The alloys 1.7386, 1.4841, 1.4959 and 2.4816 were tested under conditions simulating a thermal cracking process. Tests were performed at 480, 580 and 680 °C, for 24, 72 and 240 h in a gas atmosphere containing 3.8 vol.% HCl, 200 ppm H₂S and CO, CO₂, H₂ and N₂. It represents the partial pressures, which are present in the reactor of a thermal cracking process of anthropogenic resources. Flat samples were prepared from pipes and in order to achieve a uniform surface the samples were ground to 1000 grit SiC paper. After corrosion experiments the samples were analyzed by metallography, SEM/EDX and XRD. Additionally the mass loss of the samples during the experiment was evaluated. During corrosion the metallic phase is attacked by HCl and various corrosion products are formed by reaction with CO₂ and H₂S from the gas phase. After the tests the corroded samples showed a porous multilayered surface structure. The outer layer consisted mainly of Cr sulphide, followed by a Cr oxide layer. Below these two layers an area containing chlorine was detected. The porosity in the corrosion zone can be explained by the evaporation of FeCl₂ which was observed as colorless crystals at me colder parts of the testing equipment. This general structure of corrosion products was detected at all tested materials. At high testing temperatures the mass loss decreased with increasing Ni content. However, at lower testing temperatures this effect changed entirely, so that at 480 °C the Ni content showed no influences and the mass losses were nearly equal.