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Nickel‐free high‐nitrogen‐alloyed stainless steels like the P2000 (X13CrMnMoN18‐14‐3) were developed to enhance the strength and corrosion resistance of austenitic stainless steels like 304 and 316 while keeping the typical high ductility. The mechanical and corrosive properties of P2000 were investigated and compared with 304 and 316 to highlight the application opportunities of this new alloy. The microstructure of the solution‐annealed condition was characterised by electron backscatter diffraction and the mechanical properties were studied by uniaxial tensile tests, Charpy impact tests and hardness measurements. The passivation behaviour was analysed using the electrochemical potentiodynamic reactivation, whereas the pitting corrosion resistance was compared by pitting potentials and pitting temperatures. However, secondary thermal influences or suboptimal heat treatment can impair the corrosion resistance due to the precipitation of secondary phases and the resulting sensitisation. Thermodynamic calculations and artificial ageing treatment in the range of 500–900°C for up to 100 h were used to determine critical time–temperature parameters for sensitisation. The microstructure of the various aged states was evaluated by scanning electron microscopy and compared with the degrading corrosion resistance characterised by the KorroPad method.
Tetraethyl orthosilicate (TEOS) was used as a chemical precursor to deposit ultra-thin SiO x C y plasma polymer films onto mild steel surfaces for preventing the corrosion process. The structure–property relationships of the coatings were evaluated by X-ray Photo Spectroscopy (XPS), X-Ray Diffraction (XRD), Fourier Transform InfraRed spectroscopy (ATR-FTIR) and Energy Dispersive X-ray spectroscopy (EDX) completed with Scanning Electron Microscopy (SEM). The SEM micrographs confirmed a pinhole-free surface morphology of the low-pressure deposited plasma polymer films. The TEOS molecules become fragmented in the plasma by numerous collisions with energy-rich electrons and heavier particles. Recombination of fragments and condensation onto the steel substrate is responsible for the formation of organic SiO containing plasma polymer layers. Such thin layers consist of predominantly SiO x structures. Their properties are determined largely by the gap distance between the two samples used as electrodes in the plasma. The efficiency of the corrosion-protecting coating was compared with uncoated samples. The corrosion protection was determined by exposure of samples to 3.5% NaCl aqueous solutions. For this purpose, polarization and Electrochemical Impedance Spectroscopy (EIS) were used to monitor the corrosion. The optimal gap distance between the electrodes was determined for corrosion protection. The best protective efficiency reached more than 97% of the total protection as measured at room temperature.
The 13CrMo4-5 ferritic steel is commonly used in power plants, due to its favorable mechanical properties. According to EN 10028-2, this steel can be used at temperatures up to 570°C because of its creep behavior. The inefficient corrosion resistance limits the application of this steel to lower temperatures depending on the gas temperature and slag formation. Therefore, the application of a highly resistant Ni-based coating is proposed to extend the corrosion resistance of elements made of ferritic steel. The corrosion test was performed in an environment containing a mixture of gases, like O2, COx, and SOx, and deposited ashes with elements, e.g., Na, Cl, Ca, Si, C, Fe, and Al. The exposure time was, respectively, 240 h, 1000 h and 4500 h at a temperature of 600°C. The oxide scale formed on the 13CrMo4-5 steel was significantly thicker than on the IN686 coating. The microstructure and chemical and phase compositions of the oxide scale were investigated using light optical microscopy together with scanning and transmission electron microscopy techniques. Energy dispersive x-ray analyses were performed when appropriate.
The ferritic steel 13CrMo4-5 due to good properties with relation to attractive price is frequently use in power plants industry. According EN10028-2 this steel can be used up to 570 °C because of its creep behavior but its corrosion resistance limits the use frequently to lower temperatures, depending on gas temperature and slag formation. The corrosion test were performed in environment containing mixture of gases like: O2, COx, SOx and ashes, with elements e.g. Na, Cl, Ca, Si, C, Fe, Al. Exposure time was respectively 240 h, 1000 h and 4500 h in temperature 600 °C. The oxide scale on the 13CrMo4-5 steel was significant thicker than for In686 coating and the difference increase according for longer exposure time.
The microstructure, chemical and phase composition of the oxide scales were investigated by means of a light microscope, the electron scanning and transmission microscopes (SEM,TEM) equipped with the EDS detectors.
The copper alloyed stainless steel 1.4542 (X5CrNiCuNb16-4) is used in different areas due to its good mechanical properties and corrosion resistance. Strength and corrosion resistance can be adjusted by the heat treatment, which is of importance for the application of this alloy. The mechanical properties (strength and hardness) are attributed to the dispersive precipitation of the copper rich ε–Phase. The additional precipitation of chromi-um carbides can reduce the corrosion resistance. Different ageing states were produced to investigate the precipitation behaviour with various methods. Furthermore, the influence of cold-rolling on the precipitation behaviour was studied in comparison to a solution annealed state without deformation. The microstructure was studied by SEM and the variations of hardness and magnetic proportion were characterised. The electrochemical potentiodynamic reactivation (EPR) was used to determine the corrosion resistance and detect chromium depletion in all heat-treated states. The results show that a work hard-ening accelerates the precipitation rate, while ageing at 600 °C reduces the corrosion resistance due to chromium depletion.
The copper alloyed stainless steel 1.4542 (X5CrNiCuNb16-4) is used in different areas due to its good mechanical properties and corrosion resistance. Strength and corrosion resistance can be adjusted by the heat treatment, which is of importance for the application of this alloy. The mechanical properties (strength and hardness) are attributed to the dispersive precipitation of the copper rich ε–Phase. The additional precipitation of chromi-um carbides can reduce the corrosion resistance. Different ageing states were produced to investigate the precipitation behaviour with various methods. Furthermore, the influence of cold-rolling on the precipitation behaviour was studied in comparison to a solution annealed state without deformation. The microstructure was studied by SEM and the variations of hardness and magnetic proportion were characterised. The electrochemical potentiodynamic reactivation (EPR) was used to determine the corrosion resistance and detect chromium depletion in all heat-treated states. The results show that a work hard-ening accelerates the precipitation rate, while ageing at 600 °C reduces the corrosion resistance due to chromium depletion.
The copper alloyed stainless steel 1.4542 (X5CrNiCuNb16-4) is used in different areas due to its good mechanical properties and corrosion resistance. Strength and corrosion resistance can be adjusted by the heat treatment, which is of importance for the application of this alloy. The mechanical properties (strength and hardness) are attributed to the dispersive precipitation of the copper rich ε–Phase. The additional precipitation of chromium carbides can reduce the corrosion resistance. Different ageing states were produced to investigate the precipitation behaviour with various methods. Furthermore, the influence of cold-rolling on the precipitation behaviour was studied in comparison to a solution annealed state without deformation. The microstructure was studied by SEM and the variations of hardness and magnetic proportion were characterised. The electrochemical potentiodynamic reactivation
(EPR) was used to determine the corrosion resistance and detect chromium depletion in all heattreated states. The results show that a work hardening accelerates the precipitation rate, while ageing at 600 °C reduces the corrosion resistance due to chromium depletion.
The double loop electrochemical potentiodynamic reactivation (EPR) method is a standardised procedure for detecting and quantifying sensitisation on austenitic, ferritic and ferritic-austenitic stainless-steel grades. This sensitisation is caused by microstructural alternations, as the formation of chromium carbides, nitrides or sigma-phases, which generate local chromium depleted zones nearby. The latter strongly influence the corrosion resistance and the electrochemical response of a stainless steel during electrochemical potentiodynamic reactivation, providing important information on the degree of chromium depletion. In case of martensitic stainless steels, which are used for cutlery and surgical instruments, the heat treatment has a strong impact on the microstructure, the material properties and especially on the corrosion resistance. To study this interaction, the EPR method was modified for the application on martensitic stainless steels with about 13 wt.-% chromium. Different H2SO4 concentrations and EPR-parameters were tested and compared on two standard martensitic stainless-steel grades (AISI 420 A / X20Cr13 and AISI 420 C / X46Cr13) to define applicable parameters. Afterwards, these parameters were used to study the effect of austenitisation time and cooling rate on the corrosion resistance of both martensitic stainless steels. The response of both alloys was different due to the different carbon levels, which will be explained by microstructural investigations in detail. All results allow postulating a process window, in which chromium depletion is suppressed and an optimised corrosion resistance is guaranteed. Based on this research, the modified EPR-test is now used to control the heat treatment and its impact on the corrosion resistance of martensitic stainless steels in the cutlery industry. The EPR-test is thus an interesting alternative for manufacturers, processors, users and researchers to the time-consuming exposition test normally used to characterise the corrosion resistance of martensitic stainless steels.
Hardness and pitting corrosion resistance are the major quality criteria of cutlery. Both are achieved by the heat treatment (austenitization, quenching and tempering) of the normally used martensitic stainless steels. The established quality control method regarding the pitting corrosion resistance is an alternating immersion test in 1 % NaCl solution at 60 °C according to DIN EN ISO 8442. This standard test shows a high deviation, which limits any optimization of the heat treatment process. New approaches for corrosion testing of martensitic stainless-steels were developed and used in the last years to connect the weak pitting corrosion resistance of martensitic stainless-steels with the phenomenon of chromium depletion. The tempering temperatures used in the industrial heat treatment of cutlery are too low to explain the appearance of chromium depletion. For this reason, a systematic investigation of three heat treatment parameters (austenitization time, cooling speed and tempering temperature) were performed on the martensitic stainless-steels X50CrMoV15 (1.4116) to detect their contribution to chromium depletion. The electrochemical potentiodynamic reactivation (EPR), which is very sensitive to any change of the microstructure, was used to quantify the degree of chromium depletion. The KorroPad indicator-test was applied to correlate low pitting corrosion resistance to the presence of chromium depletion. The results of all investigations allow conclusions about the very small process window, which is necessary to achieve cutlery with high pitting corrosion resistance.
The KorroPad indicator test was invented at the Federal Institute for Materials Research and Testing to detect stainless-steel surfaces susceptible to pitting corrosion. The KorroPad indicator test is thus an interesting alternative for manufacturers, processors, users and researchers to time consuming exposition experiments and complex electrochemical investigations, which are normally used to reveal surface-related processing errors resulting in reduced pitting corrosion resistance. Furthermore, the KorroPad indicator test can be used to visualize alloy- and microstructure-related factors causing susceptibility to pitting corrosion. The detection limit of the KorroPad indicator test was characterized in this work using reference alloys with different chromium content (5 % to 18 %). The alloy-specific detection limit was shifted successfully to higher chromium content by increasing the concentrations of NaCl and K3[Fe(CN)6]. The modified KorroPads can now be used to establish an alloy-specific quality control regarding the pitting corrosion resistance of different stainless steel grades.