Objective Statement
To provide comprehensive and accurate answers to frequently asked questions about the corrosion rate calculations using OLI Systems' models, including the effects of passivation layers, scale formation, flow conditions, and material properties.
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Is it possible to manually add alloys for corrosion rate simulations?
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Does changing the dimension of a pipe’s diameter change the estimation of the corrosion rate?
1. Are the effects of a passivation layer included in the corrosion rate calculation of the general corrosion rate model?
Yes, the effects of a passivation layer are included in the general corrosion rate model. Detailed modeling of the passive layer and its impact on corrosion rates can be found in the paper "Computation of Rates of General Corrosion Using Electrochemical and Thermodynamic Models" by Anderko et al., particularly in the "Active-Passive Transition" section. This paper also describes how the model aligns with experimental data.
Reference: A. Anderko, P. McKenzie, and R. D. Young, "Computation of Rates of General Corrosion Using Electrochemical and Thermodynamic Models," CORROSION, vol. 57, no. 3, pp. 202-213, 2001.
2. Does the corrosion rate model include the effects of FeCO3/FeS scale formation in the corrosion rate calculation?
Yes, the corrosion rate model accounts for the effects of FeCO₃ and FeS scales, which form on steel surfaces under certain environmental conditions. Although these scales do not cause an active-passive transition, they influence corrosion rates by acting as surface barriers and impacting anodic and cathodic reactions. This effect is more pronounced in carbon steel than in corrosion-resistant alloys.
Reference: A. Anderko and R. D. Young, "Modeling of Corrosion Rate," CORROSION/99, Paper 31.
3. Does the corrosion rate model include the effects of other mineral scales, such as dolomite, minnesotaite, etc., in the corrosion rate calculation?
No, the general corrosion model does not include the effects of mineral scales such as dolomite or minnesotaite. The model only accounts for scales formed from electrochemical reactions with dissolved CO₂ or H₂S. The impact of carbonate, sulfate, or silicate scales is not considered due to the complexities involved in modeling their formation and effects on corrosion rates. While these scales may reduce corrosion, the extent of this reduction cannot be accurately predicted.
4. Under determined conditions, the Pourbaix diagram indicates that a passivation layer of Minnesotaite will form, yet the corrosion analysis output presents a significantly high corrosion rate. What could be the reason(s) for this? Does the software assume that the passivation layer formation is kinetically slow, and so ignore its effects?
The model does not account for the formation of a minnesotaite passivation layer in predicting corrosion rates. While thermodynamic data suggest that minnesotaite could form under certain conditions, the model does not include the effects of this layer due to the lack of a theoretical framework to quantify its impact on reducing corrosion rates.
5. If a case involves a specific carbon steel alloy, for example L-80, which is not included in the OLI material database, what can be done to predict the corrosion rates in the OLI Corrosion Analyzer Software?
For steel L-80 and similar carbon steels, the generic carbon steel should be used. In general, it is rather difficult to quantify the differences between various carbon steel grades. However, the electrochemical mechanisms are the same. Some parameters (such as exchange current densities) may differ on various carbon steel surfaces but a consistent quantification of these differences would not be practical in view of the available literature database.
6. Is it possible to manually add alloys for corrosion rate simulations?
No facility is provided for a user to do it. This is done internally at OLI.
7. In a system consisting of a mix of oil and water, with a concentration of Cl-, H2S and CO2, at a predetermined temperature T and pressure P, the general corrosion rates for static flow conditions differ significantly from the general corrosion rates for multiphase flow conditions, as is illustrated in Figure 1. Why are these calculated corrosion rates different?
Figure 1. General corrosion rate estimations using OLI static and approximate multiphase models.
OLI has extensively tested the general corrosion rate model predictions against experimental laboratory data. In fact, experimental laboratory data are particularly abundant for static conditions and for well-defined single-phase flow conditions. Thus, predictions can be considered more trustworthy for static and single-phase flow conditions than for multiphase flow conditions. We believe that the methodologically correct approach is to compare our model directly against experimental data.
In general, large differences between static and multiphase flow conditions such as the ones shown in Figure 1 are to be expected when the metal is not passive (and carbon steel is certainly not passive in typical CO2/H2S environments). Then, mass transport of reacting species (such as protons (H+), carbonic acid molecules (H2CO3) and hydrogen sulfide (H2S) is numerically important in the electrochemical reactions.
8. Does changing the dimension of a pipe’s diameter change the estimation of the corrosion rate?
Yes, changing the pipe diameter affects the estimated corrosion rate. In the software, the default pipe diameter is 10 cm (3.937 inches), but this can be adjusted. Corrosion rate calculations have shown that as the pipe diameter increases, the corrosion rate tends to decrease, as illustrated in Figure 2.
9. What are the differences between the static, pipe flow, approximate multiphase flow, and shear rate calculations? How are the corrosion rates estimated based on these different types of flow?
Corrosion rates are calculated based on the mass transfer coefficient, which influences interfacial reactions. This coefficient is linked to properties like diffusivity, viscosity, and density, and varies with flow conditions:
- Static and Single-Phase Flow: Well-established correlations are used to link the mass transfer coefficient with the fluid's physical properties.
- Pipe Flow: Specific correlations account for the flow dynamics in pipes.
- Approximate Multiphase Flow: In the absence of detailed multiphase flow data, an approximate model uses the NORSOK correlation to estimate shear stress and, consequently, the mass transfer coefficient.
- Shear Rate: In multiphase flow conditions, the rigorous approach involves calculating shear stress directly from specialized flow software, though this is not always accessible to all users.
The key references detailing these methodologies are:
- A. Anderko, P. McKenzie, and R. D. Young, "Computation of Rates of General Corrosion Using Electrochemical and Thermodynamic Models," CORROSION, vol. 57, no. 3, pp. 202-213, 2001.
- A. Anderko and R. D. Young, "Modeling of Corrosion Rate," CORROSION/99, Paper 31.
10. In an O2 containing environment, does corrosion of carbon steel decrease as the pipe diameter increase?
The limiting current density is proportional to the mass transfer coefficient. The mass transfer coefficient is obtained from the following equation:
Sh = 0.0165 Re0.86 Sc0.33
The pipe diameter d enters into both the Sherwood number (Sh) and the Reynolds number (Re) but with different exponents:
Sh = km d / D Re = V d / nu
So, calculating km from the correlation, the pipe diameter will be d0.14 in the denominator. This means that the mass transfer coefficient will decrease with an increase in the pipe diameter.
This means that the smaller the diameter, the greater the turbulence and, consequently, the greater the mass transfer near the wall (this of course would not apply to very thin capillary tubes, but it does apply to “normal” pipes).
Summary Statement
This FAQ section addresses common queries about the corrosion rate calculations in OLI Systems' software. It includes explanations on how various factors such as passivation layers, scale formation, flow conditions, and material properties influence corrosion rates. References to relevant studies and papers are provided for detailed understanding.