Abstract
Corrosion fatigue (CF) of duplex stainless steel X2CrNiMoN22-5-3 was investigated in situ under geothermal conditions of Northern German Basin (NGB) aquifer solution. The specimens, which were placed in the corrosion chamber, underwent purely alternating axial cyclic push/pull load to failure. The circulating electrolyte of the NGB was heated to 369 K to simulate geothermal environment. The experiments covered an extended range of mechanical loads, whereby the influence of a distinct electrical insulation of the experimental setup was additionally investigated. The electric insulation resulted in the stabilization of open-circuit potential (OPC), its more noble initial values, increased deterioration of the alloy and negative shift of the CF life span of the material. Corrosion fatigue damage, such as cracks and pits, were investigated via micro-sectional analysis and scanning electron microscope (SEM). Multiple fatigue cracks accompanied by pitting corrosion exhibited horizontal grain attack exposing preferable degradation of austenitic phase in the pit’s cavity. This phenomenon corresponded well with the measured Pitting Resistance Equivalent Number (PREN) of austenitic and ferritic phases.
Introduction
Technological applications e. g. geothermal energy extraction, oil and gas industries, deal with the conditions in which materials are loaded mechanically and exposed to highly corrosive environments, which may cause corrosion fatigue. Thus, the demand for materials capable of withstanding such conditions will be growing in the era of energy transition. In the field of geothermal applications components, which are mostly prone to CF, are primarily the delivery pumps as well as the components for conveying and filtration.
Among multiple factors that may influence CF there are the frequency of cyclic load, temperature and chloride concentration of the corrosive media. Moreover, corrosion fatigue may be intensified, especially in the case of low chromium content steels, by the presence of chlorides, hydrogen sulfide (H2S) and CO2.1–5 The increase of temperature and mechanical load, as well as the decreasing pH of the medium will lead to the reduction of the CF endurance limit concerning highly alloyed steels. 6–9 The internal compressive stress within the surface region and increase of chromium content on the contrary will lead to the increase of endurance limit.
