CATHODIC CORROSION PROTECTION

Coatings of more noble metals than the substrate metal (e.g., Cu on Fe) are only protective when there are no pores. In other cases severe local corrosion occurs due to cell formation (bimetallic corrosion). Cathodic protection is theoretically possible. This protection combination is not very efficient since the coating usually consumes more protection current than the uncoated steel. 

Where there is a perceived risk of crevice corrosion, cathodic protection can often be used to prevent its initiation. Once more a 100 mV cathodic polarisation will usually prove sufficient. However, it is doubtful whether cathodic protection can arrest crevice corrosion once started and, despite claims to the contrary, whether it could be effective in arresting stress-corrosion cracking. The problem lies in the fundamental difficulty of forcing cathodic current into an occluded area. 

Monitoring of the pipelines, (inspections, pigging, corrosion, cathodic protection, helicopter survey, etc.)... 

Isopotential lines may vary with electrode position for the secondary and tertiary current and potential distributions, where interfacial polarization of various types is considered. The variation of local true potential across the electrochemical interface with electrode position is of great interest in galvanic corrosion, cathodic protection, etc., since this true potential drives electrochemical reactions. 

The geometries discussed above are relevant to laboratory scale experimental cells. The geometries discussed below are more appropriate for situations encountered in industry or practical application involving galvanic corrosion, cathodic protection, or field corrosion probes. 

The scope of application of CP is enormous and continuously increasing. It is possible to protect vessels and ships, docks, berths, pipelines, deep wells, tanks, chemical apparatus, underground and underwater municipal and industrial infrastructure, reinforced concrete structures exposed to the atmosphere, as well as underground parts, tunnels, and other metal equipments using cathodic protection. Apart from reduction of general corrosion, cathodic protection reduces SCC, pitting corrosion, corrosion fatigue, and erosion-corrosion of metallic materials. 

The ( a — Ec) term can be reduced by applying a cathodic current to the corroding sample so that its potential at least reaches the equilibrium potential of the anode E, where the corroding electrode is cathodically protected. It is necessary to apply an additional current to ensure that the potential is a few millivolts more negative that the a value to achieve complete protection from corrosion. Cathodic protection can be achieved by applying the current externally or by coupUng the specimen of interest to sacrificial anodes of metals such as Al, Mg, Zn. These principles are illustrated in Fig. 14.9. 

NACE International and Institute of Corrosion, Cathode Protection Monitoring for Buried Pipelines, pub. no. CEA 54276, Houston, Tfex, NACE International, 1988. 

Electrode Potentials and Potentlometry Controlled-Potentlal Electrolysis and Voltammetry Electron-Transfer Processes Electrochemical Characterization of Molecules Industrial Electrosynthesis Batteries and Fuel Cells Corrosion Cathodic Protection... 

In addition, based on the concept of critical potential to induce pitting corrosion, cathodic protection would be a good solution if design allows (Ref 11). 

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