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Einfluss industrieller Korundschleifprozesse auf die Korrosionsbeständigkeit nichtrostender Stähle
(2018)
Im Vortrag werden Ergebnisse des AiF Forschungsvorhabens "Optimierung industrieller Korundschleifprozesse zur Sicherstellung der Korrosionsbeständigkeit nichtrostender Stähle" vorgestellt, die den Einfluss verschiedener Schleifmittel auf die Korrosionsbeständigkeit geschliffener Oberflächen nichtrostender Stähle belegen. Es konnte ein signifikanter Einfluss durch die Verwendung von Granulatschleifbändern nachgewiesen werden, der die Korrosionsbeständigkeit der Oberflächen reduziert. Grund sind auf der Oberfläche zurückbleibende Materialaufplattungen von Eigenmaterial aus dem Schleifprozess. Es wurden unterschiedliche Korrosionsuntersuchungen durchgeführt und durch Oberflächenanalytik im Rasterelektronenmikroskop untermauert.
Die Passivschicht von Anlagenkomponenten aus nichtrostendem Stahl Ist nicht nur durch den rauen Industriealltag gefährdet. Bereits im Neuzustand können Schäden entstehen, etwa durch Schleifvorgänge oder ungenügende Entfernung von Anlauffarben nach dem Schweissen. TIJV Süd Chemie Service beteiligte sich an der Prüfung eines von der Bundesanstalt für Materialforschung und -prüfung (BAM) entwickelten Schnelltests.
KorroPads liefern in 15 Minuten eine valide Aussage zur Korrosionsbeständigkeit der Passivschicht.
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 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.
Samples of the austenitic stainless steel grade X5CrNi18‐10 (1.4301, AISI 304) were ground industrially with various grinding parameters to study their influence on corrosion resistance. The ability of the mechanically ground surfaces to form a stable passive layer was evaluated by KorroPad test and a modified electrochemical potentiodynamic reactivation test based on a single loop (EPR‐SL). Furthermore, the surfaces were characterized by surface analytical methods. The main influence was determined regarding abrasive belt type. Surfaces mechanically ground with granulate abrasive belts constantly had a lower corrosion resistance than surfaces ground with single‐coated grain. The granulate abrasive belts generated more sensitized surface areas and left formations of welded sample material on the mechanically ground surfaces. A post‐treatment with a nonwoven abrasive proved to be an effective finishing process by which the surface defects and sensitized material got removed and the surfaces regained the expected corrosion resistance.
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).
Keine Panik bei der Passivschicht - Schnelltest auf Korrosionsgefahr an nichtrostenden Stählen
(2018)
Anlagenkomponenten aus nichtrostendem Stahl sind durch eine Passivschicht auf der Oberfläche vor Korrosion geschützt. Ist diese schadhaft oder unvollständig ausgebildet, geht der Schutz verloren. Korropad bietet eine Möglichkeit, dies zu überprüfen. Dabei handelt es sich um einen Schnelltest, der in nur 15 min ein valides Prüfergebnis liefert.
Mit der KorroPad-Prüfung kann die Lochkorrosionsbeständigkeit nichtrostender Stähle auf sehr einfache, schnelle und kostengünstige Weise eingeschätzt werden. Das KorroPad ist damit für Hersteller, Verarbeiter und Anwender nichtrostender Stähle eine interessante Alternative zu zeitintensiven Langzeitversuchen und komplexen elektrochemischen Untersuchungsmethoden. Die prinzipiellen Funktionsweise der KorroPad-Prüfung wird im Vortrag beschrieben, um anschließend verschiedene Anwendungen aus Industrie und Forschung vorzustellen. Exemplarisch werden dazu die Bewertung von Schleif- und Passivierungsprozessen, die Qualitätskontrolle der Wärmebehandlung und Oberflächenbearbeitung von Schneidwaren vorgestellt. Mit der KorroPad-Prüfung lassen sich aber auch werkstoff- und gefügebedingte Einflüsse auf die Lochkorrosionsbeständigkeit gezielt untersuchen. Dies wird an Beispielen aus aktuellen Forschungsarbeiten dargestellt.