Metals grain boundaries

Phosphides. Zirconium forms several phosphides ZrP [39318-19-9] 2 [ 12037-80-8] and ZrP g [12066-61 -4]-, they are part of the Zr—P phase diagram (137). The solubiUty of phosphoms in zirconium metal is low, ca 50 ppm, and at higher concentrations it collects as separate globules at the metal grain boundaries. Analysis indicates that this material is Zr P. 

The behavior of polycrystalline materials is often dominated by the boundaries between the crystallites, called grain boundaries. In metals, grain boundaries prevent dislocation motion and reduce the ductility, leading to hard and brittle mechanical properties. Grain boundaries are invariably weaker than the crystal matrix, and... 

Fig. 5 In-situ TEM view for initial oxidation of Fe-25 Cr-20 Ni (T = 650°C, pO = 10 torr) with time. Small oxides are arrayed along metal grain boundaries and randomly distributed within the grains.

CORROSION, INTERGRANULAR - Localized attack occurring on the metal grain boundaries. This is commonly found with stainless steels which have been improperly heat treated. 

The pictures imply that the inter-grain layer, formed as a result of Sn and Ca segregation at the metal grain boundaries, is more reactive and PbS04 crystals form over it. This layer has... 

Corrosion problems on process units often first show up as weld leaks. More precisely, the failures occur in the heat-affected zone adjacent to the weld. The welding procedure lowers corrosion resistance at the metal grain boundaries in the heat-affected zone. Stress relieving and improved welding techniques will not necessarily mitigate the problem. Use of low-carbon steels or Type 321 stainless (contains titanium) and Type 347 stainless (contains columbium) for replacement piping subject to frequent weld leaks will minimize the rate of failure. 

The formation of oxide pegs at the metal grain boundaries leading to a keying effect of the oxide on the metal. 

Schematic of a cross section showing the structures of internal corrosion underneath an outer oxide scale, at metal grain boundaries, and inside the metal grains.

Perusin S, Viguier B, Monceau D, Ressier L and Andrieu E (2004), Injection of vacancies at metal grain boundaries during the oxidation of nickel , Acta Mater, 52,5375-5380. 

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