In-situ Anodic Polarization With Concurrent Imaging of Precursor Sites For Pitting In UNS S32305 Duplex Stainless Steels In Chloride And Sour Environments

Sunaba, T. (INPEX Corporation Material & Corrosion Group) | Tomoe, Y. (INPEX Corporation Material & Corrosion Group) | Mendez, C. (Det Norske Veritas (U.S.A.), Inc. Materials & Corrosion Technology Center) | Garfias-Mesias, L.F. (Det Norske Veritas (U.S.A.), Inc. Materials & Corrosion Technology Center)



A new methodology to study in-situ the pitting corrosion of stainless steels at high temperature and high pressure (HT/HP) was developed. This methodology allowed the real-time in-situ anodic polarization while determining the sites for pit initiation in UNS S32305 (22% Cr Duplex Stainless Steels), in chloride containing environments with additions of H2S and CO2 at high temperature and pressure. The methodology allowed measurements at two temperatures 40oC (and 582 psi) and 180oC (and 752 psi). This methodology allowed the in-situ imaging of metastable pitting as well as stable pitting under different conditions. Pitting corrosion developed in inclusions (5-10 micrometer or larger); which are mainly composed of intermetallic particles containing significant amounts of B, N, O, Mg, Al, S, Ca and C.


In-situ characterization of corrosion resistant materials (CRAs) offers a new window of opportunities to characterize and improve materials1, particularly when they need to be qualified to perform in highly corrosive environments. It is well established that MnS inclusions are the main cause of pitting in commercial stainless steels2-5. However, in highly alloyed materials, like in the case of corrosion resistant alloys (CRAs), the cause for pitting is a more complex phenomenon, mainly because of the presence of different precursors (not only MnS inclusions). In the present work, we show that by using a novel setup that include a mini-autoclave coupled to a potentiostat and additional hardware it is possible to study, in real time, the surface of steels at high temperature and high pressure inside the mini autoclave. Additionally, the same methodology allowed the observation of metastable pitting and precursor sites for stable pitting and the location where the first pit developed in the same alloy (UNS S32305) at 180oC (and 752 psi) in a chloride containing environment with H2S and CO2.