Inhibition aluminium

Whether tubes will be either inhibited aluminium brass or aluminium bronze, the tube sheets should be of the same material as the mbes. 

Inhibited aluminium pigments are also available, and these should be added after the necessary pH adjustment of the formulation. Optimum stability for aluminium pastes is at pH 7. 

Interference pigments, micas, should be considered as a more stable alternative to inhibited aluminiums for certain colours. 

The development of acidity within an occluded cell is by no means a new concept, and it was used by Hoar s as early as 1947 in his Acid Theory of Pitting to explain the pitting of passive metals in solutions containing Cl ions. According to Hoar the Cl ions migrate to the anodic sites and the metal ions at these sites hydrolyse with the formation of HCl, a strong acid that inhibits the formation of a protective film of oxide or hydroxide. Edeleanu and Evans followed the pH changes when aluminium was made anodic in Cl solutions and found that the pH decreased from 8-8 to 5-3. 

Uhlig etal. have also determined the minimum activity of inhibiting oxy-anions fli b to inhibit the pitting of Fe-18Cr-8Ni stainless steel - and aluminium , and found that equations of the type... 

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)... 

This material can be used only in seawater or similar chloride-containing electrolytes. This is because the passivation of the silver at discontinuities in the platinum is dependent upon the formation of a film of silver chloride, the low solubility of which, in seawater, inhibits corrosion of the silver. This anode, consisting of Pt-lOPd on Ag, was tried as a substitute for rapidly consumed aluminium, for use as a trailing wire anode for the cathodic protection of ships hulls, and has been operated at current densities as high as 1 900 AmHowever, the use of trailing anodes has been found inconvenient with regard to ships manoeuvrability. 

Dissimilar metals in the same system Because of the specific action of many inhibitors towards particular metals, problems arise in systems containing more than one metal. In the majority of cases these problems can be overcome by the choice of a formulation incorporating inhibitors for the protection of each of the metals involved. With this procedure it is necessary not only to maintain an adequate concentration of each of the inhibitors but also to ensure that they are present in the correct proportion. This is because of two effects firstly, failure to inhibit the corrosion of one metal may intensify the attack on the other metal the best example of this is with aluminium and copper in the same system, and failure to inhibit copper corrosion — usually achieved with sodium mercaptobenzothiazole or benzotriazole—can lead to increased corrosion of the aluminium as a result of deposition of copper from copper ions in solution on to the aluminium surface. Secondly, an inhibitor of the corrosion of one metal may actually intensify the corrosion of another metal. Thus, benzoate is usually used to prevent the corrosion of soldered joints by nitrite inhibitor added to protect cast iron in the same system. A benzoate nitrite ratio of greater than 7 1 is necessary in these cases. 

Locomotive diesels As larger volumes of coolant are required in railway locomotives than in road vehicles, the cost of inhibition is proportionally greater. An additional factor is the possibility of cavitation attack of cylinder liners. These considerations place a restriction on the choice of inhibitors. In the past, chromates have been used at concentrations of up to 0-4%, but their use presents handling and disposal problems. Chromates cannot be used with ethanediol antifreeze solutions. A IS I borate-metasilicate at a concentration of 1 % has been used in the UK. Nitrate is added to this to improve inhibition of aluminium alloy corrosion. Tannins and soluble oils are also used, but probably to a lesser extent than in the past. The benzoate-nitrite formulation (formerly BS 3151) is effective and has been used by continental railways . ... 

Elsewhere, in a series of Japanese patents, mixtures of resorcinol + sodium nitrate, glycerine + sodium nitrate, lithium hydroxide + tungstate, etc., have been claimed to be effective. An example of the use of inhibited cooling mixtures of low toxicity is provided by a patent which describes a mixture of silicate-I- polyphosphate -I- a saccharide, e.g. sucrose or fructose, as the inhibitor formulation in a propylene glycol -I- potassium-hydrogen-carbonate mixture used in aluminium cooler boxes for ice-cream. 

Most of the information available on the mechanism of action of inhibitive anions relates to iron, which will be discussed in some detail, and followed by brief accounts of zinc, aluminium and copper. 

When aluminium is immersed in water, the air-formed oxide film of amorphous 7-alumina initially thickens (at a faster rate than in air) and then an outer layer of crystalline hydrated alumina forms, which eventually tends to stifle the reaction In near-neutral air-saturated solutions, the corrosion of aluminium is generally inhibited by anions which are inhibitive for iron, e.g. chromate, benzoate, phosphate, acetate. Inhibition also occurs in solutions containing sulphate or nitrate ions, which are aggressive towards iron. Aggressive anions for aluminium include the halide ions F ,... 

Cr, Br , I, which cause pitting attack, and anions which form soluble complexes with aluminium , e.g. citrate and tartrate, which cause general attack. Competitive effects , similar to those observed on iron, are observed in the action of mixtures of inhibitive anions and chloride ions on aluminium. The inhibition of aluminium corrosion by anions exhibits both an upper and a lower pH limit. The pH range for inhibition depends upon the nature of the anion . 

The mechanism of action of inhibitive anions on the corrosion of iron, zinc and aluminium in near-neutral solution involves the following important functions ... 

Recent developments have also been reported in the inhibition of zinc , aluminium and copper . ... 

Komrska Satava (1970) showed that these accounts apply only to the reaction between pure zinc oxide and phosphoric acid. They found that the setting reaction was profoundly modified by the presence of aluminium ions. Crystallite formation was inhibited and the cement set to an amorphous mass. Only later (7 to 14 days) did XRD analysis reveal that the mass had crystallized directly to hopeite. Servais Cartz (1971) and Cartz, Servais Rossi (1972) confirmed the importance of aluminium. In its absence they found that the reaction produced a mass of hopeite crystallites with little mechanical strength. In its presence an amorphous matrix was formed. The amorphous matrix was stable, it did not crystallize in the bulk and hopeite crystals only grew from its surface under moist conditions. Thus, the picture grew of a surface matrix with some tendency for surface crystallization. 

The addition of aluminium to the liquid slowed down the reaction. An amorphous cement was formed and there was no crystallization in the bulk of the cement. However, after some time crystallites were formed at the surface. Thus, the presence of aluminium exerts a dedsive influence on the course of the cement-forming reaction. This effect is to be attributed to the formation of aluminophosphate complexes (see Sections 6.1.2 and 4.1.1). These complexes may delay the predpitation of zinc from solution and also introduce an element of disorder into the structure, thus inhibiting crystallization. It is significant that zinc, which does not form complexes, has little effect on the nature or speed of the reaction. 

Finely divided magnesium or aluminium hydroxides (or a 3 1 combination) are currently the best smoke suppressants. They also neutralise the acidic vapours produced from halogen-containing flame inhibiters. The more finely divide they are the more efficient they become. 

Godbold DL, Jentschke G. Aluminium accumulation in root cell walls coincides with inhibition of root growth hut not with inhibition of magnesium uptake in Norway spruce. Physiol Plant 1998 102 553-560. 

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