Alloys and Alloying Elements

Although alloying other elements with aluminum can improve certain properties, it tends to have a negative effect on its corrosion resistance. Some elements, such as magnesium, however, can be alloyed in amounts of about <1% without significantly decreasing the corrosion resistance compared to pure aluminum. Common alloying elements include copper, magnesium, silicon, and zinc. Iron is not usually intentionally used as an alloying element, rather it is commonly a contaminant, and it is typically attributed as being the primary cause of pitting in aluminum alloys. Some of the general classes of aluminum alloys and their corrosion characteristics are described in the following sections. Although there are a number of specific aluminum alloys, only a few are discussed.

Aluminum (1000 Series)

The 1000 series of aluminum alloys has approximately 99% aluminum with the remaining percent consisting of other elements, which are considered impurities. Similar to pure aluminum, this series of metals has excellent corrosion resistance to many environments, but with increasing impurity content the corrosion resistance decreases.

Copper (2000 Series)

The 2000 series of aluminum alloys contains copper as the principal alloying element. These are higher strength alloys and are consequently used mainly for structural applications, but they have a much lower corrosion resistance compared to other aluminum alloys. Therefore, alloys with little or no copper are used for applications where corrosion resistance is important. This series of alloys, in general, is prone to stress corrosion cracking and exfoliation, and typically copper alloying results in the occurrence of uniform, pitting, and intergranular forms of corrosion to a greater extent. For instance, copper additions greater than 0.15% decreases the resistance to pitting corrosion. Alloys containing copper are also more susceptible to corrosion in seawater and marine environments.

If alloys in this series are slightly overaged, their resistance to SCC is improved to the point where the alloys are no longer susceptible to this form of corrosion. Solution heat treatment and artificial aging of 2000-series aluminum alloys, however results in CuAl2 precipitates at grain boundaries, which causes the alloy to be susceptible to intergranular corrosion.

Aluminum alloy 2020 in general is not suitable to be used for structural applications, but in the T651 condition it does exhibit an excellent resistance to SCC. Aluminum alloys 2024-T851 and 2219-T851 are also highly resistant to SCC.

Manganese (3000 Series)

Manganese is the primary alloying element in the 3000 series of aluminum alloys. These alloys exhibit a very good resistance to corrosion, in general, and are particularly very resistant to SCC.

Silicon (4000 Series)

Silicon is the main alloying element in the 4000 series of aluminum alloys, but it has little effect on the corrosion resistance of aluminum. In particular, this series of alloys is characteristically very resistant to SCC.

Magnesium and Silicon (5000 and 6000 Series)

Magnesium is the main alloying element in the 5000 series of aluminum alloys, and it provides extra protection against aqueous corrosion. Magnesium can also serve to increase the resistance to corrosion in salt water and under alkaline conditions compared to unalloyed aluminum. It may, however, also help to advance SCC and intergranular corrosion, if it is present in the grain boundaries as an anodic magnesium aluminum phase. If the magnesium content exceeds the specified limit, it tends to precipitate another phase with aluminum, and consequently causes an increase in susceptibility to intergranular corrosion. Aluminum-magnesium alloys also have a tendency to be susceptible to exfoliation.

Aluminum alloys 5083, 5086, and 5456 in the H30-series of conditions should not be used for structural applications since, they are very susceptible to SCC. Aluminum alloy 5454-H34, on the other hand, has an excellent resistance to SCC. Furthermore, the H116 and H117 tempers for the 5000-series of aluminum alloys offer a good resistance to exfoliation.

The 6000 series of aluminum alloys contain magnesium and silicon as the primary alloying elements. These alloys are stronger while maintaining the same excellent resistance to aqueous corrosion as the 5000-series alloys. However, silicon in amounts greater than 0.1% reduces the resistance to pitting corrosion, and decreases the corrosion resistance in marine environments. Furthermore, excess silicon decreases the resistance to intergranular corrosion. Alloys containing magnesium or magnesium and silicon tend to have the best resistance to corrosion in seawater and marine environments of any of the other aluminum alloys.

In general, similar to the 5000-series, the 6000 series alloys are susceptible to SCC. In particular these alloys with >3% magnesium can be very susceptible to SCC. Cold-worked aluminummagnesium and aluminum-magnesium-silicon alloys containing <3% Mg, however, are very resistant to SCC.

Zinc (7000 Series)

Zinc is the primary alloying element in the 7000 series of aluminum alloys, and in general, as an alloying element it only has a small influence on the corrosion resistance of aluminum. These alloys, however, are characteristically much more susceptible to aqueous corrosion. A high zinc content may result in decreased resistance to intergranular corrosion, SCC, and exfoliation corrosion. In addition, zinc may decrease the resistance of aluminum to acidic environments, but may increase the resistance to alkaline environments.

Within the 7000 series of aluminum alloys, some alloys are especially susceptible to SCC and are therefore not suitable for structural applications. Overaging in the 7000 series of aluminum, however, tends to reduce their susceptibility to SCC. Aluminum alloys 7079 and 7178 are not suitable for structural applications. The high strength aluminum alloy 7075 in the T6 condition is very susceptible to SCC and exfoliation, but in the T73 condition it has a greater resistance to SCC. 7075 in the T7351 condition has an excellent resistance to SCC. In general for the 7000-series aluminum alloys, the T76 tempers have a greater resistance to exfoliation than the T73 tempers.

Chromium

Chromium can be a beneficial alloying element because it typically provides improved corrosion resistance. For instance, Cr improves the corrosion resistance of Al-Mg and Al-Mg-Zn alloys when added in small amounts (0.1-0.3%). Furthermore, Cr increases SCC resistance in highstrength alloys, however, it does tend to increase the pitting potential in water for high purity aluminum.

Lithium

Lithium is a chemically active metal and may increase aluminum’s susceptibility to corrosion. For instance, it seems that lithium additions of <3% result in a slightly more anodic aluminum. This indicates that additions of lithium, however, may only increase the susceptibility of aluminum to corrosion marginally. Moreover, studies have shown that the susceptibility of the aluminum lithium alloy to corrosion is largely dependent on the δ phase, which is the AlLi phase. Increasing the amount of δ phase present, for example, increases the alloy’s susceptibility to corrosion.

Two of the more common aluminum lithium alloys are 2090 and 8090. 2090 is similar to 7075 aluminum in terms of resistance to SCC, and has a higher resistance to exfoliation corrosion than 7075.70 8090 aluminum with an altered surface structure (heat treatment T82551) has been shown to have a greater general resistance to corrosion than 2090 aluminum. Both 2090 and 8090 aluminum have been shown to be susceptible to pitting and intergranular corrosion. Alloy 2097 is another aluminum lithium alloy and has shown improved pitting corrosion resistance compared to an aluminum copper alloy (2124) and comparable general corrosion resistance.

Comparison of the Corrosion Resistance of Aluminum Alloys

A comparison of the relative corrosion resistance of the various groups of aluminum alloys is provided in Table 29.