de Waard-Milliams Model (1975/1991/1993)
If blank, pH is calculated from CO2 equilibrium (saturated pH).
Understand CO2 corrosion mechanisms, de Waard model derivation, protective scale formation, and mitigation strategies
The de Waard-Milliams model (1975) is an empirical correlation for predicting CO2 corrosion rate of carbon steel in wet gas and multiphase pipelines. The base equation is log(Vcor) = 5.8 - 1710/(T+273) + 0.67*log(pCO2), where the rate depends on temperature and CO2 partial pressure. Updates in 1991 and 1993 added pH correction, fugacity correction, and protective scale factors.
The main factors are: CO2 partial pressure (higher pressure = higher rate), temperature (rate increases with temperature but FeCO3 scale forms above ~140 deg F providing protection), pH (lower pH = higher rate), flow velocity (affects mass transfer and erosion), water cut (no corrosion without free water), steel type (chrome alloys resist CO2 corrosion), inhibitor efficiency, and FeCO3 scale formation.
Iron carbonate (FeCO3) scale typically becomes protective above 140 deg F (60 deg C) when supersaturation conditions are met. A dense, adherent FeCO3 layer can reduce corrosion rate by 90% or more. However, scale stability depends on flow conditions, pH, and temperature. High velocity or slug flow can strip protective scale, exposing bare metal.
NORSOK M-506 is a Norwegian standard CO2 corrosion model that uses CO2 fugacity, temperature, pH, and wall shear stress. It tends to predict higher rates than de Waard-Milliams at high pressures due to better fugacity treatment. Both models are widely used; NORSOK M-506 is preferred for North Sea applications while de Waard-Milliams is more common in North American operations.