Search

The high strength martensite steels are widely used in aerospace, ocean engineering, etc., due to their high strength, good ductility and acceptable corrosion resistance. This paper provides a review for the influence of microstructure on corrosion behavior of high strength martensite steels. Pitting is the most common corrosion type of high strength stainless steels, which always occurs at weak area of passive film such as inclusions, carbide/intermetallic interfaces. Meanwhile, the chromium carbide precipitations in the martensitic lath/prior austenite boundaries always result in intergranular corrosion. The precipitation, dislocation and grain/lath boundary are also used as crack nucleation and hydrogen traps, leading to hydrogen embrittlement and stress corrosion cracking for high strength martensite steels. Yet, the retained/reversed austenite has beneficial effects on the corrosion resistance and could reduce the sensitivity of stress corrosion cracking for high strength martensite steels. Finally, the corrosion mechanisms of additive manufacturing high strength steels and the ideas for designing new high strength martensite steel are explored.

Mechanical properties and corrosion behaviours of hastelloy X fabricated by selective laser melting (SLM) were investigated and compared with the wrought counterpart, and the anisotropy of the selective laser melted (SLMed) hastelloy X was also clarified in this work. Nano-inclusions (Mo-rich M 6 C) were widely distributed at the sub-grain boundaries in the SLMed hastelloy X alloy, whereas micron-scale precipitations existed in the wrought counterpart. The grain sizes of the hastelloy X fabricated by SLM on XOZ plane were slenderer than that on XOY plane (equiaxed grains), and the molten pool boundaries widely existed in the SLMed matrix, leading to poor plasticity and the XOY plane was worse due to the cracks perpendicular to the stretching direction. The corrosion resistance for the SLMed matrix was better than the wrought in the less aggressive solution due to the thicker oxide passive film, but the SLMed parts corroded faster in a severe environment once the voids and the molten pool boundaries started to dissolve. The corrosion depth for the SLMed hastelloy X was approximately three times larger than for the wrought and there was little difference on different planes for the SLMed matrix.

In this work, Inconel 718 alloys with different TiC contents were fabricated one time successfully by a joint powder-fed laser metal deposition (LMD) system with two independent powder delivery systems. The TiC particles, with an average size of 20 μm, distributed in the LMD matrix uniformly, and the Laves phase segregation in the LMD Inconel 718 alloys declined with increasing TiC content, which was attributed to the strong affinity between the Nb and C atoms. The Vicker hardness increased while the grain size decreased with increasing TiC content for the LMD Inconel 718 alloys. The nanohardness for the TiC particles was approximately 1.5 times greater than that of the γ matrix, thus reducing the actual contact area and consequently minimizing the frictional force during sliding compared to the matrix. The corrosion rate increased slightly with increasing TiC content in the LMD Inconel 718 alloys, and micro-pits existed mainly at the bottom of the TiC particles, which was attributed to the galvanic effect induced by TiC particles and the massive discontinuity of the passive film.

Effect of Sr on microstructure and corrosion behavior of biodegradable Mg-1Zn-1Mn-xSr alloy is investigated. Decrease in second phase spacing and linear increase of Mg17Sr2 fraction are observed with increasing Sr content. Corrosion rate increases linearly with increasing Sr content from 0 to 1.5 wt.% because of the enhanced micro-galvanic corrosion. Undermining of Mg17Sr2 and α-Mg grains results in the dramatic increase in corrosion rate at a high Sr content (3.0 wt.%). Localized corrosion is inhibited by alloying with Sr, yielding an optimum Sr content of 1.0˜1.5 wt.% with comprehensive consideration of uniform/localized corrosion resistance and biomedical adaptability.

Irregular grains, high interfacial stresses and anisotropic properties widely exist in 3D-printed metallic materials, and this paper investigated the effects of heat treatment on the microstructural, mechanical and corrosion properties of 316 L stainless steel fabricated by selective laser melting. Sub-grains and low-angle boundaries exist in the as-received selective laser melted (SLMed) 316 L stainless steel. After heat treatment at 1050 °C, the sub-grains and low-angle boundaries changed slightly, and the stress state and strength decreased to some extent due to the decrease of dislocation density. After heat treatment at 1200 °C, the grains became uniform, and the dislocation cells vanished, which led to a sharp decline in the hardness and strength. However, the ductility was improved after recrystallization heat treatment. The passive film thickness and corrosion potential of the SLMed 316 L stainless steel decreased after heat treatment, and the pitting potential also decreased due to the accelerated transition from metastable to steady-state pitting; this accelerated transition was caused by the presence of weak passive films at the enlarged pores after heat treatment, especially for an adequate solid solution treatment.

The multiple effects of ɳ-Ni3Ti precipitates and reversed austenite on the passive film stability of nickel-rich Custom 465 steel were studied. The results showed that with an increase in tempering temperature, the η-Ni3Ti precipitates gradually coarsened. The Volta potential of the ɳ-Ni3Ti/matrix interface was approximately 30 mV lower than that of the precipitates, which decreased the passive film stability. The volume of reversed austenite gradually increased at higher tempering temperatures. The Volta potential of the reversed austenite was 14 mV higher than that of the martensite, which enhanced the passive film stability and slightly increased the pitting corrosion resistance.

In this work, the influence of reversed austenite on the intergranular corrosion of AM355MSSs (martensitic stainless steels) was investigated. The combined results revealed that the reversed austenite exhibited an inhibiting effect on the intergranular corrosion of AM355MSSs. The inhibiting effect of reversed austenite on the intergranular corrosion was summarized as follows. Firstly, the formation of reversed austenite at the Cr-depleted regions reduced the number of nucleation sites for intergranular corrosion. Secondly, the reversed austenite always distributed along the martensite lath boundaries, preventing the propagation of intergranular corrosion.

Passivation and film chemistry of 2507 super duplex stainless steel in modified artificial seawater (ASW) are investigated. Removal of dissolved oxygen decreases the electric field strength and point defect diffusivity of the passive film. Passive current density in acidified ASW is higher than that in aerated ASW due to increased potential drop across film/solution interface, donor density and point defect diffusivity. The content of oxidized Cr, Fe(II) and hydroxides increases and film thickness reduces with acidification. In acidified ASW, passive film become denser from pre-passivation to passivation region and pores form accompanied with the increase of oxidized Fe after transpassivation.

Recently, the stainless steels produced by selective laser melting (SLM) have received considerable attention in biomedicine, but their corrosion behaviors in the body environment are not revealed clearly up to now. In this study, the fundamental research on influences of microstructures on corrosion behavior of 316L stainless steel (316L SS) produced by SLM was conducted simulated body fluid (SBF). Higher densities of grain boundary, sub-grain boundary and dislocation resulted in a thicker passive film forming on SLM 316L SS comparing with that of wrought sample in SBF. Composition analyses of passive films revealed that the excessive thickness was the outer layer. No sulfide and dislocation tangling around oxide inclusions in SLM 316L SS contributed to a 0.205 V nobler pitting potential of SLM 316L SS than that of the wrought in SBF.

Accelerated corrosion tests of the 7005-T4 aluminum alloy were conducted to determine a suitable service life prediction method by using alternating wet–dry cycles in three kinds of solutions. The morphology and composition analysis of the corrosion product revealed that slight corrosion occurred on the surfaces of the samples immersed in a 0.25wt% Na2S2O8 solution. However, pitting corrosion occurred on the surfaces of the samples immersed in a 3.5wt% NaCl solution, whereas exfoliation corrosion occurred on the surfaces of the samples immersed in a mixture of 0.25wt% Na2S2O8 and 3.5wt% NaCl solutions. A power exponent relationship was observed between the mass loss and exposure time of the 7005-T4 aluminum alloy immersed in the three kinds of solutions. In the mixture of 0.25wt% Na2S2O8 and 3.5wt% NaCl solutions, the mass loss of the aluminum alloy yielded the maximum value. Based on the calculation of the correlation coefficients, the alternating wet–dry procedure in a 3.5wt% NaCl solution could be used to predict the corrosion behavior of 7005-T4 aluminum alloy exposed in the atmosphere of Qingdao, China. The prediction model is as follows: T = 104.28·t0.91, where T is the equivalent time and t is the exposure time.

The correlation between the distribution/size of carbides and corrosion resistance of 17Cr-1Ni ferritic-martensitic stainless steel after different heat treatment temperatures was investigated by transmission electron microscope, electrochemical tests, and corrosion morphology observations. The results showed that the size of the precipitated phase decreased and corrosion resistance increased with an increase in the annealing temperature. When the tempering temperature was low (290 °C), carbides precipitated mainly at the phase boundaries due to a low degree of atomic matching and higher grain boundary energy. In this case, the polarization curve had a passivation interval and the pits were mainly initiated at the phase boundary. When the tempering temperature was higher than 400 °C, the carbides gradually precipitated in the martensite laths because the accelerated diffusion of Cr healed the Cr depletion zone at phase boundary. This outcome resulted in a polarization curve that had no passivation range and uniform corrosion occurred in martensitic region.

Pure copper, tin copper and beryllium copper were exposed to a harsh marine environment, and the effects of tin and beryllium addition on copper corrosion were systematically investigated and compared. The corrosion rate for pure copper was similar with that for tin copper, but significantly higher than that for beryllium copper, indicating an improvement in corrosion resistance due to the addition of beryllium. Pitting corrosion occurred in all investigated materials and the extent of pitting damage was in the order of pure copper > tin copper > beryllium copper. Flaking corrosion was prominent on tin copper plate after long-term exposure and the primary corrosion products in all materials were Cu2O, CuCl and Cu2Cl(OH)3. The tin content in the inner corrosion layer was higher than in the outer layer for tin copper, whereas beryllium accumulated at the outer layer of beryllium copper, confirmed by auger electron spectroscopy, and related corrosion mechanisms were proposed.

Fine gas-atomized powders were prepared for fabricating 316L stainless steel by selective laser melting (SLM) for medical implant applications. The effect of 3D printing laser power on the microstructure, biocompatibility, mechanical and corrosion properties of SLM 316L were systematically investigated and compared with the traditionally quenched. The SLM 316L showed irregular grains with many sub-grains and a thicker passive film with lower Mo content than the quenched 316L. SLM 316L formed at high laser power showed improved elongation, corrosion resistance, and biocompatibility, while that prepared at low power showed poorer performance and was unstable during long-term experiments in simulated body fluid. Keywords: 3D printing, 316L stainless steel, Selective laser melting, Transmission electron microscopy, Corrosion, Bio-compatibility

Microstructure and corrosion behavior of 316L stainless steel fabricated by selective laser melting (SLM) for bipolar plate were investigated and the subsequent heat treatment effect was also clarified. Results showed that sub-grains widely existed in the SLM 316L and the dislocations in the grain boundary migrated and disappeared after heat treatment. The as produced SLM 316L exhibited inferior corrosion resistance than the wrought in 0.5 M H2SO4 solution with 50 ppm Cl− and 2 ppm F− ions at 70 °C due to the SLM defects (molten pool boundaries, non-equilibrium phases etc.), and short, stress relief annealing did not homogenize the non-uniform structures. However, recrystallization heat treatment can improve the durability due to the more uniform structure and thicker passive film. Meanwhile, the proportion of the oxides in passive film formed on SLM 316L increased after heat treatment, especially for the chromic oxide, leading to a better anti-corrosive property.

The evolution of the corrosion process on Cu-Ni-Zn alloy in alkaline chloride solution was investigated by in-situ scanning electrochemical microscopy, X-ray photoelectron spectroscopy, and ex-situ laser confocal microscopy, and the effects of ambient temperature and polarization time were also discussed. The results demonstrated a higher pitting nucleation rate and lower pit growth rate at low temperature. The ratio of pit depth to mouth diameter decreased with increasing pit volume and temperature, indicating that pits preferentially propagate in the horizontal direction rather than the vertical direction owing to the presence of corrosion products and deposited copper. The surface current was uniform and stabilized at approximately 2.2 nA during the passive stage, whereas the current increased after the pits were formed with the maximum approaching 3 nA. Increasing the temperature led to an increase in porous corrosion products (CuO, Zn(OH)2, and Ni(OH)2) and significantly increased the rate of transition from pitting to uniform corrosion. Dezincification corrosion was detected by energy dispersive spectrometry, and a mechanism for pitting transition into uniform corrosion induced by dezincification at the grain boundaries is proposed.

In this study, the 5-Amino-1,3,4-thiadiazole-2-thiol grafted graphene oxide (AMT-GO) composite was synthesized and the corrosion protection properties of the neat EP, GO/EP and AMT-GO/EP coatings at the ambient and low temperatures were comparatively studied. Based on the experimental results, AMT is anchored on GO mainly via the reactions between -NH2 and -SH of AMT and -COOH and -CH(O)CH- of GO. After modification, the dispersity of the GO nanosheets is significantly improved. As the filler, both of the GO and AMT-GO nano-platforms could enhance the protective ability of the polymer coating, the latter possessing the more remarkable effect because of its better dispersity and the beneficial impact provided by exposed -NH2 and -SH. All of the three coatings exhibit worse anti-corrosion performance at the low temperature than the ambient temperature, because the diffusion of corrosive media across the polymer matrix and along the substrate/coating interface could be accelerated by freezing.

The semiconducting properties of passive films formed on copper, in anaerobic alkaline sodium chloride solution, were studied using Mott–Schottky analysis and electrochemical impedance spectroscopy, based on the point defect model. Results showed that the corrosion resistance increased with increasing potential, which was attributed to a well crystallised, refined grain structure, and a thicker passive film at higher potential. P-type semiconducting characteristics were obtained with or without chloride. The density of copper vacancies was approximately 1020 cm−3, and increased with the increasing chloride concentration, which was attributed to faster film-formation in a higher chloride environment. The diffusion coefficient of the defects, a key dynamic parameter for passive film breakdown, was in the range of 10−16–10−15 cm2 s−1, and increased with increasing chloride concentration, thus leading to a greater probability of pitting.

Influence of temperature on the electrochemical and passivation behavior of 2507 super duplex stainless steel in the simulated desulfurized flue gas condensates in thermal power plant chimney are investigated. Cathodic reactions are enhanced with temperature, accompanying with the involvement of oxygen reduction when the temperature is raised to 60 °C or higher. The unchanged semiconductor type, enrichment of Cr and thickening of the passive film are observed as the temperature increases. The temperature-induced variations of film structure and doping concentrations are responsible for the degradation of protective ability of the passive film.

The structure and property of passive film on copper are strongly dependent on the sulfide concentration; based on this, a series of electrochemical methods were applied to investigate the effect of sulfide concentration on copper corrosion in anaerobic chloride-containing solutions. The cyclic voltammetry and x-ray photoelectron spectroscopy analysis demonstrated that the corrosion products formed on copper in anaerobic sulfide solutions comprise Cu2S and CuS. And the corrosion resistance of copper decreased with increasing sulfide concentration and faster sulfide addition, owing to the various structures of the passive films observed by the atomic force microscope and scanning electron microscope. A p-type semiconductor character was obtained under all experimental conditions, and the defect concentration, which had a magnitude of 1022–1023 cm−3, increased with increasing sulfide concentration, resulting in a higher rate of both film growth and dissolution.

The effects of rare earth element Nd on the microstructure, mechanical, shape memory properties and corrosion behaviour of Cu-13.0Al-4.0Ni-xNd (wt.%) high-temperature shape memory alloys were investigated in this paper. It was observed that the grains in Cu-13.0Al-4.0Ni-xNd alloys can be refined by adding Nd, and the grain size decreased from the millimeter scale to several hundred microns. The matrix of the Cu-13.0Al-4.0Ni-xNd alloys, composed of 18R and 2H martensite, was not changed by Nd doping. However, the hexagonal (Al,Ni)Cu4Nd phase was formed as the Nd content increased. The grain refinement led to an improvement in the mechanical properties of the Cu-13.0Al-4.0Ni-xNd alloys. When x = 0.5, the compressive fracture stress and strain increased from 580 MPa to 10.5% to 940 MPa and 18.3%, respectively. The Nd addition also enhanced the shape memory effect of the Cu-13.0Al-4.0Ni-xNd alloys, and a reversible strain of 6.8% was obtained for x = 0.2 under a prestrain of 10% after heating. In addition, the results of electrochemical impedance measurement and polarization measurement showed that the corrosion resistance of Cu–Al–Ni alloy were slightly reduced by Nd addition, and the corrosion behaviour of Cu–Al–Ni alloy was mainly controlled by Cu element.

Micro-galvanic corrosion behaviors between two primary intermetallic particles (Mg2Si, α(Al-Fe-Cu)) and the Al matrix were investigated under elastic and plastic deformations in AA7075-T6. Strain localization was mainly concentrated within Mg2Si and at the interface of α(Al-Fe-Cu)/Al matrix under deformations. The experimental Volta potential differences between Mg2Si, α(Al-Fe-Cu) and Al decreased by 0–16 mV and 2–27 mV in the elastic stage but increased by 0–39 mV and 0–81 mV in the plastic stage, which were consistent with the calculation results. Accordingly, the pitting corrosion resistance was improved under elastic deformation but deteriorated under plastic deformation.

The quick-emerging paradigm of additive manufacturing technology has revealed salient advantages in enabling the tailored-design of structural components with more exceptional performances over ordinary subtractive processing routines. As a peculiar feature, sub-micro cellular structures widely exist in additively manufactured (AM) metallic materials. This phenomenon primarily appears with high-density dislocations and segregated elements or precipitates at the cellular boundaries. The discovery of novel metastable substructures in various alloys through numerous investigations has proven their substantial effects on the engineering properties of AM components. This paper reviews the most recent research momentum regarding the formation mechanisms (elemental segregation, dislocation cell and oxide inclusion), the kinetics of the size and morphology, the growth orientation and the thermodynamic stability of these cellular structures by taking AM austenitic stainless steel as an exemplary material. Another topic of concern here is the inherent correlation between the unique cellular microstructure and the corresponding mechanical properties (strength, ductility, fatigue, etc.) and corrosion responses (passivity, irradiation damage, hydrogen embrittlement, etc.) for this category of AM materials. The design, control, and optimization of cellular structures for additive manufacturing techniques are expected to inspire new strategies for advancing high-performance structural alloy development.

The corrosion behavior of the SLM and wrought Ti6Al4V are comparatively investigated. The critical NaF concentrations of SLM Ti6Al4V in the three cases of pH = 2, 4 and 6 are 0.0020 M, 0.080 M and 0.10 M, respectively, which are similar to those of the wrought sample. The repassivation process occurs when NaF concentrations are much larger than the critical values. The crystallinity and the Al and F contents decrease when the NaF concentration and pH value rise. As compared to the wrought Ti6Al4V, the SLM sample exhibits significantly worse corrosion resistance in almost all the test solutions.

Electrochemistry methods were used to investigate the influence of pH on the passive film and corrosion behavior of ultrahigh strength AM355 stainless steel in chloride-containing media. Analysis of the Pourbaix diagram indicates that AM355 stainless steel exhibits higher corrosion resistance in natural and near-natural environments than that in acidic and alkaline conditions. Electrochemistry measurements and composition analysis of the passive film show that pitting potential increased due to the enhanced repassivation capacity of AM355 stainless steel with increasing pH. The mixed MnS/oxide inclusions are the main pitting sensitive locations under all conditions. Morphological observations and energy-dispersive spectroscopy showed that the influence of the gap between the martensitic laths is significant with increasing pH. The inclusions, element concentrations, and microstructures weaken the resistance of ultrahigh-strength martensitic AM355 stainless steel against corrosion.

The influence of oxide scales on the corrosion behaviors of B510L hot-rolled steel strips was investigated in this study. Focused ion beams and scanning electron microscopy were used to observe the morphologies of oxide scales on the surface and cross sections of the hot-rolled steel. Raman spectroscopy and X-ray diffraction were used for the phase analysis of the oxide scales and corrosion products. The corrosion potential and impedance were measured by anodic polarization and electrochemical impedance spectroscopy. According to the results, oxide scales on the hot-rolled strips mainly comprise iron and iron oxides. The correlation between mass gain and test time follows a power exponential rule in the damp-heat test. The corrosion products are found to be mainly composed of γ-FeOOH, Fe3O4, α-FeOOH, and γ-Fe2O3. The contents of the corrosion products are different on the surfaces of the steels with and without oxide scales. The steel with oxide scales is found to show a higher corrosion resistance and lower corrosion rate.

In this study, the different corrosion behaviors between α and β phases of Ti6Al4V in solutions with different NaF concentrations are investigated. When the NaF concentration changes from 0.15 to 0.20 M, α phases are more likely to suffer from corrosion than β phases. However, the formation of a repassive film makes α phases exhibit superior corrosion resistance than β phases as the NaF concentration reaches 0.50 M. The mechanism analysis reveals that the formation of a repassive film on the α phases depends on not only the Al content but also on the active dissolution rate of Ti.

In this study, the pitting behaviors of wrought and SLM 316L SSs (stainless steels) are comparatively investigated under the conditions of different aggressiveness. The experimental results show that SLM 316L SS exhibits higher sensitivity to pitting corrosion in extremely aggressive solutions, while the wrought sample is more vulnerable to pitting in conditions with low aggressiveness. Gas pores are the pitting-sensitive sites in SLM 316L SS, and metastable pits initiated at covered gas pores are easier to evolve to stable growth compared to open pores due to the greater diffusion resistance.

The effect of hydrogen charging on the microstructure and durability of traditional wrought and selective laser melted 316L stainless steels (SLMed 316L SSs) in a proton exchange membrane fuel cell was studied. The results showed that superior resistance to hydrogen damage was achieved by the SLMed 316L SS due to minimal transformation of austenite into martensite and that corrosion preferentially occurred at the martensite sites. The defect density of the passive film on the hydrogen-charged wrought 316 L SS was much higher than that of the SLMed counterpart, indicating the excellent passive film stability of the SLMed 316L SS.

The microstructural optimization for selective laser melted 15-5PH stainless steel via heat treatment to enhance corrosion resistance was investigated. The results showed that after aging treatment, Cu-rich nanoparticles (about 10 nm) diffusely precipitated, and approximately 18∼25 % austenite was distributed near the molten pool boundary. The surface potential of the austenite was approximately 15 mV higher than that of martensite by scanning Kelvin probe force microscopy. However, the austenite phase disappeared and the new NbC-(Mn, Si)O duplex particles precipitated after solution treatment and aging treatment, which decreased the pitting corrosion resistance and passive film stability of the SLM 15-5PH stainless steel.

The long-term corrosion behavior of magnesium alloy AZ31 exposed to a dry tropical desert environment was investigated. The results of field-exposure test revealed that after an initial rapid period, the corrosion rate slowed down as the corrosion product layer protected the surface until cracks formed, allowing for the corrosion rate to accelerate. An accelerated corrosion test confirmed that the condensation caused by the large diurnal temperature difference, which could support the occurrence of electrochemical reactions, had a decisive effect on the corrosion of alloy AZ31 at the exposure site.

The electrochemical behavior and the characteristic of 2205 duplex stainless steel (UNS S32205) under various hydrogen charging conditions in borate buffer solution containing chloride ion were evaluated by potentiodynamic measurements, electrochemical impedance spectroscopy, capacitance measurements (Mott-Schottky approach), and anodic polarization experiments. The results indicate that the hydrogen generated by the cathodic charging process decreased the corrosion resistance of 2205 duplex stainless steel, especially the pitting resistance of passive film. As a result of the hydrogen charging, the semiconductor characteristic of the passive film changed from n-type and p-type coexisting to only n-type existing, and the pitting susceptibility increased. Along with the charging process, the critical concentration of chloride for breakdown of passive film decreased and the repassivation process was prevented by hydrogen.