Intergranular corrosion aluminium alloys

Later work on aluminium alloys has also focused more closely upon the role of hydrogen which had not previously been widely considered as an embrittling species in the stress-corrosion cracking process for these alloys. The idea was not new, however. Reports of intergranular failure under cathodic charging conditions had been made at a much earlier time . A reduction in stress-corrosion life and alloy ductility in a high purity Al-5Zn-3Mg alloy had been found in specimens pre-exposed to a 2% NaCI solution" , an effect that was accentuated if specimens were stressed". ... 

The embrittlement caused to aluminium alloys by pre-exposure to moist atmospheres or stress-corrosion environments is thought to be due to hydrogen in the atomic form. Intergranular bubbles of hydrogen, formed in association with certain precipitates, have been observed by HV and... 

Test method for determining the susceptibility to intergranular corrosion of 5XXX series aluminium alloys by mass loss after exposure to nitric acid (NAMLT test) Practice for liquid sodium corrosion testing of metals and alloys... 

Budd and Booth found the potentiostatic test best for investigating the intergranular and layer corrosion of aluminium alloys. 

Intergranular corrosion occurs in stainless steels and alloys based on nickel, aluminium, magnesium, copper and cast zinc. In the following sections we shall look at the three former groups in some detail. 

Examples of intergranular cracking [7.49] comprise i) carbon steel in caustic, nitrate, acetate and carbonate icarbonate solutions, ii) low-alloy steels in pure water, iii) stainless steels that are liable to ordinary intergranular corrosion in oxygen-containing water, iv) a-brass in ammonia solutions that cause surface films, v) aluminium alloys in water vapour and humid hydrogen gas, vi) P-titanium alloys in metanol solutions, vii) tempered martensitic stainless steels in chloride solutions and viii) nickel alloys in very pure water and alkaline solutions. 

Several aluminium alloys show very good corrosion resistance in various atmospheres. Some pitting occurs, but the pits remain small. Maximum depth seldom exceeds 0.5 mm during 6-20 years of exposure it is usually in the order of 0.1 mm. Some alloys may, however, be attacked by intergranular corrosion or exfoliation corrosion (see Section 7.7). Extensive galvanic corrosion may occur on aluminium in contact with copper, mild steel (in marine atmosphere) and graphite, less in contact with stainless steel, while aluminium is compatible with zinc [8.2]. 

Corrosion of metals and alloys—Corrosivity of atmospheres—Guiding values for the corrosivity categories Corrosion of metals and alloys—Corrosivity of atmospheres—Measurement of pollution Corrosion of metals and alloys—Corrosivity of atmospheres—Determination of corrosion rate of standard specimens for the evaluation of corrosivity Corrosion tests in artificial atmospheres—Salt spray tests Nickel-based alloys—Determination of resistance to intergranular corrosion Corrosion of aluminium alloys—Determination of resistance to stress corrosion cracking Corrosion tests in artificial atmosphere at very low concentrations of polluting gas(es)... 

Corrosion of metals and alloys—Determination of resistance to intergranular corrosion of solution heat-treatable aluminium alloys... 

Sensitivity of Aluminium Alloys to Pitting Corrosion Transgranular and Intergranular (Intercrystalline) Corrosion... 

On the other hand the localized corrosion and more specially the role of galvanic coupling is largely discussed for aluminium alloys. In these alloys, microstmctural corrosion e.g. pitting or intergranular corrosion (IGC) can be initiated at the interface between constiment irrtermetallic (IM) particles and the matrix. Most of the constituent particles contained in stractural alloys have a simple cathodic behavior towards the matrix and support reduc-tioa This reduction can be corrsidered, in a simple way, as a fotu-electron process ... 

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