The corrosion resistance of stainless steels is based on a protective oxide film forming the metal surface. This natural passive film is not resistant to attack by chlorides in warm oxidizing electrolytes. Chloride breaks the surface film, causing a potential difference between the more noble oxide film and the less noble base metal at the exposed spots. The potential difference gives rise to corrosion current.
In cathodic protection, opposite current is applied to the metal structure with a rectifier and anodes. In mild steel structures, steel will be driven by external current of a cathodic protection system to a status of immunity against corrosion whereas stainless steel will be maintained in a region of perfect passivity.
Cathodic Protection can be applied to any metallic structure that is in contact with an electrolyte. Cathodic Protection is commonly used in structures such as wharfs, jetties, ship hulls, foundation of offshore wind mills, offshore oil drilling platforms and exterior of pipelines buried in soil or water. Especially, corrosion of steel piles and steel sheet piles buried in a corrosive soil or immersed in water can be stopped by installing a properly designed and engineered Cathodic Protection System.
In picture the Sundsvall Bridge. The steel piles showed severe corrosion progress soon after installation and were protected by Savcor CP in 2019.
Under water steel piles are prone to corrosion
The splash zone and tidal zone are the most vulnerable areas of concrete piles in sea water.
Chloride ingress from de-icing salt or sea water will lead to an increased risk of corrosion. Once chlorides have destroyed the protective oxide layer from steel surface, corrosion will initiate leading to material loss. Corrosion products which take more space than original rebar, will eventually cause cracks in concrete and later spalling of concrete.
Cathodic Protection (CP) is a well-known and proven method to extend the service life of the reinforced concrete structures in case of chloride contaminated concrete corrosion. Cathodic Prevention is usually applied during the building process or the ordinary concrete repairs. Cathodic Protection System in concrete consist of external DC power source, non-soluble anodes, cabling, reference electrodes and a control unit.
With old concrete structures all the damaged concrete need to be removed before system installation. The cathodic protection system stops the corrosion progress but it does not repair already deteriorated structure. The Cathodic Protection is usually installed together with the repair work.
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