Korozní charakteristiky vysokolegovaných korozivzdorných materiálů

Abstract

The dissertation is devoted to the study of corrosion characteristics of high alloyed corrosion-resistant materials, especially stainless steels and nickel alloys, which are essential structural materials for highly demanding use in the fields of energetic, desulphurization units, waste incinerators, etc. These materials are also very important in the health sector, which requires developed of special materials with a high resistance to body fluids environment together with high demands for health protection. A common problem of high alloyed corrosion resistant materials, whose surface is considered as passive in the environment, is local, structure-dependent corrosion of various types, such as intergranular corrosion, pitting and crevice corrosion and the associated and very dangerous corrosion cracking. In the present thesis, I have attempted to demonstrate these problems in broader mutual interrelations. The susceptibility to the stress corrosion cracking was studied for six types of nickel alloys (Hastelloy C-276, INCONEL 625, INCOLLOY 825, Alloy 59, Alloy 31, Alloy 33), and three types of high-alloy steels (Alloy 800 NiCrTiAl 33-21, austenitic steel Type NiCrWTiAl 35-15-5 and duplex steel X2CrNiMoN May 3, 1922). The corrosion cracking was studied in relation to other local types of corrosion. For the study of the susceptibility of materials to stress corrosion cracking, a relatively new test DET (Drop Evaporation Test) has been applied. This test was standardized during this work as an ISO standard. The used testing equipment was gradually improved in the course of my work, e.g. by installation of a microscope to study the initiation of corrosion pits with possibility of monitoring and measurement of corrosion cracks. DET test was proved to be very suitable, not only for determination of the susceptibility of the studied materials to stress corrosion cracking, but also as a way to investigate and get a deeper understanding of the mechanism of stress corrosion cracking, especially of its initiation. The obtained results led to the formulation of a mathematical model of heat and stress changes during DET test, which significantly contributed to the understanding of the processes inside the material and determining the most important factors influencing the character of material fractures. The study of stress corrosion cracking is also accompanied by study of susceptibility of selected materials to the intergranular corrosion. This study uses electrochemical polarization method DL-EPR (based on Cihal’s Method), which was newly standardized as an ISO standard during the period of my work. The results of my thesis have been in a large extent published and it shows that they have got a positive response in the professional community.