Cathodic corrosion

The thermodynamic data pertinent to the corrosion of metals in aqueous media have been systematically assembled in a form that has become known as Pourbaix diagrams (11). The data include the potential and pH dependence of metal, metal oxide, and metal hydroxide reactions and, in some cases, complex ions. The potential and pH dependence of the hydrogen and oxygen reactions are also suppHed because these are the common corrosion cathodic reactions. The Pourbaix diagram for the iron—water system is given as Figure 1. 

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. 

In an MCFC power system, increased pressure can result in increased cathode corrosion. Cathode corrosion is related to the acidity of the cell, which increases with the partial pressure of CO2, and therefore with the cell pressure. Such corrosion is typified by cathode dissolution and nickel precipitation, which can ultimately result in a shorted cell, causing cell failure (37). Thus, the chosen pressure of the MCFC has a direct link to the cell life, economics, and commercial viability. 

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. 

Material corroding (A) Uncoupled corrosion Cathode member of 10 1 0 1... 

The corrosion phenomena commonly observed on painted metals include cathodic delamination, anodic undercutting, and filiform corrosion. Cathodic delamlnatlon results when the alkali produced by the cathodic corrosion reaction disrupts the paint-metal interface. This phenomenon has long been observed on cathodically protected painted steel (18) and has also been demonstrated to be responsible for the loss of paint adhesion that often occurs adjacent to corrosion sites on painted steel (19). The localization and separation of anodic and cathodic sites associated with corrosion at a break in a paint film on steel are schematically illustrated in Figure 7. 

CORROSION, CATHODIC - Corrosion resulting resulting from a cathodic condition of a structure usually caused by the reaction of an amphoteric metal with the alkaline products of electrolysis. 

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. 

C. E. Locke, Corrosion cathodic and anodic protection. Encyclopedia of Chemical Processing and Design, Marcel Dekker, New York, 1981, pp. 13-59, Vol. 12. 

Biofilms on passive alloy surfaces can increase cathodic kinetics by way of increasing the propagation rate of galvanic corrosion. Cathodic kinetics increased during biofilm formation on passive alloy surfaces. Crevice corrosion initiation times were reduced when natural biofilms were allowed to form on passive alloys S 30400 and S 31600. 

In fact, there are many ways that can be thought of as routine strategies that, nowadays, industries use globally to manage corrosion. Cathodic/anodic protection, application of coatings, use of the so-called corrosion-resistant alloys (CRAs), as well as improvements in design are among the most important ones. 

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. 

No. Steel Cr content Hardness HV10 Free corrosion Cathodically polarised (zinc anode) ... 

In the case of surface corrosion or pitting corrosion not microcolonies but biofilms are involved (Kunzel, 1991). Under oxygen deprivation in the biofilm, fermentative processes take place and carbonic acids (also complexing agents) are formed. Also, sulfate reduction occurs (H2S evolution). These processes cause acid corrosion as well as attack by hydrogen sulfide. In the reaction of HjS with iron(II) ions, iron sulfide is formed, and this speeds up/accelerates the oxygen corrosion (cathodic reaction) Cough-ton et al., 1988). These processes are presented in Fig. 4-6. 

This reaction is facilitated by cation counterions and a catalytic surface of metal oxide. Via this equation, we can speculate on some of the measures that should help to prevent or determine the corrosion (cathodic) process and the subsequent loss of adhesion of the coating ... 

Cathodically protective pigments As with inhibition, cathodic protection in coatings is mostly provided by additives in the primer. The main function of these additives is to shift the potential of the environment to a less-corrosive cathodic potential. Inorganic zinc-based primers are good examples of this concept. 

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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