The pitting corrosion of welded joints of lean duplex stainless steel (LDSS) UNS S32304 was studied. LDSS S32304 thick plates were welded by different processes commonly employed in the manufacture of equipment and piping: shielded metal arc welding, gas metal arc welding, and flux cored arc welding. The electrochemical behavior of different weldment zones (fusion zone, base metal, and heat affected zone (HAZ)) was characterized both independently, using an electrochemical microcell, and together by testing the whole welded area. The electrochemical testing was performed in acidified glycerin, a process fluid of the biodiesel industry.The HAZ was the most susceptible zone for pitting corrosion for all samples. The results were correlated to the microstructural features of the materials. Finally, the results from the microcell and whole weld approaches were compared. According to the results, the gas metal arc welding process with 2209 as filler metal was found to be the optimum for application of 2304 duplex stainless steels in the acidified glycerin environment. Figure 3. Optical microstructure of base metal (a) and top samples welded by different processes showing HAZ and FZ, 2209 as filler metal: (b) SMAW, (c) GMAW, (d) FCAW.

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Duplex stainless steel (DSS) has been considered as an excellent material of construction for applications where high corrosion resistance and high mechanical strength are required. The use of this material can minimize costs related to corrosion in different industries, for example, in the biodiesel industry. The objective of this study is to evaluate the electrochemical and electronic properties of passive films of thick welded joints of UNS S32304 lean duplex stainless steel (LDSS) fabricated by different welding processes, in acidified glycerin, a byproduct of the biodiesel industry. The electrochemical and electronic behavior of the passive films formed on UNS S32304 LDSS was evaluated using electrochemical impedance spectroscopy and Mott–Schottky measurements. The capacitance study shows that the passive films formed on welded DSS behave as n‐ and p‐type semiconductors above and below the flat band potential, respectively. Furthermore, the samples welded by the gas metal arc welding process exhibited better corrosion resistance than other welding process using the same filler metal.

Thick welded joints of UNS S32304 lean duplex stainless steel (LDSS) has been studied in acidified glycerin, a byproduct of biodiesel plants, using cyclic polarization technique. LDSS S32304 thick plates were welded by different processes commonly employed in the fabrication process: shielded metal arc welding, gas metal arc welding and flux cored arc welding. The electrochemical behavior of different weldment zones was characterized. Transmission electron microscopy and scanning electron microscopy (SEM) were used for sample characterization. SEM images showed that the heat affected zone was the most critical zone for pitting corrosion for all samples. The results were associated to the microstructural features of the materials. Generally, the breakdown potential of the top regions of welds was higher in acidified glycerin than the weld root for welds fabricated with two different filler metals. Basically, all the welded samples exhibited similar or higher corrosion resistance, compared with the corresponding base metal.

-The present study investigated batch kinetics and the batch equilibrium of manganese removal from acid mine drainage (AMD) using bone char as an adsorbent. Equilibrium tests revealed that the Langmuir-based maximum manganese uptake capacity was 22 mg g -1 for AMD effluents and 20 mg g -1 for laboratory solutions at a pH ranging from 5.5 to 5.7. The pseudo-second order model best described the manganese kinetics within bone char. Manganese removal was mainly influenced by the operating variables of the solid/liquid ratio and the pH of the aqueous phase. In fact, metal uptake was favored at nearly neutral pH values. The effect of particle size and temperature proved to be insignificant for the investigated operating range. This work also evaluated the mechanism for manganese removal using bone char. Results showed that intraparticle diffusion is the main rate-limiting step; however, additional contributions from boundary layer diffusion may well affect this removal when particles of smaller sizes are used. The final concentration of fluoride and other metals present in the AMD effluent was in agreement with the concentration limit set forth by Brazilian legislation. The present study demonstrated that bone char is a suitable material to be used for the removal of manganese from AMD effluents.

-In the present study, continuous fixed bed column runs were carried out in an attempt to evaluate the feasibility of using bone char for the removal of manganese from acid mine drainage (AMD). Tests using a laboratory solution of pure manganese at typical concentration levels were also performed for comparison purposes. The following operating variables were evaluated: column height, flow rate, and initial pH. Significant variations in resistance to the mass transfer of manganese into the bone char were identified using the Thomas model. A significant effect of the bed height could only be observed in tests using the laboratory solution. No significant change in the breakthrough volume could be observed with different flow rates. By increasing the initial pH from 2.96 to 5.50, the breakthrough volume was also increased. The maximum manganese loading capacity in continuous tests using bone char for AMD effluents was 6.03 mg g -1 , as compared to 26.74 mg g -1 when using the laboratory solution. The present study also performed desorption tests, using solutions of HCl, H 2 SO 4 , and water, aimed at the reuse of the adsorbent; however, no promising results were obtained due to low desorption levels associated with a relatively high mass loss. Despite the desorption results, the removal of manganese from AMD effluents using bone char as an adsorbent is technically feasible and attends to environmental legislation. It is interesting to note that the use of bone char for manganese removal may avoid the need for pH corrections of effluents after treatment. Moreover, bone char can also serve to remove fluoride ions and other metals, thus representing an interesting alternative material for the treatment of AMD effluents.

Two chitosan hydrogels (prepared by NaOH neutralization and by polyphosphate ionotropic gelation) have been tested in the dry state for Pd(II) and Pt(IV) sorption at pH 2. Similar sorption isotherms with maximum sorption capacities close to 190 mg Pd g −1 and 235 mg Pt g −1 were achieved. The sorption mechanism involves electrostatic attraction of the chloro-anionic species onto protonated amine groups; the drastic decrease of sorption capacity with the addition of chloride ions supports this hypothesis. SEM-EDAX analysis suggests that sorption proceeds, in kinetic terms, through a shrinking core mechanism. Metal ions can diffuse throughout all the sorbent volume. The main differences between the sorbents are revealed by kinetics. The hydrogels prepared by ionotropic gelation in polyphosphate (C-PPh) allows reaching equilibrium much faster than the hydrogels prepared by the neutralization process (C-NaOH). While for C-PPh sorbent the chemical reaction rate seems to control sorption profiles, in the case of C-NaOH a combination of mechanisms including intraparticle diffusion resistance controls uptake kinetics. Metal desorption from loaded sorbents is possible using thiourea alone or in association with HCl solutions. The recycling of the sorbents is possible but for a limited number of cycles.

The effect of strain-induced martensitic transformation (SIMT) and its reversion (SIMRT) on the microstructure evolution and localised corrosion of 2304 lean duplex stainless steel (LDSS) was investigated. The cold-rolled LDSS was subjected to reverse annealing at 600°C to 900°C for 1800 s. To evaluate the microstructure, scanning (SEM) and transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) were used as the main techniques. Subsequent electrochemical characterisation included cyclic polarisation and double loop-electrochemical potentiokinetic reactivation test. SIM formation was noticed after cold rolling, and a partial reverse transformation was seen after annealing at 600°C. At 600°C, SIMRT led to decrease in pitting resistance and increase in the degree of sensitisation (DOS). Increasing the annealing temperature favoured localised and intergranular corrosion resistance. The obtained results indicate that reversed austenitic and recrystallised grains have a beneficial effect on desensitisation and corrosion resistance of 2304 LDSS.