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
0
10
20
30
40
50
60
70
0 100 200 300 400 500
Co
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, M
PY
Gas Phase Partial Pressure, (ppCO2) *
Carbon Dioxide Corrosion(Water Containing Low Dissolved Solids)
* ppCO2 = ((System CO2 Volume %) X (System Pressure + Atmospheric Pressure))/100
0
1
2
3
4
5
0 5 10 15 20 25
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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
0
5
10
15
20
25
30
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Co
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ate
, M
PY
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
0
1
2
3
4
5
200 400 600 800 1000 1200 1400 1600 1800 2000
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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
100
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