Pitting Resistance Equivalent Number Research Articles

The relationship between austenite stabiliser element and post-weld heat treatment on the corrosion behaviour of the welded duplex stainless steel made by gas tungsten arc welding

The corrosion behaviour and the influence of nickel and post-weld heat treatment (PWHT) on the susceptibility to corrosion in welded duplex stainless steels were investigated. After PWHT, the amount of nickel for the weld region increases resulting in the generation of austenite and consequently, helping to obtain a favourable ferrite-to-austenite ratio. Comparing the experimental outcomes, it is noticed that the pits initiate and propagate in HAZ indicating this area is more susceptible to corrosion resistance compared to the fusion zone. The pitting resistance equivalent number alone is not sufficient reason to study the complex corrosion phenomenon, and the influence of other parameters namely microstructural evolution on the breakdown of passivity by pitting and the existence of intermetallic phases should be determined together.

Comparative statistical analysis of pitting in Two 2205 duplex stainless steel variants

The inherent variability of pitting poses challenges in accurately evaluating the pitting resistance due to potential disparities in test results. This study compares the pitting resistance of two commercial 2205 duplex stainless steels. Counterintuitively, the variant with higher Pitting Resistance Equivalent Number, exhibits a lower Critical Pitting Temperature. Through the potentiostatic pulse test and potentiostatic polarization, this variant is observed to have a greater number of pitting initiation sites. Further investigation, using SEM inclusion statistics, reveals an increased presence of inclusions rich in calcium oxides as the underlying cause of this unexpected phenomenon.

Novel high nitrogen austenitic stainless steels: From high-throughput screening to experimental validation and properties relationship

The quest for affordable and price-stable alternatives for nickel as the main austenitizing alloying element is pushing the stainless steel industries to develop new types of Ni-lean austenitic stainless steels. This work focuses on the challenges of designing austenitic stainless steels with appreciable nitrogen content using computational methods, which can still be produced at 1 atm N2 pressure and thermomechanically processed without excessive cracking or N2 degassing. CALPHAD-based computational tools enabled the identification of promising alloys by high-throughput screening from more than 2000 candidates based on predefined criteria considering composition (Ni-poor, N-rich), phase stability (austenite), degassing (far from N2 loss from liquid or solid phase), cost (Ni-lean), and pitting resistance equivalent number (PREN > AISI 201 stainless steel). The resulting alloys exhibited an austenitic microstructure with dispersed micrometric and nanometric aluminum nitrides (AlN) precipitates. Even with the occurrence of appreciable AIN precipitates, hot rolling was performed without cracking, resulting in equiaxed austenitic grains. The mechanical properties of the developed alloys exceeded those of AISI 201 stainless steel in terms of hardness, yield strength, and ultimate tensile strength, with values of 304 HV, 534 MPa, and 953 MPa, respectively, yet with considerable ductility (∼40% elongation). Even in the presence of AlN that pick up some of the nitrogen from solid solution, the corrosion resistance of the developed alloy was superior to the AISI 201 stainless steel. This research provides valuable insights for developing and processing high-nitrogen austenitic stainless steels to compete with 201 and 304 stainless steels in terms of mechanical properties and corrosion resistance.

A new index to estimate the corrosion resistance of aluminium‐containing steel

AbstractA new index that estimates the corrosion resistance of aluminium‐containing steel is established and verified. It is based on the well‐known pitting resistance equivalent number (PREN) but accounts for aluminium additions. It is shown that aluminium enhances the corrosion resistance of chromium‐containing steel, with a coefficient of 5.2. The new corrosion resistance index (CRI) is as follows CRI = Cr + 3.3Mo + 16N + 5.2Al. A minimum of 4 wt% of chromium addition is needed. In addition, the atomic ratio of chromium to aluminium addition needs to be higher than 1, for this new index to be valid. This new index allows for simple alloy design guidelines for steel applications that require improved corrosion resistance. Analysis of the data from the literature shows that this index becomes inaccurate when the corrosion environment is in the presence of NaCl, and aluminium becomes less effective in enhancing corrosion resistance.

Manufacturing process encoding through natural language processing for prediction of material properties

Knowledge of manufacturing processes is crucial to determine the final properties of a material, thus this work focuses on analyzing the relationship between final properties, chemical composition, and manufacturing process through data analysis. Furthermore, techniques of natural language processing are used to encode the manufacturing process as input in the neural network. The work consisted of two main parts: firstly, the relevant data was gathered, cleaned-up, and analyzed using statistical and probabilistic methods, K-means, and Principal Components Analysis (PCA), and secondly, a model was developed to predict elongation, yield, and tensile strength. Fully Connected Neural Network (FCNN) algorithms were used to build the aforementioned model. In addition, in order to avoid overfitting and evaluate the model dropout function and K-fold cross-validation are incorporated. Results demonstrated reasonable accuracy in elongation, yield, and tensile strength. Two cases of mechanical properties prediction of stainless steel alloy were presented, first, an existing alloy that was not in the training and test set, and second a suggestion of a new stainless steel alloy, which combines good YS, UTS, and Pitting resistance equivalent number (PREN). Additionally, it was considered an example from the TRIP family to showcase the tool’s versatility across various steel types.

Development of Thermodynamic Criteria for Determining the Composition of Duplex Stainless Steels with High Corrosion Resistance.

One of the most popular methods for ranking duplex stainless steels (DSSs) and predicting their corrosion properties is the calculation of the pitting resistance equivalent number (PREN). However, since DSSs are two-phase materials with a significant fraction of secondary phases and precipitates, the application of the PREN can be highly limited. This article attempted to use a new approach to describe the corrosion resistance of these steels. The corrosion resistance of two DSSs of the same class was investigated. Under identical solution heat treatments in the temperature range of 1050-1200 °C, the crevice corrosion resistance of one steel increased, while that of the other decreased. It was demonstrated that the amounts of austenite and ferrite changed similarly in these steels, and the different corrosion resistances were associated with the behaviors of secondary phases: niobium carbonitride and chromium nitride. SEM-EDS analysis was conducted to analyze the redistribution of elements between phases in both cases, showing good agreement with the thermodynamic modeling results. The PREN was calculated for each phase depending on the treatment temperature, and a method for calculating the effective PREN (PRENeff), accounting for phase balance and secondary phases, was proposed. It was shown that this indicator described corrosion properties better than the classical PREN calculated for the average steel composition. This study demonstrated how the calculation of critical temperatures (the temperature of equal amounts of ferrite and austenite, the temperature of the beginning of chromium nitride formation, and the temperature of the beginning of σ-phase formation) could describe the corrosion resistance of DSSs. Maximum possible deviations from these temperatures were defined, allowing the attainment of the required corrosion properties for the steels. Based on the conducted research, an approach for selecting new compositions of DSSs was proposed.

The effect of surface roughness on re-passivation and pitting corrosion of super duplex stainless steel UNS S 32760

Stainless steel is used as a battery case material because of its excellent strength and corrosion resistance. Among the stainless steels used in fields requiring superior corrosion resistance, duplex stainless steels, composed of austenite and ferrite, exhibit improved strength and corrosion resistance. In particular, super duplex stainless steel (SDSS), with a pitting resistance equivalent number (PREN) between 40 and 50, offers excellent strength and corrosion resistance, making it suitable for structural components in various applications. However, the actual corrosion resistance of a material is often lower than its theoretical value, which has led to extensive research efforts to enhance practical corrosion resistance. Recent studies reported atomic-scale measurements of corrosion resistance and identified the influence of atomic layers on corrosion. Corrosion properties were studied by Trinh et al. (2020) and Nilsson (1992) [26]; however, they did not examine the re-passivation of the SDSS. SDSS requires re-passivation because of real corrosion. Although structural stainless steels are processed to achieve surface roughness in the micrometer range and undergo passivation, research on this aspect is still lacking. To enhance the performance of SDSS, research on re-passivation before and after pitting is required. Therefore, this study analyzed the effect of surface roughness variation from 100 to 1000 nm on the corrosion resistance and re-passivation with or without pitting of SDSS. The surface conditions corresponding to different roughness levels were analyzed using field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM), while corrosion resistance was evaluated through open circuit potential (OCP) measurements, potentiodynamic polarization tests, and cyclic polarization tests at 0.6 V (before broken passivation layer) and 1.3 V (after broken passivation layer), as well as critical pitting temperature (CPT) tests. Surface roughness reduces the corrosion potential (Ecorr) and increases the corrosion current density (Icorr) for activation polarization. With passivation, as the surface roughness increased, uniform corrosion decreased, resulting in a stronger passive layer. The surface roughness lowered the OCP and increased the corrosion rate, thereby reducing the corrosion resistance. Therefore, when SDSS is used in an environment prone to corrosion, a surface roughness of less than 100 nm exhibits superior corrosion resistance.

Machine learning-based prediction of pitting corrosion resistance in stainless steels exposed to chloride environments

Aiming at the limitation of the pitting resistance equivalent number theory in qualitative evaluation of pitting resistance, a new strategy based on extreme gradient regression (XGBR) is used to predict the Epit of stainless steel (SS). The goodness of fit (R2) and root mean square error (RMSE) of the model on the test set were 0.877 and 132.51 mV, respectively. In addition, the generalization ability of the model is accepted on a validation set. This innovative strategy provides valuable insights to accurately predict the Epit of stainless steel, which is helpful to design new SS with excellent pitting corrosion resistance.

Using kinetics to avoid sigma phase formation on hyper duplex stainless weld cladding

The Hyper Duplex Stainless Steel HDSS enhanced corrosion resistance and toughness relies upon high alloying to obtain a balanced ferrite and austenite volume and pitting resistance equivalent number PREn. However, during welding, sigma phase precipitates might form, hindering corrosion and mechanical performance. Therefore, a kinetics model is developed to avoid the sigma phase’s formation during welding and validated using physical simulation, finite element analysis (FEA), welding, and SEM characterisation. The sigma phase kinetics model produced calculated and validated temperature-time-transformation (TTT) and continuous-cooling-transformation (CCT) curves from which a 4°C/s cooling rate was found as a cooling rate threshold for sigma phase formation in this new material. Three-layered gas tungsten arc welding GTAW cladded mockup with 53 beads produced 24°C/s minimum cooling rate. Moreover, microscopy, mechanical, and corrosion testing attested it as a sigma-free weld.

Optimizing annealing temperature for Duplex Stainless Steel 2205 in acidic NaCl environments according to corrosion resistance

In this article, we reveal that the optimal annealing temperature for DSS 2205 differs between acidic and neutral NaCl environments, rendering the limitation of utilizing pitting resistance equivalent number (PREN) to optimize annealing temperature. Elevating the annealing temperature leads to a better corrosion resistance of ferrite phase in the activation zone through Ni redistribution. In acidic NaCl systems, the optimal annealing temperature for DSS 2205 is co-governed by the Ni content and PREN. This conclusion offers a guidance for heat treatment process of DSS to be used in acidic environments.

Mechanical properties and biocompatibility of various cobalt chromium dental alloys

Cobalt-based metal alloys are often used in dentistry for prosthetic restorations because of their considerable wear and corrosion resistance. For this reason, the effect on the human body of three Co–Cr based alloys, named "Co–Cr", "C" and "L", differing in their concentrations, for subsequent use as dental materials, has been studied and compared. Metallographic observations of the microstructure, electrochemical and three-point bending tests were performed. The metallographic investigations showed similar dendritic microstructure of the Co–Cr and L dental alloys, as well similar porosities and defects. From the corrosion and pitting potential tests, it was found that the samples show surface passivation, reaching closed corrosion potential values from −0.280 V vs SCE to −0.250 V vs SCE. The Pitting Resistance Equivalent Number (PREN) values are above 38 for all the samples, which means that the studied alloys have a high resistance to pitting corrosion. Likewise, when applying the Electrochemical Impedance Spectroscopy technique, a slightly higher corrosion resistance was observed for sample L, since corrosion resistance tends to increase as the applied potential is more positive and as the impedance and phase angle values are higher. Moreover, in the three-point bending test, the alloy C presents the lowest values of the elastic modulus of 134 ± 13 GPa. The analyzed Co–Cr alloys are recommended for the efficient treatment of patients with dental prostheses that have metal frameworks, as shown by all of the obtained results.

Effect of alloying element content on anaerobic microbiologically influenced corrosion sensitivity of stainless steels in enriched artificial seawater

Stainless steels (SS) are not immune to microbiologically influenced corrosion (MIC) especially in the presence of sulfate reducing bacteria (SRB). It is necessary to study the influence of alloying elements on the MIC. SRB MIC behaviors of four stainless steels (2205 SS, 316L SS, 304 SS, and 410 SS), with different alloying element compositions were compared after 14 days of incubation at 37°C in enriched artificial seawater inoculated with Desulfovibrio sp. The sessile cell sequence was 410 SS > 316L SS > 304 SS > 2205 SS, inversely proportional to Cr content. The uniform corrosion rate (based on weight loss) sequence was 410 SS > 304 SS > 316L SS > 2205 SS, which matches the pitting resistance equivalent number (PREN) sequence inversely. 410 SS with the lowest Cr and Mo contents suffered the most severe pitting, with pit depth of 35 μm and weight loss of 0.75 mg/cm2 (0.91 mm/a pitting rate and 25 μm/a uniform corrosion rate). The other three stainless steels with higher Cr and Mo contents suffered only metastable pits. The semiconductor characteristics and the re-passivation abilities of the passive films were found to be affected by Cr and Mo contents.

Corrosion Resistance of Dissimilar GTA Welds for Offshore Applications

The pitting corrosion resistance of dissimilar pipeline steel API X70 and Super Duplex 2507 Stainless Steel gas tungsten arc (GTA) welds was studied. The GTA welds were fabricated using Super Duplex 2594 Filler Metal or austenitic 309L Filler Metal. The specimens extracted from the base metal, weld fusion zone, fusion boundary, and heat-affected zones (HAZs) were subjected to a potentiodynamic corrosion test in a 3.5% NaCl water solution. The weld made using the 309L Filler Metal was found to have a smaller pitting resistance equivalent number (PREN) difference between ferrite and austenite, making it more corrosion resistant than the weld fabricated using the 2594 Filler Metal. The HAZ on the X70 side was the most susceptible to pitting and galvanic corrosion for both filler metals.

The roles of Al addition and heat treatment temperature on chloride corrosion of 9Cr alloy steel

The effects of 0.7% Al and heat treatment temperatures on chloride corrosion of 9Cr alloy steel were investigated through potentiodynamic polarization measurement and salt spray test. Al promoted the formation of Al2O3-rich inclusions and AlN, thus reducing the pitting potential at initial stage. However, Al addition reduced weight loss after longer time by optimizing the α/γ * value, compactness and element distribution of rust layer. The mechanism of corrosion resistance changing with heat treatment temperature was understood from the decrease in the pitting resistance equivalent number caused by (Fe, Cr)23C6 precipitation and the microgalvanic corrosion between fresh martensite and tempered martensite/ferrite.

Ferritic-induced high-alloyed stainless steel produced by laser powder bed fusion (L-PBF) of 2205 duplex stainless steel: Role of microstructure, corrosion, and wear resistance

Stainless steels are dominant in application where combination of corrosion and mechanical resistances are requested. They have gained scientific attention in additive manufacturing (AM) field, and stainless steel products produced by AM find use in a wide array of applications. This study aims to evaluate the microstructure, the wear and corrosion resistances of a high-alloyed ferritic-induced stainless steel (FSS) obtained by laser powder bed fusion (L-PBF), from a duplex stainless steel (DSS) precursor, SAF 2205, used to produce gas-atomized powders. No heat treatment was applied to the ferritic-induced L-PBF stainless steel to restore the duplex structure. The L-PBF FSS was dense and high alloyed with Cr, Mo, and N (pitting resistance equivalent number, PREn, ~33), presenting a negligible corrosion current density in the order of 10−7 A.cm−2, with an enlarged passivation window ~ 1.2 V with respect to the corrosion potential, restraining the current density at the passivation plateau below 10−4 A.cm−2. Also, double-loop electrochemical potentiokinetic reactivation (DL-EPR) tests indicated unsensitized L-PBF FSS, with negligible degree of sensitization caused by the cyclic heat input during building up of the specimen. Besides be corrosion-resistant, the L-PBF FSS was hard (290 HV0.5) and wear-resistant against sliding wear, with a reduced specific wear rate of 5 × 10−4 mm3·N−1·m−1. The corrosion resistance of the L-PBF specimen (almost fully ferritic microstructure) was comparable to its hot-rolled precursor that displayed a duplex microstructure, albeit slight inferior. The L-PBF FSS was harder and more wear resistant than the DSS precursor. These results point out to the formation of a new high-alloyed ferritic-induced stainless steel, which are corrosion-resistant in chloride-rich medium and wear resistant in sliding conditions.

The Isolated Austenite Forming during High-Temperature Cooling and Its Influence on Pitting Corrosion Resistance in S32750 Duplex Stainless Steel

The formation mode of isolated austenite (IA) formed during high-temperature cooling and its effect on pitting corrosion resistance of S32750 DSS were investigated. As the structure formed during the high-temperature cooling process after hot rolling in S32750 duplex stainless steel (DSS), the precipitation of isolated austenite (IA) can be considered as the combined production of temperature and cooling rates. IA generated at the ferrite (α) grain boundaries or internal α and performed the K-S crystallographic relation with the α N element enriched in IA. The redistribution of elements not only increased the concentration gradient between α/IA, but also reduced the pitting resistance equivalent number (PREN) of original austenite (γ). Scanning Kelvin probe (SKP) and Gaussian fitting proved that with prolonged corrosion time, the potential difference enlarged and local corrosion aggravated. Measured by the weight loss method, pitting resistance decreased to one-third of the solid solution state due to the existence of IA; following, the quantities, sizes, and depth of corrosion pits increased significantly. In addition to α/γ boundaries and α/α boundaries, corrosion pits preferred to initiate around IA. The IA that appeared at α grain boundaries deteriorated the pitting corrosion resistance severely. Furthermore, original austenite with decreased PREN tended to be corroded. In general, the order of pitting occurrence is displayed as follows: α-γ-IA.

Stainless Steels as Sustainable Solution for Concrete Reinforcement - From Laboratory to Practice

Stainless steel reinforcing bars show excellent corrosion resistance in concrete exposed to harsh environments. In this combined electrochemical and surface analytical work, an explanation for this behavior is proposed. XPS surface analytical results (thickness, composition of the passive film and of the interface beneath the film) obtained on black steel, FeCr alloys, and a series of stainless steels after exposure to alkaline solutions simulating concrete are reported. Pitting potentials were determined in the same solutions with electrochemical experiments. It is shown that the pitting potentials of the steels can be related to the Cr (III) oxy-hydroxide and Mo (VI) content in the passive film. It is proposed to calculate a Cr and Mo oxide equivalent similar to the well-known pitting resistance equivalent number (PREN). A correlation between the critical chloride content in concrete (reported in literature for CEM II A/LL and CEM I) and the pitting potential for carbon steel, Fe12%Cr alloy, DIN 1.4301 and DIN 1.4571 stainless steels is proposed to link results of solution analysis and performance in concrete.

Unexpected failure of cast superduplex stainless steel exposed to high chlorides containing water: From failure analysis to corrosion mechanisms settlement

This work deals with the failure analysis of a super duplex valve operating in technical waters with high chlorides content up to 80 °C. A careful fracture surfaces and microstructural characterization was combined with corrosion studies (SKPFM (Scanning Kelvin Probe Force Measurements) and polarization curves) of different valve regions in order to determine the failure mechanism.It was found that the cast material presents a segregated microstructure containing ferritic phases with different PREN (Pitting Resistance Equivalent Number). This strongly affects the corrosion properties of the material, enhancing the susceptibility to localized corrosion and leading to SCC (Stress Corrosion Cracking) failure.

A duplex stainless steel (DSS) with striking tensile strength and corrosion resistance produced through wire arc-additive manufacturing (WAAM) using a newly developed flux-cored wire

The austenite ratio in the WAAM DSSs using commercial welding wire is generally higher than the acceptable range. In this work, a new flux-cored wire was especially developed for the WAAM of DSS. The chromium equivalent (Creq) and pitting resistance equivalent number (PREN) of the newly developed flux-cored wire arc additively manufactured duplex stainless steel (FCWA-AM DSS) were significantly high. Its austenite ratio was successfully modified to the acceptable range and massive scattered intragranular austenite was found in the columnar ferrite grains. Such chemical composition and microstructure promised it with good strength and corrosion resistance. Its tensile strength was far beyond the requirements of the rolled and cast DSSs and its corrosion resistance was comparable to the hot-rolled 2205 DSS.

Failure Analysis of Austenitic Stainless Steel Implant Screws and Prospection of Chemical Composition Using Artificial Intelligence

In this work, austenitic stainless steel screws employed in a locking compression plate for veterinarian use were investigated. These types of implants are widely utilized in bone fractures healing. Two surgical screws were extracted due to the observation of slight superficial red rust colorizing on one of the screw implants, visual evidence of probable screw rusting. From the same implant, another screw was extracted simultaneously without visual evidence of rusting. In order to characterize and analyze the different behavior of both screws, the chemical composition was characterized by atomic absorption and energy dispersive X-ray spectroscopy (EDS) coupled to a scanning electron microscope (SEM). Also, the screws were studied by metallography, optical microscopy (OM), Vickers microhardness tests, and SEM analysis. On the other hand, a prospection for alloy chemical composition limits of these types of implants was performed based on the Schaeffler-Delong diagram and the ASTM F-138 standard. To analyze the effect of the chemical composition, heat treatment, microstructure, pitting resistance equivalent number (PRE) and stacking fault energy (SFE), a genetic algorithm (GA) and an artificial neural network (ANN) were used. In accordance with the elemental analysis, the surgical screws do not fulfill the ranges of the chemical composition established by the ASTM F-138 standard. Furthermore, there were found differences between the microstructures of the screws. In regard to the prospection, the results of GA and ANN support the proposed chemical composition region on the Schaeffler-Delong diagram. The corrosion failure was associated with severe plastic deformation and the presence of precipitates. The proposal can minimize the cause of failures in these types of austenitic stainless steel implants.