Protection of Aluminium

Cathodic protection for aluminium is more difficult than for steel, because it takes place at a potential that is significantly more electronegative, below — 800 mV SCE. At this level, the reduction of ions H is possible according to the reaction  

If the medium, such as soil, is not replenished easily, H ions that have been reduced are not replaced quickly, and the medium becomes alkahne by an excess of OH . As the 

In all these cases, OH ions will be produced. They give rise to so-called cathodic corrosion of aluminium  

2A1 + 20H + 2H2O 2AIO2 + 3H2 which usually develops as bright craters. 

The goal of cathodic protection of aluminium is to avoid pitting corrosion. It operates at a potential very close to the dissolution potential a very low or even minute level of uniform corrosion can be accepted [24]. 

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It remains to be determined whether the previous experiments , which have been interpreted as confirming the cathodic protection of aluminium by zinc, can be truly interpreted in this fashion or whether they are due to the accumulation of Zn in the electrolyte. Under laboratory conditions, and under some practical conditions in stagnant solutions or in recirculating systems, the latter explanation is quite likely. 

Technical Committee Reports of the National Association of Corrosion Engineers, USA, on pipeline corrosion control, including Statement on Minimum Requirements for Protection of Buried Pipelines , Some Observations on Cathodic Protection Criteria , Criteria for Adequate Cathodic Protection of Coated Buried Submerged Steel Pipelines and Similar Steel , Methods of Measuring Leakage Conductance of Coatings on Buried or Submerged Pipelines , Recommended Practice for Cathodic Protection of Aluminium Pipe Buried in Soil or Immersed in Water ... 

Scott, D. J., Aluminium Sprayed Coatings Their use for the protection of aluminium alloys and steel. IMF Conference paper (1971)... 

Reininger, H., Further Developments in Metal Spraying Technique , Metalloberfldche, 15, 52, 88, 118, 148 (1961). (Translation available as TM460, Aluminium Federation, Brimingham) Scott, D. J., Aluminium Sprayed Coatings Their Use for the Protection of Aluminium Alloys and Steel , Trans. Inst. Met. Fin., 49 No. 3, 111-122 and 49 No. 4, 173-175 (1971) Sprowl, J. D., Aluminized Steel-A Description, Report of the Department of Metallurgical Research, Kaiser Aluminium and Chemical Corporation, April (1958)... 

The mitigation of corrosion can be achieved economically by the use of corrosion inhibitors. Chromate has been extensively used in an aqueous environment for the protection of aluminium, zinc and steel. Although chromates are cheap and effective, they are not acceptable because of their toxicity. Alternate inhibitors such as molybdates, organic inhibitors such as phosphonates, mixtures of phosphates, borates and silicates and surfactants like sulfonates have been used in place of chromates. Chromates are anodic inhibitors and help to form passive oxide on the metal surface. 

In recent years, the use of rare earth metal salts as inhibitors has been gaining attention. Rare earth metal salts have been found to be effective inhibitors for the protection of aluminium alloys, mild steel and zinc. 

R. Racicot, R. Brown and S.C Yang. Corrosion protection of aluminium alloys by double-strand Polyaniline. Synt. Metals, 1997, Vol. 85, No. 1-3, pp. 1263-1264. 

D.E. Tollman, K.L. Levine, C. Siripirom, V.G. Gelling, G.P. Bierwagen, and S.G. Croll, Nanocomposite of pol3 pyrrole and alumina nanoparticles as a coating filler for the corrosion protection of aluminium alloy 2024-T3 Appl Surf. Set, 254, 5452-5459 (2008). 

Sathiyanarayanan, S., Azim, S., and Venkatachari, G. (2008) Performance studies of phosphate-doped polyaniline containing paint coating for corrosion protection of aluminium alloy. /. Appl. Polym. Sci., 107, 2224-2230. 

W. Huppatz The fundamentals of corrosion protection of aluminium alloys used as structural materials. Part 1, Metall 49, 505-509 (in German) (1995)... 

Hinton BRW, Amott DR, Ryan ME (1986), Cerium conversion coatings for the corrosion protection of aluminium , A/atB/ Forum, 9, 3,162-173. 

Mansfield, F., Use of rare earth metal salt solutions for corrosion protection of aluminium alloys and mild steel. Russian Journal of Electrochemistry, 2000. 36(10) 1063-1071. 

Elamdy, A.S., Beccaria, A.M., and Traverso, R, Corrosion protection of aluminium metal-matiix composites by cerium conversion coatings. Surface and Interface Analysis, 2002. 34(1) 171-175. 

Smith, C.J.E., Baldwin, K.R., Evans, V.M., Garrett, S.A., and Smith, K.S. Research into Chromate-free Treatments for the Protection of Aluminium AUoys. in 83rd Meeting oftheAGARD SMP. 1996. Florence, Italy. 

Smith C.J.E., Baldwin K.R., Evans V.M., Garrett S.A. and Smith K.S. (1996), Research into chromate-free treatments for protection of aluminium alloys , DERA Report. 

The intensity of cathodic corrosion of aluminium does not depend on the nature of the anions or on the concentration of cations [23]. According to the Pourbaix diagram (Figure B.1.10), cathodic protection of aluminium must be carried out at highly electronegative potentials, on the order of — 1750 mV SCE, which is well below the stability domain of water. At such a potential, there will always be cathodic corrosion. 

Altenpohl D.G., Use of boehmite films for corrosion protection of aluminium. Corrosion, vol. 18, 1962, p. 143t-153t. 

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