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Investigation on defect morphology and corrosion behaviour of TiMgN hard coatings on steel substrate
(2014)
Nitride hard coatings, such as Titanium nitride and chromium nitride coatings are widely used for applications focussing on wear protection and decorative purposes. Also these coatings are often applied on low alloyed steel substrates. The inevitable contact of the coated components with the environment through pores and coating defects bears the danger of corrosion of the steel substrate followed by further delamination of the coating and a loss of function of the component. A new attempt to increase the corrosion behaviour of those physically deposited hard coating systems is the implementation of magnesium. The positive effect of magnesium could be verified already electrochemically. Performed experiments on TiMgN have shown that corrosion resistance could drastically improved with increasing Mg content of the TiMgN up to 30 at% compared to steel substrates with applied titanium nitride hard coatings. Besides the clarification of the electrochemical and phenomenological effect of the magnesium on the corrosion behaviour, an additional aim of this work is the investigation of the coating defects, defect morphologies and their influence on the local corrosion behaviour. For this the incorporation of the magnesium and its effects on the coating material, the influence of coating growth related defects and the effect of the coating structure on the corrosion behaviour should be experimental examined by using innovative surface and material analysis methods such as FIB and TEM. Furthermore the evolution of the coating defects should be investigated by combing confocal microscopy with a new developed exposure test method using the electrochemical indication test KorroPad. The function of the KorroPad test, which was developed and patented [1] at the BAM for the detection of corrosion sensitive steel surfaces by indicating dissolution of iron ions [1], allows the detection and identification of critical coating defects for further microscopic investigation. Additional to that the KorroPad test simulates an accelerated exposure test by simultaneous absence of the disadvantages of typical short time exposure test like salt spray tests. Thus it allows a monitoring and analysis of the evolution of the critical coating defects and their influence on the local corrosion behaviour and the overall corrosion mechanisms. The results of this work should contribute to the development of new hard coatings with improved corrosion protection properties and also to a better understanding of the corrosion mechanisms of coated steel substrates.
Martensitic stainless steels are commonly used in cutlery fabrication requiring high hardness and sufficient corrosion resistance. The heat treatment process affects the mechanical and electrochemical behavior of martensitic stainless steels due to the precipitation of chromium carbides. Depending on the heat treatment the corrosion resistance of these steels can vary strongly, and improper heat treatment parameters can lead to a weak pitting corrosion resistance. The aim of this work is to identify heat treatment parameters influencing the corrosion resistance of martensitic stainless steels by using three different electrochemical testing methods. To this purpose, five different heat treatments were applied to the alloys 1.4116 and 1.4034. In addition to the determination of the critical pitting potentials and the modified double-loop electrochemical potentiodynamic reactivation tests (DL-EPR) a new KorroPad indicator test was used assessing the pitting corrosion behavior. The results showed that all methods used were in good agreement for verifying the influence of the various heat treatment parameters on the corrosion behavior and to identify the effect of heat treatment conditions on the pitting corrosion resistance.
In the last years, new approaches for the evaluation of the corrosion resistance of martensitic stainless steels were developed, which allow nearly non-destructive as well as short-term corrosion testing. This work analyzes the sensitive influence of microstructure and surface treatment on the corrosion resistance. The effect of alloy composition and microstructure, which is always present in addition to the influence of various surface treatments will be evaluated and discussed. The corrosion behavior was investigated with conventional evaluation of the pitting potential, electrochemical potentiodynamic reactivation (EPR), electrochemical noise (EN), and the 'KorroPad' (KP) technique. EPR provided information about the passivation ability and the extent of chromium content reduction due to chromium carbide precipitation in the microstructure. The use of EN and KP aims to detect the influence of different surface treatment parameters on the corrosion resistance in a nearly non-destructive way. Furthermore the results shall demonstrate the functionality and convenience of the short-term corrosion test methods EN and KP in order to gain more acceptance within the scientific community. The results will allow the extraction of surface treatment effects on the corrosion behavior of the martensitic stainless steels 1.4116, 1.4034, and 1.4021.
Investigation of chloride-induced pitting corrosion of steel in concrete with innovative methods
(2016)
The combination of electrochemical and 3D computed tomography (CT) investigations offers the possibility of verifying electrochemical measurements non-destructively. Determining the steel surfaces damaged by pitting corrosion allows developing specimens having damage pattern corresponding to practice. Corrosion phenomena like decoupled pitting corrosion cannot be verified by electrochemical measurements only, but with the combination of those two kinds of investigations it is possible. Another advantage is the minimization of the number of samples. The classical experimental procedure requires the destruction of samples after each damage step.This can be avoided by the use of 3D computed tomography. As long as the corrosion phenomena are completely within the examination zone shown by the 3D computed tomography, the electrochemical measurements can be calculated on an area basis to get the specific corrosion rate or polarization resistance.
The book „Electropolishing“ by M. Buhlert is dealing with the electrolytic brightening, smoothing and deburring of technical materials like steel, copper, brass, aluminum, titanium and magnesium. The book content covers the basics and the main influencing parameters of the electropolishing process and provides detailed and application orientated procedure information for technical relevant materials.
After a brief introduction into the topic of electrolytic polishing in the first chapter, the author gives a detailed description of the basic electrochemical reaction mechanisms in the second chapter. The reader will also be informed about the physical and chemical parameters, which control the electrochemical removal process. Additionally, the author reports about the advantages and disadvantages of electropolishing.
The third chapter provides a detailed insight into the influencing manufacturing parameters affecting the results of the electropolishing process. Distinct aspects of the manufacturing e.g. workpiece preparation, electrolyte composition, polishing time, electrolyte temperature and alloy compositions will be discussed in detail by the author.
In the fourth and fifth chapter, the author gives useful hints and information about the parameter variation and the suitable handling of hull cells for the optimization of the removal process, followed by a particular overview about material specific electrolyte mixtures for common-used technical metals and alloys, like steel, aluminum, brass, magnesium, copper and titanium.
The sixth chapter of the book provides many selected manufacturing results and investigations on electropolishing of different metals and alloys, which allows the reader the opportunity to develop a systematic understanding of the topic and to adopt the knowledge on the optimization of his own electropolishing process.
Finally, the book concludes with a brief chapter about some remarks with respect to the work safety and environmental efforts.
In summary, this book contains a very detailed and clear arranged overview about the electropolishing method for the surface optimization process. For this reason, it is a suitable and useful lecture for people, which want to take an in depth look into the topic in order to start using this method or are interested in optimize their existing electropolishing processes.
The duplex stainless steel 1.4062 (X2CrNiN22-2) is used as alternative material to austenitic stainless steels in the construction industry. The corrosion resistance of welded seams is influenced by the base material, the weld filler material, the welding process and also by the final surface treatment. The scale layer next to the weld seam can be removed by grinding, pickling, electro-polishing or blasting depending on the requested corrosion resistance. Blasted surfaces are often used in the industrial practice due to the faster and cheaper manufacturing process compared to pickled or electro-polished surfaces. Furthermore blasting with corundum-grain is more effective than blasting with glass-beads which also lower the process costs. In recent years, stainless steel surfaces showed an unusually high susceptibility to pitting corrosion after grinding with corundum. For this reason, it is now also questioned critically whether the corrosion resistance is influenced by the applied blasting agent. This question was specifically investigated by comparing grinded, pickled, corundum-grain- and glass-bead-blasted welding seams. Results of the SEM analyses of the blasting agents and the blasted surfaces will be presented and correlated with the results of different corrosion tests (KorroPad-testing and pitting potentials).
. 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.
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.
Hard and wear resistant coatings deposited by PVD techniques have been characterized for decades for their capabilities to protect steel substrates from corrosion. In the present work the effect of Mg incorporated into TiN coatings is described in terms of the corrosion behavior as well as the mechanical and structural properties.
TiN and TiMgN films with Mg contents between 10 and 35 at.% were deposited onto mirror polished 100Cr6 (1.3505) steel samples with 2.5 and 5 μm thickness by using DC magnetron sputtering. The corrosion protection capabilities of the coatings were characterized by neutral salt spray (NSS) test, considering the amounts and sizes of growth defects inherent in each coated sample as determined by a recently developed optical scan method (Large Area High Resolution mapping). The defect data were statistically analyzed for improved interpretation of NSS test results. Chosen growth defects were additionally analyzed by focused ion beam technique. Furthermore the coating composition and morphology, the hardness and the tribological behavior were characterized.
Polished steel samples coated with 2.5 μm TiMgN containing about 35 at.% Mg were in the plane free of corrosion after 24 h in a NSS test. TiMgN with 10 or 20 at.% Mg only provided a slightly improved corrosion protection in relation to pure TiN coatings, which was limited to certain types of growth defects. The highest Mg containing coatings exhibited a decreased hardness down to 1200 or 1800 HV depending on type of deposition (HV 1200: Ti- and Mg-target with rotating substrate holder, 1800: Mg-plugged Ti-target with static substrate holder), but also showed a strongly improved wear resistance against Al2O3 related to pure TiN. By analyzing the NSS test results it was found that the corrosion behavior of the coated samples did not only depend strongly on the Mg content, but also on the sample individual defect concentrations. Therefore this subject is extensively discussed.