Experimental foundation has been established for improving corrosion models for stainless steels exposed to aqueous chloride media with low levels of dissolved oxygen. Accurate repassivation potentials were measured for UNS S31603 and UNS S32305 alloys in chloride electrolytes at room and elevated temperatures using Tsujikawa-Hisamatsu Electrochemical (THE) and Cyclic Potentiodynamic Polarization (CPP) methods. It was observed that the repassivation potential values measured from CPP methods were lower than the ones obtained from THE method for the same environmental conditions. Cyclic polarization scans in aqueous chloride media were conducted at different reverse scan rates resulting in varied repassivation potentials. Medium-term corrosion potentials were measured in chloride media with controlled oxygen levels ranging from 20 – 400 ppb. These results were used as inputs for improving model prediction for repassivation and corrosion potentials. With a flexible definition of the repassivation current density, the previously developed mechanistic repassivation model reproduces the experimental repassivation potentials obtained using THE and CPP methods. Dependence of corrosion potential on dissolved oxygen has been modeled using a mixed-potential model. It has been demonstrated that the effect of agitation (caused by bubbling of oxygen) needs to be accounted for to describe the corrosion potential, especially at low dissolved oxygen where mass transport is important.
In Upstream, CRAs (Corrosion Resistant Alloys) are widely selected to handle seawater and brines in piping, valves, pumps, heat exchangers, vessels, and seawater injection1-4. Also, disposal of produced water is commonly performed through injection into spent fields. Water from a variety of sources including produced water, seawater and surface/fresh water may also be injected to create pressure drive for existing fields. Usually dissolved oxygen (DO) is not fully controlled when there are multiple sources of injection water and sometimes even possibility of injection of fully oxygenated water exists. For oxygenated seawater, the PREN (Pitting Resistance Equivalent Number = %Cr + 3.3 *(%Mo + 0.5 %W) + 16 %N) shall be >40 and limits are applied to the temperature4. Other applications involve Solid CRA or cladded production pipelines which may get flooded with seawater during installation and pre-commissioning. For these cases a preservation time limit of 5 days is usually applied. Ideally, the susceptibility to localized corrosion should reliably be predicted for a CRA exposed to any combination of beforementioned environmental parameters. This would be beneficial to materials selection to the design phase as well as for risk assessment after unforeseen circumstances.
