Studium korozní odolnosti uhlíkových a nízkolegovaných ocelí používaných v energetice

Abstract

The present dissertation deals with the causes of corrosion attack on the inner surface of boiler tubes (evaporating tubes) made of L290NB structural carbon steel, which are part of the evaporator of the combustion chamber in a steam, dry bottom boiler with an operating pressure of 10MPa where there is natural circulation of deionized, chemically treated boiler water together with dosed Na3PO4 alkalizing agent to stabilize normative pH values. The outer surface of the evaporator tubes is heated by combustion gases with a temperature of up to 1 200°C, creating saturated steam which is further converted on a superheater into output superheated steam intended for electricity production. The corrosion attacks always have always occurred on the inner, heated walls of the evaporator tubes, especially in areas with uneven and inhomogeneous surfaces. Corrosion degradation of the material results in shrinkage of the inner tube walls, while tube wall cracks and leaks are thereafter initiated by the effect of the internal pressure of the medium, resulting in massive leakage of the operating medium into the combustion chamber of the boiler. The disruption of tube integrity leads to forced downtime of the plant producing superheated steam and electricity. On the basis of the results of experimental tests, it has been found that there has been an increase in the monitored amount of P2O5 due to incorrect dosing of the Na3PO4 alkalizing agent in the production of saturated steam, causing a deviation from normative pH values towards acidic and, over the long term, alkaline ranges in the boiler water, together with copper ions released from brass and copper tubes from the supporting power engineering equipment. This process results in disruption of the protective oxide layer and gradual loss of the tube wall material under a build-up of corrosion product incrustation. The results of empirical measurements have shown that the corrosion attack is also supported by the thermal load of the tube material due to combustion gas temperatures reaching 1 600°C. Stabilisation of the chemism of boiler water together with the thermal load, and quality control of the inner tube surfaces, has suppressed the corrosion attack.