Corrosion of aluminum: the effect of alloying
technical aluminum Corrosion
technical aluminum (99,00 % and cleaner) are always more resistant to corrosion, than any Aluminium alloy.
The highly concentrated acidic and alkaline solutions, it dissolves quickly, but in the pH range from 4 to 9 on its surface may appear only minor spots and local pitting.
Pure aluminum is not subject to such severe types of localized corrosion, how to intergranular corrosion and subsurface.
Aluminum Corrosion resistance decreases with increasing content of impurities – gland, silicon and copper. Copper and part of silicon in solid solution. Secondary phase particles contain either iron or iron and silicon - Al6Fe, Al3Fe and Al12Fe3Si2 - they are all cathode, more noble, to the aluminum matrix. When these particles are on the surface, oxide film over or very thin, or there are no.
These particles form a local electric circuit, that cause pitting surface, which is in a conductive electrolyte (figure 1). The number and size of such corrosive portions proportional to the area fraction, occupied by the secondary phase particles.
IN Dural-duralumin Copper is the main alloying element. Therefore they have a lower corrosion resistance, than other alloys, in which the copper is much less. Electrochemical corrosion effects may be manifested in these alloys is significantly stronger, than other alloys already because, they have higher electrode potential difference between the different surface areas of oscillation of copper in solid solution.
However, the general corrosion resistance decrease with increasing content of copper is not due to the inhomogeneity of the copper in solid solution, but because of the galvanic cells, which are formed on the smallest particles of copper, which are deposited on the alloy surface due to its corrosion.
Corrosion of aluminum alloys with magnesium
alloys Al–Mg have high corrosion resistance and is therefore widely used in construction, chemical and food industry, as well as in machines and installations, exposed to sea water.
With a low content in the alloy (somewhere up to 3 %) magnesium is in solid solution or partially in the form of particles Al8Mg5, which are evenly distributed throughout the matrix. corrosion resistance of such alloys like aluminum.
In alloys with magnesium content more 3 % particles Al8Mg5 begins to fall mainly within the grains not, and along their boundaries, that extremely adverse effect on corrosion resistance.
Corrosion of aluminum alloys with magnesium and silicon
Quite high strength and very good corrosion resistance makes these thermally hardenable alloys very suitable for use in construction, marine engineering and infrastructure. phase Mg2Si is the basis for strengthening the mechanism of these alloys. Both in solid solution, and as precipitates it has a negligible effect on the electrode potential.
copper supplements, are introduced to improve the strength properties of some of these alloys, restrict small amounts, to minimize adverse effects on corrosion resistance. When the copper content is more 0,5 % can start to manifest intergranular corrosion.
When these alloys, magnesium and silicon is balanced in the proportions to form only Mg2Si (alloys 6060/6063 and AD31), the intergranular corrosion is very low in most industrial atmospheres. If silicon alloy contains more than is necessary to form Mg2Si, as, for example, in the alloy 6005, the tendency to intergranular corrosion increases.
Corrosion of aluminum alloys with zinc
In these alloys, magnesium, copper and manganese are added to achieve different levels of strength.
Alloys of copper, for example, 7075 or B95, They have a maximum strength, and are used as a structural material, including aircraft.
Alloys without copper, for example, 7005 or 1915, have many positive qualities: high strength, high viscosity, good machinability, formability and weldability. They are increasingly used in cars (for example, in bumpers), building structures and vehicles. As these alloys contain zinc, they are the anode to the technical aluminum and other aluminum alloys. Of all the aluminum alloys are the most prone to stress corrosion.
General corrosion resistance of the alloys without copper alloys doping level reaches Al–Mn, Al–Mg and Al–Mg–Si. alloys, alloyed copper, for example, 7075 and B95, have a lower resistance to general corrosion, than copper-free alloys (7005 and 1915). All alloys Al–Zn They are more resistant to general corrosion, than alloys Al-Fromin-(Mg,Mn), but inferior to the alloys of other series.
The source: Corrosion of Aluminum and Aluminum Alloys. Edited by J.R. Davis. — ASM International, 1999.