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 in coastal and marine constructions in order to prevent corrosion induced by chloride ions penetrating into the concrete. In these highly aggressive situations, the lean duplex stainless steels have been extensively employed due to their high mechanical and corrosion resistances. However, the influence of Mo addition on pitting corrosion resistance of these steels is not clearly understood in alkaline chloride conditions even if this element is widely associated to an increasing corrosion resistance in acidic and neutral environments. Therefore, understanding Mo mechanism on corrosion resistance in alkaline media is hence of major importance to the setting of optimized alloy composition. The present work aims to study the effect of Mo addition on pitting corrosion properties of lean duplex stainless steels in alkaline environments (concrete). The results are discussed with respect to the influence of Mo addition on pitting potential for two industrial duplex alloys (1.4362 and 1.4462) in several aggressive media mainly in synthetic, chlorinated and carbonated solution simulating concrete pore environments (pH10 solution with carbonates and chlorides ions). In order to establish the real role of Mo addition on lean duplex corrosion and passivation properties, the corrosion behaviors of two specific laboratory lean duplex alloys, which the only difference between then is the amount of Mo (0 and 3% wt. Mo), are also studied. Furthermore, scanning electronic microscopy (SEM) was used to verify which phase of duplex microstructure (ferritic or austenitic) is mostly susceptible to the pit nucleation in these corrosion conditions as the role of Mo is probably different to each simple phase.

Embrittlement of duplex stainless steels after aging below 500 °C is a well documented phenomenon, 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, like NiSiMo-rich G phase or Cu-ε, are also known to participate to embrittlement, making the composition dependence of embrittlement kinetics even more intricate. In this study, long term aging treatments, up to 2 years at intermediate temperature (250–400 °C), have been performed for various duplex stainless steels grades containing different alloying contents in Cr, Mo, Si, Ni and N. Impact toughness has been measured after each aging condition, and an apparent activation energy could be deduced for the various grades. This quantity is a good indicator of embrittlement resistance. Fine microstructural characterizations have been performed in the ferrite: composition before aging was determined 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 the basis of these results and of driving force calculations performed on Thermo-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 combine elevated mechanical properties (strength, fatigue...) and an optimal resistance to many types of corrosion like Stress Corrosion Cracking or Intergranular Sensitization. Given high chromium content of the standard and super-duplex steels along with molybdenum and nitrogen alloying additions, they also exhibit an excellent resistance to pitting and crevice corrosion in halide-containing media generally equivalent to austenitic and super-austenitic with similar Pitting Resistance Equivalent. Further aspects of the question are reviewed in introduction of the present paper including passive film and metallurgical features such as two-phase microstructure, presence of non metallic inclusions, precipitates or intermetallic phases. Then, this work addresses the problems of corrosion testing of duplex stainless steels. It is intended in particular to shed light on some points of practical importance regarding electrochemical characterization best practices. Thus, a reliable ranking of materials, including new lean grades, requires a relevant choice of corrosion criteria and polarization methods. The determination of both pitting potentials and critical pitting temperatures is achieved by using a combination of several electrochemical tests based on potentiodynamic, potentiostatic as well 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 are also 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 most important property, after corrosion resistance, is their machinability. Unfortunately, one of the most known way to improve the machinability of stainless steels is to increase their sulphur level, thus forming manganese sulphides in the stainless steels which induce better chip breaking and lubrication at the chip – cutting tool interface. So the question is: can the sulphur well-known way to improve machinability of stainless steels be used on Duplex Stainless Steels, without inducing hot rolling difficulties and too important a decrease of their other main properties, i.e. localised corrosion resistance and toughness? If not, are other ways possible?

For microorganisms, the preferred living state is adhering to surfaces that can lead to the development of a biofilm. In drinking water networks, a biofilm can affect the organoleptic quality of the water as well as whether it is safe to drink. This is due to the probable presence of pathogens inside the biofilm which may be spread in the water. To keep the water quality safe, sanitization processes are regularly carried out using oxidizing biocides which can alter the physico-chemical properties of the material. Due to their surface physico-chemical properties and their high corrosion resistance, stainless steels seem to be good candidates for the making of drinking water systems. In this 2 year study, stainless steel grades were compared with materials commonly used, in respect of fouling, 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 were performed in a pilot specially designed to assess the material biocontamination on a natural drinking water loop for more than 2 years. The results confirmed the significant impact of the material on the biofilm development in terms of formation kinetic and ecology. No differences were observed between stainless steel grades whereas they displayed an intermediate behaviour compared to other materials. Corrosion attacks were only observed on galvanised carbon steel while on stainless steels, negligible release rates, much below international health requirements were measured. So stainless steels offer a good compromise to make drinking water pipes.