Aluminum corrosion potential

Overall, these results indicate that chromates inhibit corrosion by elevating the pitting potential on aluminum with respect to the corrosion potential, which decreases the probability for the formation of stable pits. In general a chromate chloride concentration ratio in excess of 0.1 is necessary to observe significant anodic inhibition. 

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. 

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... 

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. 

Fig. 7.41 Anodic polarization curve for 99.99 wt% aluminum in deaerated 0.1 M NaCl solution. Eb jt is potential at which upscan of the potential, starting at the corrosion potential, results in sudden increase in current density. Redrawn from Ref 61...

Aluminum substrates are particularly prone to anodic undermining, whereas on steel this form of corrosion usually occurs under an applied potential [88]. Under conditions of free corrosion potential. 

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. 

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... 

Cathodic control protection protects the substrate by coating with a less noble metal, for which the slopes of the cathodic polarization curves are steep. The cathodic overpotential of the surface is increased by the coating therefore, the corrosion potential becomes more negative than that of the substrate. Coating materials used for this purpose are zinc, aluminum, manganese, cadmium, and their alloys. The electrode potential of these metals are more negative than those of iron and steel. When exposed to the environment, these coatings act as sacrificial anodes for the iron and steel substrates. 

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