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.     

Stainless steel crevice corrosion testing in natural and synthetic seawater

Susceptibility of stainless steel to crevice corrosion in natural and synthetic seawater was investigated by a new test method, developed by a research group, CrevCorr, of the Marine Working Party of the European Federation of Corrosion. The method was defined on the basis of a Round Robin test performed by twenty laboratories over the world [1, 2]. Among them was our Institute at the Adriatic coast. Geometry of the crevice model, gasket materials, finishing of metal surface and the electrochemical potential increase rate were strictly defined. The monitoring of the free corrosion potential of two austenitic steels and a duplex steel, in the constant flowing conditions, as well as the redox potential, temperature, chloride and oxygen content, have been carried out during six months in the natural seawater from the Adriatic Sea at the location in Dubrovnik, Croatia. Sharp drop of the corrosion potential was used as an indication of the crevice corrosion initiation. After exposure to the seawater eventual crevice damage on a steel specimen was detected by visual and microscopic inspections. In the synthetic seawater the organic component was simulated by glucose and glucose oxidase [3, 4]. The monitoring of the free corrosion potential and the redox potential was performed at 20, 30 and 40°C during five days, and results compared with those obtained in the natural seawater at same temperatures.     

UNS S31803双相不锈钢的焊接工艺

采用气体保护钨极氩弧焊(GTAW)对UNS S31803双相不锈钢焊接,研究其焊接工艺下焊接接头的相比例、焊缝和热影响区(HAZ)的力学性能,为我国00Cr22Ni5Mo3N双相不镑钢的现场焊接提供指导.     

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.     

Evaluation of the intergranular corrosion susceptibility of UNS S31803 duplex stainless steel with thermoelectric power measurements

In this study, the use of thermoelectric power (TEP) measurements as a nondestructive method for the determination of the degree of sensitization (DOS) in UNS S31803 duplex stainless steel (DSS) is presented. Samples were aged for times up to 240 h and subjected to the double loop electrochemical potentiokinetic reactivation (DL-EPR) test to calculate the DOS induced. The correlation between TEP and the DOS evaluated by the DL-EPR method indicates that the TEP coefficient is sensitive to gradual microstructural changes produced by thermal aging and may be used to monitor the susceptibility to intergranular corrosion in UNS S31803 DSS.     

The effect of surface modification on the corrosion protection ability of the passive films of sensitized UNS S31803 duplex stainless steel

ABSTRACT

The influence of sensitisation heat treatment, surface roughness and repetitive cyclic voltammetry (RCV) procedure (that was used to create passive films on the surface of sensitised samples) on the corrosion behaviour of UNS S31803 duplex stainless steel was evaluated using sodium hydroxide etching, double-loop electrochemical potentiodynamic reactivation (DLEPR), potentiodynamic polarisation, Mott–Schottky analysis, X-ray diffraction and optical microscopy techniques. The results showed that prolonged sensitisation time leads to the formation of defective passive films on the surface. In addition, as the substrate surface roughness decreases, the defect concentration in the resulting passive film decreases. Moreover, thick passive films that were created at a high number of RCV cycles contain fewer defects than the thin ones. Finally, the specimens with smooth surfaces, thick passive films and low degree of sensitisation exhibit high corrosion resistance due to their intact passive layers formed on their surfaces.     

Microstructural changes produced by plastic deformation in the UNS S31803 duplex stainless steel

Plastic deformation by cold rolling produces important changes on microstructure of the duplex stainless steel UNS S31803. Structure refinement and martensitic transformation were detected and analyzed by microscopy, X-ray diffraction and magnetic measurements. True deformations in the range of 0.92–3.38 were applied. The maximum amount of ′ martensite was 30.2% obtained with the maximum deformation applied (3.38). The annealing at 400 °C promotes a further increase of ′ martensite content, as observed before in austenitic metastable steels. Hardness against deformation curves of AISI 304L and duplex steel were compared and analyzed. The stability of the martensite phase with temperature was investigated by magnetic measurements, and it is found that the reverse reaction ′ → γ starts between 500 and 520 °C.     

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.     

Influence of biofilms growth on corrosion potential of metals immersed in seawater

The changes of corrosion potential (E_corr) of metals immersed in seawater were investigated with electrochemical technology and epifluoresence microscopy. In natural seawater, changes of E_corr were determined by the surface corrosion state of the metal. E_corr of passive metals exposed to natural seawater shifted to noble direction for about 150 mV in one day and it didn't change in sterile seawater. The in‐situ observation showed that biofilms settled on the surfaces of passive metals when E_corr moved in noble direction. The bacteria number increased on the metal surface according to exponential law and it was in the same way with the ennoblement of E_corr. The attachment of bacteria during the initial period played an important role in the ennoblement of E_corr and it is believed that the carbohydrate and protein in the biofilm are reasons for this phenomenon. The double layer capacitance (C_dl) of passive metals decreased with time when immersed in natural seawater, while remained almost unchanged in sterile seawater. The increased thickness and reduced dielectric constant of C_dl may be reasons.     

Effect of bacterial biofilm on 316 SS corrosion in natural seawater by eis

The electrochemical behavior of AISI 316 stainless steel in natural seawater collected from Canary Archipelago was investigated. A comparative study on a large number of samples immersed during three weeks in this environment with and without sulfate-reducing bacteria (SRB) was made. Open circuit potential measurements, cyclic polarization, Scanning Electron Microscopy (SEM) and Electrochemical Impedance Spectroscopy (EIS) were used. The obtained results permitted an interpretation of the contribution of biofilms to corrosion of stainless steel and a mechanism of the process is checked.     

Effect of nitrogen on crevice corrosion in austenitic stainless steel

Corrosion properties of high nitrogen austenitic steels in chloride solutions have been investigated. Nitrogen behavior was evaluated at various electrode potentials, and analysis of the surface film was carried out with XPS. The alloy used for the experiments had a composition of 23%Cr-4%Ni-0-1%Mo-0.7-1%N and was obtained through electro-slag remelting (ESR) under high nitrogen pressure. High nitrogen austenitic steel produced in the solution by crevice corrosion under a constant potential of 0.2 V (SCE). In the transpassive region and at 0.7 V (SCE), the products in the solution were , and . The amount of dissolved and increased with the electrode potential. in the solution suppressed decreases of pH, having a re-passivation effect. For crevice corrosion under a higher electrode potential than 0.4 V (SCE), the number of crevice corrosion points and the corrosion loss decreased as the electrode potential increased. This behavior can be attributed to the corrosion suppressing effect of dissolved in the solution as a product of crevice corrosion. The presence of chromium and iron oxides in the passivation film and crevice corrosion surface film were identified from XPS analysis. N 1s spectra indicated the presence of a nitride (CrN) or NH₃.     

Rainbow fringes around crevice corrosion formed on stainless steel AISI 316 after ennoblement in seawater

The crevice corrosion occurrence probability of stainless steel (SS) AISI 316 was increased under ennoblement condition due to chemically added H₂O₂ into seawater. The H₂O₂ was used to simulate the important factor causing ennoblement in natural marine biofilm. Morphology of the crevice corrosion was observed using an incident‐light source microscopy. Some interesting “rainbow” fringes were observed around micro‐crevices. The mechanism was discussed from the ions diffusion and potential distribution during the crevice formation. This result shows that under ennoblement condition the colored fringe is a distinct characteristic of the morphology of localized corrosion for stainless steel.     

抗硫高压分离器UNS N06625堆焊工艺与腐蚀试验

分析了高含H2S,CO2的产出物的特性,确定了井口分离器采用低合金钢堆焊625合金的结构形式,并选择合适的焊接工艺方法,对625合金的焊接性进行分析。经过工艺试验,摸索出合适的焊接工艺参数,进行了单层电渣堆焊、焊条电弧焊堆焊和小管内孔自动氩弧焊堆焊3种焊接方法评定,并进行了晶间腐蚀、氯化镁应力腐蚀和模拟分离器工况条件的抗硫化氢应力腐蚀和动态腐蚀评价试验。试验结果合格,满足技术要求,并据此完成了设备制造。     

An experimental study of crevice corrosion behaviour of 316L stainless steel in artificial seawater

The effects of applied torque on corrosion behaviour of 316L stainless steel with crevices were investigated using the cyclic potentiodynamic polarization method. Three kinds of crevices (316L-to-polytetrafluoroethylene, 316L-to-fluoroelastomeric and 316L-to-316L) were tested in artificial seawater at 50 °C. Corroded surface morphology was also investigated using scanning electron microscopy. Results indicate similar trends in crevice corrosion susceptibility with increasing applied torque. Among the three crevices, the 316L stainless steel specimen, coupled to the 316L stainless steel crevice former, is the most susceptible to crevice corrosion.     

Effect of nickel alloying on crevice corrosion resistance of stainless steels

The crevice corrosion behaviour of stainless steels containing 25 mass% Cr, 3 mass% Mo and various amounts of Ni was investigated in natural seawater. The results showed that ferritic steels containing nickel were more resistant to corrosion than both ferritic steels without nickel and austenitic steels. The superiority of the Ni bearing ferritic steel over the other steels was in close agreement with the depassivation pH of those steels in acidic chloride solutions. The results showed that the addition of Ni to ferritic steel was effective in decreasing the depassivation pH and the dissolution rate in acidic chloride solutions at crevices.     

Behaviour of crevice corrosion in iron

Crevice corrosion was investigated in iron exposed to a strong-buffered acetate solution (0.5 M CH₃COOH + 0.5 M NaC₂H₃O₂), pH = 4.66. The current and the potential gradient within the crevice were measured at crevice depth (L) = 7.35, 8, 10, and 15 mm, for a crevice that was positioned facing the electrolyte in a downward position. A remarkable shift in potential (>1.2 V) in the active direction was measured inside the crevice cavity, when the potential at the outer surface was held at 800 mV(SCE). Experimentation showed that there is a critical depth value, above which little changes occur on the transition boundary between passive and active regions on the crevice wall, x_pass, and below which x_pass location shifts sharply towards the crevice bottom. Steeping of the potential gradient occurred with time indicating enhancement of crevice corrosion, which was seen by the gradual increase in the current. These findings were in close agreement with the IR voltage theory and related mathematical model predictions. Morphological examination showed an intergranular attack around the active/passive boundary (x_pass) on the crevice wall.     

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.