Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-31 Cooperative journals: 《中国腐蚀与防护学报》
Abstract： The influence of steam on the corrosion behavior of 439 stainless steel for automotive mufflers was studied by means of cyclic hot air oxidation-immersion in condensate test and cyclic hot air oxidation-immersion in condensate-exposure to steam test as well as electrochemical measurements, SEM, XRD and pitting depth analyses. The results show that all the specimens, after cyclic hot air oxidation-immersion in condensate test and then with and without subsequent exposure to steam test, suffered from pitting corrosion with similar corrosion products. However, the specimens show lower corrosion resistance and deeper corrosion pits when they had experienced the exposure to steam test rather than those only experienced the cyclic hot air oxidation-immersion in condensate test. The exposure to steam may facilitate the growth of corrosion products and pits.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-31 Cooperative journals: 《中国腐蚀与防护学报》
Abstract： One oxidation-condensate immersion cyclic method was used to simulate muffler internal service environment under the long-distance driving conditions. The corrosion behavior of type 409, 429, 436, 439 and 441 stainless steels was investigated in the condensate solutions. The results show that the oxidation/corrosion product films on the surfaces of these five steels are mainly composed of Cr2O3 and Fe2O3. The alloying elements Cr and Mo play the most important role in the resistance of product films and charge transfer processes on steel surfaces. Pit depth decreases in order of 409, 439, 441, 429 and 436, but there is small depth difference between 439 and 441 as well as 429 and 436. Moreover, these five stainless steels show good pit resistance in the simulated processes of long-distance driving trips.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-31 Cooperative journals: 《腐蚀科学与防护技术》
Abstract： The corrosion behavior of type 304 and 441 stainless steels was investigated in a simulated environment of diesel exhaust selective catalytic reduction (SCR) system by using a two step tests, i.e. first the steels were suffered from hot exhaust gas oxidation at 600 ℃ for 100 h and then immersed in a simulated condensates solution at 80 ℃ to reveal their electrochemical corrosion characteristics. The results indicate that, after oxidation at 600 ℃ in the exhaust gases with and without addition of urea respectively, 441 stainless steel shows passive corrosion state in the condensates solution, whereas 304 stainless steel is in active corrosion state. After polarization curve measurements, some pits formed on 441 stainless steel surface, and intergranular corrosion feature was observed on parts of 304 stainless steel surface. After the oxidation in hot exhaust gas with addition of urea, the aforementioned localized corrosion for both 304 and 441 stainless steels can be accelerated during the subsequent corrosion in the condensates solution.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： The heat-affected zone (HAZ) produced by welding in stainless steel has higher susceptibility to intergranular corrosion, which is attributed to the Cr depletion induced by grain-boundary carbide-precipitation. The grain boundary engineering can be used to control over the grain boundary structure, which has significant influence on the carbide precipitation and the associated Cr depletion and hence on the susceptibility to intergranular corrosion. The grain boundary network in a 304 austenite stainless steel can be controlled by grain boundary engineering (GBE) with 5% tensile deformation and subsequent annealing at 1100 ℃ for 30 min. The total length proportion of Σ3n coincidence site lattice (CSL) boundaries was increased to more than 75%, and the large-size highlytwinned grain-cluster microstructure was formed through the treatment of GBE. Specimens were welded by gas tungsten arc- welding. Then the microstructure and the corrosion resistance of HAZ were characterized. The result showed that the high proportion of low ΣCSL boundaries and the optimum grain boundary character distribution were stable in the HAZ of the grain boundary engineered stainless steel, and the grain size was nearly the same. The weld-decay region of GBE samples performed better intergranular corrosion resistance during the intergranular corrosion immersion experiment and electrochemical potentiokinetic reactivation (EPR) test. The reported results indicated that the grain boundary engineering can effectively improve the intergranular corrosion resistance of the heat-affected zone in 304 austenitic stainless steel.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Zirconium alloys with low alloying content are mainly used in the nuclear industry as structural materials because of their superior properties in terms of thermal neutron transparency, mechanical strength and corrosion resistance. They are used for fuel cladding tubes and channels. The reaction between zirconium and water at high temperature forms oxide film on the surfaces. In order to further improve the corrosion resistance of Zrbased cladding tubes, research has continued on developing new zirconium alloys. The corrosion resistance of Zr-0.72Sn-0.32Fe-0.14Cr-xNb alloys (x=0, 0.12, 0.28, 0.48, 0.97, mass fraction, %) was investigated in a superheated steam at 500 ℃ and 10.3 MPa by autoclave tests. All the plate specimens of zirconium alloys with thickness of 2.8 mm have a similar texture. The microstructure of alloys and oxide films on the corroded specimens were observed by TEM and SEM. The results showed that no nodular corrosion appeared on these alloys for 500 h exposure. The thickness of oxide layers developed on the rolling surface (SN), the surface perpendicular to the rolling direction (SR) and the surface perpendicular to the transversal direction (ST) after 500 h exposure was close to each other. There was no anisotropic corrosion resistance for these alloys. The corrosion rate of the alloys increased with the increase of Nb content after 250 h exposure when the Nb content exceeded 0.28%. In the alloy with low Nb content, the fcc-Zr(Fe, Cr)2 or fcc-Zr(Fe, Cr, Nb)2 precipitate was mainly formed, while the hcp-Zr(Fe, Cr, Nb)2 precipitate was frequently observed in the alloy with high Nb content. The corrosion resistance of Zr- 0.72Sn- 0.32Fe-0.14Cr-xNb alloys was improved by decreasing the Nb/Fe ratio. From a point of view for the improving corrosion resistance, the addition of Nb no more than 0.3% is recommended.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Zr-4 coarse-grained specimens were corroded in static autoclave at 360 ℃, 18.6 MPa in 0.01 mol/L LiOH aqueous solution for 70 and 160 d exposure. EBSD, SEM and HRTEM were used to investigate the microstructures and crystal structures of oxide layers, and the relationships between the oxide thickness and the grain orientation of the metal matrix. The results showed that the oxide layers formed on the grain surfaces with the orientations nearby basal plane (0001) were thicker, and exhibited a prominent anisotropic for the oxide growth when Zr-4 specimens were corroded in LiOH aqueous solution for 160 d, but this was not the case for 70 d. The grains with the surface orientation nearby (0001), (1010) and (1120) were selected from the specimens corroded for 70 d to investigate the effect of metal grain orientation on the microstructure of oxide layers. The results showed that the crystal structure and microstructure of oxide layers formed on different metal grains were obviously different, and the scattering of m-ZrO2 columnar grain orientations in the oxide layers formed on near basal plane (0001) was wider than that on near prismatic plane (1010) and (1120). Besides the majority of m-ZrO2, c-ZrO2, t-ZrO2 and sub- oxide phase Zr3O were also detected at the oxide/metal interface, and it showed that the microstructure and crystal structure of oxide layers were very complex. The microstructural evolution of oxide layers will affect the diffusion of oxygen and subsequently the growth of oxide. Therefore, the microstructural evolution of oxide layers, which was affected by the different microstructure of oxide layers formed initially on grains and the water chemistry of corrosion tests, resulted in the anisotropic growth of oxide layers when Zr- 4 specimens were corroded in LiOH aqueous solution in subsequent corrosion tests.
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