Anodic protection of steel

Introduction of electrOcheiiiicaUy active cathodes that facilitate passivation Raise potential by external e.m.f Additions of Pt. Pd and other noble metals to Ti, Cr and stainless steels Anodic protection of steel, stainless steel and Ti... 

Anodic protection of steel can be achieved by applying a small anodic current using a less expensive metal cathode. However, it is necessary to make sure that the protective, passive film on the surface is not attacked by tmce impurities such as chloride. Anodic protection of steel storage tanks for 50% NaOH is successful at a current density of 1A m , but it is not used with diaphragm-cell caustic because of the presence of salt. 

More recently Ahmad and MacDiarmid [30] published a fundamental study on corrosion inhibition of steel with chemically deposited polyaniline. The study concludes that the corrosion protection for iron provided by this conducting polymer mostly arises from anodic protection, which requires much less current density (micro- to milliamperes per square centimeter) than a polymer can provide to the metal. Anodic protection of steel is achieved by the formation of a passive oxide layer, which can be induced by applying an anodic potential of about 0.1 V vs. SCE. The study showed that the minimum potential required for passivation of a sam-... 

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. 

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)... 

In principle, cathodic protection can be used for a variety of applications where a metal is immersed in an aqueous solution of an electrolyte, which can range from relatively pure water to soils and to dilute solutions of acids. Whether the method is applicable will depend on many factors and, in particular, economics — protection of steel immersed in a highly acid solution is theoretically feasible but too costly to be practicable. It should be emphasised that as the method is electrochemical both the structure to be protected and the anode used for protection must be in both metallic and electrolytic contact. Cathodic protection cannot therefore be applied for controlling atmospheric corrosion, since it is not feasible to immerse an anode in a thin condensed film of moisture or in droplets of rain water. 

Table 10.35 Effect of temperature on different acids on the operating variables for anodic protection of different steels (after Walker and Ward )...

Not only is the anodic protection of a mild-steel tank cheaper than one with a glass or phenolic lining , but, because the steel conducts heat, it can be used for heat exchangers, and in addition it may be more stable at high... 

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. 

Examples of cathodic protection with impressed current are, at the present time, protection of steel pipelines in maritime environments or in subsoil. An important example of anodic protection is in the storage of acids in steel tanks—the anodic current passivates the steel (see Fig. 16.1a). 

The more negative the potential, the greater the cathodic reaction and the smaller the anodic reaction the metal is more cathodic, which is the basis of cathodic protection of metals. By applying more positive potentials the system moves into the passive region where the corrosion rate may be reduced. This is particularly the case for some steels in particular environments and other metals, which forms the basis of anodic protection of metals. Thus, it is seen that changing the potential of a system in an environment which cannot be altered leads to effective corrosion control by cathodic or anodic protection as the case may be. 

W. H. A. Peelen, R. B. Polder, Throwing power of the zinc-hydrogel anode for sacrificial protection of steel in concrete , ICC 15 Int. Corrosion Congress, Granada,... 

The anode must be a metal that is more active than the metal to be protected. Commercial sacrificial anodes for the protection of steel include magnesium, zinc, and aluminum. 

Figure 10.21 Internal anodic protection of a steel tank.

CPs are able to raise the surface potential and provide anodic protection of the substrate material. Tallman et al. [61] reported that the redox potential of PANI is 0.4 to 1.0 V [vs. standard hydrogen electrode (SHE) at pH 7] and that of polythiophene is 0.8 to 1.2 V. Both values are higher when compared to the corrosion potential of steel and aluminum. This indicates that both PANI and poly thiophene are able to passivate the surface of both steel and aluminum. Anodic protection alone can be fatal if the... 

---