Aluminum alloy materials are well known for their light weight and strength-to-weight ratios. They are also susceptible to several forms of corrosion. Relative to other metals, their natural electrochemical potential places them adjacent to other very susceptible (active) metals such as magnesium (Mg) and zinc (Zn) in a traditional galvanic series in seawater. However, Al alloys naturally form a very thin, protective (passive) oxide film on their surface when exposed to oxygen. This film prevents corrosion as long as it is maintained over the surface. Pitting corrosion is a common form of attack on aluminum. This is typically caused by exposure to halide ions, chloride ions being the most common type found in corrosive media, that breakdown the passive film at discreet spots. This permits pits to initiate and grow.
Besides prevalent galvanic attack and pitting, other forms of corrosion that different Al alloys may suffer include deposition attack, intergranular attack (IGA), stress-corrosion cracking (SCC), exfoliation and filiform attack. General corrosion can also occur but it is usually a lesser problem. Aluminum and its alloys are typically immune or experience only minor corrosion at pH values from about 4 to 9. However, rates are often high at pH values below 4 and above 9. This response to both acidic and alkaline pH values is generally unique to aluminum and its alloys.
There are Aluminum Association (AA) designations for the many wrought and cast aluminum alloys. The wrought alloys are designated by four numerical values without a decimal, i.e., 1XXX, 2XXX, 3XXX, 4XXX, 5XXX, 6XXX and 7XXX. The 1XXX series is essentially all aluminum while the other wrought series have widely varying amounts of different alloying elements such as Cu, Mn, Si, Mg and Zn. Iron (Fe) is an impurity element that is very detrimental to corrosion resistance. The maximum percentage of Fe permitted is defined for many Al alloys. Cast alloys are designated by the AA with three-digit numerical values with a decimal point, e.g., 356.0. There are corresponding Unified Number System (UNS) designations for all the commercial wrought and cast Al alloys.
High strength, heat treatable alloys in the 2XXX and 7XXX series have significant amounts of either alloyed Cu, Mg or Zn. This fact makes them much more susceptible to the various forms of corrosion. Their susceptibilities are primarily addressed by the use of specialized heat treatments. These are designated with "T" for a given temper and a number. The different tempers provide increased corrosion resistance balanced against other desirable properties. Wrought alloys in the other series have little or no Cu, Mg or Zn as alloying elements. Alloys without these elements provide the best corrosion resistance but typically have lower strengths.
Deposition corrosion occurs when heavy metal ions, e.g., ions of Cu, Pb, Hg, Ni, Sn or Fe, in a solution exposed to the Al adsorb onto the surface and accelerate corrosion. This occurs because these ions are more noble, i.e., much less active than the Al, and a galvanic effect is created. The heavy metal ions often occur in a corrosive medium after being generated by corrosion of other metals elsewhere in a system.
SCC of Al alloys has been and often continues to be a major problem. As in other forms of attack, the alloys with significant amounts of alloyed Cu, Mg, Zn and Si are particularly susceptible. Cracking is typically intergranular and thus control of SCC by use of different tempers is also effective for control of IGA. The direction of acting tensile stresses necessary to SCC is very important to the probability of occurrence. Stresses that are perpendicular to the thickness of thick metal sections of wrought and forged parts are the most detrimental. This is the short-transverse direction. It is perpendicular to the direction of rolling of flat, thick plate. The short-transverse effect does not occur in thin cross sections. ASTM Standard G64 provides a listing of relative resistances of different 2XXX and 7XXX alloys to SCC according to different tempers, stress levels and stress directions. Castings are usually resistant to SCC with the exception of those high in alloyed Cu and Zn.
Typically the rates of corrosion of Al alloys by immersion in seawater and due to atmospheric exposure decrease with time. Alloys of the 5XXX series are particularly resistant to seawater and those in the 3XXX and 6XXX series also provide good resistance. Biofouling on the Al hulls of boats is a common problem in seawater. This is combated by the use on anti-biofouling paints.
Gerald O. Davis, PE, President and co-owner of DM&ME, has over 40 years experience in Materials Engineering and Business. Mr. Davis is a Forensic Expert in Materials Usage, Corrosion, Metallurgy, Mechanical Failure, & Root-Cause Failure Analysis. His recent background includes work as a corrosion researcher, senior engineer, and program manager for Battelle Memorial Institute, DNV, Inc., Henkels & McCoy, Inc., respectively and, since 2004, as president of DM&ME.
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