Corrosion effects on pipes made of Cr-Mo steel from central heating systems

The paper refers to the analysis of the factors contributing to the occurrence of corrosion, one of the main degradation mechanisms affecting the reliability and service life of Cr-Mo steel pipes used in thermal energy production facilities. These steels, appreciated for their resistance to high temperatures and good creep behavior, are often exposed to aggressive environments characterized by high temperature (160-450oC) and pressure (140-200 bar), as well as by the presence of oxygen, water vapor and corrosive compounds, such as sulfates and chlorides. In most cases the combined action of these factors leads to the formation of oxide layers, the occurrence of pitting corrosion and the thinning of the pipe walls, which can compromise structural integrity and operational safety. The paper investigates the aspect and amplitude of corrosive attacks observed in Cr-Mo steel pipes in service, under various working conditions. Pipe samples taken from decommissioned district heating installations, which had operated for long periods of time ranging from 27 to 41 years, were examined in cross-section, measuring the thicknesses of the oxidized layers and the microhardness. It was found that the thickness of the oxide crusts depends on the alloy class of the steels, the nature of the working environment and the operating temperature.