Hot corrosion of metallic materials in humid acid atmosphere in the presence of molten salts

摘要

The world problem of the pollution makes in thelast few years particularly interesting the plants for theincineration of the urban solid refuse. These plants are able toeliminate clearly considerable quantities of refuse producingcontemporaneously energy at low cost. The industrialtechnology and the careful study of the operating conditionshave done a strong increasing of the efficiency, but someproblems, due to the complex nature of the refuse, to the heatingpower and to the physico-chemical environment, are stillunsolved. The aim of the present paper is the study of thebehaviour of three metallic materials in gaseous atmosphere andin the presence of molten salts, conditions similar to theoperating conditions of a modern incinerator The chosenlaboratory conditions were: T=600 deg C, atmosphere ofnitrogen containing hydrogen chloride, sulphur dioxide, carbondioxide and water vapour, presence of the salts as eutecticmixture of potassium chloride and calcium chloride. The testshave been performed by continuous weight changedeterminations by means of a thermobalance, anddiscontinuously in tubular furnace in order to evaluate the attackmorphologies. The studied metallic materials were a simplecarbon steel (COA2S) characterised by a low price, and twostainless steels, AISI304 (austenitic stainless steel) andSAF2304 (austenoferritic stainless steel).The obtained results show that the employed laboratoryconditions produce an intergranular corrosive process on thethree considered materials. The oxidation and chlorinationreactions cause a different kinetic of the corrosion process: thecarbon steel (COA2S) shows a linear corrosion kinetic with aconsiderable weight gain due to the oxidation of the iron. TheAISI304 steel corrosion kinetic is also linear; the surface film isnot very protective, in consequence of the thermodynamicinstability of the formed oxides. The austenoferritic SAF 2304stainless steel shows a pseudo-logarithmic corrosion kinetic,which tends to become steady after 40 hours.At long times is possible to note some variations due to thegrowth of the surface oxide spalling. In this way the chlorinationprocess becomes easier and the formation of volatile chlorideproduces the next weight less.The microscopy observation shows for the COA 2S theformation of non-protective oxide layer able to decreasepartially the corrosion rate. AISI 304 stainless steel during theexposure suffers an intergranular corrosion process, withformation of chromium carbide. In the case of SAF 2304 theattack is also intergranular but concentrated on the ferritic grainsand the corrosion propagation is limited by the discontinuity inthe matrix of the ferritic phase.The results obtained during this work show that from theeconomical point of view the employ of more expensivematerial (as the austenoferritic stainless steel) can produce atlong-term a decreasing of the operating costs.