Transcript Effects of Oxygen, Temperature and Salinity on Carbon

Effects of
Oxygen, Temperature and Salinity
on Carbon Steel Corrosion
in Aqueous Solutions Model Predictions versus Laboratory
Steven L. Grise
Brian J. Saldanha
Oct 23, 2007
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Brine Corrosion of Carbon Steel
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Water and saline solutions (brine) with oxygen can be
found in essentially any industrial site
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Cooling water ~0-300 ppm Cl(-)
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Waste headers, WWT operations, brine refridge systems, etc
Cost is a significant issue
For example, a carbon steel waste header which needs to
accept a new brine stream
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Seawater ~ 3.4% NaCl
We all have this problem !
Much of the pipe which contains these solutions is carbon
steel
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Could include oxidizing biocides
Near neutral pH
Low concentration of NaCl
Seems innocent enough, right ?
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Corrosivity of Brine
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NaCl solutions are corrosive because
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Good electrolyte
Presence of chloride – forms metal chlorides
Affected by
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Metals that display lowest corrosion rates
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pH
Temperature
Oxygen
Velocity
Copper alloys in the absence of oxygen (low strength is issue)
High Ni-Cr-Mo alloys to minimize SCC and crevice corrosion,
Hastelloy C-276® ($$)
Titanium ($$)
Stainless steels suffer from pitting attack
Decision becomes financial
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Corrosion Modeling
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Multi-disciplinary. Requires knowledge of
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Solution chemistry
Surface electrochemistry
Mass transport / Fluid flow
Metallurgy
Still an “infant” technology
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1st comprehensive approach was Pourbaix (1950’s)
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Some commercial products available
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Tend to be industry specific (i.e., Oil & Gas)
OLI CorrosionAnalyzer® (CA or CSP)
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Purely thermodynamic
Most comprehensive
Solution based, broad chemistry coverage
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Three Rules of Corrosion Modeling
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“All models are wrong, but some are useful”
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Models are not completely predictive
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Prof George E.P. Box, Univ. of Wisconsin, 1979
Data are required to develop rate parameters
Need to incorporate BOTH thermodynamics and kinetics
Since data are always required, models will
never replace laboratory and field corrosion
testing !
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Effect of pH on CS Corrosion in Brine
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Extremes of pH have profound effect on
corrosion rate
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Brines concerned primarily in pH 5-9 range
Reference:
Kirby, G.N.,
Chemical Engineering,
March 12, 1979
Pg 72-84
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Effect of pH on CS Corrosion in Brine
Slow moving H2O or 3.5 wt% Brine;
3.9 ppmw O2
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Corrosion Rate (mpy)
30
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Kirby data @ 22 C
Kirby data @ 40 C
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CA - Water @ 22 C
CA - Water @ 40 C
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CA - Water @ 55 C
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CA - 3.5% Brine @ 22 C
CA - 3.5% Brine @ 40 C
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CA - 3.5% Brine @ 55 C
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5
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pH
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Corrosion Rate as a Function of Temperature
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Temperature will affect corrosion rate
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Effect is multi-faceted
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Kinetics
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O2 solubility
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O2 solubility decreases with temperature
Increases rate of diffusion through layers of Fe(OH)x
pH
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Corrosion rate (kinetics) increase with temperature
Usually follows Arrhenius kinetics
Temperature can affect equilibrium composition of solution
Not significant through reasonable temperature ranges
Mass transfer will also increase with temperature
Difficult to experimentally keep all other
variables constant
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O2 Solubility vs Temperature
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p(O2) = 0.2094 atm; NaCl conc ~ 0.34 g/kg soln
O2 Solub Reference:
1) Carpenter, J.H., Limnology and Oceanography , Vol. 11, No. 2 (1966), pg. 264
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O2 Solubility (mg/kg soln)
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Carpenter data
OLI Prediction
Lab Data - Water
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6
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5
10
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Temperature ( C )
30
35
40
45
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pH vs Temperature
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7.8
p(O2) = 0.2094 atm
NaCl conc ~ 0.34 g/kg soln
7.6
7.4
pH
7.2
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OLI Calc'd pH
6.8
6.6
6.4
6.2
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5
10
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20
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Temperature ( C )
30
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40
45
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Equilibrium Concentration of O2 in Brine
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Brines exposed to air will absorb O2
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Amount of O2 absorbed will depend on
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For example, splash zones in seawater
exposures up to 10x higher corrosion rates
than subsurface exposure
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p(O2)
Salt concentration
Temperature
Brine / gas surface area
High surface area for O2 transfer
Concentration of NaCl due to evaporation of splash
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Comparison of OLI Predictions to Data
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O2 Solubility (mg/l)
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p(O2) = 0.2094 atm; 37 C
References:
1) Lang, W. and Zander, R., IEC Fundamentals , V 25, No. 4, pg 775 (1986)
2) Iwai, Y., et al, Fluid Phase Equilibria , V 83, pg 271 (1993)
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NOTE: MSE framework does not yet include Setchenov-type correlations.
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Lang
Iwai
OLI Aq
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5
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wt% NaCl
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25
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Corrosion Rate as a Function of O2 Concentration
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O2 provides the cathodic reaction for corrosion
Metal + ½ O2 + H2O  Metal(2+) + 2 OH(-)
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Higher O2 conc  higher corrosion rates
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…but as we have discussed, the variables are
interdependent. So what happens in real
experiments ?
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Lab Data vs OLI Predictions
8.00
Tap water in air
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OLI
Tap + 3.5% NaCl in air
6.00
OLI
Corrosion Rate (mpy)
Tap water w/ N2 purge
OLI
5.00
Tap + 3.5% NaCl w/ N2 purge
OLI
4.00
3.00
CorrosionAnalyzer assumptions:
- Static flow conditions
- Liquid saturated with O2
- p(O2) = 0.21 atm
- p(O2) = 0 for N2 purge cases
2.00
1.00
0.00
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20
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30
35
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Temperature (deg C)
45
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55
60
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Corrosion Rate as a Function of Velocity
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Limiting current density is a function of species
movement to/from the surface
ii ,lim  ni  F  km,i  (ai,bulk )
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Role of velocity and flow conditions is to define
mass transfer coefficient, km
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Velocity also affects adherency of passive films
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Higher velocity can remove films, leaving base metal
It can also cause erosion or mechanical removal of metal
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Different Flow Conditions
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Flow conditions available in Corrosion Analyzer
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Static
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Rotating Disk
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km d/D = 0.0165 Re0.86 Sc0.33
Complete Agitation
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km d/D = 0.0791 Re0.70 Sc0.356
Pipe Flow
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km = 0.6211 D2/3 w1/2 / n1/6
Rotating Cylinder
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km ~ 0
km = very large number ( infinity)
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Compare Predictions, Similar Conditions
Flow Conditions
~ 1/2
in/sec
~ 20
ft/sec
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Static
Pipe flow (d = 2 m; v = 0.0127 m/s)
Rotating Disk (d = 10 cm; 2.5 cycle/min)
Rotating Cyclinder (d = 10 cm; 2.5 cycle/min)
Pipe flow (d = 2 m; v = 6.1 m/s)
Rotating Disk (d = 10 cm; 1165 cycle/min)
Rotating Cyclinder (d = 10 cm; 1165 cycle/min)
Completely Agitated
Water
1018 CS
40oC
pH ~ 6.8
p(O2) = 0.21 atm
Water
1018 CS
40oC
pH ~ 6.8
p(O2) = 0 atm
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10.3
8.7
9.5
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515
211
0.44
0.44
0.44
0.44
0.81
0.52
0.77
3.9
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CorrosionAnalyzer Velocity vs Data at 23°C
1000
Melchers/LaQue
Corrosion Rate (mpy)
OLI Pipe flow
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OLI Rotating Disk
OLI Static
OLI Complete
Agitation
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T = 23oC, Mild Steel
Assume p(O2) = 0.2094 atm and 3.5% NaCl brine
References:
1) Melchers, R.E. and Jeffrey, R., Corrosion , V 60, No. 1, pg 84 (2004)
who references LaQue, F.L., in Uhlig's Corrosion Handbook
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Velocity (m/s)
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Summary
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Velocity and oxygen concentration appear to
have most significant impact on corrosion in
near neutral brines
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Both in experimental and OLI prediction
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Temperature is secondary effect
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pH is not a significant factor between 5 and 9
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OLI CorrosionAnalyzer® predictions
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Very good for static conditions
Velocity effect needs further development
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Summary
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Corrosion modeling is multi-faceted and
interdisciplinary
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Modeling and experimental programs are
interactive
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Model suggests experiment conditions
Experimental data validate and/or feed model
Improve the model
Suggest new experiments
Etc…
Did not discuss MIC and microbiological,
including impact of oxidizing biocides
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Use all your resources
Study all the variables
Very important considerations for water corrosivity!
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