Intergranular oxidation

Intergranular Oxidation Intergranular penetration of oxide can be a serious problem particularly when thin-walled components are used in load-bearing applications such as cooled turbine blades. Mass change data often do not adequately reflect the extent of this type of attack, and examination... 

Pfeiffer contends that, in undeoxidised nickel at least, the low sulphur contents normally found are insufficient to cause grain-boundary embrittlement and that the latter is, generally, due to intergranular oxides. In the presence of sulphur-containing gases, however, the level of sulphur required. 

Unalloyed steel can be directly oxidized by steam at temperatures over 750 °F (399 °C). Extensive cracks may occur, the steel surface may resemble tree bark, and magnetic oxide deposits are generated at the failure site. Intergranular oxidation may take place. In the thermal oxidation reaction, hydrogen is directly released as shown ... 

A number of intermetallic compounds, which form protective alumina or silica scales at high temperature, undergo accelerated degradation at intermediate temperatures. This subject has been recently reviewed [29]. The observations actually involve several different, but related, phenomena which may be subdivided into accelerated oxidation , internal oxidation , intergranular oxidation and disintegration . The following definitions will be used throughout this paper. 

The intergranular oxidation can be followed or accompanied by internal oxidation of NiAl.Thc internal oxidation must be initiated by a rapid oxidation causing considerable Al-depletion but without formation of a protective scale [5], Then oxygen can diffuse into the NiAl matrix and reacts under formation of inward growing A1 03 precipitates. In the ampoule experiments with various metal/oxide or oxide/oxide mixtures the attack is mostly localized and inward and outward growth of pocks or cones is observed on the material (Fig. 3), but also general attack can occur, especially in the... 

An X-Ray diffraction analysis was carried out to identify the crystalline phases constituting the material. The silicon carbide (SiC) is in its moissanite form, whereas the intergranular oxide phase is essentially composed of silica. 

The ageing mode essentially consists of the infiltration of the material by zinc, and of possible changes in the structure of the intergranular oxide phase. In the upper, colder part of the column, the structure of the material seems not significantly affected by ageing. Only metallic zinc can be found, and it probably migrated through the porosity of the material. In this case, only the cristobalite form of silica was detected in the upper part of the column. 

For the used material as well as for the as-received one, an increase in temperature always induces a decrease in the elastic modulus (Table III). This observation is generally explained by a progressive weakening of the intergranular oxide phase. The damage created in... 

It should be noted that the term pesting is sometimes erroneously used to describe other clearly different oxidation phenomena. It is also often used more broadly, with respect to intermediate-temperature degradation in an oxidizing environment, than is justified by the present definition. For example, the end result of rapid degradation of an intermetallic alloy by internal or intergranular oxidation can seem to resemble that of pesting, but is certainly not the same as the... 

Figure 11.17 Intergranular oxidation. (From Source NALCO Bulletin No. 61, Boiler Metal Failure Analysis, NALCO Chemical Co., Oak Brook, IL, USA)...

For T91, when temperature is above 550°C, internal oxidation occurs with chromium-rich oxide precipitates localized along the grain boundaries. For austenitic steels, a pronounced intergranular oxidation is observed. 

Any heat treatment of stainless steel should be preceded and followed by cleaning. Steel should be cleaned before heat treating to remove any foreign material that may be incorporated into the surface during the high-temperature exposure. Carbonaceous materials on the surface could result in an increase in the carbon content, causing carbide precipitation. Salts could cause excessive intergranular oxidation. Therefore, the stainless steel must be clean before it is heat treated. 

Besides the formation of an external oxide scale, internal corrosion m occm imder technical service conditions. In some cases, internal corrosion occims preferentially along grain boundaries (intergranular corrosion). Of course, internal corrosion, particularly intergranular oxidation, is undesirable, since it usually enhances intergranular fracture resulting in premature brittle failure. 

Oxidation of Ni-base superalloys after exposure to laboratory air at 1000°C oxide scale and intergranular oxidation zone of (a) Inconel 625SI after 90 h and (b) Inconel 718 after 140 h exposure. 

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