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Corrosion, Wear, and Degradation of Materials
Published in Mahmoud M. Farag, Materials and Process Selection for Engineering Design, 2020
Intergranular attack is often strongly dependent on the mechanical and thermal treatment given to the alloy. For example, unstabilized stainless steels are susceptible to intergranular corrosion when heated in the temperature range of 550°C−850°C (1000°F−1550°F). In this sensitizing temperature range, chromium combines with carbon to form chromium carbides, which precipitate at the grain boundaries, and this depletes the neighboring areas of chromium. In many corrosive environments, the chromium depleted areas are attacked. Intergranular attack may take place in welded joints in the areas that were heated to the sensitizing temperature range, as shown in Figure 3.5. Dissolving chromium carbides by solution heat treatment at 1060°C−1120°C (about 1950°F−2050°F) followed by water quenching eliminates sensitization. Susceptibility of stainless steels to sensitization can also be reduced by reducing the carbon content to <0.03% as in the case of extra low carbon grades, for example, 304L, or by adding sufficient titanium and niobium to combine with all the carbon in the steel, for example, 347 or 321 stainless steels. The following case study illustrates how intergranular corrosion occurs in practice and how to avoid it.
Open-Circuit Metal Dissolution Processes
Published in Madhav Datta, Electrodissolution Processes, 2020
Intergranular corrosion is usually related to the segregation of specific elements or the formation of a compound in the grain boundary. Corrosion then occurs by a preferential attack on the grain-boundary phase, or in a zone adjacent to it that has lost an element necessary for adequate corrosion resistance – thus making the grain-boundary zone anodic relative to the remainder of the surface. The attack usually progresses along a narrow path along the grain boundary and, in an extreme case of grain-boundary corrosion, an entire grain may be dislodged due to the complete deterioration of its boundaries. In such a case, the mechanical properties of the structure will be seriously affected. If materials with incorrect heat treatment enter service, they are liable to crack or fail by intergranular corrosion much more rapidly than properly treated materials.
Introduction to Corrosion
Published in S.K. Dhawan, Hema Bhandari, Gazala Ruhi, Brij Mohan Singh Bisht, Pradeep Sambyal, Corrosion Preventive Materials and Corrosion Testing, 2020
S.K. Dhawan, Hema Bhandari, Gazala Ruhi, Brij Mohan Singh Bisht, Pradeep Sambyal
Intergranular corrosion is a special form of corrosion which generally occurs at grain boundaries or region next to its boundaries. It is also known as the intergranular attack or interdendritic corrosion. The main reasons for intergranular corrosion are the formation of precipitates and segregate in the specific region of grain boundaries. The presence of precipitates and segregates make the grain boundaries physically and chemically different from the grains, causing selective dissolution of grain boundaries or the region close to the grain boundaries. Intergranular corrosion is generally confined in a very small area, but in some cases, the complete grain gets dislodged due to the total destruction of the boundaries. It severely affects the mechanical properties of the metal substrates. The well-known example of intergranular corrosion is the sensitization of stainless steels or weld decay. In this case, chromium gets precipitates at grain boundaries that lead to a depletion of Cr concentrations in the region next to these precipitates, making these areas susceptible to corrosive attack. Identification of this corrosion is usually done under microscopic examination, but in some cases, it is even visible to the naked eyes.
Quantitative analysis of susceptibility to intergranular corrosion in alloy 625 joined by friction stir welding
Published in Corrosion Engineering, Science and Technology, 2023
E. B. Fonseca, A. Z. Fatichi, M. Terada, A. F. S. Bugarin, J. Rodriguez, Isolda Costa, A. J. Ramirez
Intergranular corrosion is typically caused by the precipitation of carbides at grain boundaries. Precipitation consumes Cr, leaving a depleted zone along grain boundaries. Thus, the corrosion rate is affected by the volume fraction of the precipitated carbides per unit area of the grain boundaries. As shown in the time–temperature-transformation diagram of the Alloy 625 [17], carbides form when the material is exposed to temperatures ranging from 800°C and 1000°C for about 1 hour. Liu et al. [18] conducted thermomechanical simulation of the TIG welding HAZ in alloy 617 and concluded that M23C6 carbides were found along grain boundaries in HAZ at lower temperatures (1150°C) and reduced between 1300°C and 1350°C. However, due to the temperature increase from 1150°C to 1350°C the yield and ultimate tensile stresses decrease due to the presence of coarse carbides. In conventional welding, the sensitisation phenomena are significantly delayed in the coarse-grained specimens. This is attributable to the lower driving force available in the coarser microstructures for the diffusion of elements, such as Cr, Mo, and Nb from the grain to the boundary [19,20]. Pulsed current GTAW also produces fine equiaxed dendrites in the fusion zone, and CO2 laser welding causes micro-level segregation of Cr, Mo, and Ti into interdendritic regions in the fusion zone [21,22]. However, FSW occurs in the solid state and the cooling rate is fast enough to hinder carbide coarsening, forming a shallow Cr depletion zone [2]. It has been reported that FSW of Alloy 625 reaches a peak temperature of 1150°C with a cooling time between 800°C and 500°C (t8/5) below 20 s [23]. Therefore, there is not enough time for significant carbide coarsening during FSW. Thus, the area of the grain boundaries has a major influence on sensitisation in this case [24].
Effects of the corrosion mechanism evolution of low silicon-cast aluminium alloys in service
Published in Philosophical Magazine, 2023
Tengfei Cheng, Guotong Zou, Xin Mao, Yitao Yang
There are mainly α-Al, eutectic(α + β) and a small amount of Si in low silicon-cast aluminium alloy. The reason for intergranular corrosion is that various compounds form numerous corrosion micro-cells in the corrosion medium, forming anodes along the grain boundary, thus dissolving the channel. According to Guillaumin and Mankowski [8], over-ageing will lead to the equilibrium between the corrosion potential of the matrix and the element depletion zone around the grain boundary. Svenningsen et al. [9] show that the solid solution aluminium alloy is not sensitive to intergranular corrosion, due to the uniform distribution of alloy elements in the grain interior and grain boundary region. Mg and Si depletion zones are formed around the grain boundary of the T6 heat-treated aluminium alloy. The Mg- and Si-depleted regions on the grain boundary promote the susceptibility of intergranular corrosion of the T6 heat-treated aluminium alloy [10,11]. At the same time, the materials treated with T4 are prone to intergranular corrosion. Artificial ageing reduces the susceptibility of intergranular corrosion of T4-treated materials, but it is found that ageing caused the transformation of the corrosion mechanism to pitting corrosion [12]. In order to further study the effect of different element distribution on corrosion resistance of the aluminuum alloy without thermal exposure and after thermal exposure, the corrosion microstructure of the 5.8% Si aluminum alloy without thermal exposure and after thermal exposure for 50 h was observed by SEM, as shown in Figure 7 below. The corrosion solution was 75% HCl and 25% HNO3, and the corrosion time was 90 s. EDS analysis of corrosion pits and grain boundaries in the microstructure of aluminum alloy samples exposed at 200°C for 50 h and without thermal exposure was carried out, as shown in Figure 8.