Evaluation of aged duplex stainless steel UNS S32750 susceptibility to intergranular corrosion by optimized double loop electrochemical potentiokinetic reactivation method

Optimized double loop electrochemical potentiokinetic reactivation (DL-EPR) in combination with microstructure observation were successfully applied to study the sensitization of super duplex stainless steel UNS S32750 aged at 900 °C for different duration to intergranular corrosion (IGC). The results indicated that the degree of sensitization increased gradually with the aging time increased from 10 min to 4 h due to the sigma phase precipitation, and I_r:I_a value reached the maximum value of 17.1%. However, further increasing aging time slightly decreased the sensitization due to the healing effect incurred by the diffusion of chromium from adjacent grains to chromium-depleted zone.     

Influence of strain-induced martensite in the anodic dissolution of austenitic stainless steels in acid medium

The anodic dissolution of austenitic stainless steel bars in an acid medium (0.5 M HCl + 2 M H₂SO₄) under different polarizations is studied. The material comes from bars formed by hot working (HW) and by cold working (CW). UNS S30403 and S31603 grades are considered. The determining effect of alloying elements and working method are proved. The morphology of the attack at different anodic potentials is analyzed. The important influence of the amount and distribution of strain-induced martensite on corrosion is demonstrated. The effect of the martensite on the anodic polarization curves is quantified using specifically designed software.     

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 grain orientation spread on the pitting corrosion resistance of duplex stainless steels using electron backscatter diffraction and critical pitting temperature test at the microscale

The corrosion behavior of UNS S32202 duplex stainless steel was studied by combining electron backscatter diffraction (EBSD) measurements and critical pitting temperature tests at the microscale. The grain orientation spread (GOS) value was determined in grains of both phases from EBSD data. It was shown that austenitic sites containing extremely small ferrite grains having a GOS value greater than 1.3° were precursor sites for pitting in 4 M NaCl. The critical pitting temperature range was 45–90 °C. All the other sites of both phases remained passive up to 100 °C.     

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.     

Improvement of the cavitation erosion resistance of UNS S31803 stainless steel by duplex treatment

A duplex surface treatment consisting of High Temperature Gas Nitriding (HTGN) followed by Low Temperature Plasma Nitriding (LTPN) was carried out in an UNS S31803 duplex stainless steel. The HTGN treatment was intended to produce a relatively thick and hard fully austenitic layer giving mechanical support to the thinner and much harder expanded austenite layer. HTGN was performed at 1200 °C for 3 h, in a 0.1 MPa N₂ atmosphere while LTPN, was carried out in a 75% N₂ + 25% H₂ atmosphere, at 400 °C for 12 h, under a 250 Pa pressure, and 450 V. An expanded austenite γ_N layer, 2.3 μm thick, 1500 HV0.025 hard, was formed on top of a 100 μm thick, 330 HV 0.1 hard, fully austenitic layer, containing 0.9 wt% N. For comparison purposes LTPN was carried out with UNS S30403 stainless steel specimens obtaining a 4.0 μm thick, 1500 HV 0.025 hard, expanded austenite layer formed on top of a fully austenitic matrix having 190 HV 0.1. The nitrided specimens were tested in a 20 kHz vibratory cavitation-erosion testing equipment. Comparison between the duplex treated UNS S31803 steel and the low temperature plasma nitrided UNS S30403 steel, resulted in incubation times almost 9 times greater. The maximum cavitation wear rate of the LTPN UNS S30403 was 5.5 g/m²h, 180 times greater than the one measured for the duplex treated UNS S31803 steel. The greater cavitation wear resistance of the duplex treated UNS S31803 steel, compared to the LTPN treated UNS S30403 steel was explained by the greater mechanical support the fully austenitic, 330 HV 0.1 hard, 100 μm layer gives to the expanded austenite layer formed on top of the specimen after LTPN. A strong crystallographic textured surface, inherited from the fully austenitic layer formed during HTGN, with the expanded austenite layer showing {101} crystallographic planes//surface contributed also to improve the cavitation resistance o f the duplex treated steel.     

Effect of oxygen and biofilms on crevice corrosion of UNS S31803 and UNS N08825 in natural seawater

The effect of oxygen and biofilms on crevice corrosion of UNS S31803 and UNS N08825 in seawater was studied. Passivity breakdown occurred through crevice corrosion in UNS N08825 and through transpassive dissolution in UNS S31803 although both alloys displayed crevice corrosion under potentiodynamic conditions. The most severe crevice corrosion occurred in the absence of oxygen and the presence of a biofilm. Microbial adhesion as investigated by fluorescence microscopy occurred mainly outside the crevice and DNA sequencing revealed a shift in biofilm composition as a function of substratum surface and oxygen pressure.     

EIS assessment of critical pitting temperature of 2205 duplex stainless steel in acidified ferric chloride solution

In this study, critical pitting temperature (CPT) of 2205 duplex stainless steel (DSS2205) was assessed using electrochemical impedance spectroscopy (EIS) in ferric chloride solution. In order to verify the results other methods such as ASTM G 48, potentiodynamic and potentiostatic polarisation and zero resistance ammeter (ZRA) were also employed. The results show a strong close relation between the results of this method by those of previous methods. CPT of the alloy is 40 °C based on standard method and 44 °C, 49 °C according to the ZRA and potentiostatic methods. Both potentiodynamic and EIS methods give an almost identical CPT value.     

Nondestructive assessing of the aging effects in 2205 duplex stainless steel using thermoelectric power

The microstructural transformation of ferrite into secondary austenite and sigma phase during long term exposure to high-temperatures (650–900 °C) in a 2205 duplex stainless steel has been investigated using the thermoelectric power (TEP) technique, scanning electron microscopy (SEM), Charpy-impact test (CIT), equivalent ferrite and sigma phases content measurements. The possibility of using the TEP coefficient as a nondestructive assessment technique to characterize the aging kinetics of 2205 duplex stainless steel is discussed. Experimental results indicate that TEP coefficient is sensitive to the gradual microstructural transformation of ferrite phase experienced by the 2205 duplex stainless steel during the aging treatments.     

Corrosion resistance of duplex stainless steel subjected to long-term annealing in the spinodal decomposition temperature range

The effect of thermal annealing up to 15,000 h between 300 °C and 500 °C on the corrosion resistance of the duplex stainless steel (DSS) 7MoPLUS has been investigated by using the DLEPR test. Spinodal decomposition in 7MoPLUS is unabated even after annealing for 15,000 h and no healing has been observed. The possible healing mechanisms in this temperature range (back diffusion of Cr atoms from the Cr-rich ferrite (α_Cr) and diffusion of Cr atoms from the austenite) and its absence in the present steel have been discussed.     

Passivation behaviour of Alloy 31 (UNS N08031) in polluted phosphoric acid at different temperatures

The influence of temperature (20–80 °C) and chloride concentration (0.06–0.42 wt.% KCl) on the electrochemical behaviour of the UNS N08031 was studied in 40 wt.% polluted phosphoric acid solution.Passivation behaviour was investigated by using potentiostatic tests at different potentials. From the linear regions of the log i vs. log t transients, the parameter n was obtained. The results showed that the applied potential hardly affects on the passivation rate n. However, n values decreased when temperature increased. The values of n demonstrated that the passive film formed on Alloy 31 was compact and highly protective.     

Theoretical studies of electronic structure and hole drift mobility of host hole transporting material 4,4′-N,N′-dicarbazol-biphenyl

The most commonly used host hole transport material, 4,4′-dicarbazole-1,1′-biphenyl (CBP), its structure, electronic transition mechanism and hole mobility were studied by means of ab initio HF, DFT B3LYP methods and Marcus theory. The lowest singlet excited state (S1) has been studied by the singles configuration interaction (CIS) method and time-dependent density functional theory (TD-DFT). The lowest singlet electronic transition (S0 → S1) is π–π* electronic transitions between carbazole and biphenyl parts involving the charge transfer from N atom to biphenyl. TD-B3LYP calculations predict an emission wavelength of 403.3 nm. This is comparable to 400 nm observed experimentally for photoluminescence. Using an incoherent transport model we calculated its hole mobility (μ). Both its reorganization energy and electronic coupling, especially the electronic couplings are considered and calculated in detail. It has high hole transport efficiency (μ = 8.60 × 10^?2 cm²/(V s)) and the result was rationalized in terms of the spatial extent of the highest occupied molecular orbital (HOMO) and molecular structural character.     

Practical aspects on the use of on-site Double Loop Electrochemical Potentiodynamic Reactivation Technique (DL-EPR) for Duplex Stainless Steel

This work discusses on-site use of Double Loop Electrochemical Potentiodynamic Reactivation Technique (DL-EPR) for Duplex Stainless Steel. It is suggested that, instead of altering the standard parameters for each different material tested, the procedure adopted is to determine in laboratory all the parameters for a specific material tested beforehand and then use those results as standard criteria to be compared with the results obtained on-site. Analysis is performed at two commercial super-duplex steels, UNS S 32750 and UNS 32760 and in steel 22Cr6Ni2Mo with low nitrogen content. Then, a welded joint of UNS S 32750 steel is analysed.     

Memory effects of transformation textures in steel and its prediction by the double Kurdjumov–Sachs relation

The phenomenon that the transformation texture near the initial texture reproduces after the phase transformation cycle such as ferrite (α, body-centered cubic) → austenite (γ, face-centered cubic) → α is called a texture memory. In this study, the texture change in a 0.1% C–1% Mn hot-rolled steel sheet during the α → γ → α transformation cycle was studied via neutron diffraction and the transformation texture prediction based on a variant selection rule that we call the double Kurdjumov–Sachs (K–S) relation. The texture change observed by neutron diffraction, which clearly showed the texture memory, could be quantitatively reproduced by the proposed variant selection rule adopted into the calculation method based on the spherical harmonics expansion of orientation distribution functions. Therefore, it is most likely that the texture memory in steel is caused by the preferential selection of those K–S variants that reduce the interfacial energy between a precipitate and two adjoining parent phase grains at the same time, which we call the double K–S relation.     

Brazing of Mo to a CuZr alloy for the production of bimetallic raw materials for the CLIC accelerating structures

Future linear accelerators, as CLIC (Compact LInear Collider), are extremely demanding in terms of material properties. Traditionally accelerating structure is made of brazed OFE copper parts. For the high conducting regions submitted to mechanical fatigue, CuZr would represent an improved selection than pure copper while for regions where the highest electric field is applied a refractory metal, i.e. Mo, could result in a better performance. The feasibility of joining such materials, namely CuZr (UNS C15000) and pure Mo has been investigated. The joining method developed and investigated here consists in a vacuum brazing process exploiting a Cu-based brazing filler applied under appropriate vacuum conditions.Apparent shear strength (adapted from ASTM B898) on the joined samples was about 200 MPa.     

Effect of B-S co-doping on large diamonds synthesis under high pressure and high temperature

Large single-crystal diamonds with n-type semiconductor were synthesized from S/B-S co-doping FeNiCo-C system under high pressure and high temperature (HPHT) in this paper. It was found that the slight variation of the additive S content had not made obvious change for the color of diamonds synthesized from FeNiCo-C system. The B-S co-doping samples became more transparent and yellow than the samples added alone by S. The analysis of X-ray photoelectron spectroscopy (XPS) spectra and Fourier transform infrared (FTIR) spectroscopy showed the presence of B and S in the obtained diamonds. The electrical properties of large diamond crystals were tested by Van der Pauw method with a four-point probe. The highest value of the hall mobility was 628.726 cm²/vs. And the lowest value of the resistivity was 9.33 × 10⁵ Ω·cm with boron additive of 0.8 wt.% and sulfur of 2 wt.% doping to diamond which was confirmed as n-type. This work indicated that B-S co-doping to synthesize diamond crystals was a trend to promote the electrical properties of large diamond crystals.     

Hydrogen diffusion and trapping in a precipitation-hardened nickel–copper–aluminum alloy Monel K-500 (UNS N05500)

Hydrogen uptake, diffusivity and trap binding energy were determined for the nickel–copper–aluminum alloy Monel K-500 (UNS N05500) in several conditions. The total atomic hydrogen (H) concentration increased from 0 to 132 wppm as the hydrogen overpotential decreased to ?0.5 V in alkaline 3.5% NaCl electrolyte at 23 °C. The room-temperature H diffusion coefficient ranged from 0.9 to 3.9 × 10^?14 m² s^?1 for single-phase solid solution, aged, and cold worked then aged microstructures. Diffusivity was independent of lattice H concentration but depended weakly on metallurgical condition, with slower H diffusion after aging. The apparent activation energy for H diffusion was in the range of 29–41 ± 1.5 kJ mol^?1 at the 95% confidence level. The lower value approached nearly perfect lattice transport, while the high value was strongly influenced by traps of low-to-intermediate strength. Atomic hydrogen trapping at metallurgical sites, strongly suggested to be spherical-coherent γ′ (Ni₃Al) precipitates, was evident in the aged compared to the solution heat treated + water-quenched condition. Both thermal desorption and classical Oriani trap state analyses confirmed that the apparent hydrogen trap binding energy interpreted as Ni₃Al (10.2 ± 4.6 kJ mol^?1) interfaces was significantly less than the activation energy for perfect lattice diffusion (25.6 ± 0.5 kJ mol^?1) in this nickel-based alloy system.     

Localised effects of macrofouling species on electrochemical corrosion of corrosion resistant alloys

Abstract

Interactions between macrofouling and corrosion have been studied on two duplex stainless steels (UNS S31803 and UNS S32760), an austenitic stainless steel (UNS S31354), a nickel base alloy (UNS N06625), and a cobalt base alloy (UNS R30006) in long term tests conducted in natural sea water off the west coast of Scotland. After an 18 month exposure period, the specimens were heavily fouled primarily with barnacles and mussels and all the materials exhibited crevice corrosion at the specimen/resin interface, although this was less extensive on the nickel base alloy. Localised corrosion was observed under the base of live barnacles on UNS S31603 stainless steel. Direct current electrochemical anodic polarisation tests undertaken after the 18 month exposure period yielded unusually high currents in the range of potentials between the free corrosion value and the breakdown potential. However, long term exposure and associated macrofouling had little effect on the overall breakdown of general surface passivity as indicated by values of the breakdown potential. The higher currents were associated with the appearance, after the anodic polarisation, of black sulphide corrosion products at the specimen/resin crevices, around barnacles, and around mussel byssus threads.     

Conductive polymer-coated textiles: The role of fabric treatment by pyrrole-functionalized triethoxysilane

Polypropylene (PP) and viscose (VS) textiles were modified by the in situ synthesis of a conducting polypyrrole (PPy) overlayer. To improve adhesion of the conducting layer to the textile surface, a pyrrole-functionalized silane (SP) was synthesized and bonded onto the surface before polypyrrole formation. Moreover, to introduce hydroxyl groups into the surface, PP was pretreated by grafting vinyltrimethoxysilane by means of a radiofrequency plasma discharge. The study is focused on the influence of SP on the washing fastness of a PPy layer and, consequently, on the overall conductivity of the textiles after washing. In the case of viscose, PPy was found to penetrate the substrate. A compromise was found between the influence of SP and penetration phenomenon (best conductivity after washing: wVS–0.2SP/25Py = 3 × 10⁻⁵ S/square). In the case of polypropylene the effect of pretreatment with SP is much better than for viscose, and a higher concentration of SP leads to improved fastness of the conductive layer (wmPP–0.2SP/25Py = 3 × 10⁻⁵ S/square; wmPP–1SP/25Py = 8 × 10⁻⁵ S/square), which indicates that the coating promoted by means of SP is more favoured than for viscose.