Corrosion behaviour of HVOF sprayed SUS316L stainless steel in seawater

SUS316L stainless steel was coated onto the SUS316L plate by the high velocity oxy-fuel (HVOF) spray technique. Its corrosion behaviour in seawater was investigated by the electrochemical method and the microscopy. The coating had corrosion resistance inferior to the bulk plate. The corrosion of the HVOF sprayed SUS316L coating was related to both its porosity and its oxygen content. Depending on them, the corrosion took place at the small pore and the boundary between the spray particles on the surface.     

Corrosion of nickel-containing stainless steel in concentrated sulphuric acid

The corrosion of UNS S30403 stainless steel was investigated in 93.5 wt% sulphuric acid at temperatures from 40 to 100 °C. Time-dependent free corrosion potential measurements demonstrate that the steel is subject to spontaneous active-passive oscillation. Studies on the effect of temperature and electrode rotation speed indicate that the oscillation is activation and transport controlled over different segments of the oscillations. It is concluded that the oscillation is caused by reversible changes in the exchange current for the cathodic reduction of sulphuric acid, depending on the presence or absence of nickel sulphide on the steel surface.     

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.     

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.     

Erosion-corrosion of 2205 duplex stainless steel in flowing seawater containing sand particles

2205 duplex stainless steel (UNS S31803) was tested in flowing artificial seawater containing sand particles using a jet impingement apparatus. Erosion-corrosion was measured under a range of hydrodynamic conditions by recording the increase in anodic current density that occurred when the passive film was damaged by particle impacts. The current density increase was shown to be linearly related to the mean kinetic energy of the sand particles. When the stainless steel was held at anodic potentials the surface oxide film developed optical interference colours and these colours were used to identify the film thickness. The highest rate of erosion-corrosion occurred in the stagnation region, immediately beneath the jet, where the particles impacted the surface at angles close to 90°. The results are discussed in terms of the rates of particle impacts and their effects on the processes of film damage and repassivation.     

Corrosion potential oscillation of stainless steel in concentrated sulphuric acid: I. Electrochemical aspects

The spontaneous oscillation of corrosion potential between the active and passive states of UNS S30403 stainless steel has been studied in 93.5 wt.% sulphuric acid at 60 °C. Detailed electrochemical analysis of the kinetics of the oscillating potential has suggested the presence of a nickel salt which is stable over a limited potential range. The nickel salt is subject to dissolution at potentials more positive than −0.25 V (MSE), while at lower potentials—less than −0.35 V (MSE)—a stable salt film is formed. A critical coverage of the salt on the surface is the prerequisite for the mixed potential to be shifted into the passive range by an increase in the exchange current density for the sulphuric acid cathodic reaction. However, when the coverage exceeds a critical value, such as can be produced by a potentiostatic hold, the salt effectively blocks the formation of a passive film.     

Potentiodynamic polarization behaviour of AISI type 316 stainless steel in NaCl solution

Potentiodynamic polarization behaviour of AISI type 316 SS in NaCl solution was investigated in terms of the potential scan rate effect. The critical pitting potential, E_pit, of the stainless steel appeared to be strongly dependent on the potential scan rate. A cumulative anodic electric charge density of the steel was defined as the total charge density from the open circuit potential, E_ocp, and calculated using the potentiodynamic polarization curves. It was found that, plotted as a function of the polarization time, the values of the cumulative charge density consisted of two lines with different slopes. It was confirmed that the deflection of the cumulative charge density vs. time plots corresponded to E_pit and the values of the cumulative charge density at the deflection were little dependent on the applied scan rate. The cumulative charge density at the deflection was defined as a critical electric charge density for the stable pitting. Also, it was suggested that this electric charge density should be associated with the critical condition for the stable pitting and the critical electric charge for stable pitting should be a representative parameter for the pitting resistance of a material.     

Critical pitting temperature for Type 254 SMO stainless steel in chloride solutions

The variation with time of the open circuit potential of high molybdenum containing stainless steel (Type 254 SMO) was measured in 4% sodium chloride solution in the temperatures range 30-100 °C. The plot of steady state potentials as function of temperature showed an inflection at 50 °C, attributed to the decrease of oxygen solubility in test solution above 50 °C. Potentiodynamic cycling anodic polarization technique was used to determine the critical pitting potential (E_pit) and the critical protection potential (E_prot) of the steel in 4-30% NaCl solutions at temperatures between 30 and 100 °C. By plotting the two values versus solution temperature, the corresponding critical pitting (CPT) and the critical protection (CPrT) temperatures were determined. Both parameters decreased with increasing chloride content. Above the CPT, E_pit and E_prot decreased linearly with log[Cl⁻]. The addition of bromide ions to the solution shifted both E_pit and E_prot towards positive values. In 4% NaCl, E_pit increased linearly with pH in the range 1-10. The combined effect of chloride ion concentration and pH on the morphology of the pits was examined by scanning electron microscopy (SEM) following potentiodynamic cycling anodic polarization.     

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.     

Microstructural evolution and localized corrosion resistance of an aged superduplex stainless steel

The present investigation was undertaken to analyze the effects of isothermal ageing treatments, carried out between 700 and 900 °C for a variety of times up to two weeks and followed by water quenching, on the microstructure and on the localized corrosion resistance of a superduplex stainless steel, SAF 2507.The quantitative metallography coupled with X-ray diffraction techniques was adopted to follow the microstructural evolution, together with SEM microscopy.Electrochemical potentiodynamic tests, as cyclic polarization curves recorded in sodium chloride solutions, together with weight loss measurements were employed to evaluate the susceptibility of the aged specimens to pitting corrosion.The influence of the transformation of ferrite into secondary austenite and sigma phase and of other microstructural variations, as chromium nitrides precipitation, on the stability of the passive film is shown. The susceptibility of the aged alloy to pitting corrosion phenomena, is related to sigma phase precipitation in association to the secondary austenite formation, which lead to a noticeable Cr depletion at grain boundaries.     

Corrosion behavior of TiC particle-reinforced 304 stainless steel

TiC particle-reinforced 304 stainless steels were prepared using a new developed in situ technology and their corrosion behavior was compared with that of 304SS in 5 wt.% HCl solution. As compared to 304SS, E_corr, E_pit and E_rp values had shifted to a more negative region in 304SS containing TiC, indicating faster corrosion rate by TiC addition. The addition of TiC particles to 304SS resulted in no rapid pit propagation but maintained a high corrosion rate in the whole immersion time investigated.     

Assessing the tribocorrosion performance of Ti–6Al–4V, 316 stainless steel and Monel K500 alloys in artificial seawater

The electrochemical and tribocorrosion behaviors of Ti–6Al–4V, 316 stainless steel and Monel K500 alloys sliding against Al₂O₃ in artificial seawater are investigated in this paper. It can be observed that the open circuit potential drops down to more negative values due to the removal of passive film. And a rapid dissolution occurs in the wear track compared with the unworn area. The wear loss polarized in cathodic potential is lower than that in anodic potential and open circuit potential conditions, because the material deterioration is enhanced by corrosive attack. The wear volumes of 316 stainless steel are much higher than the ones measured for Ti–6Al–4V and Monel K500 alloys. Friction coefficients are significant large in cathodic polarization compared with anodic polarization for all alloys. Moreover, the 316 stainless steel exhibits large friction coefficients compared with Ti–6Al–4V and Monel K500 alloys.     

Acoustic emission during pitting corrosion of 304 stainless steel

Acoustic emission (AE) during pitting corrosion of 304 stainless steel (304 SS) in H₂SO₄ solutions with different pH values and Cl⁻ concentrations was studied. Two types of AE signals are detected in all solutions. Each type of signals is characterized by AE parameters (rise time, counts number, duration and amplitude) and waveform carefully. It is believed that the hydrogen bubbles evolution inside the pits is the AE source.     

Effects of shielding gases on the microstructure and localized corrosion of tube-to-tube sheet welds of super austenitic stainless steel for seawater cooled condenser

The effects of shielding gas on the microstructure and localized corrosion of tube-to-tube sheet welds of SR-50A super austenitic stainless steel for seawater cooled condense were investigated in highly concentrated chloride environments. The localized corrosion resistance of the weld metal after welding with an Ar shielding gas supplemented with N₂ increased due to a decrease in the pitting resistance equivalent number (PREN) difference between the PREN_IR of interdendritic region and the PREN_DC of dendrite core. The localized corrosion was selectively initiated at the dendrite core because the PREN of the dendrite core was smaller than that of the interdendritic region.     

Electrochemical investigation of chloride induced pitting of stainless steel under the influence of a magnetic field

The effect of a magnetic field on chloride induced pitting of stainless steel was studied by potentiodynamic measurements in aqueous NaCl solution in a cylindrical cell with the field perpendicular to the surface under test. Compared to identical tests without field, a significant shift of the repassivation potential E_r in the cathodic direction was observed, together with the formation of small pits at high density in the periphery of the electrode. These pits develop under the influence of a vortical flow induced by the magnetohydrodynamic effect. The shift of E_r is explained as the consequence of the occluded morphology of these pits.     

海水淡化泵用2507超级双相不锈钢铸件的焊接修复工艺

使用2507超级双相不锈钢铸造海水淡化泵的承压铸件,通过焊条电弧焊对铸件出现的缺陷进行焊接修复。确定海水淡化泵用2507超级双相不锈钢铸件的合理焊接修复工艺参数,用于指导海水淡化泵用2507超级双相不锈钢铸件的生产。结果表明,采用E2594-16焊条,热输入控制在1 800 J/cm以下,层间温度控制在≤100 ℃,焊后经过1 080 ℃固溶处理可以得到较优的修复质量。     

A correlation between phosphorous impurity in stainless steel and a second anodic current maximum in H2SO4

Phosphorous as a minor element (0.03%) in AISI 304 austenitic stainless steel greatly affects the polarization and corrosion behavior in sulfuric acid solution. The presence of P in stainless steel created a second current maximum in the anodic polarization curve and the current increased with increasing aging in the solution. An adhesive corrosion surface layer, rich with phosphate, formed on the surface of a P-containing steel during active dissolution. The layer lowered the cathodic Tafel slope at low current densities, and is likely, due to a change in hydrogen evolution mechanism. Phosphorous increases the H-adsorbed (and/or absorbed) atoms on the surface, leading to the appearance of a second anodic current peak that is interpreted as re-oxidation of hydrogen atoms. Also, P shifted the corrosion potential to the noble side, decreased effectively the active anodic dissolution, and lowered the corrosion rate.     

Investigation of intergranular corrosion of 316L stainless steel diffusion bonded joint by electrochemical potentiokinetic reactivation

Electrochemical potentiokinetic reactivation technique (EPR) was employed to assess degree of sensitization in 316L stainless steel diffusion bonded joint (DBJ). The result showed the degree of sensitization of DBJ was much smaller than that of base material (BM). No chromium carbides precipitated at grain boundaries in DBJ after 100 h treatment at 650 °C, while chromium carbides could be seen clearly in the BM after 8 h treatment, indicating that DBJ has better intergranular corrosion resistance than BM. Diffusion bonding technique will not increase intergranular corrosion susceptibility of 316L DBJ. Reactivation potential has the biggest effect on sensitization.     

Anodic behaviour of stainless steel S43000 in concentrated solutions of sulphuric acid

Electrochemical, AES and XPS techniques were employed to characterize the anodic behaviour of S43000 stainless steel in concentrated sulphuric acid (90.0-96.4 wt.%). Electrochemical experiments showed that passivity is not spontaneous and requires anodic polarization in the acids studied. Rotating cylindrical electrode experiments showed that the corrosion rate is controlled by the mass transfer rate of FeSO₄ from a saturated surface salt. AES and XPS analyses provided evidence that passivity involves the formation of a chromium-rich oxide-hydroxide film. The passivation mechanism and passive state stability are considered to relate to the manner in which undissociated H₂SO₄ molecules participate in the corrosion process. The findings have meaningful implications regarding the development of more corrosion resistant stainless steels for acid service.