Corrosion Inhibitors and Corrosion Inhibitor Selection Aberdeen, 15th March 2011
Classification of Corrosion Inhibitors
Scavengers
- remove O2 and H2S
Biocides
- remove microbes
Organic Film Forming
- prevents corrosive substance interacting with metal
Anodic
- passivating, inhibits oxidation reactions
Cathodic
- inhibits reduction reactions
Vapour Phase
- volatile inhibitors
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Organic Film Formers
Generally contain nitrogen, phosphorus, oxygen or sulphur polar groups
Examples are amines, phosphate esters, imidazolines, betaines
Polar group attaches to metal surface
Hydrophobic end dispels water
Changes wettability of metal surface
This provides a barrier between the corrosive water phase and the metal surface
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Colloidal Aspects
Corrosion Inhibitors are SURFACTANTS
They can form colloidal dispersions (MICELLES)
Increased dose rate does NOT always equal increased corrosion inhibition – the formation of micelles means less inhibitor at the metal surface
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Two-phase System
Oil Phase OIL DISPERSED MOLECULES
OIL SOLUBLE MOLECULES
Interface
Water Phase
WATER SOLUBLE MOLECULES
OIL FILM
ADSORBED FILM
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WATER DISPERSED MOLECULES
Partitioning
We want the inhibitor to partition into the water phase
The partitioning co-efficient of the inhibitor can be derived experimentally
This helps determine the suggested dose rate
Water Cut (%)
90
20
10
5
2
Total Fluids Dose Rate to Give 30 ppm in Water Phase (ppm)
31.0
11.2
7.5
5.7
4.6
A change in water cut will require a change in dose rate
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Formulating - Composition of a Corrosion Inhibitor
Corrosion Inhibitor bases – Can be combinations to achieve synergistic effects, or to protect against more than one type of corrosion.
Salting agents – MEA, GAA – To make bases more water soluble
Solvents – Water, Glycols, Aromatics – To improve viscosity, improve thermostability, alter dose rate, reduce cost
Synergists – generally sulphur based – To improve performance
Other bases – anti-foams, demulsifiers, scale inhibitors, hydrate inhibitors
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Project design Field Modelling Laboratory Test Design
Deployment
Field Trial Corrosion Test Techniques Field Testing Third Party Confirmation / Specialist Testing Integrity & Hydrocarbons Group, Oil & Mining Services, Application Development (Copyright Clariant. All rights reserved.)
Final Confirmation by Laboratory / Specialist Testing
Project design
A full system review should be performed and a questionnaire completed
Relevant system information includes: – water chemistry (including organic acids) – CO2 and H2S partial pressure = (mol% / 100) * total pressure – system temperatures – production rates, pipeline diameters, flow regime – materials used in the system – sand production – weld corrosion (galvanic corrosion risks)
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Project design
Other information required: – are there any compatibility issues with other system chemicals? – are there any environmental restrictions? – is there a restriction on dose rate? – are there any cost constraints? – is a sample of the incumbent available? – is oil in water or foaming a major concern? – is an oil sample available?
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Target corrosion rates
mm/year, or mmpy
(millimetre per year)
mils/year, or mpy
(milli-inches per year)
1 mm/year = 39.4 mils/year
A combination of electrochemical techniques and coupon weight loss measurements used to measure corrosion rate
A low corrosion rate is considered as below 0.1 mm/year (4 mils/year)
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Performance Requirements
General (CO2) corrosion
Pitting corrosion
Sour (H2S)
High temperature
High pressure
Under deposit
Weld
High shear
Scale, antimicrobial, scavenger etc. in combined products
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Standard techniques – wheel test
Wheel Test – 10 spaces available – Ambient to 90 oC (194 oF) – Weight loss coupons – Microscope evaluation – Ambient pressure
Can be used to test: – Inhibitor screening – Partitioning – Batch treatment – Persistency
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Linear Polarisation Resistance
2 or 3 electrode probe inserted into process system
Electrodes are electrically isolated from each other
A small potential is applied (up to 20mV) between the electrodes
Resultant current is measured
Polarisation resistance is the ratio of the applied potential and resulting current level
Measured resistance is inversely related to the corrosion rate
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Standard techniques – LPR bubble cell test
Bubble Test – Ambient to 90 oC (194 oF) – Linear Polarization Resistance – Electrochemical Impedance – Galvanic corrosion
Can be used to test: – Inhibitor screening – Partitioning – Persistency
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LPR bubble cell test graph 6
5 Crude Oil Added
Corrosion Rate (mm/yr)
4 Corrosion Inhibitor Added
3
2
1
0 0
2
4
6
8
10
12
14
Time (hours) 50ppm CI
100ppm CI
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75ppm CI
16
18
20
22
24
Standard techniques – dynamic RCE test
Rotating Cylindrical Electrode (RCE) – Ambient to 90 oC (194 oF) – Linear Polarization Resistance – Electrochemical Impedance – Shear stress up to 80 Pa
Can be used to test: – Shear conditions (<80 Pa) – Partitioning – Persistency
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Dynamic RCE test graph 7
6
Corrosion Rate (mm/yr)
5
4
3
2
1
0 0
5
10
15 Time (hours)
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20
25
Standard techniques – HP/HT Autoclave
Autoclaves –
Maximum Temperature 350 oC (660 oF)
–
Maximum Pressure 200 kgf/cm2 (2845 psi)
–
Weight loss coupons or electrochemical
Can be used to test: –
High temperatures
–
High pressure CO2/H2S
–
Partitioning
–
Localised corrosion (sour)
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Specialised techniques – weld test
Segmented Weld Testing –
Customised electrode fabricated specifically for individual projects
–
Parents materials, weld consumables and welding technique specified
–
Ambient to 90 oC (194 oF)
Can be used to test: –
LPR corrosion rate
–
Galvanic current
–
Partitioning
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Weld test LPR corrosion rate graph 6 Addition of 30 ppm MULTITREAT 9347
Corrosion Rate (mm/yr)
5
4
3
2
1
0 0
2
4
6
8
10
12
Time (hours) Weld
HAZ
Parent 1
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Parent 2
14
16
18
20
Weld test galvanic current density graph 0.2
Addition of 30 ppm MULTITREAT 9347
0.15
Current Density (mA/cm2)
0.1
0.05
0 0
2
4
6
8
10
12
-0.05
-0.1
-0.15
-0.2 Time (hours) Weld
HAZ
Parent 1
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Parent 2
14
16
18
20
Specialised techniques – under-deposit test
Under Deposit Testing –
Developed in IFE JIP
–
Deposit specific i.e. sand, FeS, scale
–
Ambient to 90 oC (194 oF)
Can be used to test: –
Inhibitor performance with solids
–
Galvanic currents
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Test matrix Wheel Test
Bubble Test
Product Screening
Partitioning
Persistency
CO2 / H2S High Shear
RCE Test
Weld Test
Under Deposit
Autoclave
Weld Solids
High Temperature
High Pressure
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Third party laboratories Autoclave
Flow Loop
Jet Impinge
Microbial Testing
H2 S Scavenger
JIP
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Criteria
Performance
Cost
Environmental
Material compatibility – Brine, oil, other chemicals, elastomers, metals
System compatibility – Foaming, emulsion, water quality
Physical properties – Viscosity, flash point, gunking
Stability – High temperature, low temperature
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Secondary performance
Includes: – Emulsion tendency – Foaming tendency – Brine compatibility – Chemical compatibility – Thermal stability – Elastomer compatibility – Viscosity – Flash point – Residual analysis
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A typical study Product
Stability
Compatible with incumbent
Foaming
(Ambient)
Performance < 0.1 mm/yr
Stability
Viscosity
A
PASS
PASS
PASS
PASS
PASS
FAIL
N/A
N/A
B
PASS
PASS
FAIL
N/A
N/A
N/A
N/A
N/A
C
PASS
PASS
PASS
PASS
PASS
PASS
FAIL
PASS
D
PASS
PASS
PASS
PASS
PASS
FAIL
PASS
N/A
E
FAIL
N/A
N/A
N/A
N/A
N/A
N/A
N/A
F
PASS
PASS
FAIL
PASS
PASS
PASS
N/A
N/A
G
FAIL
N/A
N/A
N/A
N/A
N/A
N/A
N/A
H
PASS
PASS
PASS
PASS
PASS
PASS
PASS
PASS
I
PASS
FAIL
N/A
PASS
PASS
PASS
PASS
PASS
J
PASS
PASS
FAIL
PASS
PASS
PASS
N/A
N/A
K
PASS
PASS
PASS
PASS
PASS
FAIL
PASS
N/A
L
PASS
PASS
PASS
PASS
PASS
PASS
PASS
FAIL
(low temp)
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Elastomer
Cost
Compatibilit y
Summary
Corrosion inhibitors are surfactants and partition between oil and water phases
They inhibit corrosion by – Removing factors that cause corrosion (O2, H2S, microbes, acidity) – Forming films on the metal surface
Formulating is required
Products need to be tailored to suit specific applications
Increased dose rate does NOT always equal increased corrosion inhibition
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Summary Part II
A full system review is essential to understand the corrosion risk and system requirements
A number of routine and specialised techniques are available
Third party evaluations may be necessary depending on the system conditions
The correct choice of tests are essential and these are based on the system conditions and requirements
Secondary performance influences selection of inhibitor
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Integrity & Hydrocarbons Group Oil & Mining Services Application Development 19.10.2010
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