMODYNAMIC

The corrosion reaction can take place if it is thermodynamically possible

At pH = 0 in de-aerated water,

potential of H+/H couple is 0 (nil)

potential of Fe++/Fe couple is about - 0.4 to - 0.44

At pH zero, Iron will corrode.

At pH = 9 in de-aerated water,

potential of H+/H couple is - 0.53

potential of Fe++/Fe couple is about - 0.44

At pH 9, in de-aerated water Iron/steel will not corrode.

At pH = 9, in the aerated water:

potential of O/O-- couple is 0.63

potential of Fe++/Fe couple is about - 0.44

At pH 9, in aerated water Iron/steel will corrode.

O2 + 4 H+ 2 H2O E O/O-- = 1.23 + (RT/4F) Log (a

4H+ + aO2)



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Corrosion under scale or biofilm is well known in oxygenated

water lines.

STEEL

WATER

SCALE ORBIOFILM

Typical corrosion under corrosion products in

water injection lines with insufficient oxygen

control, low velocity and no cleaning.

GEOLOGICAL CELLGEOLOGICAL CELL


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Pipe

Clay Sand

Anodiczone

Cathodiczone

V

Clay

Sand

Anodiczone

Cathodiczone

Casing

GEOLOGICAL CELLGEOLOGICAL CELL

The oxygen content in clay is lower than in sand due

to low oxygen refurbishment possibility in clay.

CORROSION CELLS ON MARINE STRUCTURESCORROSION CELLS ON MARINE STRUCTURES


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CORROSION CELLS ON MARINE STRUCTURESCORROSION CELLS ON MARINE STRUCTURES

Riser or jacket

Anodic zone

Cathodic zone Sea

Air

Oxygen content

Depth

WELDING DEFECTSWELDING DEFECTS


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Anodic zone Anodic zone

Anodic zoneSupport

WELDING DEFECTSWELDING DEFECTS

CORROSION OF STAINLESS STEELSCORROSION OF STAINLESS STEELS


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Iron

Stainless steel

Iron oxide

IronoxideCracks if

stress ispresent

CORROSION OF STAINLESS STEELSCO OS ON O S N SS S S

Stainless steels need oxygenated environment to remain passivated

CORROSION - ELECTROCHEMICAL ASPECTCORROSION - ELECTROCHEMICAL ASPECT


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Fe Fe++ + 2 e-

IA = Anodic current

IK= Cathodic current

Without the contribution of

an external current,

IA = - IK

AT METAL - SOLUTION INTERFACE

2 H2O + O2 + 4 e- 4 OH-

H+ + e- 1/2 H2

H2O + e- 1/2 H2 + OH-

Reduction at cathode

Oxidation at anode

HETEROGENEITY ON THE METAL SURFACEHETEROGENEITY ON THE METAL SURFACE


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STEEL

Breakdown of the protective filmMesa corrosion by CO2

Non-metallic inclusion

STEEL STEEL

EXAMPLES OF OTHER HETEROGENEITIES:

Precipitated metallic compounds and impurities at the grain

boundaries

Junction of two or more different metals (galvanic corrosion)

ANODIC AND CATHODIC ZONE ON A CORRODED WELDANODIC AND CATHODIC ZONE ON A CORRODED WELD


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At the beginning of experience

Anodic zones : yellow

Cathodic zones : blue

After a few hours of exposure

to corrosive

environment

Samples are immersed in water containing 0.1 % NaCl under 1 bar of CO2

SUMMARY - INHIBITIONSUMMARY - INHIBITION


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ALVANIC CORROSION & INTERGRANULAR CORROSION OF S. S.ALVANIC CORROSION & INTERGRANULAR CORROSION OF S. S.


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Precipitation of Cr23C6 if

C>0.03% and T=600-900C

Chromium content

13 %

18 %

Cr23C6

GALVANIC CORROSION

Carbonsteel

Stainlesssteel

Precipitation of metallic compoundsGalvanic corrosion

Water

HETEREOGENEITY ON THE METAL SURFACE - MESA CORROSION BY CO2ETEREOGENEITY ON THE METAL SURFACE - MESA CORROSION BY CO2


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METAL LOSS : UNIFORM AND/OR LOCALISED CORROSION

LOCALISED CORROSION UNIFORM CORROSION

Fe Fe ++ + 2 e-H + + e- HANODIC ZONECATHODIC ZONE

e-

Fe ++

Na +H+

Cl -H 2

STEELFeCO3

WATER

GALVANIC CORROSIONGALVANIC CORROSION


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Zinc

A

Na+

Cl -

e-

Steel

Zn++

O2 + 2e- + H2O 2OH

- Zn Zn++ + 2e-

Steel ZincAnodic zones

Electrical connection

Water

Water

STANDARD POTENTIAL OF METALSSTANDARD POTENTIAL OF METALS


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Eo (V) Standard

Na / Na+

Fe / Fe ++

Zn / Zn ++

Ni / Ni ++

H2 / H+0.0

- 0.25

0.34

- 0.44

-1.66

- 0.763

1.5

Cu / Cu ++

Au / Au +++

- 2.71

Cr / Cr +++- 0.74

Al / Al +++

Mg / Mg ++- 2.37

I

Cr. passivated

Al passivated

Ni / Ni ++

E practical

Cr / Cr+++

Fe / Fe ++

V

ECr Passivation

EFe

Eo Cr

EMF : ELECTROMOTIVE FORCE(TENDENCY TO LOOSE AN ELECTRON)

THE SCALE OF GALVANIC POTENTIALSTHE SCALE OF GALVANIC POTENTIALS


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-1.4

Magnesium-1.6

Copper , Copper alloys 90/10 and 70/30

Aluminium bronze

Carbon steel, low alloy steel

Aluminium, Zinc

Nickel Aluminium bronzeStainless steels 304, 316, 321, 347, Monel K500Nickel - Chromium - Molybdenum alloys, Titanium0.0

-0.2

0.2

-0.4

-1.0

-1.2

-0.8

-0.6

0.3

Platinum

POTENTIAL (V) / CALOMEL REFERENCE ELECTRODE

0.0

- 0.05

- 0.1

0.05Graphite

In sea water

CO2 service

CORROSION POTENTIALCORROSION POTENTIAL


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Electrical connection Electrical connection

Steel potential V1 Steel potential V2

I1 # I2

V1 # V2

Steel

THE POTENTIAL OF A METAL IS DEPENDING ON :

* the electrochemical reaction taking place on the metal.

* the current passing through the electrode surface

For a given anode/cathode ratio higher the potential between two

metals, higher the galvanic current .

I1 I2

POURBAIX DIAGRAM THERMODYNAMIC DEFINITIONPOURBAIX DIAGRAM THERMODYNAMIC DEFINITION


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pH1370

- 0.2

0.6

0.4

0.2

0.0

- 0.4

- 0.6

- 0.8

- 1.0

- 1.2

1.2

1.0

0.8

3 4 5 6 981 2 121110 14

H2O

H2

H2OO2

Fe2O3

Passivation

Fe++

Corrosion

FeArea of Immunity

Fe3O4

Fe+++

V

ENERGY pH relationshipH+ /H2

electrode

Ag/ AgCl

electrode

- 0.45

0..35

0.15

- 0.05

- 0.25

- 0.65

- 0.85

- 1.05

- 1.25

- 1.45

0.95

.75

0.55

GALVANIC CORROSIONGALVANIC CORROSION


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O2 + 2e- + H2O 2OH

-

Zinc

A

Na+

Cl -

Zn Zn++ + 2e-

e-

Steel

Zn++

Anodic zones Steel Zinc


CATHODIC PROTECTION PRINCIPALCATHODIC PROTECTION PRINCIPAL


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Water

- 730 mV

- 720 mV

- 725 mV

- 650 mV

- 720 mV

- 725 mV


Steel Zinc

SUMMARYSUMMARY


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Corrosion takes place if:

Free water is present

A corrosive (oxidising) agent is present A heterogeneity is present either :

in the electrolyte

in the metal structure

on the metal surface

STEEL

WATER

Corrosion control:

Dehydration

Removal of oxygen

or acids

Cleaning of the pipes

Avoiding precipitation at the

grain boundaries

Cathodic protection

Coating/painting

Inhibition

corrosion mechanism - 2

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