The role of indium in the activation of aluminum alloy galvanic anodes

Abstract

Despite six decades use of aluminum as a galvanic (sacrificial) anode, there remains a need for a better understanding of the underlying mechanisms for enhancing its efficient performance in cathodic protection systems. A few mechanisms have been proposed for the role of indium in the activation of Al‐Zn‐In anodes and there appears to be no general agreement on whether this element plays its depassivating role by modifying the bulk microstructure of the anode, chemical composition of its surrounding electrolyte or directly through doping the structure of the passive oxide film. These mechanisms have been critically reviewed to achieve a more comprehensive understanding of the role of indium in such applications. Moreover, the novel solidification processing called controlled diffusion solidification (CDS) has been introduced as an efficient way to surmount the poor castability of the anode alloy without any need for the addition of elements with detrimental effects on the electrochemical properties of the anode.

Cathodic Protection of Mild Steel Using Aluminium-Based Alloys

“…For the special case of indium, a double effect of indium on the sacrificial aluminium anode's surface is observed, first modifying the oxide layer on Al, then forming agglomerates of indium (see In distribution on mapping, Figure 10). It significantly favours the reactivity of aluminium, impeding its passivation, which is consistent with the literature [23,24]. Similarly, Sn promotes a similar effect.…”

“…The corrosion product mainly consists of Al 2 O 3 /alloying element-oxides after galvanic coupling to steel in 3.5 wt.% NaCl. The porous regions described above are promoted due to the accumulation of the alloying particles underneath the corrosion-product layer, preventing the aluminium from forming an oxide layer as reported in [18,24,52]; this becomes visible by the lower distribution of O in that area from the element mapping (Figure 10). This phenomenon was most visible in the Al-Sn, Al-Si, Al-Zn, Al-Zn-Ti-Mn-In and Al-In alloys.…”

“…Therefore, the most efficient way to produce sacrificial aluminium-based alloy anodes is by adding 5 wt.% of zinc, since the formation of Al-Zn phases significantly enhances not only the metallurgical, but also the electrochemical properties of the anodes [19,20]. The presence of indium in Al-based alloy increases the adsorption of chloride ions at the electrode surface, leading to an activation of Al [21][22][23][24]. On the other hand, minor additions of Cr possess moderate corrosion resistance properties [25].…”

Influence of Sn, Cd, and Si addition on the electrochemical performance of Al–Zn–In sacrificial anodes

“…Accompanied by a negative shift in the aluminum anode potential by 0.1–0.3 V, Zn can make the aluminum anode alloy composition uniform and corrosion products to fall off easily . It has the ability to slow down the pitting rate of the aluminum anode, which makes the surface corrosion uniform and the potential of aluminum anode more negative . Studies have been conducted to find the synergistic interaction between In and Zn in activating aluminum anodes .…”

“…[11][12][13] It has the ability to slow down the pitting rate of the aluminum anode, which makes the surface corrosion uniform and the potential of aluminum anode more negative. [14,15] Studies have been conducted to find the synergistic interaction between In and Zn in activating aluminum anodes. [16][17][18][19] While refining the grains and reducing the intergranular segregation phase, Sn can form a solid solution with aluminum to break the passive layer and lower the potential of aluminum.…”