Acid Cleaning Corrosion
Background
Acid cleaning corrosion is a key factor to considering during the chemical cleaning activities. A few months after the completion of a combined cycle, the plant was forced to shut down due to operational instabilities and loss of pressure in the steam generator.
An exhaustive inspection was conducted to find out multiple leaks in the tube side of the evaporator and an uncommon yellowish pigment at the bottom header. Some tube samples were taken and sent to laboratory to determine the metal morphology, surface chemical composition and the nature of the deposit.
Analysis
- The spectroscopy X-ray results indicated the presence of a significant enrichment of carbon along with bonded oxygen and ferrous ions which suggests the presence of organic compounds.
- X-ray diffraction revealed that most of the carbon and oxygen were combined in an aliphatic structure, suggesting the existence of citrates, which would explain the yellowish aspect.
- The microscopy analysis showed an acid corrosion pattern with jagged, roughness, undercut pinholes and general pitting appearance
Conclusions
During pre-commissioning, the cleaning of the boiler was carried out by the execution of a traditional one-phase citric cleaning.
In this case, the system was not effectively washed out after the chemical cleaning, and great amount of deposits, with high content of ferrous citrate remained, in the evaporator.
The ferrous citrate, in combination with remaining water, created under deposit cells and regions of low pH which generated the corrosion damage shown in the photo.
