Aluminum alloys corrosion potentials

Uses. In spite of unique properties, there are few commercial appUcations for monolithic shapes of borides. They are used for resistance-heated boats (with boron nitride), for aluminum evaporation, and for sliding electrical contacts. There are a number of potential uses ia the control and handling of molten metals and slags where corrosion and erosion resistance are important. Titanium diboride and zirconium diboride are potential cathodes for the aluminum Hall cells (see Aluminum and aluminum alloys). Lanthanum hexaboride and cerium hexaboride are particularly useful as cathodes ia electronic devices because of their high thermal emissivities, low work functions, and resistance to poisoning. 

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. 

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

Galvanostatic methods for localized corrosion. At constant chosen currents, the evolution of potential as a function of time is recorded until the rate of change in potential with time approaches zero. This technique is under development for aluminum alloys in ASTM Gl4 as a test method for application to aluminum alloys. (Scully)14... 

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. 

Pitting corrosion is usually associated with active-passive-type alloys and occurs under conditions specific to each alloy and environment. This mode of localized attack is of major commercial significance since it can severely limit performance in circumstances where, otherwise, the corrosion rates are extremely low. Susceptible alloys include the stainless steels and related alloys, a wide series of alloys extending from iron-base to nickel-base, aluminum, and aluminum-base alloys, titanium alloys, and others of commercial importance but more limited in use. In all of these alloys, the polarization curves in most media show a rather sharp transition from active dissolution to a state of passivity characterized by low current density and, hence, low corrosion rate. As emphasized in Chapter 5, environments that maintain the corrosion potential in the passive potential range generally exhibit extremely low... 

Interface Potential and Pit Initiation. It is generally accepted that pit initiation occurs when the corrosion potential or potentiostatically imposed potential is above a critical value that depends on the alloy and environment. However, there is incomplete understanding as to how these factors (potential, material, and environment) relate to a mechanism, or more probably, several mechanisms, of pit initiation and, in particular, how preexisting flaws of the type previously described in the passive film on aluminum may become activated and/or when potential-driven transport processes may bring aggressive species in the environment to the flaw where they initiate local penetration. In the former case, the time for pit initiation tends to be very short compared with the initiation time on alloys such as stainless steels. Pit initiation is immediately associated with a localized anodic current passing from the metal to the environment driven by a potential difference between the metal/pit environment interface and sites supporting cathodic reactions. The latter may be either the external passive surface if it is a reasonable electron conductor or cathodic sites within the pit. 

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... 

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. 

Magnesium and zinc are the predominantly used galvanic anodes for the cathodic protection of pipelines [13—16]. The corrosion potential difference of magnesium with respect to steel is 1 V, which Umits the length of the pipeline that can be protected by one anode. Economic considerations have led to the use of aluminum and its alloys as anodes. However, aluminum passivates easily, decreasing current output. To avoid passivation, aluminum is alloyed with tin, indium, mercury, or gallium. The electrochemical properties of these alloys, such as theoretical and actual output, consumption rate, efficiency, and open circuit (corrosion) potential, are given in Table 15.1. 

Typically, titanium alloys have been the materials of choice for medical implants. The Ti-6A1-4V alloy is generally considered chemically inert, compatible with human tissue, and resistant to corrosion by human body fluids. However, the small percentages of vanadium and aluminum contained in the alloy are potentially toxic. Pure titanium is chemically and biologically more compatible with human fluids and tissue, but it is too weak for prostheses that must bear heavy loads, such as leg or hipbone implants. 

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