Corrosion potential of aluminum

Annual Book ofASTM Standards, Standard Practice for Measurement of Corrosion Potentials of Aluminum Alloys, ASTM, Philadelphia, PA, 1994, G 69-81. 

G69-97, Standard practice for measurement of corrosion potentials of aluminum alloys. Annual Book of ASTM Standards, ASTM International, Philadelphia, Pa., 2000, p. 268, Vol. 3.02. 

Practice for Measurement of Corrosion Potentials of Aluminum Alloys... 

Figure 3-2. Effects of alloying elements on the corrosion potentials of aluminum alloys.

Anodic oxidation of valve metals, particularly, aluminum, has attracted considerable attention because of its wide application in various fields of technology. Traditionally, aluminum is anodized in order to protect the metal against corrosion, to improve its abrasion and adsorption properties, etc.1 The more recent and rapidly growing applications of anodic aluminas in electronics are due to their excellent dielectric properties, perfect planarity, and good reproducibility in production. Finally, ways have recently been found to use the energy potential of aluminum oxidation for chemical power sources of the metal-air type2,3 and other electrochemical applications. 

Figure 1. Schematic representation of potential profile and charge distribution across an anodic oxide film of thickness S on aluminum (a) hypothetical situation in the absence of any current (b) in the presence of an anodic current caused by corrosion or by an external source. RE, reference electrode to which the potential of aluminum is referred.

Note the potentials of the graphite and the aluminum alloy that you determined. If these two are connected with an electrical contact, their potentials should move toward each other. Further, since the solution is relatively conductive, and assuming that the electrical lead connecting them was highly conductive, they would come to the same potential. Therefore connect the leads of the two electrodes together and connect them both to the positive (or V) lead of the voltmeter. Measure the potential of this galvanic couple relative to one of the reference electrodes and confirm that the couple potential does indeed rest somewhere in between the corrosion potentials of the two materials. 

Corrosion resistance of metallic coatings is dependent on the composition and nature of the electrolyte, oxygen concentration, polarization characteristics, ratio of cathodic to anodic area and the surface contaminants. If the corrosion potentials of two metals such as iron and aluminum are close to each other in a particular environment there may be reversal of the galvanic couple. 

Pitting potential increased with increase in chromium contents >20 wt%, and molybdenum of 2-6 wt%. Recent results, applying microelectrochemical techniques, confirmed that even in the superaustenitic stainless steels molybdenum strongly improves the repassivation behavior but has no influence on pit initiation.27 The corrosion resistance of aluminum alloys is totally dependent on metallurgical factors.52, (Frankel)5... 

The galvanic series of metals and alloys in seawater is given in Table 7.20. From this series it is clear that steel and 2024 aluminum are in close proximity. From their positions it is inferred that steel is cathodic and aluminum is anodic in seawater. The corrosion potentials of iron and aluminum measured after immersion in various media for 24 h are given in Table 7.21. It is seen from these data that the corrosion potentials of iron and aluminum are very nearly the same in 0.1M sodium chloride. Some studies on the galvanic action of the steel-aluminum couple in fresh waters such as pure, river, lake and underground water and salt solutions are noted in Table 7.22. In one of the studies, the... 

The first step of SAIE in the case of corrosion protection of aluminum alloys is the preparation of oxides. The top layer of an aluminum alloy is generally covered with hydrated mixed oxides. Either alkaline cleaning or a combination of alkaline cleaning and deoxidization removes major organic contaminants and this potentially unstable oxide layer. A thin layer of plasma polymer is deposited on the stabilized oxide layer thus created. 

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... 

The line representing reaction (1) of Table 2.17 indicates the proteetion potential of aluminum. At lower potentials, the rate of corrosion of aluminum metal is negligible. The lines that correspond to reactions (3) and (4) of Table 2.17 define the limits of stability for gibbsite as a function of pH. 

Due to its simplicity, open circuit corrosion potential measurements (see Chapter 20 of this manual) have been used in MIC studies for many years. Corrosion potential measurements as a function of time have been used to obtain information on MIC of steel, aluminum alloys, stainless steels, and other passive alloys. By itself, the corrosion potential of plain carbon and low alloy steels indicates very little because these steels can corrode at a wide range of potentials. Rapid changes in the corrosion potential, however, can be used to indicate cathodic depolarization, or an enhancement of the anodic reaction, or to the formation of a semi-protective film. 

Galvanic corrosion and the factors affecting it have been discussed in Chapter 20. However, a few precautionary comments are in order for aluminum, since it is anodic to most common materials of construction, with the exception of magnesium and zinc. In the presence of a good electrolyte, as little as 15 mV difference in corrosion potential of the two metals can have an effect, and if the difference is 30 mV or greater the anodic material will definitely corrode sacrificially to protect the contacting cathodic metal. A recently revised report on galvanic corrosion, with emphasis on automotive applications is available [73]. 

It is recognized that elements in solid solution are less detrimental to the corrosion of aluminum, but that the existence of secondary phases in the mass are harmful, because a discontinuous and non-protective oxide film is often formed at the matrix-particle interface. The harmful extent of the secondary phases depends on the kinds and amounts of the particles. It is important elec-trochemically to know the potential of the microstructural particle phases. The potential difference between aluminum matrix and secondary phase is of primary importance in the corrosion behavior of aluminum and its alloys. The potentials of solid solu-... 

The corrosion resistance of 3xxx wrought aluminum alloys is very high. The manganese is present in the aluminum matrix as submicroscopic precipitates. The secondary particulate phases are intermetallic compound particles such as MnAl. Good resistance to corrosion of these series is primarily explained electrochemically - the corrosion potential of MnAl is almost the same as that of the aluminum matrix. 

In the 2XXX system, corrosion potential measurements were conducted on high-purity aluminum and various binary Al-Cu alloys up to and beyond the limit of solid solubility of 5.65% copper. Also, it was possible to produce large particles of the stoichiometric precipitates CuAla and CuMgAla so that their corrosion potentials could be measured. Figure 19.2 is a plot of the corrosion potential of the various materials as a function of copper content, showing that significant potential differences of as much as... 

Plot of corrosion potentials of pure aluminum and binary aluminum plus copper alloys, plus the two stoichiometric precipitates. The binary alloys were fully solution heat treated and quenched as rapidly as possible to retain the maximum amount of copper in solid solution. Note that the addition of copper raises the corrosion potential of pure aluminum by about 0.14 V. (Courtesy of Alcoa Technical Center, Marsha Egbert.)... 

Effect of alloying elements on the electrode potential of aluminum. (From E.H. Hollingsworth and H.Y. Hunsicker. 1988. Aluminum alloys, in Corrosion and Corrosion Engineering, 2nd ed., P.A. Schweitzer, Ed., New York Marcel Dekken)... 

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