Corrosion potentials of aluminum alloys

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.

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 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. 

Copper. It reduces the corrosion resistance of aluminum alloys and increases the susceptibility to pitting. The corrosion potential of Al-Cu alloys become cathodic in direct proportion to the amount of Cu in soUd solution. 

A marked increase in aluminum Ep values towards anodic potentials was observed in the presence of marcescens, even in the sterile medium, suggesting the metal surface may experience some protective action by these bacteria. Local acidification enhanced by adhesion processes taking place at the metal/mycelia interface accounts for some of the specific effects of resinae in the corrosion process of aluminum alloys in fuel/water systems (Salvarezza et al., 1979). The acidification also has been reported in the literature as a differential effect between two Pseudomonas spp. in relation to aluminium corrosion. Acidity can prevent repasivation and may hinder the formation of a protective oxide film. Therefore, under acidic conditions, pitting of the metal by chloride anions occurs at more cathodic potentials than in neutral solution (Salvarezza et al., 1983). 

Corrosion is an electrochemical process and hence the corrosion potentials achieved at the surface of different aluminum alloys is of considerable importance. While the major forms of corrosion are covered elsewhere in this monograph, it is noted that Al alloys are susceptible to all forms of corrosion, and hence a brief mention of these manifestations is given in the context of Al. Also, the difference between the potential of aluminum alloys and other metals is important, as is the relationship between the potential of microstructural constituents of a single alloy. The major forms of corrosion of Al are as follows ... 

Because of the electrochemical nature of most corrosion processes, relationships among solution potentials of different aluminum alloys, as well as b ween potentials of aluminum alloys and those of other metals, are of considerable importance. Furthermore, the solution-potential (or corrosion-potential) relationships among the microstructural constituents of a particular alloy significantly affect its corrosion behavior. 

Effect of Electrode Potential. Stress-corrosion cracking of aluminum alloys can be dramatically reduced by cathodic polarization rotection. This is shown in Fig. 14 where the application of a negative potential reduces the growth rate of stress-corrosion cracks in the plateau region by a factor of more than one thousand. 

DeKiibes aprocedureto determine the repassivation potential of aluminum alloy 3003- HI4 as a measure of relative susceptibility to pitting corrosion by conducting agalvanostatic polarization See also ASTM G100 summarized later in this table. 

Describes a procedure for measurement of corrosion potential (or open-circuit solution or rest potential) of aluminum alloys in aqueous solution of sodium chloride with enough hydrogen peroxide added to provide ample supply of cathodic reactant. See Chapter 2 for listings of corrosion potentials... 

Inhibiting the corrosion of aluminum alloys by adding 1-5% of transition metals is a dramatic case of corrosion protection because of the small amounts of additives that are successful in reducing the corrosion rate by 1-2 orders of magnitude. It turns out that the alloying materials shift the pzc toward the positive side on the potential scale. Thus, in many practical situations, the alloys of the transition metals are in a... 

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. 

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... 

J.R. Galvele, S.M. De Micheli, I.L. Muller, S.B. de Wexler, and I. Alanis, Critical Potentials for Localized Corrosion of Aluminum Alloys, Localized Corrosion NACE 3, B.F. Brown, J. Kruger, and R.W. Staehle, Ed., National Association of Corrosion Engineers, 1974, p 580-598... 

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. 

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... 

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