Aluminium alloys, corrosion

If Ihe main incoming male contacts are made ol aluminium alloy, which is normally a eompnsilioii of aluminium-magnesium and silicon, they must be provided wiili a coat of bron/e. copper and tin to give it an adequate mechanical hardness and resistance to corrosion. For more details refer to Section 27.2..5. 

So aluminium alloy is good it resists all the fluids likely to come in contact with it. What about GFRP The strength of GFRP is reduced by up to 20% by continuous immersion in most of the fluids - even salf water - with which it is likely to come into contact but (as we know from fibreglass boats) this drop in strength is not critical, and it occurs without visible corrosion, or loss of section. In fact, GFRP is much more corrosion-resistant, in the normal sense of loss-of-section, than steel. 

Metals which owe their good corrosion resistance to the presence of thin, passive or protective surface films may be susceptible to pitting attack when the surface film breaks down locally and does not reform. Thus stainless steels, mild steels, aluminium alloys, and nickel and copper-base alloys (as well as many other less common alloys) may all be susceptible to pitting attack under certain environmental conditions, and pitting corrosion provides an excellent example of the way in which crystal defects of various kinds can affect the integrity of surface films and hence corrosion behaviour. 

Fig. I.I2 Curves showing the relationship between strength, stress-corrosion susceptibility and heat treatment for a high-strength precipitation-hardening aluminium alloy...

Murray, G. A. W., Artificial Pits for Quantitative Studies of Corrosion of Aluminium Alloys in Natural Waters , Corrosion, 20, 329 (1964)... 

The general indication of the results in this table is that the corrosion rates of non-ferrous metals increase with depth in spite of lower temperatures and lower oxygen concentrations than at the surface. It was noted in the paper by Kirk that the results at depth were typical of the variation of performance of these materials experienced on numerous occasions in surface sea water. A notable exception was for aluminium alloys of the 5000 (Al-Mg) and 6000 (Al-Mg-Si) series which had good resistance to corrosion... 

In gasoline engines lead halides accumulate in the lubricant occasionally these give rise to the corrosion of aluminium-alloy pistons and very rarely to corrosion on aluminium-tin bearings. ... 

Aluminium and the aluminium alloys lend themselves to many engineering applications because of their combination of lightness with strength, their high corrosion resistance, their thermal and electrical conductivity and heat and light reflectivity, and their hygienic and non-toxic qualities. The variety of forms in which they are available also enhances their utility. 

Composites of aluminium alloy with a thin cladding on one or both surfaces of a more anodic aluminium alloy or pure aluminium, enable sheet, plate and tube to be produced with special combinations of strength and corrosion resistance appropriate to service conditions. Although originally applied to high strength aircraft alloys, this principle of cladding is now utilised in several important industrial applications. 

This appears as a random non-branching white tunnel of corrosion product either on the surface of non-protected metal or beneath thin surface coatings. It is a structurally insensitive form of corrosion which is more often detrimental to appearance than strength, although thin foil may be perforated and attack of thin clad sheet (as used in aircraft construction) may expose the less corrosion resistant aluminium alloy core. Filiform corrosion is not commonly experienced with aluminium, as reflected by the insignificance afforded it in reviews on the phenomena (Section 1.6). 

Processing and storage equipment for many chemicals, including acetaldehyde, formaldehyde, nylon salt, methyl methacrylate, carbon tetrachloride, glycerol, triacetin, proprionic acid, acetic acid and acetic anhydride, is manufactured from aluminium alloys, primarily because of their excellent corrosion resistance. 

Bucci has produced a useful and extensive report of value for the selection of suitable aluminium alloys to resist both SCC and corrosion fatigue while Khobaib discusses a range of beneficial inhibitors suitable under conditions of corrosion fatigue. 

Corrosion rates of 0.1-0.2 mg/cm /day under ASTMB117 salt fog conditions are typical for WE54 and the high purity AZ91 alloys which are comparable with those of some aluminium alloys. This considerable... 

Sandoz, G., In Stress Corrosion Cracking in High Strength Steels and in Titanium and Aluminium Alloys, Ed. B.F. Brown, Naval Research Laboratory, Washington, pp. 79-145, (1972)... 

Stress-corrosion Cracking of Titanium, Magnesium and Aluminium Alloys... 

Stress-corrosion cracking occurs in certain aluminium alloys which have been developed for medium and high strength by employing variations in composition, cold work and heat treatment The main alloys are based upon Al-Mg, Al-Mg and Al-Cu, but stress corrosion also occurs in Al-Ag, Al-Cu-Mg, Al-Mg-Si, Al-Zn and Al-Cu-Mg-Zn alloys. It has... 

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