Vliv prestupu tepla na nerovnomerne formy koroze korozivzdornych oceli

Abstract

Susceptibility of metals to corrosion attack under conditions of heat transfer is in most cases evaluated regarding the surface temperature of the metal only and the influence of the heat flux itself is disregarded. This thesis studies the effect of heat transfer on corrosion of stainless steel in water environments under non-boiling conditions, when the influence of the heat flux and the surface temperature is separated on principle. The theoretical part concerns possible factors affecting processes that influence the corrosion rate of metallic materials under conditions of heat transfer. Effect of heat temperature and heat flux on mass transfer, kinetics of surface reactions and environment composition are discussed here, as well as effects of temperature gradient in the metal and possibility of inhomogeneity formation in the system at present of heat flux. This part does not concern the localized forms of corrosion only, but it deals with the heat transfer influence on corrosion processes in general. All the available sources concerning exparimental studies of heat transfer influence on corrosion are summarized in the annex. Requirements on construction of basic types of experimental devices are considered as well. Corrosion tests were aimed at verifying the influence of heat transfer on initiation and propagation of intergranular and pitting corrosion and stress corrosion cracking of austenitic stainless steel under conditions close to real systems or well correlating with these. Three devices in four modifications were constructed. The samples were exposed at surface temperatures from 40 to 105 C and heat fluxes from -60 to 300 kWm-2, while the value of one of these variables was kept constant. With the exception of corrosion cracking, exposure and electrochemical tests were performed. Measurements of the heat transfer influence have revealed that the material resistance depends on the surface temperature as well as on the heat flux. Increasing the surface temperature had a negative effect in all cases and led to increase in susceptibility to the given type of corrosion attack. Effect of heat flux depended on its direction, intensity, corrosion system and surface temperature, and it differed as well in different stages of corrosion attack. The heat flux showed to be positive (suppressing corrosion) in case of propagation of the intergranular corrosion and initiation and repassivation of pitting corrosion. In the initiation stage of localized corrosion, heat flux affects most significantly the passive film resistance, namely in changing its quality (for example thickness), as well as it causes a change of the environment aggressivity (the aggressive ions activities, oxidizing power of the environment, flow velocity). In the propagation stage of localized attack, presence of the temperature gradient in the metal itself has the most significant influence, thanks to which the temperature inside pits or cracks differs from the temperature on the free surface. The mentioned factors are partially connected and their influence takes place at the same time, while they affect the metal resistance often in opposite ways. Higher reliability of operating the heat exchange devices made of stainless steels can be reached, regarding the localized corrosion, above all by decreasing the surface temperature from the medium side that may cause the attack, which can be possibly done by decreasing input as well as output medium temperature and increasing the heat flux by flow intensification. From the corrosion point of view it is suitable to construct the exchangers as parallel-flow ones and to prevent formation of deposits on the heat exchanging surfacesAvailable from STL Prague, CZ / NTK - National Technical LibrarySIGLECZCzech Republi