Investigation of deposits in a water-cooled exhaust pipe
During the investigation of a water-cooled exhaust pipe that had become leaky due to hole formation, soot deposits were found inside that contained not only carbon but also iron oxides such as goethite (Fe₂O₃·H₂O) and hematite (Fe₂O₃). The aim of the investigation was to determine the chemical and structural composition of the deposits and to identify possible causes of the damage to the pipe.
Analysis methods used
Various methods were used for the investigation:
- Elemental analysis using EDX (energy dispersive X-ray micro-area analysis)
EDX was used to determine the elemental composition of the deposits. The X-rays emitted by the sample atoms were measured by the electron beam in order to record the spectrum of the characteristic X-ray lines. This enabled a precise determination of the chemical elements in the deposits. - X-ray powder diffraction (XRD)
X-ray powder diffraction (XRD) was used to analyse the crystalline structure of the deposits. Based on the diffraction patterns, which were compared with a reference database, the following phases were identified:- exhaust residues: hematite (Fe₂O₃), goethite (Fe₂O₃ H₂O)
- water residues: Hematite (Fe₂O₃), Magnetite (Fe₃O₄), Calcite (CaCO₃)
These phases provide information about the chemical processes that took place during deposition and corrosion.
- scanning electron microscopy (SEM)
The scanning electron microscope (SEM) was used to examine the surface structure and the distribution of elements in the deposits in detail. In combination with EDX, the chemical composition of the samples was further refined.
Results of the investigation
- internal deposits (exhaust gas residues):
The deposits inside the exhaust pipe consist mainly of carbon (soot), iron (in the form of hematite and goethite) and oxygen. The low crystallinity of hematite and goethite indicates incomplete reactions, which is confirmed by the low reflections in the XRD spectrum. The deposits inside also show the typical characteristics of hot gas corrosion. - external deposits (water residues):
The deposits on the water-cooled exterior contain hematite, magnetite and calcite. Magnetite forms at temperatures above 180 °C from dissolved iron ions and water molecules, indicating a higher temperature and a more complete chemical reaction. Calcite is an end product of calcium carbonate crystallization and shows that the chemical processes on the exterior are complete.
Conclusion and cause of hole formation
The results suggest that the hot gas corrosion inside the exhaust pipe is the cause of the hole formation. Probably sulfur-containing compounds in the exhaust gas, combined with moist exhaust gas (T < 950 °C) or condensation (e.g. during cyclic shutdowns of the plant) is responsible for the damage to the pipe. The corrosive effect could have been intensified by contact with water and the chemical substances present there.
Recommendations for preventing hot gas corrosion
- continuous operation: For continuous operation, the application of a Cr₂O₃ top layer help to increase the corrosion resistance of the exhaust pipe.
- Cyclic operation: When operating with frequent temperature fluctuations, plasma spray coatings advantageous because they are less susceptible to cracking.
- exhaust gas composition: Better control of exhaust gas composition could help prevent hot gas corrosion.
In addition, typical materials for exhaust systemsthat have a higher resistance to these types of corrosion should be considered.