The Concept of Corrosion SACOR Engineering, September/1999

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1 The Concept of Corrosion SACOR Engineering, September/1999 Corrosion is the deterioration of metals and alloys through environmental action. Weather this environment is the sea water or the soil, metals and alloys submitted to it will be under severe action of corrosion processes. Electrochemical Corrosion Electrochemical corrosion is the destruction processes that develop, by the electrolytes action on the metal. Such electrolytes comprise: sea water, soil, acid aqueous solutions, alkaline solutions, salts, atmospheric air with humidity, etc. Among the several types of corrosion of electrochemical nature, the most expressive for metals submerged in sea water is the Galvanic Corrosion. It results from the electric contact of different materials in presence of the electrolyte. This type of corrosion will be as intense as distant are the materials in the table of potential electrochemicals. When this happens, the metal that releases current to the electrolyte corrodes, asuming anodic behavior called anode. The metal that receives the electrolyte current is protected and is called cathode as it assumes cathodic behavior. The formation of anodic and cathodic areas in the vessels depends on several factors, as for example: heterogeneity of the steel, coating deterioration, sea water salinity and temperature, etc. We can mention as consequence of this corrosion the increase in the roughness of the hull surfaces, with consequent increase in the fuel consumption used for the vessel displacement. Types of Protection Based on the same principle, which is the injection of a protective electric current in the structure through the electrolyte, there are two methods of protection: Galvanic Cathodic Protection and Cathodic Protection by Impressed Current. Galvanic Cathodic Protection: In this process the flow of the electric current is supplied by the difference of potential existent between the metal to protect and the other metal, chosen as anode, as result of its different electrochemical potentials. The materials used in the practice as galvanic anodes are zinc or aluminum alloys, widely used for the protection of small and medium vessels hulls Sacor Siderotécnica S.A. Todos os Direitos Reservados 1/5

2 Due to the fact that its current capacity is higher and its weight lower, in comparison with the zinc, the aluminum anodes, in the recent years, have been very well accept in the market. Today magnesium is not very used in protecting vessels, once it became uneconomic due to the low efficiency and high cost. The galvanic anodes life varies around two to three years. The galvanic systems are also used for the internal protection of ballast tanks. Cathodic Protection by Impressed Current: The use of Protection Systems by Impressed Current is the most modern trend for the external protection of the vessels hulls or Offshore rigs, especially the medium and large size. In this process the anodes are lighter and more cost efficient than the galvanic, once they are designed for a life of about tem to fifteen year and as there is no need to replace them during docking. The system by impressed current typically comprises the following main parts: a) Rectifying Unit Converts the alternating current onboard to a direct current that will be drained by the anodes due to the level of hull protection potential, monitored by the electrodes. b) Inert Anodes Are typically made from Titanium with coating of Noble oxides, mounted on hull with the purpose to distribute the protective current along the submerged surface of the hull. c) Reference Electrode Has the purpose to generate the potentials to the hull. This information is sent to the Rectifying Units so as they control the protective currents. Protection Criteria To evidence that a vessel is effectively protected against corrosion, the minimum potential criterion is used between the vessel and the electrolyte, potential that can be measured with a reference electrode (semi-cell). For the case of vessels, the most used reference electrodes are Silver/Silver Chloride (Ag/AgCl) and the Electrolytic Zinc. In table I we see the minimum protective potentials of three different types of electrodes. TABLE I 1999 Sacor Siderotécnica S.A. Todos os Direitos Reservados 2/5

3 PROTECTIVE POTENTIALS FOR THE STEEL HULL MEASURED REGARDING DIFFERENT TYPES OF REFERENCE ELECTRODES Reference Electrode Electrolyte Protective Potential in Sea Water with Electric Resistivity of 20 ohm.cm at 20.C Ag/AgCl Sea Water V Cu/CuS04 (Solution) Sea Water V Zinc Sea Water V Cathodic Protective System Dimensioning The dimensioning of a Cathodic Protective System must be based on the data survey of the vessel or structure to be protected. Once defined the main data, the system is dimensioned. The dimensioning observes some basic premises among which we can mention: a) Emerged Surface or Live Works: The LARGEST surface is considered, between the calculated and the indicated by the Shipyard. b) Protective Current Density: It is one of the most important parameters for the dimensioning of a cathodic protective system. The protective current density value depends on many factors, the most significant being the following: the electrical resistivity of the electrolyte (influenced by salinity and temperature), the speed developed by the vessel, the quantity of oxygen in the water and the conditions and quality of the applied protective coating. Table II presents the protective maximum current densities in quiet sea water mar, for bronze, the steel without coating and the steel with new coating, well applied Sacor Siderotécnica S.A. Todos os Direitos Reservados 3/5

4 TABLE II DENSITY OF THE PROTECTIVE CURRENT IS QUIET SEA WATER METAL DENSITY OF THE CURRENT (ma/m 2 ) Bronze 540 Steel (no coating) 108 Steel (new coating, well applied) 10 c) Maximum Current Released by the Anodes The maximum current released by an anode is determined by the resistances existent in the electric circuit and by the life that the anode was dimensioned. The electric resistances are: - The voltage collapse in the electrical conductor by its internal resistance (ohm/km); - The electric resistivity of the electrolyte (sea water is approximately 25 ohm.cm.); - And the maximum density of the allowed current, which is determined by the type of anode alloy (for titanium with noble oxides it is in the range of 600 A/m 2 ), and for the designed life. d) Current Distribution: This phase requires experience and skills from the designer, so as not to have excess of protection in one areas and failures in others. The quantity of current necessary for the hull protection, at any time, is automatically supplied by the system control unit of impressed current. Bibliographic References - GENTIL, V., Corrosão. Almeida Neves Editores Ltda. - HUMBLE, H.A., The Cathodic Protection of Steel Piping in Sea Water. Corrosion 5 (9). Set SPELLER, T.N., Corrosion, Causes and Prevention. Mc Graw-Hill, COMPTON, K.G., Cathodic Protection of Strutures in Sea Water. Corrosion/75. Paper nr Sacor Siderotécnica S.A. Todos os Direitos Reservados 4/5

5 - MORGAN, J.H., Automatic Control and Monitoring Equipament for Cathodic of Offshore Strutures. Corrosion/78. Paper nr FINN, J., Corrosion and Protection of Offshore Steel Structures. Corrosion/76. Paper nr DUTRA, A.C., Proteção Catódica - Técnica de Combate à Corrosão Editora Técnica Ltda. 1. Edição BS 7361 : Part 1 : 1991 Cathodic Protection BSI - British Standard Sacor Siderotécnica S.A. Todos os Direitos Reservados 5/5