NACECorpus Christi Section

Lunch & LearnJanuary 16,2019

Fundamentals of Internal Corrosion

Tom Pickthall

EnhanceCo, Incorporated

Corrosion Principles

• Corrosion Circuit• Anode

• Cathode

• Metallic Path

• Electrolyte

Corrosion Circuit

• Eliminate one part of the circuit and corrosion stops.

• Anode - More anodic (CP), CRA’s

• Cathode - Coating , inhibitor film, CRA’s

• Metallic Path - Use non-metallics

• Electrolyte - Get rid of water, use inhibitors & coatings

Acid GassesCarbon Dioxide (CO2 )

• CO2 forms a weak acid

that attacks metal

• Often takes the form of

“Mesa” pitting type

attack

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70

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Gas Phase Partial Pressure, (ppCO2) *

Carbon Dioxide Corrosion(Water Containing Low Dissolved Solids)

* ppCO2 = ((System CO2 Volume %) X (System Pressure + Atmospheric Pressure))/100

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Carbon Dioxide, mg/L

Corrosion Rates vs. Carbon Dioxide & Oxygen(Surface Water)

0 mg/L O2

10 mg/L O2

6 mg/L O2

Estimation: ppm O2 = 10 – 0.555 (XoF - 30oF), where XoF is the water temperature

Acid GassesHydrogen Sulfide (H2S)

Has characteristic “rotten egg” smell

Creates a black deposit FeS

Usually a shallow dish-type attack, but may have pitting

H2S forms a weak acid that attacks metal

Usually has etching in the bottom of the pits

Courtesy- Corrosion of Oil and Gas Well Equipment, API cc 1958

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Dissolved Hydrogen Sulfide, mg/L

Hydrogen Sulfide Corrosion(Water Containing Low Dissolved Solids)

pH 9.0

pH 8.0

pH 7.0

pH 6.25

pH 5.0

pH 4.5

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Sulfides, mg/L

Hydrogen Sulfide Corrosion With Respect to pH

Acid Gasses Oxygen (O2)

• Strong depolarizer

• Forms deep pits

• Usually found in surface equipment where air has entry point

• Can greatly accelerate other corrosive agents

Effect of O2 on Sour CorrosionWith and Without Inhibitor

0

50

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150

200

250

300

No Air/No

Inhibitor

Inhibitor/No

Air

Air/No

Inhibitor

Inhibitor/Air

MPY

Acid Gas Corrosion Rates

Acid Gas “Worry Levels”

Sweet

Sour

Oxygen Assisted

Any amount of CO2 with 5 ppm or less H2S

Any amount of CO2 with 10 ppm or more H2S

Any amount of CO2

and/or H2S with 10 -20 ppb or more O2

* Levels relative to measurements made on fresh samples.

Reality

Galvanic Corrosion Brass and Steel

Corrosion Coupons

• ADVANTAGES

• Easy to Use

• Allows Examination

• If located properly, very representative of system

• Inexpensive

• DISADVANTAGES

• Long time needed to collect data

• Time consuming

• If not located properly NOT representative of system

Rod Coupons

Corrosion Coupon 2

Bacteria Monitoring

• Planktonic– Serial Dilution

– Rapid Check

• Sessile– Serial Dilution

– “Robbins Device”

– Screen

– Swabs

Serial Dilution Vials

Electrical Resistance Probes

Linear Polarization Resistance Probes

Pipeline Monitor

Remember

•RUST

•NEVER

•RESTS

References• Corrosion Control in Petroleum Production

– Byars, Harry, NACE International, 2nd Edition

• Microbiologically Influenced Corrosion Handbook – Susan Watkins Borenstein, Woodhead Publishing

• Applied Water Technology C. Patton, Campbell Petroleum Series, Norman OK

• Oilfield Water Technology– M. Davies, PJB Scott, NACE Press

• Pipeline Rules of Thumb– E.W. McAllister, Gulf Publishing Co., Houston TX