PRINCIPALES MICROORGANISMOS FERMENTADORES

Intergranular Corrosion (IGC) is a form of localised attack that selectively takes place at grain boundaries. In the presence of stresses in the material, IGC can develop into Stress Corrosion Cracking (SCC), which involves crack propagation by a mechanical stress as well as a corrosive environment. It is also referred to as intercrystalline corrosion because there is the selective corrosion of grain boundaries, while the grains or crystals themselves suffer little attack.

In general, IGC comes about when the grain boundaries are more anodically reactive than the grain interiors. This is usually because the composition of phases found on the grain boundary is different to that of the matrix, forming a localised galvanic couple. The bulk matrix between the grain boundaries is more cathodic and therefore is not affected. A slight variation exists in Al-Cu alloys, where Al2Cu precipitates on the grain boundaries act cathodically to a Cu-depleted region just outside of it, which itself undergoes anodic dissolution [63]. In the case of 5xxx series alloys, the highly reactive grain boundary β-phase (Al3Mg2) acts anodically to the Al-Mg matrix when it is precipitated onto grain boundaries. This model was demonstrated in a simulated β and Al-matrix model by Ren, where bulk precipitates were melted and cast according to their chemical proportion, then mounted in an epoxy resin with an electrical connection [64]. As discussed already, the dissolution and subsequent precipitation of β-phase onto grain boundaries comes about through exposure to elevated temperatures for extended periods of time (sensitisation). However it is only once on the grain boundary that its anodic character becomes an influence.

2.3.3.1. Propagation of IGC

IGC propagates through a material by attacking the reactive grain boundary phase in preference to the matrix. Once the attack has initiated on the susceptible boundary at the surface, subsurface dissolution events take place, which become governed by the electrolyte composition and the material that is present. As penetration moves deeper into the material, the pH of the electrolyte within the crevice has been observed to drop [61], while a continuous path of β-phase is expected to give a higher penetration rate.

Many studies have looked into IGC of Al-Mg alloys and have concluded that the reactive β-phase is central in causing susceptibility to IGC and subsequently SCC in the 5xxx series [8, 14, 65-76]. What is not known however is the exact mechanism of IGC propagation along grain boundaries where the β-phase does not form a continuous path. One hypothesis is that the corrosion product built up at the leading edge of the fissure causes a ʹwedging stressʹ which extends the fissure by stress-assisted corrosion processes [77]. Another method may result from a change in the alloy chemistry close to the grain boundary through sensitisation and precipitation, leading to a change in the local corrosion properties. This has been reported for high Mg binary Al alloys [78], however no Mg depleted regions have been seen in the 5083 alloy [32]. The final hypothesis involves the dissolution of the precipitate changing the local chemistry at the fissure tip [39]. The β-phase dissolution would occur as the following reaction:

Al3Mg2 3Al3+ + 2Mg2+ + 13e-

The metallic ions that are released into solution increase until a critical concentration is reached, at which point reduced water activity can impair repassivation and facilitate the propagation of the corrosion. Whichever hypothesis is closest to the true explanation, the

spreading has been confirmed recently to spread along a β-phase precipitate chain [79], Figure 12.

Figure 12 (a) and (b) SEM images of sensitised AA5083 (100 °C for 30 days) after 1 hr potentiostatic hold at -0.73 VSCE. (c) and (d) SEM images of same area after etching in

phosphoric acid to indicate remaining grain boundaries containing β-phase [79].

2.3.3.2. Characterisation of IGC

Very early work to characterise the sensitisation process and corrosion performance related to alloy composition found the open-circuit potential (OCP) of bulk β-phase to be very active (-1.15 VSCE) [40]. Searles, Gouma and Buchheit [73] went further to establish that the

breakdown potential of β-phase was -0.94 VSCE, which they observed to be considerably more negative than the pitting potential or OCP of AA5083.

As part of work on a generalised theory of stress corrosion of alloys, Mears et al.[80] demonstrated the importance of β-phase dissolution in the IGC of Al-Mg alloys. In a 1M NaCl + H2O2 solution, β-phase had a breakdown potential 0.2 V below that of the matrix, subsequently causing the rapid failure of a sensitised Al-10 % Mg sample by SCC when stressed. The conclusion of this body of work is that β-phase particles corrode preferentially to the matrix, which is in turn cathodically protected by the β-phase [68].

Several studies have investigated IGC susceptibility and its dependence on sensitisation parameters [71, 81, 82]. All came to the conclusion that susceptibility depends on sensitisation time and temperature. Davenport et al. saw Nitric Acid Mass-Loss Test (NAMLT) values in AA5182 increase with increasing ageing temperature and time Susceptibility became problematic at temperatures of 150 °C, and worsened with increasing exposure time. Similar results were observed in a separate study of AA5083 [82], where the material became sensitised after 20 hours at 150 °C. The same study showed that a higher Mg alloy (AA5456) showed a much higher corrosion rate through NAMLT values after exposure to comparable conditions. They concluded that the higher Mg concentration was the critical factor, and linked this to an increased β-phase occurrence, which showed up under a phosphoric acid etch.

Sensitisation of AA5083 between 125 and 325 °C has shown how the variation of temperatures can affect corrosion behaviour based on where the β-phase precipitates [83], Figure 13. Low temperature treatment led to β-phase precipitating onto grain boundaries and

showing low breakdown potentials, however higher heat treatments (275 °C and above) precipitated β-phase both on the grain boundaries and within the grain.

Figure 13 TEM images showing AA5083 following sensitisation for 2 hours at (a) 175 °C, with arrows indicating β-phase on grain boundaries, (b) 275 °C, where much less β- phase has precipitated [83].

The amount of β-phase that precipitates onto any given grain boundary has been found to be related to the misorientation of the grain boundary. Work by Yuan on AA5182 found boundaries that had a misorientation angle <20° were free from β-phase precipitates, while those with boundary angles >25° generally displayed either continuous or discontinuous precipitates, and on occasion no decoration [18]. The misorientation angle of the boundaries and the amount of β-phase which they contained showed very good agreement with corrosion tests that showed low angle boundaries to be resistant to IGC.