Schwingungsrißkorrosionsverhalten des Duplexstahles X 2 CrNiMoN 22 5 3 unter definierten Wärmeübergangsbedingungen

Abstract

Bei zahlreichen in der Praxis korrosiv beanspruchten Bauteilen kommt neben der Temperatur des Korrosionsmediums dem Warmeubergang zwischen dem Werkstoff und dem Umgebungsmedium eine entscheidende Bedeutung hinsichtlich des Korrosionsverhaltens zu. Um definierte Warmeubergangsbedingungen bei gleichzeitiger Scliwingungsriakorrosionsbeanspruchung zu ermiiglichen, wurde eine entsprechende Versuchseinrichtung entwickelt und vorgestellt .Fur den Versuchswerkstoff 1.4462 ergeben sich unter Warmeubergangsbedingungen in 3%iger NaCI-Losung Zeitschwingfestigkeiten, die oberhalb der isotherm bei Raumtemperatur ermittelten Werte liegen. So betragt bei einer Temperatur der Probe von 58 "C sowie einer Elektrolyttemperatur von 36 "C und der daraus resultierenden Warmestromdichte von 45 kW/m2 die 107-Schwingfestigkeit 410 N/mm2 gegenuber 380 N/mm2 unter isothermen Bedingungen bei Raumtemperatur. Noch deutlicher 1st der Festigkeitsanstieg im Vergleich zu isothermen Versuchen bei 7 0 T , bei denen eine Schwingfestigkeit von 340 N/mm2 beobachtet wurde.Die verbesserten Schwingungsriakorrosionseigenschaften unter Warmeubergangsbedingungen sind auf giinstigere Verhaltnisse bei der Ausbildung der Passivschicht auf der Werkstoffoberflache zuruckzufuhren. Es entsteht eine etwa doppelt so dicke Passivschicht wie bei Raumtemperatur, wodurch die RiBentstehung fur den Schwingungsbruch erschwert wird. Aufgrund lokaler Korrosionserscheinungen wird dieser positive Effekt bei hoheren Warmestromdichten (100 und 150 kW/m2) jedoch etwas abgeschwacht. Corrosion fatigue behaviour of duplex stainless steel X 2 CrNiMoN 22 5 3 under heat transfer conditionsThe corrosion behaviour of metallic components is not only affected by the temperature of the corrosive environment but also by the heat transfer conditions between the heated material and the cooling agent. Therefore the corrosion fatigue behaviour of the austenitic-ferritic stainless steel X 2 CrNiMoN 22 5 3 (german material-number 1.4462) in 3% NaCI-solution is investigated for isothermal conditions and three different heat transfer conditions. The specimens are tested under cyclic tension load (R = a,/ao = 0) with a frequency of f = 25 Hz up to NGrenz = 10'. The isothermal fatigue strengths are 380 N/mm2 for room temperature and 340 N/mm2 for a temperature of 70°C.For heat transfer conditions between the sinusoidal loaded specimens and the corrosive agent a new developed testing equipment is presented. The corrosion fatigue strength for a heat flux of 45 kW/m2 reaches a value of 410 N/mm*, while the improvement relative to the isothermal room temperature strength is lower for higher heat flux values (100 and 150 kW/m2).The better corrosion fatigue behaviour for heat transfer conditions bases on the favourable conditions for the formation of the passive layer.The thickness of the layer is nearly twice as high as for isothermal room temperature corrosion and therefore the crack initiation is delayed. For higher values of heat flux local corrosion attack is found. With that the positiv effect on corro...

Initiation and propagation of stainless steel pitting corrosion under heat flux

“…Better protective ability of the thicker passive film is explained by necessity of higher potential formation to reach such potential gradient, which allows the chloride ions to pass through the film. The increase of the passive film thickness at the presence of positive heat flux was reported by Kaluzina [10,11], Vollmar [13] and under certain conditions (chloride concentration, temperature gradient value) by Parshin [12]. On the basis of our measurements it seems to be likely that the increase of the breakdown potential and lowered density of pits on the steel surface resulted from the passive film increased protective ability, most likely by means of its increased thickness.…”

“…It was reported that the presence of heat flux in different systems affected composition [10], defectiveness [11,12], electrical conductivity [12] and thickness [10 -13] of passive layers. Higher resistance of the passive film against irregular corrosion attack in the presence of positive heat flux was observed on iron [10] and stainless steel [13], the opposite effect on copper [11] and aluminium alloys [12]. The composition and/or thickness of the passive film can be affected by higher solubility of oxygen in the bulk of the water environment, as the presence of positive heat flux at constant surface temperature is accompanied with a decrease of the bulk water temperature.…”

Influence of heat flux and surface temperature on the intergranular corrosion of stainless steel

“…It is possible that better availability of oxygen at the metal surface in the phase of formation of the passive layer influenced its composition and protection ability. However, such an explanation of easier reactivation of stainless steel in the presence of heat flux is also rather unlikely, because (1) no significant differences were detected in stainless steel passivation under various experimental conditions (the passivation charge was practically independent of the heat flux), and (2) based on the available literature, an adverse effect of increased oxygen availability on the passive layer resistance may be assumed [10,27]. The most likely explanation of the reactivation charge increase may be the change of hydrodynamic conditions in the system, i.e.…”

Influence of heat flux on localised corrosion of stainless steel under boiling conditions

“…The presence of solid substances or gas bubbles establishes a different surface temperature, which leads to the formation of thermomacrocells 9–14. With heat flux present, superficial processes can be affected, such as adsorption 7, 15, change of a reaction order 8, 15 as well as the reaction itself (formation of different products) 11, change of a reaction mechanism (e.g.…”