Intercrystalline attack

In the case of the aluminium-magnesium system, most commercial alloys are usually supersaturated, so that elevated service temperatures and inexpert heat treatment are inadvisable, since any resultant grain boundary precipitation may induce susceptibility to intercrystalline attack. The extent of this susceptibility may be approximately deduced from the continuity of... 

Trace elements which adversely affect intercrystalline attack are normally controlled at a safe level. Copper is particularly pertinent in this respect since relatively small additions can cause a marked increase in intercrystalline attack in some alloy systems (Sections 1.3 and 1.7). 

The impurities likely to be present in nominally pure tin are unlikely to affect its corrosion resistance, except for minor effects on the rate of oxidation in air. Small aluminium contents, however, may result in a severely embrittling intercrystalline attack by water. The addition of antimony counteracts this effect. Although 0-1% magnesium appears to be tolerable, larger amounts produce effects similar to those of aluminium. 

Other factors are temperature, ionic strength, rate of flow of corrosive fluid, etc. At pH >11 corrosion rate decreases due to the formation of a protective film of ferric hydroxide/oxide. Although general corrosion rate decreases above this pH, the metal becomes susceptible to intercrystalline attack (at defects in oxide film) and thus fails due to caustic embrittlement. 

The boiler water contains silica which directs the attack to grain boundaries leading to intercrystalline attack of an embrittling nature. 

Magnesium and magnesium alloys are almost immune to intercrystalline attack (Makar and Kruger, 1993). Corrosion does... 

The susceptibihty to intercrystalline attack of austenitic stainless steels on contact with zinc or lead above their melting points, or with many lead and zinc compounds at similarly elevated temperatures. 

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