Method Principle
The active cathodic protection with impressed current constitutes the most reliable corrosion protection system for boilers.This method has been applied for more than 50 years and is considered the absolute method of protection of steel structures in any form of corrosive environment. In addition to boilers, this method is also applied for protection of fuel transfer pipelines, ships, oil drilling platforms, potable water distribution networks, steel tanks, etc. The impressed current cathodic protection method is based on the uniform distribution of predetermined direct current voltage on the protecting structure. The steel structure surface is connected to the source cathode and therefore it is entirely protected.
For the same reason current distribution is not feasible on pipe networks, for the protection of which the system requires the combination with a respective electrolytic aluminum dissolution unit. The aluminum ions that are imparted in water are strong corrosion inhibitors and form a thin protective coherent layer of aluminum and silicon oxides with maximum thickness of 0.1 to 0.2mm along the inner surface of steel pipes. Corrosion protection is achieved, while at the same time the formation of carbonate deposits (scale) is prevented.
The system consists of electronic control units that induce current between the boiler tank walls and special anodes made of titanium coated with mixed oxides (Mixed Oxide Titanium Anodes). These anodes are installed on the interior of the boiler tanks, based on a particular design that is modified depending on the requirements of each system with regards to the anode shape (rods, wire, etc.), and the installation location. The installation is performed with the use of equivalent connectors, specifically designed and made of titanium, stainless steel and Teflon.
Corrosion protection is controlled with the use of an Ag/AgCl reference electrode from Teflon which is installed in a particular place inside the boiler tank, according to each system design. The impressed current allows the capacity maintenance of the boiler at a level lower than -800mV vs Ag/AgCl value, which is the specified limit in all international standards (ASTM, DIN, BS) for the achievement of complete steel corrosion protection.
The impressed current cathodic protection method has been applied for more than 50 years on all structures that are subjected to intensely corrosive environments (ships, platforms, pipelines, cooling systems, boilers) and constitutes the only active corrosion protection method available for steel structures.
Regarding the method of electrolytic aluminum dissolution, it is noted that the very first system was installed in 1924 and is still in operation. Nowadays, more than 40.000 such systems have been installed worldwide. Planning authorities in Denmark, where the method was invented, consider erroneous the lack of corrosion protection of the hot water distribution pipe network in new buildings, with the use of electrolytic aluminum dissolution.
Boiler cathodic protection systems are fully automated and provide a series of operation condition indication parameters for each boiler (impressed current, voltage, potential/degree of protection) through an LCD digital screen integrated on the central control unit. Additionally, the option of monitoring and automatically adjusting the system operation is provided, when and if necessary, with the provision and programming of a special electronic control card that is connected directly and without interference on a specified location in front of the central control unit.
The anticipated operational life of anodes exceeds 20 years, essentially ensuring an unlimited operational life for the protected boilers.
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