Abstract
The application of thermal spray coatings has been effective in preventing corrosion of steel and iron products. It has been used in a wide range of applications spreading from the petroleum to the food industry. In this work, the performance and effectiveness of a two-layered aluminum-based thermal spray coating applied to an ASTM A387 G11 steel was evaluated. The coating structure was comprised of an inner Al-Fe-Cr layer and an outer layer of aluminum. Coated samples were tested in the reactor zone of a fluid catalytic cracking unit (FCCU) of a petrochemical plant for 2.5 years. The reactor zone temperature was about 793 K (520 °C) and the environment was a mixed gas containing sulfur, oxygen and carbon. Laboratory-scale tests were also conducted on the coated samples in order to gain a better understanding of the corrosive effect of the gaseous species present in the FCCU atmosphere. Porosity present in the thermal spray coatings allowed the penetration of the atmosphere corrodents, which instigated intergranular corrosion of the steel substrate. The presence of an inner Al-Fe-Cr layer did not prevent coating spallation, which further contributed to the internal corrosion process.
Highlights
Materials used in petrochemical plants are continuously exposed to the corrosive effects produced by the petroleum processing units
The first consisted of an in-situ degradation where the samples were exposed to a mixed environment in the reactor zone of a fluid catalytic cracking unit (FCCU) of a petrochemical plant for 2.5 years
The presence of S, C and O in the samples corroded in the FCCU, showed that those elements were able to diffuse to the interior of the coatings, via short-circuit paths, reaching the steel substrate and initiating internal corrosion there
Summary
Materials used in petrochemical plants are continuously exposed to the corrosive effects produced by the petroleum processing units. These effects are enhanced for processes operating at high temperatures, leading to a faster degradation of mechanical components[1, 2]. Research, aimed at producing more resistant materials as well as protective coatings has been conducted to allow for the extended use of alloy components in severe corrosion conditions[3, 4, 5]. Mixed gas environments[6, 7, 8] are very characteristic of metallurgical and chemical processes, where the presence of molecular species such as oxygen, sulfur, carbon, hydrogen and nitrogen often requires the use of coatings to avoid the accelerated degradation these mixtures are able to promote[10,11].
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