Aluminum anodic protection

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... 

Production platforms are coated only in exceptional cases or for the purposes of investigation because the life of the structure is greater than the life of the coating. Therefore in the design of the cathodic protection, only the protection potential Us of the steel need be considered. Steels with an ultimate tensile strength of up to 350 N mm are used for these structures, which are weldable even in thick sections, and the hardness of the welded material can be kept to 350 HV (see Section 2.3.4 [2,10]). Aluminum anodes with the same protection effect and life as zinc anodes have much less weight. This is a very important advantage for... 

The electrolysis protection process using impressed current aluminum anodes allows uncoated and hot-dipped galvanized ferrous materials in domestic installations to be protected from corrosion. If impressed current aluminum anodes are installed in water tanks, the pipework is protected by the formation of a film without affecting the potability of the water. With domestic galvanized steel pipes, a marked retardation of the cathodic partial reaction occurs [15]. Electrolytic treatment alters the electrolytic characteristics of the water, as well as internal cathodic protection of the tank and its inserts (e.g., heating elements). The pipe protection relies on colloidal chemical processes and is applied only to new installations and not to old ones already attacked by corrosion. 

Figure 20-12 shows schematically the arrangement of aluminum anodes in vertical and horizontal boilers with heating tubes. More than 33% of the aluminum anodes must be sited in the upper third of the tank for the formation of protective films in the tubing [18], Cathodic currents needed to protect the tubing may be three times as high as the current requirement for cathodic protection [19]. 

A higher content of AI2O3 and SiOj is critical for the composition of the protective films in the tubing, assuming the water contains silicates or silicic acid. The protective films have a maximum thickness of 1.5 mm and cannot grow further. The corrosion process can be stopped even in copper pipe networks with type I pitting [21] by providing a reaction tank with impressed current aluminum anodes. 

Pure aluminum is used in the electrolysis protection process, which does not passivate in the presence of chloride and sulfate ions. In water very low in salt with a conductivity of x < 40 yUS cm" the polarization can increase greatly, so that the necessary protection current density can no longer be reached. Further limits to its application exist at pH values < 6.0 and >8.5 because there the solubility of Al(OH)3 becomes too high and its film-forming action is lost [19]. The aluminum anodes are designed for a life of 2 to 3 years. After that they must be renewed. The protection currents are indicated by means of an ammeter and/or a current-operated light diode. In addition to the normal monitoring by service personnel, a qualified firm should inspect the rectifier equipment annually. 

Six caustic soda evaporators were anodically protected against stress corrosion in the aluminum industry in Germany in 1965 [27]. Each evaporator had an internal surface area of 2400 m. The transformer-rectifier had a capacity of 300 AJ 5 V and was operated intermittently for many years. Automatic switching on of the protection current only took place in case of need when the drop in potential reached... 

In recent years, a number of protective installations have come into operation, especially where new installations must be maintained, or where older and already damaged installations have to be saved and operating costs have to be lowered. Worldwide, equipment, tanks and evaporators in the aluminum industry and industries using caustic alkalis with a capacity of 60,000 m and a surface area of 47,000 m are being anodically protected. Equipment for electrochemical protection has been installed with a total rating of 125 kW and 12 kA. 

A particular advantage of impressed current systems is the ability to control the output voltage of the rectifier. Also, there are the comparatively low installation costs and relatively uniform current distribution. The costs of impressed current protection compared with aluminum anodes are 0.8 1. With ships this ratio depends on the length of the ship with larger ships it is 1 2.5 since the calculation is made in comparison with zinc and aluminum anodes. The order of magnitude of the annual costs depends on the structure and the investment costs. 

Cathodic protection of a hot water tanks using magnesium as the sacrificial anode is shown in Figure 1.71. Some characteristic properties of zinc, magnesium and aluminum anodes are listed in Table 1.30. 

Aluminum spraying is used to coat less corrosion-resistant alloys. In the case of some composites, corrosion is due to the galvanic action between the aluminum matrix and the reinforcing material. Aluminum thermal spraying has been successfully used for the protection of the discontinous silicon carbide/aluminum composites, and continuous graphite/aluminum. Other protection procedures include sulfuric acid anodizing and iron vapor deposition on aluminum.44... 

Aluminum and aluminum-zinc alloy anodes have become the preferred sacrificial anodes for the cathodic protection of offshore platforms. This preference is because aluminum anodes demonstrate reliable long-term performance when compared with magnesium, which might be consumed before the platform has served its useful hfe. Aluminum also has better current/weight characteristics than zinc. Weight can be a major consideration for large offshore platforms. The major disadvantage of aluminum for some applications, for example, the protection of painted ship hulls, is that aluminum is too corrosion resistant in many environments. Aluminum alloys will not corrode reliably onshore or in freshwater [37]. In marine... 

Cathodic protection has been traditionally used for a long time to protect structures in seawater. Zinc bracelet anodes are installed on pipelines immersed in seawater to provide sacrificial CP to the outer surfaces of the steel pipe. Offshore structures for gas and oil exploration, drilKng, and production are commonly protected by the use of aluminum anodes on the steel structures. Cathodic protection is employed by several power plants located near coastal sites for protecting steel structures submerged in seawater. 

Jetties are individual or multiple piles interconnected together to form a structure in the seabed and support a deck. The piles of a jetty usually have half of their length in the seabed and the rest in the high tide and splash zones up to the jetty deck. They are often concrete structures reinforced with steel. Cathodic protection using sacrificial zinc or aluminum anodes is installed after the completion of the jetty. With a deepwater jetty the suspension of more than a single anode or placing of alternate anodes at different levels is necessary. A few and larger anodes are necessary while impressed current method is employed. An... 

Other metals that have favorable reversible Flade potentials and form passive film on their surfaces include titanium, silicon, aluminum, tantalum, and niobium. Naturally formed aluminum oxide protects the underlying aluminum metal at pH between 4 and 8. Titanium possesses very high oxidizing potentials and is used to manufacture anodes for cathodic protection systems for the chlorine-alkafi process (production of hydrogen, chlorine, and sodium hydroxide) and many other appfications. 

Calculate the mass of an anode and the number of anodes required for the protection of an iron tank for 7 years if an aluminum anode weighs 13.7 kg. Maintenance current demand in amps (Inj) = 1.52 A, utilization factor (u) = 0.85, and electrochemical capacity of the anode material ( )=2670 Ah/kg. The number of hours per year is 8760. 

With other metals, in particular with aluminium and its alloys, the ennobling mechanism is not clear-cut. Recent work by Yan et al. [60] carried out with PPy doped with electroinactive 1,3-benzene-disulfonic acid and electroactive sodium 4,5-dihydroxy-l,3-benzene-disulfonate and using a two-compartment cell similar to that of Rammelt et al. [64] seems to prove that the anodic protection model cannot be applied to the aluminum surface. Although the recharging of PPy by O2 was demonstrated, they found that a small area of exposed aluminium (AA 2024-T3) simulating a defect in a coating and immersed in dilute Harrison s solution (0.35 wt% (NH4)2S04 + 0.05 wt% NaCl in water) did not... 

Aluminum and ferrosilicon anodes are suitable for the protection of hot water tanks. The use of aluminum anodes in a hot water apparatus has additional effects, an indirect protective action on subsequent piping, since a protective film (1-1.5 mm thick) is built up by anode-formed aluminum oxide hydrates. In addition to alkalization at the cathode, a shift in the calcium carbonate (CaCOj)-CO2 equilibrium leads to partial softening of water, with CaC03 being deposited at the cathode. 

Corrosion inhibition of chemically and electrochemically synthesized coatings of PANI, poly (o-methoxyaniline), and their copolymers on stainless steel and on aluminum alloy (6061-T6) was evaluated by immersion in 3% NaCl (steel) and 0.1 M NaCl (A1 alloy). These authors concluded from polarization studies that protection involved an anodic protection mechanism [206]. 

Anodic protection is apphcable only to metals and alloys (mostly transition metals) which are readily passivated when anodically polarized and for which /passive is very low. It is not apphcable, for example, to zinc, magnesium, cadmium, silver, copper, or copper-base aUoys. Anodic protection of aluminum exposed to high-temperature water has been shown to be feasible (see Section 21.1.2). 

The beneficial effect of cathodic areas can be explained [7] as one of anodic passivation or anodic protection of aluminum in the same manner as alloying additions or coupling of platinum or palladium to stainless steels or to titanium passivate these metals in acids (Section 6.4). 

---