Stainless steels are often used instead of painted carbon steels or weathering steels in architecture or construction for aesthetic reasons and/or when maintenance cost reduction is considered. With duplex stainless steels the investment cost can also be significantly reduced using their better yield and tensile stresses. Then the choice of material depends on the environment corrosivity. Usual accelerated corrosion tests as salt spray type may help to rank stainless steels but not to select a cost effective material for an outdoor location. Considering the lack of available data, Industeel launched five years ago an extensive field exposure program with the aim to collect representative data. Six locations were chosen in North America and Europe to cover a wide range of humidity, temperature, pH, chloride ions and pollutant contents. In this way more than 200 coupons with and without weld were exposed in marine, industrial, urban and rural atmospheres or combinations of them. This paper shows some of the first results obtained on duplex (lean duplex included), austenitic, martensitic and ferrito-martensitic stainless steel specimens after several years of exposure. Correlation between environmental parameters, alloy chemical composition, microstructure and localized corrosion resistance are discussed.

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.

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®.

Crevice corrosion chemistry is modeled in galvanostatic mode using finite element method calculations. The results lead to the formulation of critical conditions for crevice propagation in terms of applied current and depassivation pHd. For a given stainless steel grade, the model predicts a linear relation IC(pHd) between a “critical propagation current” IC and the depassivation pHd. Detailed computing is performed to compare the behavior of duplex 2304, ferritic AISI 430 and austenitic AISI 304 stainless steels. The model is shown to capture correctly the major asymptotic behaviors and to fairly predict the better corrosion resistance of the duplex grade, which is attributed to its slower dissolution kinetics in acidic environment. When comparing duplex stainless steels to ferritic or austenitic ones, it is now evident that the dissolution law and the depassivation pHd are the key factors that control the resistance to crevice corrosion propagation.

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.

Duplex stainless steels are known to have a good resistance to localised corrosion,especially thanks to their quiet high level of chromium (and for some of them molybdenum),but also because of their very low level of sulphur. These very low levels of sulphur arealso useful to guaranty a good toughness and a good hot workability for these grades.Concerning long products like bars made in Duplex Stainless steels, one of their mostimportant property, after corrosion resistance, is their machinability. Unfortunately, oneof the most known way to improve the machinability of stainless steels is to increasetheir sulphur level, thus forming manganese sulphides in the stainless steels which inducebetter chip breaking and lubrication at the chip – cutting tool interface. So the questionis: can the sulphur well-known way to improve machinability of stainless steels be used onDuplex Stainless Steels, without inducing hot rolling difficulties and too important adecrease of their other main properties, i.e. localised corrosion resistance andtoughness? If not, are other ways possible?

For microorganisms, the preferred living state is adhering to surfaces that can lead tothe development of a biofilm. In drinking water networks, a biofilm can affect theorganoleptic quality of the water as well as whether it is safe to drink. This is due tothe probable presence of pathogens inside the biofilm which may be spread in the water. Tokeep the water quality safe, sanitization processes are regularly carried out usingoxidizing biocides which can alter the physico-chemical properties of the material. Due totheir surface physico-chemical properties and their high corrosion resistance, stainlesssteels seem to be good candidates for the making of drinking water systems. In this 2 yearstudy, stainless steel grades were compared with materials commonly used, in respect offouling, biofilm development and composition. The tested stainless steels include ferritic(444/EN 1.4521), three austenitic (304L/EN 1.4307, 316L/EN 1.4404, and 316LN/EN 1.4429)and two duplex (2304 /EN 1.4362, and 2205/EN 1.4462) grades. The experiments wereperformed in a pilot specially designed to assess the material biocontamination on anatural drinking water loop for more than 2 years. The results confirmed the significantimpact of the material on the biofilm development in terms of formation kinetic andecology. No differences were observed between stainless steel grades whereas theydisplayed an intermediate behaviour compared to other materials. Corrosion attacks wereonly observed on galvanised carbon steel while on stainless steels, negligible releaserates, much below international health requirements were measured. So stainless steelsoffer a good compromise to make drinking water pipes.