Oilfield Metallurgy and Corrosion

Fundamentals of Corrosion

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FUNDAMENTALS OF CORROSION

• Corrosion Principles– Corrosion Cell

– Environmental Cracking

• Types of Corrosion

• Mitigation Techniques

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Corrosion Principles

• Why do metals corrode?

• An electrochemical description of corrosion

• Cracking mechanisms

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Why do metals corrode?

Interaction of metal with environment Very few metals occur naturally Natural state for most engineering metals is

combined in various metal salts (chlorides, oxides, sulfides).

Corrosion is the result of metals moving towards their lowest energy state (thermodynamic explanation)

Tendency for corrosion can be predicted through thermodynamics - electrochemistry

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THERMODYNAMICS

• Metal + oxygen = metal oxide + change in free energy.

• If free energy is reduced metal will oxidize.

• If free energy is increased metal does NOT oxidize.

• In order of free energy reduction during oxidation: Al >Ti > Cr> Fe

• Al and Ti used to deoxidize molten steel

• Al anodes cathodically protect steel

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Kinetics

• Al is considered corrosion resistant in cans, kitchenware, aircraft, etc.

• Cr plating protects steel

• Cr additions to steel produce “stainless” steel

• Ti widely used in specialized highly corrosive applications

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FUNDAMENTALS OF CORROSION

• Corrosion Principles Corrosion Cell Environmental Cracking

• Types of Corrosion

• Mitigation Techniques

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Corrosion Principles

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Cracking Mechanisms

• Stress Corrosion Cracking

• Hydrogen Embrittlement

• Hydrogen sulfide

• HIC & SOHIC

• Liquid Metal Embrittlement

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Cracking Mechanisms

• Cracking Mechanisms - material & environment Stainless Steels and high temperature chlorides Brasses and ammonia Titanium and anhydrous alcohol moderate to high strength steels and hydrogen sulfide

• Stress Corrosion Cracking Stress Environment and conditions (specific corrodents,

temperature, pH, etc.) Susceptible material

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Understanding Corrosion – Cracking Mechanisms

• Stress Corrosion Cracking

• Hydrogen Embrittlement

• Hydrogen sulfide

• HIC & SOHIC

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Table 2-5 Some environment-alloy combinations known to result in SCC.

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Cracking Mechanisms

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Failure Time as a Function of HRC & H2S

10152025303540

1 10 100 1000 10000Time to Failure, hrs

Hard

ness

, HR

C

3000ppm 15ppm 1ppm 0.1ppm

Cracking Mechanisms

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Understanding Corrosion - Cracking Mechanisms

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Understanding Corrosion - Cracking Mechanisms

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Understanding Corrosion - Cracking Mechanisms

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Understanding Corrosion - Cracking Mechanisms

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Understanding Corrosion - Cracking Mechanisms

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Cracking Mechanisms

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FUNDAMENTALS OF CORROSION

• Corrosion Principles Corrosion Cell Environmental Cracking

• Types of Corrosion

• Mitigation Techniques

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Types of Corrosion

General Corrosion Pitting Corrosion Crevice Corrosion Stress Corrosion Cracking Hydrogen Embrittlement Erosion Corrosion Corrosion Fatigue Galvanic Corrosion Liquid Metal Embrittlement

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pH versus Corrosion Rate

0

50

100

150

200

250

300

350

2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7pH

Co

rro

sio

n R

ate,

MP

Y

CO2 - no Cl

CO2 + 50000 ppm Cl

CO2 + 100000 ppm Cl

General Corrosion

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Understanding Corrosion – Localized Corrosion

Local breakdown of protective filmLocal concentration of environmentFilm reforming kineticsGeometric factors

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Understanding Corrosion - Localized Corrosion

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Understanding Corrosion - Localized Corrosion

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Understanding Corrosion - Localized Corrosion

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Understanding Corrosion – Other Corrosion Mechanisms

Corrosion Fatigue Erosion Corrosion Galvanic Corrosion Liquid Metal Embrittlement High Temperature Processes (Oxidation etc)

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Understanding Corrosion – Other Corrosion Mechanisms

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Understanding Corrosion –Other Corrosion Mechanisms

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Understanding Corrosion – Other Corrosion Mechanisms

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FUNDAMENTALS OF CORROSION

• Corrosion Principles Corrosion Cell Environmental Cracking

• Types of Corrosion

• Mitigation Techniques

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MITIGATION TECHNIQUES

Modify the Environment Select material resistant to environment Barrier coatings Cathodic protection/sacrificial anodes Modify design Corrosion Allowance

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Mitigation Techniques – Modification of environment

– Dilution

– Temperature control

– Modify pH

– Corrosion inhibitors

– Limit exposure time

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Mitigation Techniques – Barrier Coatings

• Oils/greases/waxes

• Paints

• Epoxy, phenolic, polyurethane etc

• PTFE, other fluorocarbons

• Liners - thermoplastics PE, PP, Nylon etc

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Mitigation Techniques – Resistant Materials

• Select material for environment

• Metallic plating

• Metallic liners/cladding

• Weld overlays

• Metal sprayed coatings

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Mitigation Techniques – Design Techniques

• Avoid crevices/modify crevice geometry

• Isolate susceptible materials

• Move or isolate seal surfaces

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What Factors Influence Material Selection

• Design constraints (strength, stiffness etc)

• Identification of true environment

• Fabrication

• Cost

• Knowledge of material in environment (unknowns)

• Life or exposure time

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Examples - Atmospheric Corrosion

Carbon steels - allow corrosion tolerance Copper bearing steels (“Weathering” steels) Chromium, Nickel coating – cathodic to steel Zinc coating anodic to steel Stainless steels - high Cr Martensitic, Ferritic

& Austenitic Painted & coated structures Phosphated + Corrosion Inhibitor or barrier

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Examples

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REFERENCES

• ASM Material Engineering Institute, Course 0131 “Corrosion”

• ASM Metals Handbook, “Corrosion”, Ninth Edition, Volume 13, 1986.

• “Damage Mechanisms Affecting Fixed Equipment In The Refining Industry”, API Recommended Practice 571, First Edition, 2003

• “Principles and Prevention of Corrosion”, D. A. Jones, Prentice Hall, Second Edition, 1996

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QUESTIONS?