We were asked to investigate corrosion in various structures of a refinery and make subsequent recommendations for remediation.
During the inspection of the conveyor and other structures, heavy corrosion was discovered – in some areas over 30% thickness loss of the base material. The facility was placed in service in 2002 and was in continuous services until a scheduled turnaround in 2007. Continuous service means that the facility is scheduled to receive warm petroleum coke from a coke pit for least part of a 24 hour day, 365 days a year.
The tube sample sections, coated panel sections, corrosion deposits and metallographic specimen were evaluated using a scanning electron microscope equipped with an energy dispersive x-ray spectroscopy (SEM-EDS). An accelerating potential of 20 kiloelectron volts (keV) was employed in the analyses.
In general, iron (Fe) and oxygen (O) containing compounds are the primary corrosion products on the outer surface, and zinc (Zn) and oxygen (O) containing compounds on the inner surface of the tube sample. Extensive amounts of sulfur (S) and chloride (Cl) containing corrosion products were detected in the corrosion deposits of the tube sample and coated panel samples. Zinc (Zn) coating layer and iron (Fe) and zinc (Zn) containing intermetallic layer for the tube sample and zinc prime inner layer for coated panels (MC 8610 and MC 8604) are identified by the combination of SEM-EDS and cross section microscopy.
The refinery service water is pervasive in the facilities. The petroleum coke being conveyed is near saturated with service water from the coke pit. During the cleanup, the service water is sprayed on the interior surfaces of the galleries. During these intervals when additional service water is not being introduced to the surfaces, it remains there.
Even when the water evaporates, the corrosive contaminants of the water remain on the surfaces. Later during higher humidity periods, these contaminants can absorb moisture from the air. Therefore, all interior surfaces of the galleries can be considered to be continuously exposed to a corrosive electrolyte.
The corrosion inhibitor currently being added to the service water was of minimal value for protection of the steel structures, due to presence of coke particles and corrosion products. Generally, the coating system has failed at many areas of the interior surfaces of the conveyor galleries. The failure mode is primarily corrosion activity at the pinholes, resulting in delamination of the coatings by cathodic corrosion products. This will eventually fracture the surface of the coatings and expose more substrate.
The corrosion is further aggravated by the abrasive effects of the petroleum coke material being transported by the conveyors. The material presents a very abrasive surface to the bottom of the conveyor galleries during cleanup operations.
The abrasion effect is a double-edged sword, as it is capable of eventually removing coatings, especially at locations where there is an upset in the interior surface, such as at a welded joint and is also easily capable of removing scale and other corrosion products, exposing the steel to the effect of increased contact with the reactive service water.
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