The Influence of Nb Addition on the Passivation and Pitting Behavior of CoCrNiNb <sub>x</sub> Medium Entropy Alloys
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The effect of Nb addition on the passivation and pitting behavior of CoCrNiNbx (x=0~0.5) medium-entropy alloys (MEAs) were investigated withsurface characterization and electrochemical techniques. With the addition of Nb, the Nb-riched HCP dendrite phase gradually precipitates from the original FCC phase. The pitting corrosion resistance of CoCrNiNbx MEAs increases firstly and then starts to decrease, reaching an optimal value at x = 0.2. XPS depth profiles and TOF-SIMS characterization of the surface passive films demonstrate that the film structure is transformed from a single layer to the bilayer which consists of a Cr-enriched outer layer and an Nb-enriched inner layer. Analysis of the pitting and passivation behaviour of the CoCrNiNbx MEAs suggests that a moderate content of Nb thickens the passive film, promotes the production of Nb oxides and Cr2O3, and thus stabilizes the passivation film and finally reduces pitting sensitivity.Keywords:
Passivation
Pitting Corrosion
Stability of the passive 18-8 steel in acid solution is found to depend on the potential during the etching treatment as well as the passivation treatment. The potential of the steel during the passivation treatment is controlled by changing the concentration of nitric acid solution or by using a potentiostat. Also a constant potential of the steel during the etching treatment is attained in a dilute nitric acid solution or in a concentrated sulphuric acid solution with or without applying the external polarization. Stability which is decided by measuring the self-activation time in oxygen-free sulphuric acid solution increases with increasing the potential of the passivation treatment, and a critical potential, at which the stability changes abruptly, is admitted irrespective of the method of the passivation treatment, i. e., chemical passivation or potentiostatically controlled passivation. Structural changes of the passive film are concluded to take place at this critical potential of 0.4 volt (vs. SCE). Etching potential, also, changes the self-activation time of the passive steel treated at a constant passivation condition. The maximum stability is obtained at -0.32V of the etching potential. This fact is explained by assuming the selective enrichment of chromium on the surface bofore passivation.
Passivation
Nitric acid
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Passivation
Duplex (building)
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22% Cr-3% Mo duplex stainless steels in which Ni and N contents were varied independently were prepared and base metal and TIG weld metal were subjected to pitting corrosion tests in 6% FeCl3. The critical pitting temperature at which pitting corrosion occurred was evaluated and the pitting site was observed. The pitting test results were examined metallurgically from the viewpoint of the precipitation of Cr carbide and nitride and the distribution of elements between ferrite phase and austenite phase.Since the pitting corrosion resistance of austenite phase is mainly dominant in base metal, the decrease of Ni and the increase of N cause the increase of N in austenite phase and improve the total corrosion resistance. As the decrease of Ni and N reduces the generation of austenite phase and results in the precipitation of Cr carbide and nitride in weld metal, the decrease of Ni and N deteriorates the pitting corrosion resistance of weld metal.
Pitting Corrosion
Base metal
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Passivation
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As stainless steel is mnormally selected for the equipment in petrochemical plants,the equipment is subject to pitting corrosion.In the last several years,a high nitrogen steel has been developed abroad,which not only improves the steel’s pitting corrosion resistance and strength but also replaces some nickels,which is more economical.The evaluation methods of properties of high nitrogen stainless steel are introduced.Based upon ASTMG48,the tested critical pitting corrosion temperature(CPT) and crevice corrosion temperature(CCT) are related to pitting corrosion resistance equivalent(PRE).When PRE = Cr + 3.3Mo + 16 ～ 30 N,it is indicated that nitrogen has played a very important role in improvement of pitting corrosion resistance of stainless steel.The improvement of pitting corrosion resistance of stainless steel is the result of formation of NH4+ and nitrate radical ions and stabilization of surface film,etc.The impacts of other elements on pitting corrosion resistance of stainless steel are also studied.Cr ad Mo have a very good impact.W and Cu have a positive impact.Mn,sulfur and Ni have a negative impact.The high-N stainless steels such as 200 series Mn-Ni—N stainless steel,deplex steel and super austenitic stainless steel,etc are studied.
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Crevice corrosion
Austenitic stainless steel
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Floating junction (FJ) passivation is a relatively recent passivation scheme which has both experimentally and theoretically demonstrated superior passivation than either oxide or back surface field passivation. In addition, it is suited not only to high efficiency laboratory cells, but also to commercial solar cells. The improvement in surface passivation for commercial cells is an especially critical issue in achieving lower cost solar cells through reducing substrate thickness while simultaneously increasing efficiency. Despite the many advantages of FJ passivation, its applicability has been limited by the apparent inability to translate the excellent modelling results into actual solar cells. The objective of this paper is to present a complete analysis of FJ passivation and to demonstrate a method by which the problems with FJ passivation can be eliminated. Experimental evidence as well as theoretical modelling demonstrates that a solar cell with an optimized rear FJ is insensitive to parasitic effects.
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In order to get the fundamental knowledge about the pitting corrosion in austenitic stainless steel welds, some experiments with TIG welds of nine kinds of steels were conducted. The pitting phenomenon occurring in ferric chloride solution was observed minutely, and the localized corrosion potential distribution at the weld was measured to. explain the location where the pitting corrosion is apt to occur. The following results were obtained: There were three types of forms in a weight loss of austenitic stainless steel welds by pitting corrosion in ferric chloride solution. When a delta ferrite phase appeared in weld metals, the pitting corrosion concentrated on the weld metal, and this resulted in a large weight loss. The measurement of the localized corrosion potential distribution revealed that the locations which were likely to be anodic were a slaglike oxide-carbide deposited on the weld metal surface, the interface between the slaglike oxide-carbide and a matrix, and the ripple lines on the weld metal surface.
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Austenitic stainless steel
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The electrochemical performance of 317L stainless steel used in medicine under different conditions of passivation (different contents of HNO3 solution, different passivation time and different passivation temperatures) was studied. The results show that the pitting potential of 317L stainless steel used in medicine can reach about 1.0 V (SCE) when electrochemically tested in 0.9% NaCl solution after the steel was passivated in 30% HNO3 solution at 35℃ for 6 h, which indicates that the passivation film has a relatively strong resistance to corrosion. The results also show that the corrosion resistance of the passivation film on the surface of 317SS can be increased after suitable amount of K2Cr2O7 is added into HNO3 passivation solution.
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An environmentally acceptable Cr-and P-free passivation agent SF-563 suitable for coating of Al wares was developed.The optimized passivation technology was established,and the properties of as-obtained passivation coating were evaluated.As the results,the optimized passivation parameters are suggested as bath concentration of 2%(mass fraction),pH value of 2.5～3.5,passivation time of 60～90 s,and passivation temperature of 10～35 ℃.Upon completion of passivation,the treated Al alloy surface does not need to be washed with water and can be directly dried at a maximum temperature of 120 ℃.Besides,the high-performance chemical conversion coating obtained under the optimized passivation parameters possesses excellent performance comparable to that of hexavalent Cr passivation coating,showing promising application in pre-treatment of Al alloy wares before coating.
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Hexavalent Chromium
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Anodizing
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