Aluminium aqueous corrosion

Whilst cathodic protection can be used to protect most metals from aqueous corrosion, it is most commonly applied to carbon steel in natural environments (waters, soils and sands). In a cathodic protection system the sacrificial anode must be more electronegative than the structure. There is, therefore, a limited range of suitable materials available to protect carbon steel. The range is further restricted by the fact that the most electronegative metals (Li, Na and K) corrode extremely rapidly in aqueous environments. Thus, only magnesium, aluminium and zinc are viable possibilities. These metals form the basis of the three generic types of sacrificial anode. 

Figure 47. Effect on aqueous corrosion of 8001 aluminum at 260°C, 7 m/sec velocity of added surface of aluminium or stainless steel (82)...

Thompson, G. E. and Wood, G. C., Anodic Films on Aluminium , in Corrosion.-Aqueous Processes and Passive Films, by J. C. Scully (ed.), Academic Press (1983)... 

Troutner V.H., The mechanism and kinetics of aqueous aluminium corrosion. I. Role of corrosion product film in the uniform aqueous corrosion of aluminium. Corrosion, vol. 15, 1959, p. 9t-12t. 

As a matter of principle, in the absence of humidity, a dry powder has no action on aluminium, because corrosion as an electrochemical reaction can develop only in an aqueous medium. Salts that are very insoluble in water, therefore, generally have no action on aluminium. 

Richardson, J. A. and Wood, G. C., The Interpretation of Impedance Charges on Oxide-coated Aluminium Produced by Immersion in Inhibitive and Corrosive Aqueous Media , J. Elecirochem. Soc., 120, 193 (1973)... 

Aluminium is a very reactive metal with a high affinity for oxygen. The metal is nevertheless highly resistant to most atmospheres and to a great variety of chemical agents. This resistance is due to the inert and protective character of the aluminium oxide film which forms on the metal surface (Section 1.5). In most environments, therefore, the rate of corrosion of aluminium decreases rapidly with time. In only a few cases, e.g. in caustic soda, does the corrosion rate approximate to the linear. A corrosion rate increasing with time is rarely encountered with aluminium, except in aqueous solutions at high temperatures and pressures. 

Common packaging materials are a potential source of aggressive substance s, and careful selection is recommended to avoid surface deterioration. Where paper is in contact with aluminium, the chloride content should be below 0-05 7o, sulphate content below 0-25 7o, copper content below 0-01% and the pH of aqueous extracts in the range pH 5-5-7-5, in order to avoid corrosion in damp conditions. Papers and felts used in building applications should also conform to this specification as a minimum requirement and be of the highest quality, since metallic copper found in materials of inferior origin can result in severe local galvanic attack of aluminium. 

Resistance to stress-corrosion cracking Commercially pure titanium is very resistant to stress-corrosion cracking in those aqueous environments that usually constitute a hazard for this form of failure, and with one or two exceptions, detailed below, the hazard only becomes significant when titanium is alloyed, for example, with aluminium. This latter aspect is discussed in Section 8.5 under titanium alloys. 

In more recent work embrittlement in water vapour-saturated air and in various aqueous solutions has been systematically examined together with the influence of strain rate, alloy composition and loading mode, all in conjunction with various metallographic techniques. The general conclusion is that stress-corrosion crack propagation in aluminium alloys under open circuit conditions is mainly caused by hydrogen embrittlement, but that there is a component of the fracture process that is caused by dissolution. The relative importance of these two processes may well vary between alloys of different composition or even between specimens of an alloy that have been heat treated differently. 

Although aluminium is a base metal, it spontaneously forms a highly protective oxide film in most aqueous environments, i.e. it passivates. In consequence, it has a relatively noble corrosion potential and is then unable to act as an anode to steel. Low level mercury, indium or tin additions have been shown to be effective in lowering (i.e. making more negative) the potential of the aluminium they act as activators (depassivators). Each element has been shown to be more effective with the simultaneous addition of zinc . Zinc additions of up to 5% lower the anode operating potential, but above this level no benefit is gained . Below 0 9 7o zinc there is little influence on the performance of aluminium anodes . Table 10.10 lists a number of the more common commercial alloys. 

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. 

A well established technique for reducing the rate of corrosion of metals in aqueous solutions is the use of corrosion inhibitors these are chemicals which interfere with either the anodic or cathodic corrosion reaction and thus slow the overall corrosion rate. These inhibitors may function by various mechanisms but three types are particularly relevant in the case of aluminium ... 

Both tinplate and aluminium sheet may be used to produce shallow drawn containers, the choice depending on cost versus product resistance. Dry products and nonaqueous creams are usually satisfactory in tinplate, whereas aqueous creams are better packed in aluminium. Alternatively, lacquered tinplate can be used for aqueous creams but there is always a chance of corrosion at any raw edges. Tinplate is stronger than aluminium for a given thickness. 

Two dissimilar metals in contact in an aqueous environment does not necessarily give rise to galvanic corrosion. Turner [1990] cites an example of a shell and tube heat exchanger (steam condenser), containing aluminium brass tubes expanded into carbon steel tube plates. It could be anticipated that with the relatively large area of the noble aluminium brass, severe galvanic corrosion would occur on the smaller area of exposed steel resulting in a short service life. In fact the condenser had been in service for 26 years when the examination took place ... 

The more important reactions of phenol have been given in the section above. Phenol is a poison, and has a corrosive action on the skin. Ferric chloride produces a violet coloration in a dilute aqueous solution of phenol. It is of interest to note that phenol has been prepared from benzene by the action of oxygen in the presence of palladium-black or aluminium chloride. 

MF, PC, DME), their mixtures, and of further additives (CQ, diglyme, THF, bromo-benzene, pyridine and others), using DTA. DME and PC exhibited the highest exothermic initiation temperatures, 425 °C and 244 °C respectively. Selim and Bro stated that any polar solvent is intrinsically reactive toward lithium . This may possibly be undetectable by static experiments but is crucial in deposition and reanodization experiments (see Sect. 10.5.). Either lithium or aluminium can be plated from non-aqueous mixed lithium-aluminium electrolyte solutions, depending only on the composition of the solution this amazing fact led Peled Ho suggest that alkaline and alkaline earth metals are always protected by a film formed by reaction with the electrolyte controlling corrosion and deposition-dissolution processes. 

Electrodeposition of PANI derivatives was achieved by Shah and Iroh [108] in the case of Al alloy (2024) and N-ethylaniline in aqueous oxalic acid solution. The coating was electrodeposited by cyclic voltammetry by application of an unusually large potential range (-1V to -I-3V/SCE). Corrosion current was measured in chloride solutions from Tafel plots and was found to be significantly lower (about one order of magnitude) than that of bare Al-2024. Multifunctional coatings (PPy/PANI) were also achieved on aluminium by coelectropolymerization of aniline and pyrrole (in a feed ratio of 3 7) in oxalic acid solution, but protection performances were not given [109,110]. 

---