Gregor Mori

Corrosion properties of Co-based cemented carbides in acidic solutions

Abstract The corrosion properties of cemented carbides with cobalt binder phase have been examined in HCl and H 2 SO 4 solution at room temperature. Potentiodynamic polarization technique with saturated calomel reference electrode was employed in this study. Air and inert argon were used as a circulating media. The effect of magnetic saturation property of cemented carbide on corrosion behavior is described. Specimens were prepared in industrial sinter furnaces under various conditions to obtain different magnetic saturation at various binder contents. According to aerated experiment, there was a difference of anodic behavior of cemented carbides between HCl and H 2 SO 4 solution. The specimen in H 2 SO 4 solution shows lower current density than in HCl by up to two orders of magnitude. This can be explained by the effect of anion on corrosion behavior of cemented carbides. A large difference between aerated and deaerated acidic solution was not observed. There was a small change of polarization curve in cathodic regime due to different extent of cathodic reaction. In addition, free corrosion potential was slightly shifted to more noble values in aerated solution. In anodic polarization, both curves were almost identical. This shows that dissolved oxygen has small influence on anodic behavior of cemented carbides. Chronoamperometric measurement as well as electrochemical investigations showed that pseudopassivity is caused by a diffusion controlled process, which is in contradiction to literature where coverage of surface is claimed. Unstable precipitates are formed in cemented carbides with high tungsten containing binder during anodic dissolution.

Corrosion Mechanisms and Cosmetic Corrosion Aspects of Zinc-Aluminium-Magnesium and Zinc-Chromium Alloy Coated Steel Strip

ZusammenfassungKorrosionsmechanismen und Eigenschaften bezüglich kosmetischer Korrosion von Zn-Al-Mg (ZM/HDG) und Zn-Cr (ZCE/EG) Legierungsüberzügen werden im Vergleich zu konventionellen feuerverzinkten Stahlband (Z/HDG) dargestellt. Die verbesserte Korrosionsschutzleistung der neuen Legierungsüberzüge basiert auf der Bildung spezieller korrosionsinhibierender Schichten. Im Falle von ZM/HDG enthält diese Hydrotalzit. Besonders die Verbesserung der kosmetischen Korrosion (z. B. Lackunterwanderung) ist ein wesentlicher Produktvorteil dieser neuen Beschichtungen.SummaryCorrosion mechanisms and cosmetic corrosion behaviour of Zn-Al-Mg alloy coating (ZM/HDG) and Zn-Cr alloy coating (ZCE/EG) in comparison to conventional zinc coating (Z/HDG) are presented. The outstanding corrosion protection performance of the new zinc-alloy type coatings is based on the formation of special corrosion inhibiting layers. In the case of ZM/HDG the nature of the corrosion product layer is based on Hydrotalcite. Up to six fold corrosion protection was observed even with half of the coating thickness of ZCE/EG coatings and paint creepage is reduced down to 50 % in comparison to conventional zinc coatings (Z/HDG, ZE/EG). This clear improvement in corrosion protection is surely one of the most important product properties.

Influence of the PH Value On the Corrosion of Zn-Al-Mg Hot-dip Galvanized Steel Sheets In Chloride Containing Environments

Previous studies of our group showed that Zn-Al-Mg hot-dip galvanized steel sheets have a superior corrosion resistance compared to conventional hot-dip galvanized steel sheets. This work focuses especially on the stability of this material at different pH values and the formation of corrosion products. Therefore experiments based on the normal salt spray test according to DIN EN ISO 9227 were conducted. During those tests the solution was adjusted to different pH values from acidic to alkaline. All other parameters were left untouched. The samples were compared to conventional hot-dip galvanized steel sheets of the same coating thickness. The results showed that the corrosion resistance is severely influenced by the pH value of the testing medium. Also the amount and the type of corrosion products formed in the course of the tests varies greatly. While in severely acidic conditions mainly Simonkolleite is formed. In mildly acidic to neutral conditions, Simonkolleite and Hydrozincite are formed, while, in strongly basic conditions, Simonkolleite and ZnO are formed. Corrosion products were analyzed by SEM, EDX, XRD, IR, and RAMAN spectroscopy. A cross section analysis in SEM showed that the severity of the attack of the coating is very much dependent on the pH value of the testing solution. The corrosion products could be identified as Simonkolleite and Hydrozincite. In alkaline conditions the formation of Zincoxide and Zn-Al-Hydrotalcite was also observed.Previous studies of our group showed that Zn-Al-Mg hot-dip galvanized steel sheets have a superior corrosion resistance compared to conventional hot-dip galvanized steel sheets.This work focuses especially on the stability of this material at different pH values and the formation of corrosion products. Therefore experiments based on the normal salt spray test according to DIN EN ISO 9227 were conducted. During those tests the solution was adjusted to different pH values from acidic to alkaline. All other parameters were left untouched. The samples were compared to conventional hot-dip galvanized steel sheets of the same coating thickness.The results showed that the corrosion resistance is severely influenced by the pH value of the testing medium. Also the amount and the type of corrosion products formed in the course of the tests varies greatly. While in severely acidic conditions mainly Simonkolleite is formed. In mildly acidic to neutral conditions, Simonkolleite and Hydrozincite are formed, while, in strongly basic conditions, Simonkolleite and ZnO are formed.Corrosion products were analyzed by SEM, EDX, XRD, IR, and RAMAN spectroscopy. A cross section analysis in SEM showed that the severity of the attack of the coating is very much dependent on the pH value of the testing solution. The corrosion products could be identified as Simonkolleite and Hydrozincite. In alkaline conditions the formation of Zincoxide and Zn-Al-Hydrotalcite was also observed.ZusammenfassungVorhergehende Studien haben gezeigt, dass mittels Zn-Al-Mg feuerbeschichtete Bleche ausgezeichnete Korrosionseigenschaften aufweisen.Bei dieser Arbeit lag der Fokus auf der Stabilität des Beschichtungswerkstoffs bei unterschiedlichen pH-Werten und der Bildung von Korrosionsprodukten. Dazu wurden Versuche, basierend auf dem in DIN EN ISO 9227 beschriebenen Salzsprühtest, durchgeführt. Es wurde der pH-Wert bei den Tests von basisch zu sauer verändert, jedoch wurden alle anderen Testparameter nicht verändert. Die untersuchten Beschichtungen wurden mit konventionellen Beschichtungen verglichen.Die Ergebnisse zeigen, dass eine Veränderung des pH Werts einen beträchtlichen Einfluss auf die Korrosionsbeständigkeit hat und unterschiedliche Korrosionsprodukte entstehen. In saurer Umgebung wird größtenteils Simonkolleit gebildet. In neutraler bis mild saurer Umgebung werden Simonkolleit und Hydrozincit und in basischer Umgebung Simonkolleite und ZnO gebildet.Korrosionsprodukte wurden mittels REM, EDX, XRD, IR und RAMAN Spektrometrie untersucht. Querschliffe wurden mittels REM untersucht und zeigten, dass die Form des Angriffs sehr stark vom pH-Wert im Salzsprühest abhängt. Die Korrosionsprodukte konnten als Simonkolleit und Hydrozincit im Sauren und Zinkoxid und Zn-Al-Hydrotalcit im Basischen identifiziert werden.

Erosion-Corrosion of Carbon Steels in a Laboratory: Three-Phase Flow

Abstract A new testing facility for a high-velocity, three-phase flow consisting of a gas flow loop and a jet impingement rig is described. Flow velocities between the nozzle and specimen have been determined through computational fluid dynamics (CFD) simulations and by particle image velocimetry. Tests were conducted on typical carbon steels (J55 and C95) that are used in tubings for the gas and oil industry. Flow conditions of a sweet gas condensate well have been applied. Mass-loss rates have been determined after testing times of between 4 h and 168 h using optical profilometry. Damaged surfaces were investigated using optical and scanning electron microscopy. The effects of material and flow velocity on the mass-loss rate have been investigated. Mass loss of specimens under given conditions is determined by siderite formation and increasing degradation of siderite layer by impacts of sand and fluid droplets. Degradation happens by erosion-enhanced corrosion. Normalized steel J55 behaves like a ductil...

Investigation of the Corrosion Behavior of Zn-Al-Mg Hot-Dip Galvanized Steel in Alternating Climate Tests

In this work the hot-dip galvanizing coating ZnAl2Mg2 was tested against the reference material ZnAl0.2 in various standardized cyclic corrosion tests with alternating climates. Those tests were th...

A New Testing Method for Lifetime Prediction of Automotive Exhaust Silencers

The purpose of this paper is to highlight the problems associated with daily routine corrosion tests performed in an automotive exhaust industry. Estimation of the life time of a complete system under real conditions is always uncertain and often leads to a disagreement. A new testing setup was built in which simulation of external and internal corrosion with additional thermal cycles can be performed simultaneously. Simulation of all real conditions makes this test totally versatile and unique among all the existing testing methods. All test results were investigated quantitatively and a direct comparison was made between some field systems with different mileage and total life. Conformity was accomplished between the results from corrosion tests and parts from the vehicles. Studies carried out on the silencers have shown that the new component testing method could be used for life time estimation of parts having different material and design combinations. On the basis of obtained results it can be stated that the new testing setup can be applied for different materials and design rankings.

INFLUENCE OF REFRACTORY METAL CARBIDE ADDITION ON CORROSION PROPERTIES OF CEMENTED CARBIDES

ABSTRACT Cr 3 C 2 is added as a grain growth inhibitor to produce submicron cemented carbide. In a corrosive environment, the cobalt binder suffers from corrosion while tungsten carbide (WC) itself remains intact. In this study, cemented carbide specimens have been sintered to attain different magnetic saturation and grain growth inhibitor additions. Corrosion behavior of cemented carbides has been investigated by means of electrochemical measurements. A platinum plate and saturated calomel electrode were the counter and reference electrodes. Pseudopassive current density was altered by the grain growth inhibitor and magnetic saturation during sintering. The effects of microstructure and the presence of grain growth inhibitors on corrosion resistance of cemented carbide have been described. Higher amounts of tungsten and/or chromium dissolved in the binder improve corrosion resistance of a cemented carbide.

Evaluation of CPT values determined by ASTM G48 practice: Report on round robin tests of the Corrosion Committee of the Austrian Society of Metallurgy

A round robin test for determination of critical pitting temperature (CPT) according to ASTM G48 produced unsatisfactory repeatability and reproducibility of results in agreement with literature. Investigations on experimental procedure, identification of pits and gravimetric evaluation of samples have been performed to identify crucial parameters for improvement of results. Several recommendations within ASTM G48 practice are proposed such as wet grinding of specimens, cold drying of specimens, additional gravimetric evaluation and optical evaluation only by skilled persons. A further suggestion is a finer ground surface of samples. In order to confer statistical reference to CPT values an evaluation of medians with 50% probability of pitting is proposed.

Erosion-Corrosion Rates of Corrosion-Resistant Alloys Under High-Velocity Multiphase Conditions

The erosion-corrosion behavior of two corrosion-resistant alloys (UNS S42000 and UNS N08028) has been assessed under gaseous-liquid-solid impingement conditions. Erosion-corrosion impingement tests were conducted at three different impact angles and at three different impact velocities up to 60 m/s, and furthermore, pure erosion and pure corrosion impingement tests were run for UNS S42000, and carbon dioxide (CO2) at a pressure of 1,500 kPa was used as the gas phase. The sand content, with grain size below 150 μm, was 2.7 g/L brine. Artificial brine with a sodium chloride (NaCl) content of 2.7% was used as liquid phase. The damaged surfaces of samples exposed to the high-velocity multiphase flow were investigated using scanning electron microscopy (SEM) and an optical device for 3D surface measurements to assess the depth of attack. Electrochemical investigations according to ASTM G61 were performed to determine electrochemical behavior of tested materials including critical pitting potentials (Epit) and ...

Interactions of different types of localized corrosion in surgical implants

Surgical implants often show different types of localized corrosion such as corrosion fatigue cracking, pitting and crevice corrosion on the same part. Interactions of these different corrosion phenomena were investigated. This was done by cyclic loading of electropolished tensile specimens at different constant and changing potentials. Material investigated was a surgical implant steel X2CrNiMo18-15-3 which was immersed in physiological NaCl solution. Pitting and repassivation potentials were determined. Samples with and without artificial cracks as well as masked specimens were tested. Incubation period for first damage, density and size of pits by coulometric and volumetric method were determined. The fracture surfaces were then investigated by SEM. Results show that not in all cases pitting corrosion was the cause for corrosion fatigue cracking. Also pitting is favoured by crack formation. Density of pits increases by a factor of 5 without any change to pitting potential. There are primary pits formed prior to crack initiation and secondary pits formed after crack initiation. At samples without crack there is almost no difference between the optically measured value of total pit volume and the coulometrically determined value. At samples with cracks coulometric volume of pits is much larger than optical one. This proves that there is a significant amount of crevice corrosion in the crack. The corrosion current density in the crack increases by two orders of magnitude when comparing it to electropolished surface of the sample. Results of laboratory experiments are confirmed by failure of a real implant.