Cathodic protection of steel

Silver reduces the oxygen evolution potential at the anode, which reduces the rate of corrosion and decreases lead contamination of the cathode. Lead—antimony—silver alloy anodes are used for the production of thin copper foil for use in electronics. Lead—silver (2 wt %), lead—silver (1 wt %)—tin (1 wt %), and lead—antimony (6 wt %)—silver (1—2 wt %) alloys ate used as anodes in cathodic protection of steel pipes and stmctures in fresh, brackish, or seawater. The lead dioxide layer is not only conductive, but also resists decomposition in chloride environments. Silver-free alloys rapidly become passivated and scale badly in seawater. Silver is also added to the positive grids of lead—acid batteries in small amounts (0.005—0.05 wt %) to reduce the rate of corrosion. 

Current control can be more advantageous where rail/soil potentials are predominantly positive. Current control is also preferred in the cathodic protection of steel-water construction if the anode resistance fluctuates due to changes in electrical conductivity. 

Considerable alterations have been made in the chapters concerned with technical applications which are the result of advances in electrochemical corrosion protection in general practice. Here also, abbreviation and omission of less relevant parts of the older editions have had to be made to create space for more recent information. Recent applications in the chemical industry have necessitated a complete rewriting of the industrial chapter. A new chapter is included on the cathodic protection of steel reinforcement in concrete. 

Fig. 10.2 Schematic illustration of partial cathodic protection of steel in an aerated environment. Note that one of the anodic reactions shown in Fig. 10.1 has been annihilated by providing two electrons from an external source an excess of OH ", ions over Fe now exists at...

A conductive polymer electrode has been designed specifically for the cathodic protection of steel reinforcing bars in concrete and is marketed under the trade name Ferex . The anode consists of a 16 AWG stranded copper conductor surrounded by a carbon-loaded polymeric coating similar to that used on the Anodeflex system ) to provide a nominal anode diameter of 8 mm The manufacturer claims that at the maximum recommended current density of 0 08 Am the anode life in concrete will be 32 years with a proportionately longer life at lower current densities. 

Conductive paints (resins) have recently been used for the cathodic protection of steel reinforcing bars in concrete, but they are always used in conjunction with a primary anode material, e.g. platinised-niobium or platinised-titanium wire or a conductive polymer rod. 

The most recently developed anode for the cathodic protection of steel in concrete is mixed metal oxide coated titanium mesh The anode mesh is made from commercially pure titanium sheet approximately 0-5-2mm thick depending upon the manufacturer, expanded to provide a diamond shaped mesh in the range of 35 x 75 to 100 x 200 mm. The mesh size selected is dictated by the required cathode current density and the mesh manufacturer. The anode mesh is supplied in strips which may be joined on site using spot welded connections to a titanium strip or niobium crimps, whilst electrical connections to the d.c. power source are made at selected locations in a suitably encapsulated or crimped connection. The mesh is then fitted to the concrete using non-metallic fixings. 

In recent years, there has been interest in using zinc as a power-impressed anode for the cathodic protection of steel in concrete. The zinc is flame sprayed onto a grit blasted concrete surface to a final film thickness of approximately 250 m. A primary anode is necessary. Early systems used brass plates as the primary anode, but more recent systems used platinised titanium or niobium wire anodes as the primary current conductor. 

Wyatt, B. S., Anode Systems for Cathodic Protection of Steel in Concrete , paper 23, Cathodic Protection Theory and Practice, 2nd International Conference, Stratford-upon-Avon, UK, June (1989)... 

The features of instances of such potential reversals have been described and include tin coatings on steel in various foodstuffs, particularly acid fruits , cadmium coatings on steel in hard waters , and zinc and aluminium for cathodic protection of steel in natural ground-waters . 

It is apparent that since the electrode potential of a metal/solution interface can only be evaluated from the e.m.f. of a cell, the reference electrode used for that purpose must be specified precisely, e.g. the criterion for the cathodic protection of steel is —0-85 V (vs. Cu/CuSOg, sat.), but this can be expressed as a potential with respect to the standard hydrogen electrode (S.H.E.), i.e. -0-55 V (vs. S.H.E.) or with respect to any other reference electrode. Potentials of reference electrodes are given in Table 21.7. 

G. Bech-Nielsen, P. T. Tang, and D. Ulrich, proceedings (in Danish), SkanZink 91 (Scandinavian Conference on Cathodic Protection of Steel), Centre of Advanced Electroplating, The Technical University of Denmark, 1991, p. 11. 

P. Pedeferri, G. Mussinelli and M. Tettamanti, Experiences in anode materials and monitoring systems for cathodic protection of steel in concrete , in Corrosion of Reinforcement in Concrete, edited by C. L. Page, K. W. J.Treadaway and P. B. Bamforth, Elsevier Applied Science, London, UK, 1990, pp 498-506. 

M.Y. El-Shazly, Cathodic Protection of Steel in Concrete, 24th Annual Conference on Corrosion Problems in Industry, Egyptian Corrosion Society, 5-8 December 2005, Les Rois Hotel, Egypt. 

LaQue, F.L. and May, T.P., 1965, Experiments relating to the mechanism of cathodic protection of steel in sea water. Proc. 2nd Int. Congress on Metallic Corrosion, NACE. 

In cathodic protection of steel tanks and pipes, a more reactive metal than iron is connected to the item to be protected. The active metal is then oxidized instead of the iron of the tank or pipe. 

Cathodic Protection of Steel in Chloride-contaminated Concrete... 

R. Cigna, O. Fumei, On the cathodic protection of steel in reinforced concrete , (in Italian), L lndustria Italiana del Cemento, 1981, 9. 

Effects of the Circulation of Current 353 (a) Cathodic protection of steel in soil or in seawater (immunity protection)... 

P) Cathodic protection of steel in chloride-containing concrete... 

Cathodic protection of steel-rein-forced-concrete structures — Results from 15 years experience . Materials Performance, December, 22—26,... 

P. Pedeferri, E. Redaelli, Cathodic protection of steel in concrete and cathodic prevention . Final project report 15, COST 521, Luxembourg,... 

M. Tettamanti, P. Pedeferri, Cathodic protection of steel in concrete , Materials Performance, 1988, 27 (9), 18. 

Fig. 12 The relationship between potential and the possibility of corrosion and cathodic protection of steel [3].

Coatings and cathodic protection of steel structures used in exploitation of oil in offshore locations, harbors, parts, and vessels. 

D.A. Hausmann, Criteria for cathodic protection of steel in concrete structures. Mater. Prot. 8 (1969) 23-25. 

Examples of corrosion due to SRB and other bacteria are dealt with in Chapter 8. SRB must be taken into aeeount in design of systems for cathodic protection of steel (Section 10.4). 

Table 10.13 Mtial/final design values of current density for cathodic protection of steel in different climatic zones and at different ranges of seawater depth. The given temperatures are sea surface temperatures in average during the year [10.25].

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