ALUMINUM ALLOYS

21.2 ALUMINUM ALLOYS

The usual alloying additions to aluminum in order to improve physical properties include Cu, Si, Mg, Zn, and Mn. Of these, manganese may actually improve the corrosion resistance of wrought and cast alloys. One reason is that the compound

MnAl 6 forms and takes iron into solid solution. The compound (MnFe)Al 6 settles to the bottom of the melt, in this way reducing the harmful infl uence on corrosion of small quantities of alloyed iron present as an impurity [27] . No such incorpora- tion occurs in the case of cobalt, copper, and nickel, so that manganese additions would not be expected to counteract the harmful effects of these elements on corrosion behavior.

The Duralumin alloys (e.g., types 2017 and 2024) contain several percent copper, deriving their improved strength from the precipitation of CuAl 2 along slip planes and grain boundaries. Copper is in solid solution above the homoge- nizing temperature of about 480 ° C (900 ° F) and, on quenching, remains in solu- tion. Precipitation takes place slowly at room temperature with progressive strengthening of the alloy. Should the alloy be quenched from solid – solution temperatures into boiling water, or, after quenching should it be heated (artifi -

cially aged) above 120 ° C (250 ° F), the compound CuAl 2 forms preferentially

394 ALUMINUM AND ALUMINUM ALLOYS

along the grain boundaries. This results in a depletion of copper in the alloy adjacent to the intermetallic compound, accounting for grain boundaries anodic to the grains, and a marked susceptibility to intergranular corrosion . Prolonged heating (overaging) restores uniform composition alloy at grains and grain inter- faces, thereby eliminating susceptibility to this type of corrosion, but at some sacrifi ce of physical properties. In practice, the alloy is quenched from about 490 ° C (920 ° F), followed by room - temperature aging.

Exfoliation is a related type of anodic path corrosion in which attack of rolled or extruded aluminum alloy results in surface blisters followed by separation of elongated slivers or laminae of metal. It occurs in various types of aluminum alloys in addition to the copper - bearing series. Proper heat treatment may allevi- ate such attack.

Exfoliation is commonly experienced on exposure of susceptible aluminum alloys to marine atmospheres. Simulation in the laboratory is accomplished by controlled intermittent spray with 5% NaCl containing added acetic acid to pH

3 at 35 – 50 ° C (95 – 120 ° F) [28] . In practice, severe exfoliation corrosion has been experienced in water irrigation piping constructed of type 6061 alloy [29] . The cause in this instance was ascribed to pipe fabrication methods and presence of excess amounts of impurity elements (e.g., Fe, Cu, and Mn).

21.2.1 Stress-Corrosion Cracking

Pure aluminum is immune to stress - corrosion cracking (S.C.C.). Should a Duralu- min alloy, on the other hand, be stressed in tension in the presence of moisture, it may crack along the grain boundaries. Sensitizing the alloy by heat treatment, as described previously, makes it more susceptible to this type of failure. In aging tests at 160 – 205 ° C (320 – 400 ° F), maximum susceptibility was observed at times somewhat short of maximum tensile strength [30] . Hence, in heat - treatment procedures, it is better practice to aim at a slightly overaged rather than an underaged alloy.

Magnesium alloyed with aluminum increases susceptibility to S.C.C., espe- cially when magnesium is added in amounts > 4.5%. To avoid such failures, slow cooling (50 ° C/h) from the homogenizing temperature is necessary supposedly in

order to coagulate the β phase (Al 3 Mg 2 ), a process that is promoted by addition of 0.2% Cr to the alloy [31] . Microstructures that are resistant to S.C.C. are those either with no precipitate along grain boundaries or with precipitate distributed as uniformly as possible within grains [32] .

For structural components in automotive applications, the Aluminum Asso- ciation guidelines [33, 34] recommend alloys with a maximum of 3% Mg where there is exposure for long periods to temperatures greater than about 75 ° C (167 ° F). If an alloy of higher Mg content is being considered, the thermal expo- sure of the part during its lifetime should be established, and realistic, full - component testing should be carried out as part of a detailed materials assessment to evaluate and qualify the specifi c alloy under consideration. Following these guidelines has led to successful use of, for example, alloy 5182, a 4.5% Mg alloy,

ALUMINUM ALLOYS

in structural components of the Honda NSX with no known record of fi eld fail- ures caused by SCC [35, 36] .

Edeleanu [37] showed that cathodic protection stopped the growth of cracks that had already progressed into the alloy immersed in 3% NaCl solution. On aging the alloy at low temperatures, maximum susceptibility to S.C.C. occurred before maximum hardness values were reached. This behavior paralleled that of

a Duralumin alloy cited previously. Accordingly, Edeleanu proposed that the susceptible material along the grain boundary which caused cracking was not the equilibrium β phase responsible for hardness, but instead was made up of mag- nesium atoms segregating at the grain boundary before the intermetallic com- pound formed. On this basis, susceptibility to S.C.C. decreased on continued aging of the alloy because the separated β phase consumed the original segregated

grain - boundary material responsible for susceptibility. A similar mechanism probably applies to the copper – aluminum alloy series.

High concentrations of zinc in aluminum (4 – 20%) also induce susceptibility to cracking of the stressed alloys in the presence of moisture. Traces of H 2 O, for example, contained in the surface oxide fi lm are suffi cient to cause cracking; carefully baked - out specimens in dry air do not fail [38] . Oxygen is not necessary, nor is a liquid aqueous phase required. These conditions, plus the susceptibility of a high - strength aluminum alloy (7075) to failure in organic solvents [39] , suggest that a possible mechanism of failure may be one of stress - sorption crack- ing caused by adsorbed water or organic molecules on appropriate defect sites of the strained alloy. Nevertheless, the mechanism of S.C.C. of aluminum alloys is complex, involving several time - dependent interactions that are diffi cult to evaluate independently, such as the roles of anodic dissolution, hydrogen embrit- tlement, and grain boundary precipitates in the initiation and propagation of cracks [40 – 42] . Understanding has been suffi cient, however, to permit new alu- minum alloys to be developed with high strength combined with improved resis- tance to corrosion, including S.C.C., and such alloys are now used, for example, on Boeing 777 aircraft [41] .

Many high - strength aluminum alloys are available (some are listed in Table

21.1 ); specifi c composition ranges and heat treatments for these alloys are usually chosen with the intent of minimizing susceptibility to S.C.C. Solution heat - treatment temperature affects stress - corrosion susceptibility by altering the grain - boundary composition as well as the alloy metallurgical microstructure [42, 43] . Service temperatures — especially those above room temperature — that can cause artifi cial aging sometimes induce susceptibility followed by premature failure in the presence of moisture or sodium chloride solutions. Susceptibility of any of the wrought alloys is greatest when stressed at right angles to the rolling direction (in the short transverse direction), probably because more grain - bound- ary area of elongated grains along which cracks propagate comes into play.

As mentioned earlier, cladding of alloys can serve to cathodically protect them from either intergranular corrosion or S.C.C. Compressive surface stresses are effective for avoiding S.C.C.; hence, practical structures are sometimes shot peened.

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