Stress-corrosion cracking in aluminum beverage can ends-issues

Tag : alloy rivet

Stress-corrosion cracking failures are predominately intergranularin aluminum alloys. However, transgranular SCC has been observedfor a few alloys under highly specific environmentalconditions.

Stress-corrosion cracking failures in the older end designs (stay-on-tab [SOT] or ecology end) were always observed to betransgranular. With the introduction of the newer end design (largeopening end [LOE] design), failures were observed to be eitherintergranular or transgranular, depending on specific end andenvironmental factors.
The fracture surface for transgranular SCC failure in 5182 ends hasa unique appearance, and is shown in the SEM fractograph (Figure5). Fracture surfaces display a delta or fan type morphology thatpropagates across the grain structure from the exterior to theinterior surfaces of the end. The fracture propagated from a seriesof simultaneous initiation sites. A typical feature seen in thesetransgranular fractures is that the stable crack growth phase canpropagate up to 75 to 80 percent of the score residual before finalfracture occurs. This indicates a relatively low stress state fortransgranular SCC crack propagation for this system.

The stresses along the perimeter of the score panel also vary. Theolder SOT score design typically showed the initiation of SCCfailures at the 4 o'clock and/or 8 o'clock positions (with the 12o'clock orientation being the tab rivet). Stress-corrosion crackingfailures in this SOT end were always transgranular.

With the introduction of the newer large-opening end (LOE) scoredesign, the typical site for SCC initiation changed to the 6o'clock position. The LOE design is approximately 38 percent largerthan the SOT design. This end design experiences eithertransgranular or intergranular SCC failure, depending on the levelof stress.
These SCC initiation sites are the areas of highest tensilestresses (residual plus applied stress). These sites appear to beassociated with the positions on the score panel that are adjacentto the end countersink with the lower radii of curvature (thestraightest sections of the perimeter). The SCC initiation sitesare shown in Figure 6.
The high strain region for the LOE is the 6 o'clock location, andthe high strain regions for the SOT are the 4 o'clock position,followed by the 8 o'clock position. Also, the degree of maximumlateral strain in the LOE design is significantly greater thanshown in the SOT design. Note that these locations for maximumlateral strain coincide with the initiation sites for SCC fracturesin each of the two end designs.

The intergranular failure in 5182H39 alloy was first observed withthe introduction of the LOE end design in 2001. The stressconcentration factor for initiating SCC failures depends on thedesign of the end itself, the score residual, and score knife wear(now less of a factor with current manufacturing practices). Inaddition, the "openability" of the end is enhanced by stresses(residual and applied) along the score. Stress-corrosion crackingoccurs at specific high-stress points along the score panel.

Figure 7 shows the effect of stresses (applied stress incombination with residual stresses) on the SCC failure rate for theLOE end design. The test used a series of ends made on the sameconversion press using the same tooling. Also, all ends wereproduced from a single coil of coated end stock (5182-H39). Fivedifferent residual ratios (the ratio of the score residual, t, tothe starting end stock gauge, T) were produced and varied from0.250 to 0.477. For each residual ratio, two differentmetallurgical conditions were tested: as-formed (5182-H39) andstress-relieved (5182-H28,163[degrees]C for 30 min.)

The testing consisted of an exposuretype test using ends double-seamed onto filled beverage cans, carbonated to an internalpressure of 276 kPa. The ends were exposed to a weak solution ofNaOCl (140 ppm) at room temperature (22[degrees]C). The duration ofthe test was 40 days. The aggregate or mean time to failure of thetest samples were determined for the 40 day test. Mean time tofailure for the as-formed ends varied from 11 days to 24 days. Themean time to failure for the stress-relieved samples varied from 35to 40 days,

The most significant outcome of this test was the change of failuremode to intergranular SCC for the as-formed samples with the lowerresidual ratios (Figure 7). Prior to the introduction of the LOEend design, only transgranular SCC failures had been observed inaluminum beverage can ends. This is one of the rare occurrences oftransgranular SCC in aluminum alloy systems.9 The transition fromtransgranular to intergranular SCC corresponds to an increase ineffective stress at the point of initiation (thinner cross-sectional area for a constant internal pressure).

Figure 8 shows the rate of failure for the as-formed endsexperiencing intergranular SCC, as shown in Figure 7. Note thedramatic shift in the failure rate and the total percentage failedby the slight change in the residual ratio from 0.307 to 0.250. Forthe residual ratio of 0.250, all the ends failed within 3 days. Butfor the residual ratio of 0.307, only about 70 percent of thesamples failed over a period of 13 days. This correlates to anincrease in critical stresses at the point of initiation.

The fractograph in Figure 9 shows the intergranular SCC failuremode. The failure initiates at the score residual and score flankjunction and then propagates toward the internal surface byintergranular dissolution. The elongated grain structure is clearlyapparent. By contrast, the fractograph in Figure 10 shows theunique combination of mixed-mode SCC in the 5182 lid. The fracturesurface shows adjacent areas of transgranular and intergranularcrack propagation. The observation of two different paths of crackpropagation implies a change of crack mechanism in the SCC process.

The internal pressure within the headspace of the can produces abiaxial tensile stress within the can end. This pressure produces anominal tensile stress within the end (sigma^sub non^). At thescore, the stress intensity factor, K^sub t^, yields a stresswithin the residual (sigma^sub max^) that is significantly higherthan the nominal stress. This is schematically illustrated inFigure 11. Based on typical cross-sectional dimensions, K^sub t^has a value of about 3. This does not include a shape factorcontribution. Note that a lower score residual (i.e., a thinnercross section) results in higher stresses and higher susceptibilityto SCC.

The contribution of metallurgical structure is only now beingappreciated in the sensitivity of SCC failures in aluminum beveragecans. The high magnesium 5000 alloy series (> -3% magnesium) issensitive to SCC whereas the 3000 alloy series (manganese-basedbody stock) is not considered sensitive to SCC. With reference toalloy composition, material with a higher Si:Fe ratio appears to bemore sensitive to SCC failure.

In summary, a change in SCC failure mode from transgranular SCC tointergranular SCC has been observed in 5182 aluminum can beverageends. This occurred with the introduction of a new end design.Intergranular SCC in this end design is associated with higherstress conditions (from both applied and residual stresses) for theinitiation and propagation phases of the SCC fractures. Stress-corrosion cracking failure in aluminum beverage cans is an ongoingindustrial issue. Much effort continues to be made to heipdetermine the safe operating procedures for the aluminum producers,aluminum can manufacturers, beverage fillers, and distributors.