Time-dependent in situ measurement of atmospheric corrosion rates of duplex stainless steel wires

Cem Örnek,Samuel A. McDonald,Dirk L. Engelberg,Anuj Prajapati,Fabien Léonard,Philip J. Withers

doi:10.1038/s41529-018-0030-9

Abstract

Corrosion rates of strained grade UNS S32202 (2202) and UNS S32205 (2205) duplex stainless steel wires have been measured, in situ, using time-lapse X-ray computed tomography. Exposures to chloride-containing (MgCl2) atmospheric environments at 50 °C (12–15 M Cl− and pH ~5) with different mechanical elastic and elastic/plastic loads were carried out over a period of 21 months. The corrosion rates for grade 2202 increased over time, showing selective dissolution with shallow corrosion sites, coalescing along the surface of the wire. Corrosion rates of grade 2205 decreased over time, showing both selective and pitting corrosion with more localised attack, growing preferentially in depth. The nucleation of stress corrosion cracking was observed in both wires.

Highlights

A better understanding of corrosion kinetics with exposure to atmospheric conditions has vital importance for assessing the application of duplex stainless steels (DSS) for structural components.[1,2,3,4,5,6] The UK’s intermediate-level radioactive nuclear waste (ILW) is currently stored in grade AISI 316L/304L austenitic stainless steel containers in above-ground storage facilities, until a deep-underground geological disposal facility becomes available.[5]
Without the presence of strong oxidising agents, the ferritic phase is typically selectively dissolved in grade 2205 DSS with exposure to chloride-containing environments, due to its lower corrosion potential.[33,35,36,41,42,46,47]
Similar observations of selective attack on the ferrite phase have been reported on lean DSS 2101 and 2202.2,48,49 In lean DSS 2202, the measured corrosion depthto-area ratio was low, indicating a strong tendency for repassivation of active corrosion sites

Summary

INTRODUCTION

A better understanding of corrosion kinetics with exposure to atmospheric conditions has vital importance for assessing the application of duplex stainless steels (DSS) for structural components.[1,2,3,4,5,6] The UK’s intermediate-level radioactive nuclear waste (ILW) is currently stored in grade AISI 316L/304L austenitic stainless steel containers in above-ground storage facilities, until a deep-underground geological disposal facility becomes available.[5]. Corrosion data obtained from ex situ measurements may not reflect in-operando conditions, and in situ, corrosion assessments are needed This shortcoming can be coped with the X-ray CT approach, being useful in measuring, in situ, the atmospheric corrosion behaviour and visualising the corrosion progress in 3D space. The atmospheric corrosion performance of DSS has been studied as a function of strain, stress, and deposition density, and so far, a good understanding of the relationship of ferrite and austenite in DSS microstructures has been established.[28,29,30,31,32,33,34,35,36,37] CRs and SCC growth rates, in particular under atmospheric exposure conditions, have not been reported yet. The wires were contaminated with MgCl2 aqueous solution and exposed to 50 °C and 30% relative humidity (RH), and timedependent, measurements of CRs obtained using time-lapse, in situ X-ray CT observations

RESULTS

DISCUSSION

Findings

METHODS