NACE Corpus Christi Sectionnace-corpuschristi.org/images/NACE_CC_Section_L_L_2019-01-16.pdf ·...
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NACECorpus Christi Section
Lunch & LearnJanuary 16,2019
Fundamentals of Internal Corrosion
Tom Pickthall
EnhanceCo, Incorporated
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Corrosion Principles
• Corrosion Circuit• Anode
• Cathode
• Metallic Path
• Electrolyte
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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
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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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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
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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)
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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
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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
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Effect of O2 on Sour CorrosionWith and Without Inhibitor
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No Air/No
Inhibitor
Inhibitor/No
Air
Air/No
Inhibitor
Inhibitor/Air
MPY
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Acid Gas Corrosion Rates
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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.
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Reality
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Galvanic Corrosion Brass and Steel
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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
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Rod Coupons
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Corrosion Coupon 2
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Bacteria Monitoring
• Planktonic– Serial Dilution
– Rapid Check
• Sessile– Serial Dilution
– “Robbins Device”
– Screen
– Swabs
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Serial Dilution Vials
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Electrical Resistance Probes
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Linear Polarization Resistance Probes
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Pipeline Monitor
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Remember
•RUST
•NEVER
•RESTS
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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