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. Hard coated steel components are used in a wide application range, mostly for protective, wear resistant and decorative purposes. Despite of these coatings being regarded as relatively dense, there is always a high risk of localized corrosion when a coated low alloyed steel component encounters a surrounding high humidity atmosphere or even an aqueous medium. An approach to enhance the corrosion properties is the addition of magnesium to physical vapor deposited hard coatings, like TiN. It has been found that there is a remarkable increase in corrosion resistance in dependence of magnesium content of the TiMgN coating and its surface properties. In this work the authors will explain the underlying corrosion protection mechanisms by means of electrochemical and analytical studies. The positive impact of magnesium in the coating relates on its preferred dissolution vs. steel. This causes the potential to shift to more negative direction with respect to the steel substrate and additionally leads to a temporarily passivation of the steel due to alkalization of the surrounding electrolyte by formation of magnesium hydroxide.
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.
The influence of isothermal ageing on microstructure, sensitisation and pitting corrosion resistance of the lean duplex stainless steel (LDSS) X2CrNiN23-4 was investigated with various electrochemical methods. The aging at 600 °C (from 0.1 h up to 20 h) lead to the formation of precipitations at the ferrite-ferrite (α/α) and ferrite-austenite (α/γ) grain boundaries, inducing sensitisation due to chromium depletion. The degree of sensitisation was evaluated with the double loop electrochemical potentiokinetic reactivation method (DL-EPR) according to ASTM G108 and correlated with critical pitting potentials (Epit) as well as critical pitting temperature (CPT) measured in an electrolyte according to ASTM G48 using electrochemical noise. Up to an ageing time of 1 h, the sensitisation did rise significantly, stabilising at a nearly constant level with a slight drop at 20 h. This behaviour correlated perfectly with the potentiodynamically determined pitting potentials Epit and sensitisation. The CPT showed a higher sensitivity at short ageing times compared to the DL-EPR and Epit. Finally, the KorroPad method was applied to visualise the sensitisation induced reduction of pitting corrosion resistance. The “KorroPad” is an agar-based gel-electrolyte containing 0.1 mol/l sodium chloride (NaCl) and 0.1 mol/l potassium ferricyanide III (K3[Fe(CN)6]), invented at the Federal Institute of Materials Research and Testing in Berlin (Germany) to detect surfaces of stainless steel prone to pitting corrosion. The standard configuration of the KorroPad showed no differentiation for the various aging conditions. Increasing the concentration of both NaCl and potassium ferrocyanide III to 0.5 M shifts the detection limit of the KorroPad method to stainless steels with higher corrosion resistance, producing the same trends detected by standard electrochemical pitting corrosion values (Epit, CPT) and sensitisation (DL-EPR). By that, the KorroPad method was successfully adjusted to the lean-duplex stainless steel X2CrNiN23-4, enabling short-time testing to detect sensitization.
Geothermal brine is a complex system containing a wide variety of dissolved salts resulting from the condition s in a geothermal well. These fluids lead to corrosion in pipes and other parts of geothermal system construction and necessitate intense research efforts in finding new suitable materials. Carbon steel is susceptible to corrosion in geothermal brine especially when it is exposed to a high temperature and high-pressure medium, which is considered to be an aggressive environment. An artificial geothermal water, bas ed on a brine composition found in Indonesia, was used to investigate the performance of high alloyed materials. The electrolyte has pH 4 and contains 1,500 mg/l Cl-, 20 mg/l SO4 2-, 15 mg/l HCO3 -, 200 mg/l Ca 2+, 250 mg/l K+, and 600 mg/l Na+. In order to protect the bare material in geothermal application, it is necessary to either use high alloyed material s or coatings. In this research, a coating system consisting of polyaniline and silicon dioxide was investigated regarding its behavior to protect carbon steel. In detail, the effect of SiO2 and polyaniline (PANi) addition was evaluated by exposure and electrochemical tests for 7 days, i.e. electrochemical impedance spectroscopy (EIS) and open circuit potential (OCP ) at room temperature and 150 °C with 1 MPa pressure .
Electrochemical study of polyaniline/silicon dioxide containing coatings in geothermal solution
(2018)
Polyaniline/Silicon Dioxide containing coatings were electrochemically investigated in a saline geothermal solution.
With the increase of exposure time, impedance values of coated specimens decreased at low frequency, which could be caused by the decrease of pore resistance due to electrolyte or water uptake.
Coating system needs further optimization work.
Geothermal brine is a complex system containing a wide variety of dissolved salts resulting from the condition s in a geothermal well. These fluids lead to corrosion in pipes and other parts of geothermal system construction and necessitate intense research efforts in finding new suitable materials. Carbon steel is susceptible to corrosion in geothermal brine especially when it is exposed to a high temperature and high-pressure medium, which is considered to be an aggressive environment. An artificial geothermal water, bas ed on a brine composition found in Indonesia, was used to investigate the performance of high alloyed materials. The electrolyte has pH 4 and contains 1,500 mg/l Cl-, 20 mg/l SO4 2-, 15 mg/l HCO3 -, 200 mg/l Ca 2+, 250 mg/l K+, and 600 mg/l Na+. In order to protect the bare material in geothermal application, it is necessary to either use high alloyed material s or coatings. In this research, a coating system consisting of polyaniline and silicon dioxide was investigated regarding its behavior to protect carbon steel. In detail, the effect of SiO2 and polyaniline (PANi) addition was evaluated by exposure and electrochemical tests for 7 days, i.e. electrochemical impedance spectroscopy (EIS) and open circuit potential (OCP) at room temperature and 150 °C with 1 MPa pressure.
Hard coated steel components are used in a wide application range, mostly for protective, wear resistant and decorative purposes. Despite of these coatings being regarded as relatively dense, there is always a high risk of localized corrosion when a coated low alloyed steel component encounters a surrounding high humidity atmosphere or even an aqueous medium. An approach to enhance the corrosion properties is the addition of magnesium to physical vapor deposited hard coatings, like TiN. It has been found that there is a remarkable increase in corrosion resistance in dependence of magnesium content of the TiMgN coating and its surface properties. In this work the authors will explain the underlying corrosion protection mechanisms by means of electrochemical and analytical studies. The positive impact of magnesium in the coating relates on its preferred dissolution vs. steel. This causes the potential to shift to more negative direction with respect to the steel substrate and additionally leads to a temporarily passivation of the steel due to alkalization of the surrounding electrolyte by formation of magnesium hydroxide.