Aluminum owes its excellent corrosion resistance and its usage as one of the primary metals of commerce to the barrier oxide film that is bonded strongly to its surface and, that if damaged, re-forms immediately in most environments.
On a surface freshly abraded and then exposed to air, the barrier oxide film is only 1 nm thick but is highly effective in protecting the aluminum from corrosion.
The natural film can be visualized as the result of a dynamic equilibrium between opposing forces-those tending to form compact barrier layer and those tending to break it down.
If the destructive forces are absent, as in dry air, the natural film will consist only of the barrier layer and will form rapidly to the limiting thickness. If the destructive forces are too strong, the oxide will be hydrated faster than it is formed, and little barrier will remain.
Between these extremes, where the opposing forces reach a reasonable balance, relatively thick (20 to 200 nm) natural films are formed.
Pitting corrosion
Corrosion of aluminum in the passive range is localized, usually manifested by random formation of pits. The pitting potential principle establishes the conditions under which metals in the passive state are subject to corrosion by pitting.
For aluminum, pitting corrosion is most commonly produced by halide ions, of which chloride (Cl –) is the most frequently encountered in service. Pitting of aluminum in halide solutions open to the air occurs because, in the presence of oxygen, the metal is readily polarized to its pitting potential.
Generally, aluminum does not develop pitting in aerated solutions of most nonhalide salts because its pitting potential in these solutions is considerably more noble (cathodic) than in halide solutions and it is not polarized to these potentials in normal service.
Solution Potentials
Because of the electrochemical nature of most corrosion processes, relationships among solution potentials of different aluminum alloys, as well as between potentials of aluminum alloys and those of other metals, are of considerable importance.
Furthermore, the solution potential relationships among the microstructural constituents of a particular alloy significantly affect its corrosion behavior. Compositions of solid solutions and additional phases, as well as amounts and spatial distributions of the additional phases may affect both the type and extent of corrosion.
The solution potential of an aluminum alloy is primarily determined by the composition of the aluminum rich solid solution, which constitutes the predominant volume fraction and area fraction of the alloy microstructure. Solution potential is not affected significantly by second phase particles of microscopic size, but because these particles frequently have solution potentials differing from that of the solid solution matrix in which they occur, localized galvanic cells may be formed between them and the matrix.
Since most of the commercial aluminum alloys contain additions of more than one of these elements; effects of multiple elements in solid solution on solution potential are approximately additive. The amounts retained in solid solution, particularly for more highly alloyed compositions, depend highly on fabrication and thermal processing so that the heat treatment and other processing variables influence the final electrode potential of the product.
Solution potential measurements are useful for the investigation of heat treating, quenching, and aging practices, and they are applied principally to alloys containing copper, magnesium, or zinc.
In aluminum-copper and aluminum-copper-magnesium (2xxx) alloys, potential measurement can determine
the effectiveness of solution heat treatment by measuring the amount of copper in solid solution. Also, by measuring
the potentials of grain boundaries and grain bodies separately, the difference in potential responsible for intergranular corrosion, exfoliation, and stress corrosion cracking (SCC) can be quantified. Solution-potential measurement of alloys containing copper also show the progress of artificial aging as increased amounts of precipitates are formed and
the matrix is depleted of copper.
Potential measurement are valuable with zinc-containing (7xxx) alloys for evaluating the effectiveness of the solution heat treatment, for following the aging process, and for differentiating among the various artificially aged tempers.
These factors can affect corrosion behavior.
In the magnesium containing (5xxx) alloys, potential measurements can detect low-temperature precipitation and are useful in qualitatively evaluating stress-corrosion behavior. Potential measurement can also be used to follow the diffusion of zinc or copper in alclad products, thus determining whether the sacrificial cladding can continue to protect the core alloy.
Atmospheric Corrosion
Most aluminum alloys have excellent resistance to atmospheric corrosion (often called weathering), and in many outdoor applications, such alloys do not require shelter, protective coatings or maintenance.
Corrosion in Waters
Aluminum alloys of the 1xxx, 3xxx, 5xxx and 6xxx series are resistant to corrosion by many natural waters. The more important factors controlling the corrosivity of natural waters to aluminum include water temperature, pH, and conductivity, availability of cathodic reactant, presence or absence of heavy metals, and the corrosion potentials and pitting potentials of the specific alloys.
Effects of Composition and Microstructure on Corrosion
1xxx Wrought Alloys. Wrought aluminums of the 1xxx series conform to composition specifications that set maximum individual, combined, and total contents for several elements present as natural impurities in the smelter – grade or refined aluminum used to produce these products.
Corrosion resistance of all 1xxx compositions is very high, but under many conditions, it decreases slightly with increasing alloy content. Iron, silicon and copper are the elements present in the largest percentages. The copper and part of the silicon are in solid solution.
2xxx wrought alloys and 2xxx casting alloys, in which copper is the mayor alloying element, are less resistant to corrosion than alloys of other series, which contain much lower amounts of copper.
Alloys of this type were the first heat-treatable high-strength aluminum base materials and have been used for more than 75 years in structural applications, particularly in aircraft and aerospace applications. Much of the thin sheet made of these alloys is produced as an alclad composite, but thicker sheet and other products in many applications require no protective cladding.
Electrochemical effects on corrosion can be stronger in these alloys than in alloys of many other types because of two factors: greater change in electrode potential with variations in amount of copper in solid solution and, under some conditions, the presence of no uniformities in solid solution concentration. However, that general resistance to corrosion decreases with increasing copper content is not primarily attributable to these solid-solution or second phase solution-potential relationships, but to galvanic cells created by formation of minute copper particles or films deposited on the alloy surface as a result of corrosion.
2xxx Wrought Alloys Containing Lithium. Lithium additions decrease the density and increase the elastic modulus of aluminum alloys, making aluminum-lithium alloys good candidates for replacing the existing high-strength alloys, primarily in aerospace applications.
3xxx Wrought Alloys. Wrought alloys of the 3xxx series (aluminum-manganese and aluminum-manganese-magnesium) have very high resistance to corrosion. The manganese is present in the aluminum solid solution, in submicroscopic particles of precipitate and in larger particles of Al6(Mn,Fe) or Al12(Mn,Fe)3Si phases, both of which have solution potentials almost the same as that of the solid solution matrix.
4xxx Wrought Alloys and 3xx.x and 4xx.x Casting Alloys. Elemental silicon is present as second-phase constituent particles in wrought alloys of the 4xxx series, in brazing and welding alloys, and in casting alloys of 3xx.x and 4xx.x series.
Corrosion resistance of 3xx.x castings alloys is strongly affected by copper content, which can be as high as 5% in some compositions, and by impurity levels. Modifications of certain basics alloys have more restrictive limits on impurities, which benefit corrosion resistance and mechanical properties.
5xxx Wrought Alloys and 5xx.x Casting Alloys. Wrought Alloys of the 5xxx series (aluminum-magnesium-manganese, aluminum-magnesium-chromium, and aluminum-magnesium-manganese-chromium) and casting alloys of the 5xx.x series (aluminum-magnesium) have high resistance to corrosion, and this accounts in part for their use in a wide variety of building products and chemical-processing and food-handling eguipment, as well as applications involving exposure to seawater.
6xxx Wrought Alloys. Moderately high strength and very good resistance to corrosion make the heat-treatable wrought alloys of the 6xxx series (aluminum-magnesium-silicon) highly suitable in various structural, building, marine machinery, and process-equipment applications.
7xxx Wrought Alloys and 7xx.x casting alloys contain major additions of zinc along with magnesium or magnesium plus copper in combinations that develop various levels of strength. Those containing copper have the highest strengths and have been used as constructional materials, primarily in aircraft applications, for more than 40 years.
The copper-free alloys of the series have many desirable characteristics: moderate-to-high strength, excellent toughness, and good workability, formability, and weldability. Use of these copper-free alloys has increased in recent years and now includes automotive applications, structural members and armor plate for military vehicles, and components of other transportation equipment.
The 7xxx wrought and 7xx.x casting alloys, because of their zinc contents, are anodic to 1xxx wrought aluminums and to other aluminum alloys. They are among the aluminum alloys most susceptible to SCC.
Resistance to general corrosion of the copper-free wrought 7xxx alloys is good, approaching that of the wrought 3xxx, 5xxx and 6xxx alloys. The copper-containing alloys of the 7xxx series, such as 7049, 7050, 7075, and 7178 have lower resistance to general corrosion than those of the same series that do not contain copper. All 7xxx alloys are more resistant to general corrosion than 2xxx alloys, but less resistant than wrought alloys of other groups.
Although the copper in both wrought and cast alloys of the aluminum-zinc-magnesium-copper type reduces resistance to general corrosion, it is beneficial from the standpoint of resistance to SCC.