Several austenitic alloys were tested under conditions simulating a pyrolysis process of post-consumer plastics. Impurities in the plastic feedstock like chlorine containing materials or organic components yield HCl and H2S, respectively, during the cracking process. The reactor material must be able to withstand these harsh corrosive conditions.
To evaluate the applicability of commonly used construction materials, high-temperature corrosion experiments were performed in lab-scale test equipment at 580 °C for 240 hours. The gas atmosphere consisted of either 0.02 vol% or 2 vol% H2S and 3.8 vol% HCl, 1.9 vol% CO2, 0.3 vol% CO, 2.8 vol% H2, balance N2, denoted as low-H2S and high-H2S mixture, respectively. After the corrosion experiments the samples were analyzed by metallography, SEM/EDX and XRD. Additionally, the corrosion rates were evaluated.
Results showed that the corrosion rates of the materials increased with rising H2S content in the gas atmosphere. Additionally, the corrosion behavior of the materials changed when going from the low-H2S mixture to the high-H2S mixture. In the low-H2S mixture the alloying element nickel was identified to be helpful for corrosion protection. In the high-H2S mixture chromium was beneficial alloying element against corrosive attack at the given conditions.
Pyrolysis processes of post-consumer plastics are a promising chemical recycling route and a good alternative to disposal. Nevertheless, these processes are challenging for metallic materials since chlorine containing materials or biological components inside the feedstock can yield HCl and H2S, respectively, during cracking. In combination with high temperatures of the reactor zone metallic construction materials can be attacked by high-temperature corrosion. [1-4]
Most of the commonly used high-temperature alloys rely on the formation of oxide scales to resist high-temperature corrosion, but these materials are often not able to do so in atmospheres that contain chlorine. Chlorine-induced high temperature corrosion of alloys is strongly correlated with the formation of volatile metal chlorides beneath the initial passive layer of the materials and their subsequent outward diffusion and conversion to other corrosion products, e.g. to the corresponding oxides or sulfides. The result is the formation of non-protective oxide or sulfide scales. [1, 5-12] Haanappel et al. [13] exposed various steels to an atmosphere simulating coal gasification processes with and without the addition of 500 ppm HCl. The results showed that the addition of 500 ppm HCl to the oxidizing-sulfidizing environment increased the corrosion rate significantly and yielded a to thick, porous and non-protective sulfide scales.
