Algorithm of the protectiveness of corrosion layers : 1 - Protectiveness mechanisms and CO2 corrosion prediction

A reliable corrosion prediction requires to simultaneously understand when high or low corrosion rates were encountered (field survey), how they can occur (corrosion layer protectiveness) and why they did so (link between field data and protectiveness mechanisms).
The concept of protectiveness is revisited in depth, and its leading parameters are emphasized : in-situ pH, potential corrosivity, iron solubility limit as an added corrosion product, Fe_s, and the corresponding saturation pH.
For sweet wells, the previous prediction tool (NACE paper 89466) of a low, medium or high risk is upgraded into a firm prediction of low corrosion for the two former cases, and high for the latter, and the former free HAc threshold is moved up from 0.1 to 1 mM. The implicit clauses related to sweet wells are also emphasized, so that the present tool can be extended to lines with similar leading parameters.
In summary, dissolved CO₂ is basically non-corrosive, because its weak acidity also supplies a cathodic corrosion product, HCO₃^- , and "normally" Fe⁺⁺ insolubilization. The "exceptions" are when free HAc takes the lead (fully soluble iron acetate) or in condensed waters under high P_CO2 (far too high Fe_s), or at low temperature (far too low precipitation rates).