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.