Understanding the corrosion environment
Different methods for corrosion control
Any method be made more effective…
Corrosion rates vary with process conditions
5.5% NaCl
5.5% NaCl, 5 atm
5.5% NaCl, 85 °C
5.5% NaCl, 10 °C, 15 atm
not
OLI gets the chemistry right
?
Understand what’s happening in your system
Active Corrosion (dissolution)
Protective Scale
Passive Film pH
Determine the rate limiting redox processes
Passive region
Activation controlled
Rate-limiting cathodic process
Determine pitting potential and max growth rate
No Pitting Pitting
Pro-active Analysis
Test Corrective Actions
• Determine optimum pH
• Screen alloys and inhibitors
• Assess process changes
Focus Lab work
Eliminate potential problems before they occur
The Corrosion Analyzer
Tool for understanding the corrosion environment
Mechanistically-based software tool
Speciation
Kinetics of uniform corrosion
Partial anodic and cathodic processes
Transport properties
Repassivation
The Corrosion Analyzer
Based on the OLI Engine
Complete speciation model for complex mixtures
Phase and chemical reaction equilibria
Accurate pH prediction
Redox chemistry
Comprehensive coverage of industrial chemical and petroleum systems
The Corrosion Analyzer
Based on the OLI Engine
Thermophysical properties prediction
Phenomenological and unique aqueous process models including kinetics and transport
“Out-of-the-box” solution and technical support
The Corrosion Analyzer
What It Does…
Predict metal dissolution regime, passive films, and surface deposits
Predict uniform corrosion rates and the potential for pitting corrosion
Generate real solution stability (Pourbaix)
Diagrams
Produce theoretical polarization curves
The Corrosion Analyzer
So you can gain insight on …
Corrosion mechanisms
Rate-limiting partial processes for your operating conditions
Effects of process and materials changes
Therefore
Focusing lab time
Reducing risky plant/field testing
Managing design, operation, and maintenance
Today’s seminar
“Hands-on” and “How-To”
Using example problems
Examining plots and diagrams
Understanding the basis of the predictions
Perform “Single point” calculations
Construct / interpret real solution Pourbaix Diagrams
Calculate corrosion rates
Evaluate the effects of pH, T, comp / flow
Evaluate polarization curves
Gain insight to corrosion mechanisms
See rate limiting steps
Can I read them? Can I trust them?
Determine the likelihood of pitting to occur
For your actual field or lab conditions
Welcome to the
CORROSION TEACH-IN
Simulating Real World Corrosion
Problems
Gas Condensate Corrosion
Scope
Gas condensates from alkanolamine gas sweetening plants can be highly corrosive.
Purpose
Diethanolamine is used to neutralize
(sweeten) a natural gas stream. This removes carbon dioxide and hydrogen sulfide. The off gas from the regeneration is highly acidic and corrosive
Gas Condensate Corrosion
Objectives
Determine the dew point of the acid gas
Remove the condensed phase and perform corrosion rate calculations
Mitigate the corrosion
Sour Gas Absorber
Gas Sweetening
Acid Gas
Absorber liquor regenerator
Acid Gas Concentrations
Species
H
2
O
CO
2
N
2
H
2
S
Methane
Ethane
Propane
Temperature
Pressure
Amount
Concentration (mole %)
5.42
77.4
0.02
16.6
0.50
0.03
0.03
38 o C
1.2 Atm.
100 moles
Application Time
Dew Point
•Dew Point = 37.6 o C
•pH = 3.93
•ORP = 0.576 V
Corrosion Rates: Flow
Conditions
Flow conditions have a direct effect on mass-transfer
Static
Pipe flow
Rotating disk
Rotating cylinder
Complete agitation
Application Time
Carbon Steel Corrosion @ Dew Point
HS = ½ H
2
+ S 2- e
H
2
CO
3(aq)
= ½ H + + HCO
3
- e
Corrosion Rate = 0.7 mm/yr
Corrosion Potential = -0.43 V
Repassivation Potential = > 2 V
Current Density = 60.5 A/cm 2
H
2
S
(aq)
= ½ H
2
+ HS - e
H + = ½ H
2
- e
Mitigation
Adjusting solution chemistry
Temperature profiling
Alloy screening
Cathodic protection
Adjusting the Solution
Chemistry
Changing operating pH
Add acid or base
Application Time
Adjusting solution pH = 8.0
Screening Alloys
Select an alloy that has a preferential corrosion rate
13% chromium
304 Stainless
Application Time
13 % Cr Steel Corrosion @ Dew Point
Corrosion Rate = 0.06 mm/yr
Corrosion Potential = -0.32 V
Repassivation Potential = > 2 V
Current Density = 5.7 A/cm 2
H
2
CO
3(aq)
= ½ H + + HCO
3
- e
HS = ½ H
2
+ S 2- e
304 Stainless Steel Corrosion @ Dew Point
Corrosion Rate = 0.0036 mm/yr
Corrosion Potential = -0.15 V
Repassivation Potential = > 2 V
Current Density = 0.3 A/cm 2
304 Stainless Steel Stability @ Dew Point
Passivation is possible due to
Cr
2
O
3
Explaining common observations using Stability Diagrams
Basics
Iron is inherently unstable in water & oxidizes via the following reactions to form rust
3 H
2
O
3 e
3
2
H
2
Fe o
Fe
3
3 e
3 OH
Fe o
3 H
2
O
Fe ( OH )
3
3
2
H
2
Its severity depends on (among others)
Conditions (T/P),
Composition, pH, and oxidation potential
These four can be plotted on a single chart called a stability diagram
Start example
Explaining the EH-pH diagram using Fe, showing solid and dissolved species over range of pH’s and oxidation potentials
White area is region of iron corrosion
Elemental iron (gray region) corrodes in water to form one of several phases, depending on pH. At ~9 pH and lower, water oxidizes Fe 0 to Fe +2 which dissolves in water (white region of the plot). As the oxidation potential increases (high dissolved O
2
) Fe +2 precipitates as FeOOH, or rust (green region). The lower the pH, the thicker the white region and the greater driving force for corrosion
At higher pH (10-11), Fe 0 forms Fe
3 protecting it from further attack.
O
4
, a stable solid that precipitates on the iron surface,
Fe(III) 3+ is the dominant ion
Fe(II) 2+ is the dominant ion
FeO(OH), rust is stable in water at moderate to high pH’s
Fe
3
O
4 coats the iron surface, protecting it from corrosion
Elemental iron, Fe(0) oxidizes to Fe(II) in the presence of water
Elemental iron, Fe(0) o , is stable and will not corrode in this region
Q: We all know O
2 is bad…But how much is bad?
H
2
O
1
2
O
2
2 H
2 e
H
2
O
e
1
2
H
2
OH
500 ppm O
2
10 ppm O
2
3 ppb O
2
0.1 ppT O
2
0.1 ppT H
2
0.1 ppb H
2
0.1ppm H
2
80 ppm H
2
Pure water is here…
No air, no acid, no base
Iron and water react because they are not stable together
2 H
2
O
2 e
H
2
Fe o
Fe
2
2 OH
2 e
Fe o
2 H
2
O
Fe
2
H
2
2 OH
The reaction generates 2OH , which increases the pH
Region of instability
The reaction generates H
2
, which puts the EH near the bottom line
Elemental Iron (Fe o )
Cr will oxidizes, but the reaction goes through a tough Cr
2 protective layer.
O
3
Ni3Fe2O4 is stable in the corrosion region, and will also protect the surface.
Welcome to the
CORROSION TEACH-IN
Simulating Real World Corrosion
Problems
Corrosion in Seawater
Scope
Metals used for handling sea water face both general and localized corrosion.
Various grades of stainless steels have been used to mitigate the problems.
Stainless steels owe their corrosion resistance to a thin adherent film of oxides on their surface.
Disruption of the films can lead to localized corrosion and premature failure.
Corrosion in Seawater
Purpose
Chlorine and oxygen in sea water can attack the films used to passivate the steels.
The CorrosionAnalyzer will be used to model the effects of chloride and oxygen on the rates of uniform corrosion and the possibility of pitting on the surface of the metals.
Corrosion in Seawater
Objectives
Reconcile a sea water sample for electroneutrality
Reconcile a gas analysis
Calculate uniform rates of corrosion for
• 304 stainless steel
• 316 stainless steel
• S31254 stainless steel
Corrosion in Seawater
Objectives (continued)
Determine the probability of pitting using the localized corrosion feature.
Metal
Surface film
Kinetic Model of General
Corrosion: Mass-Transfer
All reactions take place on the metal surface.
Solution
Films are a diffusion barrier to corrosive species
Reduce mass-transfer-limited currents.
Mass-transfer from solution is calculated from a concentrationdependent diffusion coefficient.
Chemistry
The rates of corrosion use a subset of the OLI
Chemistry
Neutral Species
• H
2
O, O
2
, CO
2
, H
2
S, N
2 and all inert gases, Cl
2
, SO
2
, S o and
NH
3
, organic molecules that do not undergo electrochemical reactions
Anions
• OH , Cl , Br , I , HCO
3
, CO
NO
2
, NO
3
, MoO
4
2, CN -
3
-2 , HS , S 2, SO
, ClO
4
, ClO
3
, ClO
4
-
2, HSO
4
, SO
3
2,
, acetate, formate,
Cr(VI) anions, As(III) anions, P(V) anions, W(VI) anions,
B(III) anions and Si(IV) anions.
Chemistry
Cations
• H + , alkali metals, alkaline earth metals, Fe(II) cations, Fe(III) cations, Al(III) cations, Cd(II) cations, Sn(II) cations, Zn(II) cations, Cu(II) cations, Pb(II) cations and NH
4
+ .
Corrosion of 304 Stainless Steel in
Deaerated Sea Water
Species Concentrati on (mg/L)
Cl -
Na +
Mg +2
Ca +2
SO
4
-2
HCO
3
pH
19000
10700
1300
400
2750
150
Temperatur e
8.0
25 o C
Pressure 1 atm.
LabAnalyzer used to reconcile electroneutrality
NaOH/HCl Used to adjust pH
Application Time
Screening Considerations
Some alloys do not perform well in seawater
We will evaluate 3 stainless steels
Uniform corrosion rates
Pitting possibility
Considering both deaerated and aerated conditions
Corrosion of 304 Stainless Steel in
Deaerated Sea Water
300 years to lose 1 mm of metal
.0033 mm/yr @ 25 o C
Corrosion of 304 Stainless Steel in
Deaerated Sea Water
Large difference means that pits are unlikely to form
Repassivation Potential
Corrosion Potential
Difference = 0.05 V
Or if a pit forms, then it will passivate
Application Time
Corrosion of 316 SS in
Deaerated Water
.00053 mm/yr @25 o C
1886 years to lose 1 mm of metal
Much better corrosion rate than
304 ss
Corrosion of 316 SS in
Deaerated Water
Difference = 0.086 V
Application Time
Corrosion of 254 SMO in
Deaerated Water
Corrosion rate = 0.00033 mm/yr @ 25 o C
> 3000 years to lose
1 mm of metal
Corrosion of 254 SMO in
Deaerated Water
Difference = 2.7 V
Summary in Deaerated Water
Stainless Rate @ 25 o C (mm/yr)
304 0.0033
Potential difference
(V)
0.05
316 0.00053
254 SMO 0.00033
0.086
2.7
Adding Air/Oxygen
The CorrosionAnalyzer allows you to add a gas phase based only on partial pressures
You can set the water/gas ratio
Species
N
2
O
2
CO
2
WGR
Partial
Pressure
(atm)
0.7897
0.21
0.0003
0.01 bbl/scf
Application Time
304 SS in Aerated Solutions
304 SS in Aerated Solution
The corrosion potential is greater than the passivation potential = .37 V at max O
2
Pitting will occur
Application Time
316 SS Corrosion in Aerated
Water
Pitting occurs at higher oxygen concentrations =
.21V at max O
2
Application Time
S31254 Corrosion in Deaerated
Water
Pitting should not occur
Stability Diagram for 316L SS
Stability Diagram for 316 L
Nickel Only
Mitigation
Change Alloys
S31254 seems the best at 25 o C
S31254 increased potential for pitting at higher temperatures
Cathodic Protection
Shifting of potential to less corrosive potentials via a sacrificial anode.
Analyzers do not model CP
Polarization curves can help determine the change in potential.
Welcome to the
CORROSION TEACH-IN
Simulating Real World Corrosion
Problems
Dealloying of Copper Nickel
Alloys
Scope
A copper-nickel pipe made of Cupronickel 30 has been preferentially dealloyed while in contact with a 26 weight percent calcium chloride brine. It appears that the nickel in the alloy has been preferentially removed.
Dealloying of Copper Nickel
Alloys
Purpose
The OLI/CorrosionAnalyzer will be used to show the relative stability of nickel and copper in the cupronickel alloy in an aqueous solution. It will show that protective films were not present as originally thought.
Dealloying of Copper Nickel
Alloys
Objectives
Input information into the software and perform calculations
Use stability diagrams to display information about the alloy and the protective films
Change the diagrams to view different aspects of the stability of the alloy
Application: Dealloying of
Copper-Nickel Alloys
Dealloyed cupronickel pipe.
A cupronickel 30 pipe (30 mass % copper) was used.
26 wt % CaCl
2 solution was in contact with the pipe.
Nickel was preferentially removed.
Questions?
Why did the nickel dealloy from the pipe?
What could we do to prevent this from occurring?
Which tools are available to understand this phenomenon?
Which Tools are Available?
A Pourbaix diagram can help us determine where metals are stable.
CorrosionAnalyzer
Creating the First Stability
Diagram
We will use the CorrosionAnalyzer
to create a stability diagram for this system.
Features of CorrosionAnalyzer
diagrams
Real-solution activity coefficients
Elevated temperatures
Elevated pressures
Interactions between species and overlay of diagrams.
The Pourbaix Diagram
Application Time
Time to start working with the OLI Corrosion Analyzer
The Pourbaix Diagram
There are quite a few things to look at on this diagram.
Stability field for water
Stability fields for nickel metal and copper metal
Stability fields for nickel and copper oxides
Stability fields for aqueous species.
We will now break down the diagram in to more manageable parts.
Stability Diagram Features
Subsystems
A base species in its neutral state and all of its possible oxidation states.
• Cu o , Cu +1 , Cu +2
• Ni o , Ni +2
All solids and aqueous species that can be formed from the bulk chemistry for each oxidation state.
Stability Diagram Features
For each subsystem
Contact Surface
• Base metals
• Alloys
Films
• Solids
Solid Lines
Aqueous Lines
Stability Diagram Features
Natural pH
Prediction based on the bulk fluid concentrations
Displayed as a vertical line
Solids
All solids included by default
The chemistry can be modified to eliminate slow forming solids.
Stability Diagram Features
Passivity
Thin, oxidized protective films forming on metal or alloy surfaces.
Transport barrier of corrosive species to metal surface.
Blocks reaction sites
Water Stability
Water can act as an oxidizing agent
Water is reduced to hydrogen, H
2
Water can act as a reducing agent
Water is oxidized to oxygen, O
2
To be stable in aqueous solution, a species must not react with water through a redox process.
Copper Pourbaix Diagram
Stable copper metal in alloy extending into water stability field.
The solution pH is in a region where the copper metal will be stable.
Copper pipes are used for potable water for this reason.
Nickel Pourbaix Diagram
No Nickel metal extends into the water stability field
The solution pH is in a region where nickel is expected to corrode
Ni Overlaid on Cu
We need to know the
Oxidation/Reduction potential
CuCl
(s) may form to protect the alloy at the solution pH.
Since the nickel is part of a copper-nickel alloy, it is possible that copper could provide a protective film
Application Time
CorrosionAnalyzer Calculation
CorrosionAnalyzer Calculation
The oxidation reduction potential is 0.463 V
Ni Overlaid on Cu
The potential of 0.463 V lies above the passivating film.
Dealloying can occur.
Conclusions
Why did dealloying occur?
No protective film at the operating pH and oxidation/reduction potential of the process fluid.
Copper lies within the region of water stability
Nickel does not lie within the region of water stability
The presence of Cu + ions in equilibrium with copper metal promotes replating of copper metal driven by the oxidation of nickel.
Chemistry
Standard OLI Chemistry
7400 components
9100 individual species
82 Elements of the Periodic Table fully covered
• 8 additional elements partially covered.
Stability diagrams have access all of this chemistry
Chemistry
Alloys
6 predefined classes supported
• Cu-Ni
• Carbon Steels – Fe, Mn, and C
• Ferritic Stainless steels – Fe, Cr, Ni, Mo and C
• Austenitic stainless steels - Fe, Cr, Ni, Mo and C
• Duplex stainless steels FCC phase - Fe, Cr, Ni,
Mo, C and N
User defined alloys
Limits to the Standard OLI
Chemistry
Aqueous Phase
X
H2O
> 0.65
-50 o C < T < 300 o C
0 Atm < P < 1500 Atm
0 < I < 30
Non-aqueous Liquid
Currently no Activity Coefficient Model (i.e., no NRTL,
Unifaq/Uniqac)
Fugacity Coefficients are determined from the Enhanced SRK
Limitations of Pourbaix
Diagrams
No information on corrosion kinetics is provided.
Diagram is produced from only thermodynamics.
Diagram is valid only for the calculated temperature and pressure
Oxide stability fields are calculated thermodynamically and may not provide an actual protective film.
Dealloying cannot be predicted from the diagram alone.