The use of stainless steels as concrete reinforcement is becoming increasingly popular incoastal and marine constructions in order to prevent corrosion induced by chloride ionspenetrating into the concrete. In these highly aggressive situations, the lean duplexstainless steels have been extensively employed due to their high mechanical and corrosionresistances. However, the influence of Mo addition on pitting corrosion resistance ofthese steels is not clearly understood in alkaline chloride conditions even if thiselement is widely associated to an increasing corrosion resistance in acidic and neutralenvironments. Therefore, understanding Mo mechanism on corrosion resistance in alkalinemedia is hence of major importance to the setting of optimized alloy composition. Thepresent work aims to study the effect of Mo addition on pitting corrosion properties oflean duplex stainless steels in alkaline environments (concrete). The results arediscussed with respect to the influence of Mo addition on pitting potential for twoindustrial duplex alloys (1.4362 and 1.4462) in several aggressive media mainly insynthetic, chlorinated and carbonated solution simulating concrete pore environments (pH10solution with carbonates and chlorides ions). In order to establish the real role of Moaddition on lean duplex corrosion and passivation properties, the corrosion behaviors oftwo specific laboratory lean duplex alloys, which the only difference between then is theamount of Mo (0 and 3% wt. Mo), are also studied. Furthermore, scanning electronicmicroscopy (SEM) was used to verify which phase of duplex microstructure (ferritic oraustenitic) is mostly susceptible to the pit nucleation in these corrosion conditions asthe role of Mo is probably different to each simple phase.

Increasing interest in duplex stainless steels is due to their ability to combineelevated mechanical properties (strength, fatigue...) and an optimal resistance to manytypes of corrosion like Stress Corrosion Cracking or Intergranular Sensitization. Givenhigh chromium content of the standard and super-duplex steels along with molybdenum andnitrogen alloying additions, they also exhibit an excellent resistance to pitting andcrevice corrosion in halide-containing media generally equivalent to austenitic andsuper-austenitic with similar Pitting Resistance Equivalent. Further aspects of thequestion are reviewed in introduction of the present paper including passive film andmetallurgical features such as two-phase microstructure, presence of non metallicinclusions, precipitates or intermetallic phases. Then, this work addresses the problemsof corrosion testing of duplex stainless steels. It is intended in particular to shedlight on some points of practical importance regarding electrochemical characterizationbest practices. Thus, a reliable ranking of materials, including new lean grades, requiresa relevant choice of corrosion criteria and polarization methods. The determination ofboth pitting potentials and critical pitting temperatures is achieved by using acombination of several electrochemical tests based on potentiodynamic, potentiostatic aswell as galvanostatic experiments together with a statistical analysis when needed.

Embrittlement of duplex stainless steels after aging below 500 °C is a well documentedphenomenon, usually attributed to the unmixing of the ferritic Fe-Cr solid solution,either by spinodal decomposition or by nucleation and growth of Cr-rich α′ phase,depending on aging temperature. Additional precipitation taking place during aging, likeNiSiMo-rich G phase or Cu-ε, are also known to participate to embrittlement, making thecomposition dependence of embrittlement kinetics even more intricate. In this study, longterm aging treatments, up to 2 years at intermediate temperature (250–400 °C), have beenperformed for various duplex stainless steels grades containing different alloyingcontents in Cr, Mo, Si, Ni and N. Impact toughness has been measured after each agingcondition, and an apparent activation energy could be deduced for the various grades. Thisquantity is a good indicator of embrittlement resistance. Fine microstructuralcharacterizations have been performed in the ferrite: composition before aging wasdetermined by EPMA, and precipitation state before and after aging was observed on TEM.The influence of alloying elements on the embrittlement phenomenon is analyzed on thebasis of these results and of driving force calculations performed onThermo-Calc®.