Welding Heat Input Influence on UNS S82441 Lean Duplex Stainless Steel Corrosion Resistance Assessed by Scanning Vibrating Electrode Technique (SVET)

The precipitation of the intermetallic phases can impair mechanical and corrosion properties. In the case of lean duplex stainless steels, they present solidification mode as primary ferrite, with austenite precipitation in the solid state, during cooling. This means the heat-affected zone balanced microstructure can be changed during the welding by the cooling rate, which is associated to the welding heat input, the thickness of the plate and the physical properties of the material. This work intends to present the influence of welding heat input on the corrosion resistance of UNS S82441 lean duplex stainless steel. The gas metal arc welding process was used to make the weld beads, using ER2209 as filler metal, which characterizes a dissimilar welded joint. The corrosion resistance of these specimens was analyzed through scanning vibrating electrode technique. It was observed that welding heat input variation distinctly influences corrosion in different zones of the weld beads over the specimen exposure time. The weld bead under intermediate welding heat input showed the highest corrosion rate and the one under the highest welding heat input, the lowest corrosion rate intensity.     

In situ investigation of crevice corrosion on UNS S32101 duplex stainless steel in sodium chloride solution

The crevice corrosion of UNS S32101 in neutral 0.1 M NaCl solutions at room temperature was investigated directly by a facile method. Experimental results showed that both delayed and immediate crevice corrosion can be initiated. Morphology study indicated that the heaviest corrosion attack happened just below the passive/active boundary on the crevice wall. The relocation of the active dissolution regions during crevice corrosion was observed and explained by established theory. The mechanisms of the delayed and immediate types of crevice corrosion on UNS S32101 duplex stainless steel were discussed.     

Influence of the microstructure and alloying element on the polarization behaviour within the crevice of UNS S32304 duplex stainless steel

The influence of microstructure evolution and alloying element redistribution of UNS S32304 duplex stainless steel induced by annealing treatment on the crevice corrosion behaviour was studied. As the annealing temperature was increased from 1030 to 1150 °C, the crevice corrosion resistance was decreased and the active peaks were not only increased in magnitude but also shifted towards the more noble direction. Austenite and ferrite have greatly different polarization behaviour within the crevice. This study provides guidance to the material design and usage in industry field in consideration of different polarization behaviour induced by the evolution of microstructure and alloying elements.     

Effect of thermal cycles on the corrosion and mechanical properties of UNS S31803 duplex stainless steel

The microstructure and properties of UNS S31803 duplex stainless steel had been investigated following isothermally aging in a temperature range of 450–1000 °C for 10 min, which simulates a fairly long time relative to a weld thermal cycle. The results showed that specimens aged between 600 and 950 °C are subjected to precipitation of secondary phases, such as Cr₂N, σ and χ. And the most significant reduction in pitting corrosion resistance and impact energy occurs at 850 °C. Moreover, at aging temperatures below 600 °C, austenite is the preferred site for pitting initiation, which changes to precipitates modified ferrite phase at higher temperatures above 600 °C.     

Effects of Leptothrix discophora on the potential behavior of stainless steel

Biofilm formation and electrochemical behavior of stainless steel have been studied in well water, in tap water and in water taken from fire extinguishing system. Some of the waters have been modified by adding manganese ions as well as in some cases acetate and yeast extract as a nutrient. Manganese oxidizing bacteria Leptothrix discophora has been used in these studies. To eliminate crevices in the specimen holder and to simulate reasonably well the conditions in fire‐extinguishing systems where the water is stagnant most of the time, a tape test arrangement with very slow flowing rate has been used in most of these studies. Also specimens with heat tint areas produced by heating inductively a small section of the specimen under flowing argon shielding gas were used in these studies. The results of these tape tests show that manganese oxidizing bacteria are able to increase the potential of both the base material (stainless steel UNS S30400) and of specimens with heat tint areas in well water, in tap water and in water taken from fire extinguishing system. Manganese oxidizing bacteria are also able to increase the cathodic reaction and thus enhance the initiation of corrosion. However, the increase of potential is not alone enough to start the corrosion. In these tests corrosion was initiated only in the specimens with heat tint areas in one of the waters taken from fire extinguishing system. No corrosion was detected in the specimens exposed to the well water or to the tap water used in these tests.     

Electrochemical corrosion behavior and microstructural characteristics of electron beam welded UNS S32205 duplex stainless steel

Different electrochemical techniques were used to study the corrosion behavior of UNS S32205 duplex stainless steel (DSS) welded autogenously using a single-pass by electron beam welding process, supplemented by microstructural characterization. Furthermore, a comparative study was also performed between multipass gas tungsten arc (GTA)-welded and EB-welded DSS for their microstructure and corrosion behavior. The differences in weld thermal cycle and chemical composition influenced the fusion zone microstructure of both the welds and eventually their corrosion properties. The general corrosion resistance of the EB weld was lower than the base metal and higher than the GTA weld despite its weld zone being characterized by a relatively unbalanced phase ratio (α/γ) in comparison to the GTA weld. However, the EB weld showed relatively higher susceptibility to pitting corrosion than the base metal and GTA weld due to its poor repassivation characteristics and poor resistance to pit growth.     

Effects of solution heat-treatment and nitrogen in shielding gas on the resistance to pitting corrosion of hyper duplex stainless steel welds

The effects of solution heat-treatment and shielding gas on the pitting corrosion of hyper duplex stainless steel (HDSS) welds were investigated in highly concentrated chloride environments. The pitting resistance of a solution heat-treated HDSS after welding with an Ar shielding gas supplemented with N₂ was greatly increased due to the dissolution of Cr₂N in α-phase, which followed the diffusion of N atoms from the α-phase to the γ-phase and an increase of the γ-phase in the weld metal and heat affected zone. It was also attributed to a decrease of the pitting resistance equivalent number difference between the two phases.     

Corrosion behaviour of sensitized and unsensitized Alloy 900 (UNS 1.4462) in concentrated aqueous lithium bromide solutions at different temperatures

Duplex stainless steels can undergo microstructural changes if they are heated improperly. When that happens, duplex stainless steels are sensitized and intermetallic phases appear. The high Chromium and Molybdenum content promotes the formation of secondary phases as a consequence of the heat treatment. These secondary phases, which are rich in alloying elements, such as Cr and Mo, deplete these elements from the neighbouring phases, leading to a reduction in corrosion resistance. In order to study the influence of the secondary phases on the corrosion parameters, samples of duplex stainless steel, Alloy 900 (UNS 1.4462), have been heated in argon atmosphere at 825 °C for 1 h. The corrosion behaviour of sensitized and unsensitized Alloy 900 has been analyzed in a concentrated aqueous lithium bromide (LiBr) solution of 992 g/L by means of cyclic potentiodynamic curves. Secondary phase presence reduces the pitting potential value of Alloy 900. Besides, the pitting potential decreases with temperature. On the other hand, the corrosion potential and open circuit potential values increase with temperature and sensitization.     

Effect of laser surface melting on intergranular corrosion behaviour of aged austenitic and duplex stainless steels

In the present study, the effect of laser surface melting (LSM) on intergranular corrosion behaviour of aged austenitic stainless steels (UNS S30400, S31603, S32100 and S34700) and aged duplex stainless steels (UNS S31803 and S32950) were investigated. LSM of the aged stainless steels was carried out using a 2.5 kW CW Nd:YAG laser. The microstructure of the aged stainless steels after LSM depends on their compositions. After LSM, the aged austenitic stainless steels mainly contain austenite (γ) with some ferrite (δ) as the minor phase, but the carbide phases are completely eliminated. For the aged duplex stainless steels after LSM, δ becomes the major phase and the δ/γ phase balance is disturbed, whereas the sigma (σ) phase is eliminated. The degree of sensitization (DOS) and corrosion morphology of the aged stainless steels before and after LSM were determined by the double loop electrochemical potentiokinetic reactivation (DL-EPR) using a potentiostat and SEM observation, respectively. Desensitization of the aged stainless steels has been successfully achieved by LSM and their intergranular corrosion resistance is found to be significantly enhanced as reflected by the decrease in DOS due to dissolution of the carbides or σ phase, which reduced Cr depletion or the possibility of solute segregation at the grain or phase boundaries, despite the presence of δ and disturbance of δ/γ phase balance.     

Role of microstructure on corrosion of duplex stainless steel in presence of bacterial activity

This paper presents the results on microbiologically influenced corrosion (MIC) of UNS S31803 duplex stainless steel (DSS) in as-received, thermally aged and welded conditions in 3.5% NaCl-based medium containing bacteria. Crevice corrosion was noticed underneath the biofilm. For as-received DSS, the corrosion was generally limited to the austenite phase, whereas in thermally aged condition a uniform dissolution of the matrix was noticed and the secondary phases were remained as skeleton. Detachment of intragranular austenite was observed in the heat affected zone (HAZ) of the weldment. The MIC behavior was found to be influenced by the microstructural changes which occurred due to the thermal treatments.     

A general model for the repassivation potential as a function of multiple aqueous species. 2. Effect of oxyanions on localized corrosion of Fe–Ni–Cr–Mo–W–N alloys

A recently developed model for predicting the repassivation potential has been applied to stainless steels and nickel-base alloys in aqueous environments containing chlorides and various inhibiting anions. The model accounts for the effects of solution chemistry and temperature on the repassivation of localized corrosion by considering competitive dissolution, adsorption, and oxide formation processes at the interface between the metal and the occluded site solution. An extensive database of repassivation potentials has been established for six alloys (UNS 31603, N06600, N06690, S31254, S32205, and UNS S41425) in contact with solutions that combine chlorides with hydroxides, molybdates, vanadates, sulfates, nitrates, and nitrites at various concentrations and temperatures. Also, repassivation potentials are reported for four alloys (UNS N08367, N08800, N06625, and N10276) in chloride solutions. The database has been used to establish the parameters of the model and verify its accuracy. The model quantitatively predicts the transition between concentrations at which localized corrosion is possible and those at which inhibition is expected. It is capable of predicting the repassivation potential over wide ranges of experimental conditions using parameters that can be generated from a limited number of experimental data. The parameters of the model have been generalized as a function of alloy composition, thus making it possible to predict the repassivation potential for alloys that have not been experimentally investigated.     

The influence of microstructure on the oxidation of duplex stainless steels in simulated propane combustion products at 1000 °C

A low nickel Type S32101 duplex stainless steel has been oxidised in simulated industrial reheating conditions. The surfaces have been studied using optical microscopy, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Observations show that local breakaway regions (LBRs) form on the austenitic regions whereas thinner oxides are observed on the ferritic regions of the substrate. The reason proposed for these differences is the formation of a continuous oxide layer on the ferrite region and a discontinuous layer on the austenitic region during the early stages of oxidation. The chemical composition of these LBRs have been shown to be oxide islands of iron and manganese and oxide craters of chromium rich oxides. The more protective regions consist of chromium and manganese rich oxides. A silica layer formed below the oxide which may be attributable to a slight enrichment of silicon in the ferritic regions or due to faster rates of diffusion in ferrite.     

The influence of the cathodic process on the interpretation of electrochemical noise signals arising from pitting corrosion of stainless steels

The use of electrochemical noise (EN) measurements for the investigation and monitoring of corrosion has allowed many interesting advances in the corrosion science in recent years. A special advantage of EN measurements includes the possibility to detect and study the early stages of localized corrosion. Nevertheless, the understanding of the electrochemical information included in the EN signal is actually very limited. The role of the cathodic process on the EN signals remains uncertain and has not been sufficiently investigated to date. Thus, an accurate understanding of the influence of the cathodic process on the EN signal is still lacking. On the basis of different kinetics of the oxygen reduction it was established that the anodic amplitude of transients arising from pitting corrosion on stainless steel can be decreased by the corresponding electron consumption of the cathodic process. Therefore, the stronger the electron consumption, the weaker the anodic amplitude of the EN signal becomes. EN signals arising from pitting corrosion on stainless steel can be measured because the cathodic process is inhibited by the passive layer. This was confirmed by means of EN measurements under cathodic polarisation. Since the cathodic process plays a decisive role on the form of transients arising from pitting corrosion, its influence must be considered in the evaluation and interpretation of the EN signals.     

Effect of ageing heat treatments on the microstructure and intergranular corrosion of powder metallurgy duplex stainless steels

The influence of ageing heat treatments (675 and 875 °C for 1.5 to 48 h) on the microstructure and intergranular corrosion resistance of sintered in nitrogen duplex stainless steels was investigated. The materials were obtained by sintering mixtures of austenitic AISI 316L and ferritic AISI 430L powders. Corrosion behaviour was evaluated by using electrochemical techniques. The beneficial effect of nitrogen on corrosion behaviour of solution annealed samples was established. During ageing, secondary phases were precipitated and the intergranular and transgranular corrosion resistance significantly decreased though repassivation was observed in specimens aged at 875 °C for times up to 8 h.     

Influence of pH on the passivation behavior of 254SMO stainless steel in 3.5% NaCl solution

The potentiodynamic polarization measurement of 254SMO stainless steel (UNS 31254) was conducted in 3.5% NaCl solutions with pH ranging from 0.1 to 5. The results indicated that this stainless steel offered excellent pitting corrosion resistance in corrosive environments. Further, it also exhibited various features on the polarization curves in different pH solutions. The electrochemical constant-potential passivation treatment performed at different pH followed by XPS analysis revealed that the primary constituents of the outermost layer of the passive films formed in the weak (pH 5) and strong (pH 0.8) acid solutions are iron oxides and Cr₂O₃ and Cr(OH)₃, respectively. Molybdenum oxides, primarily in the six-valence state, existed in the outermost layer of the passive film. Only very weak signals corresponding to that of nickel oxides were detected in the film formed in the weak acid (pH 5) solution. The ICP-MS analyses indicated selective dissolution of a significant amount of Fe and a few Mo and Ni ions during the passivation treatment in the strong acid (pH 0.8) solution. No Cr dissolution was observed; this indicated that the Cr in the film is relatively stable. XPS depth profiling results showed that a similar bilayer-structured film was formed in both the solutions (pH 0.8 and 5); the outer layer of this film is primarily composed of Cr(OH)₃ and Mo(VI), and the inner layer, Cr₂O₃ and Mo(IV). The results of the examinations of passive film formations and dissolution by XPS and ICP-MS were consistent with the polarization curves.     

The effect of CeCl3 inhibitor on the localized corrosion of stainless steel in chloride solutions

Stainless steels have been used extensively in many sectors such as medical and household appliances as well as construction. This has been primarily due to their high resistance to corrosion attacks, reasonable cost, and excellent mechanical properties. However, when placed in corrosive media stainless steel is susceptible to localized corrosion attacks, especially when placed in chloride solutions. The paper explores the issue of corrosion liability of austenitic (AISI 316) and duplex (UNS S32205) stainless steels in et aconditions of seawater environment as well as under the influence of an inhibitor in the same environmental conditions. The behavior of stainless steels was examined via electrochemical testing relying on DC and AC techniques, optical metallographic analysis, scanning electron microscopy, and X‐ray diffraction analysis. Duplex stainless steel showed better resistance to localized corrosion but a higher tendency toward general corrosion in all examined solutions. Cerium chloride in a chloride solution showed inhibiting properties for both the AISI 316 and the UNS S32205.     

Study of passive film on stainless steels and high grade nickel base alloy using X-ray photoelectron spectroscopy

Abstract

In the present study an assessment has been made of the air formed passive film and the passive film existing after a short exposure to a synthetic sea water environment on austenitic (UNS S31603), superaustenitic (UNS S31254 and UNS S32654) and superduplex (UNS S32760) stainless steels and a Ni base alloy (UNS N06625) using X-ray photoelectron spectroscopy. Assessment of their corrosion behaviour has been made through use of electrochemical accelerated tests supported by post-test microscopy. The passive film on the Ni base (low Fe) UNS N06625 alloy is remarkably similar to those formed on the high Cr and Mo stainless steels. It has been demonstrated that the Fe/Cr ratio in the passive film is dependent on alloy composition and on the conditions to which the material has been exposed, and a general correlation between passive film constitution and corrosion resistance is evident.     

Intergranular corrosion of welded joints of austenitic stainless steels studied by using an electrochemical minicell

An intergranular corrosion study of welded joints of austenitic stainless steels (AISI 304 and 316L) has been addressed. A specific small-scale electrochemical cell (minicell) has been used. Four different weldment zones have been studied. The electrochemical methods applied were the electrochemical potentiokinetic reactivation test and electrochemical potentiokinetic reactivation double loop test. These techniques showed that the HAZ was the most critical zone to intergranular corrosion for both materials. The weld metal was susceptible to interdendritic corrosion and the fusion line showed a mixture of intergranular and interdendritic corrosion. The effect of pre- and post-welding heat treatments for AISI 316L was analyzed. The HAZ was again the most critical zone in every heat treatment condition. The results were correlated to the microstructural features of the materials.     

Effect of chlorination on stainless steels in seawater

The effect of chlorination on the corrosion resistance of different stainless steels in North Sea water has been studied, the sea-water exposures being performed at two Norwegian seawater laboratories. In one laboratory test specimens, simulating various components used in a seawater cooling system, were immersed in the water. In the other laboratory a pipe system containing real components was studied. Seawater temperature, chlorine concentration and chlorination procedure have been varied at both laboratories. The steels investigated include highly alloyed grades like UNS S31254 (AVESTA 254 SMO), UNS S44635 (Avesta Monit) and UNS NO8028 (Sandvik Sanicro 28). The investigation shows that continuously chlorinated seawater is considerably more aggressive than unchlorinated or intermittently chlorinated seawater and that high temperature increases the risks of localized corrosion at the same chlorine concentration. The highest alloyed steel grades were very resistant to crevice corrosion even in continuously chlorinated water but certain kinds of metal-metal crevices have to be avoided. When welded, duplex UNS S31803 was sensitive to pitting corrosion in chlorinated water. Contrary to crevice corrosion, the risk for galvanic corrosion decreases considerably if the seawater is chlorinated.     

Evaluation of Alloy 146, 279, 900, and 926 sensitization to intergranular corrosion by means of electrochemical methods and image analysis

Properties of stainless steels are affected by improper heat treatment. When that happens, the material can become sensitive to intergranular corrosion. This loss of properties is due to a microstructural changes. In this paper, electrochemical potentiokinetic reactivation tests (single and double loop) were used to evaluate steel sensitization to intergranular corrosion. Four alloys, Alloy 146 (UNS 1.4331) and 279 (UNS 1.4429) (austenitic stainless steels), Alloy 900 (UNS 1.4462) (duplex stainless steel), and Alloy 926 (UNS N08926) (high alloyed stainless steel), were employed as working electrodes. The specimens were sensitized at different temperatures (600–825 °C) for periods of 1 or 2 h in argon atmosphere. Electrochemical tests were conducted using the electro-optical devices P-200002525 and P-200002526. These devices permit to obtain images of the electrode surface and electrochemical data simultaneously. In addition, these results were compared to standardized etching with oxalic acid (ASTM A-262), scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDX). Two alloys showed sensitization to intergranular corrosion, Alloy 900 and Alloy 926 at 725 and 825 °C. Additionally, the single and double loop methods showed different passivation results. In the double loop test, the formation of a layer over the entire electrode surface was visible during the process of activation. The film remained until the end of experiment. However, in the single loop method no significant variations were observed on the electrode surface during passivation process, but if material is sensitized a film was formed during reactivation branch.