Aluminum alloys corrosion potentials measured

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. 

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... 

A study involving corrosion potential measurements taken aooss wddments (through base metal, HAZ, and weld metal) has identified certain aluminum-lithium alloy compositions as being suscqrtible to galvanic attack in a saline environment (Ref 4). In particular, two experimental alloys with high lithium content (2.9 wt% Li and 3.0 wt% Cu), welded with either 2319 or 4043 fillers, displayed a narrow region widiin the HAZ... 

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. 

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

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... 

As the nature of the electrified interface dominates the kinetics of corrosive reactions, it is most desirable to measure, e.g., the drop in electrical potential across the interface, even where the interface is buried beneath a polymer layer and is therefore not accessible for conventional electrochemical techniques. The scanning Kelvin probe (SKP), which measures in principle the Volta potential difference (or contact potential difference) between the sample and a sensing probe (which may consist of a sharp wire composed of a conducting, stable phase such as graphite or gold) by the vibrating condenser method, is the only technique which allows the measurement of such data and therefore aU modern models which deal with electrochemical de-adhesion reactions are based on such techniques [1-8]. Recently, it has been apphed mainly for the measurement of electrode potentials at polymer/metal interfaces, especially polymer-coated metals such as iron, zinc, and aluminum alloys [9-15]. The principal features of a scanning Kelvin probe for corrosion studies are shown in Fig. 31.1. 

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. 

Many users do not recognize that aluminum alloys themselves span a range of about 400 mV in their respective corrosion potentials. In aerated sodium chloride solutions, pure aluminum, 3xxx alloys, and many other alloys have a potential of about —740 mV when measured with a saturated calomel reference electrode (SCE). Aluminum alloys with high magnesium or zinc contents will be more anodic by as much as 260 mV, while high copper content alloys will be more cathodic up to about 140 mV. Care must be taken therefore, that all alloys and tempers are compatible, even in an all aluminum structure. 

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

The lowest potential is measured in the center where corrosion (i.e., anodic dissolution of iron) attacks most aggressively. At the edges the potential increases somewhat in this zone oxygen reduction proceeds. The potential changes around the drop imply the presence of an ultrathin electrolyte film because the potential reaches values of the bare iron surface only at a considerable distance from the edges of the macroscopically observed drop [135]. Filiform corrosion of automotive aluminum alloy AA6016 has been studied with SKP [143]. 

Figure 7-67. Filiform corrosion on polymer coated aluminum alloy, (a) photograph corrosion potential distributions in air (b) and nitrogen (c) as measured with the SKP at constant RH (90%). From black to white increasing corrosion potential.

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... 

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