Corrosion features acid conditions

Uses Corrosion inhibitor for waterborne coatings on ferrous surfe., e.g., metal primers, flat or gloss disp. paints, polyacrylics, polyvinyl acetates, PS acrylics and terpolymers, waterborne alkyds, aq. stoving enamels temporary rust inhibitor for metal surfs. in-can corrosion inhibitor, esp. under acid conditions Features Fights flash rusting... 

Structure-activity considerations. Predictions of the adverse effects that a substance may have can sometimes be made on the basis of a knowledge of the chemical structure of the substance and an association of various features of that structure with particular types of toxicity. It must be emphasised that this is a field for the expert and uninformed structure-activity predictions can be dangerously misleading. There are some areas, however, in which experts may make such predictions with a fair degree of accuracy. Perhaps the simplest example would be the prediction of irritant or corrosive properties for a molecule likely to give rise to acidic conditions in contact with water. Predictions of mutagenic activity may also in some cases be made by experts on the basis of the potential of the molecule to interact with DNA. 

Sulfuric acid is a stronger acid than sulfurous [pAa(l) < 0, p7fa(2) = 1.99 at 25 °C and infinite dilution] rain as acidic as pH 2.1 has been recorded at Hubbard Brook, New Hampshire, and the pH of water droplets in clouds can be as low as 1.5 (for comparison, the pH of rainwater saturated with atmospheric CO2 is about 5.6 at 15 °C). Acid rain destroys building materials (especially marble), kills fish and vegetation, accelerates metallic corrosion (Sections 16.5 and 16.7), and can be directly harmful to humans (e.g., it causes the alligator skin condition reported in Cubatao, Brazil). Sulfate rain is not completely without redeeming features, as many soils (e.g., in southern Alberta, Canada) are sulfur-deficient. On balance, however, its acidity is unacceptable, and sulfur oxide emissions must be controlled at the source. Several control measures are possible ... 

Terephthalic Acid from Toluene. Both carbon monoxide and methanol can react with toluene to yield intermediates that can be oxidized to terephthalic acid. In work conducted mainly by Mitsubishi Gas Chemical Company (62,63), toluene reacts with carbon monoxide and molar excesses of HF and BF3 to yield a jtanz-tolualdehyde—HF—BF3 complex. Decomposition of this complex under carefully controlled conditions recovers HF and BF3 for recycle and ra-tolualdehyde, which can be oxidized in place of para-xyiene to yield terephthalic acid. One drawback of the process is the energy-intensive, and therefore high cost, decomplexing step. The need for corrosion-resistant materials for construction and the need for extra design features to handle the relatively hazardous HF and BF3 also add to the cost. This process can be advantageous where toluene is available and xylenes are in short supply. 

Many reactions that occur in water are sensitive to acidity (pH), concentration and to the relative oxidising or reducing conditions in the neighbourhood. This is especially true of corrosion. However, corrosion does not always occur, and under certain combinations of acidity and reduction potential iron, copper, zinc and other metals can resist corrosion. This feature is called passivation. 

Conducting polymers can be prepared by chemical or electrochemical techniques. Electrochemical synthesis provides easier routes when compared with chemical synthesis and allows control over film formation, especially relevant if polymers are required as thin films deposited on the surface of metallic substrates. However, electrochemically synthesized polymers are usually more porous, a feature that requires consideration when a barrier effect is necessary. Another important aspect in the corrosion field is that the application of potential/current necessary to promote electropolymerization may accelerate dissolution (corrosion) of the metal. In some cases, an oxide pre-layer is deposited between the metal and the polymer to promote adhesion and hinder metal dissolution during the electropolymerization process (Tallman et al., 2002 Spinks et al., 2002). Alternatively, the application of layered coatings based on different conducting polymers can be a strategy to overcome the problem of metal dissolution. In the work of Lacroix et al. (2000), a layer of PPy was firstly deposited on zinc and mild steel in neutral conditions, followed by deposition of PANi in an acidic medium, because the direct deposition of PANi on those metallic substrates was not possible in an acidic medium, causing dissolution of the metal. 

---