Acidic conditions/corrosion

Trichloroethylene [79-01-6J, trichloroethene, CHCL=CCL2, commonly called "tri," is a colorless, sweet smelling (chloroformlike odor), volatile Hquid and a powerhil solvent for a large number of natural and synthetic substances. It is nonflammable under conditions of recommended use. In the absence of stabilizers, it is slowly decomposed (autoxidized) by air. The oxidation products are acidic and corrosive. Stabilizers are added to all commercial grades. Trichloroethylene is moderately toxic and has narcotic properties. 

Severe concentration cell corrosion involves segregation of aggressive anions beneath deposits. Concentrations of sulfate and chloride, in particular, are deleterious. Acid conditions may be established beneath deposits as aggressive anions segregate to these shielded regions. Mineral acids, such as hydrochloric and sulfuric, form by hydrolysis. The mechanism of acid formation is discussed in Chap. 2. 

Corrosion of industrial alloys in alkaline waters is not as common or as severe as attack associated with acidic conditions. Caustic solutions produce little corrosion on steel, stainless steel, cast iron, nickel, and nickel alloys under most cooling water conditions. Ammonia produces wastage and cracking mainly on copper and copper alloys. Most other alloys are not attacked at cooling water temperatures. This is at least in part explained by inherent alloy corrosion behavior and the interaction of specific ions on the metal surface. Further, many dissolved minerals have normal pH solubility and thus deposit at faster rates when pH increases. Precipitated minerals such as phosphates, carbonates, and silicates, for example, tend to reduce corrosion on many alloys. 

The application of lignin as an adhesive is possible in principle. The first attempt needed very long press times due to the low reactivity (Pedersen process) [161]. This process was based on lignin polycondensation under strong acidic conditions, which led to considerable corrosion problems in the plant [161]. The particles had been sprayed with spent sulfite liquor (pH = 3-4) and pressed at 180°C. After this step, the boards were tempered in an autoclave under pressure at 170-200°C, whereby the sulfite liquor became insoluble after splitting off water and SO2. 

Boilers and steam systems Steel steam lines can be inhibited by the use of a volatile amine-based inhibitor such as ammonia, morpholine or cyclohexylamine introduced with the feedwater. It passes through the boiler and into the steam system, where it neutralizes the acidic conditions in pipework. The inhibitor is chemically consumed and lost by physical means. Film-forming inhibitors such as heterocyclic amines and alkyl sulphonates must be present at levels sufficient to cover the entire steel surface, otherwise localized corrosion will occur on the bare steel. Inhibitor selection must take into account the presence of other materials in the system. Some amine products cause corrosion of copper. If copper is present and at risk of corrosion it can be inhibited by the addition of benzotriazole or tolutriazole at a level appropriate to the system (see also Section 53.3.2). 

The Krupp work had shown interesting improvements in acid resistance resulting from molybdenum and copper additions, and the use of 2-3% Mo for more difficult acid conditions was soon established. Other early additions were made to overcome susceptibility to intercrystalline corrosion, culminating in the general use, by the early thirties, of titanium additions for carbide stabilisation, followed shortly after by the alternative use of niobium. 

In addition to impurities, other factors such as fluid flow and heat transfer often exert an important influence in practice. Fluid flow accentuates the effects of impurities by increasing their rate of transport to the corroding surface and may in some cases hinder the formation of (or even remove) protective films, e.g. nickel in HF. In conditions of heat transfer the rate of corrosion is more likely to be governed by the effective temperature of the metal surface than by that of the solution. When the metal is hotter than the acidic solution corrosion is likely to be greater than that experienced by a similar combination under isothermal conditions. The increase in corrosion that may arise through the heat transfer effect can be particularly serious with any metal or alloy that owes its corrosion resistance to passivity, since it appears that passivity breaks down rather suddenly above a critical temperature, which, however, in turn depends on the composition and concentration of the acid. If the breakdown of passivity is only partial, pitting may develop or corrosion may become localised at hot spots if, however, passivity fails completely, more or less uniform corrosion is likely to occur. 

Oxidising conditions severely reduce molybdenum s corrosion resistance, and aeration of the above acids causes a pronounced increase in the corrosion rate. It is rapidly attacked by oxidising acids such as nitric acid, and by reducing acids containing oxidisers such as HNO3, FeCh, etc. It is less resistant at 100°C, particularly in 10% acetic acid (the corrosion rate being 0-33 mm/y), 10% formic acid (0-2 mm/y) and 0-25% benzoic acid (0-25 mm/y). 

Acetic acid released corrosivity dependent on conditions and formulation (degree of hydrolysis and presence of stabilisers and inhibitors)... 

Copper alloys are particularly prone to attack by long-chain fatty acids which are often present in sealing compositions, temporary protectives and as trace additives in many plastics under acid conditions ester plasticisers may saponify in the presence of copper giving rapid corrosion of the copper and accelerating degradation of the polymer. 

Wood is particularly valuable for many conditions which are corrosive to common metals (e.g. acids and external exposure), and for contact with foodstuffs and beverages. It is not subject to corrosion in the electrochemical sense of the term applied to metals, but in saline conditions it can be attacked by the products of metal corrosion (alkali and iron salts) where poor technology or unsuitable wood species are used. Although wood is attacked by both extremely alkaline and acid conditions, particularly those which are oxidising, it can be employed over a wider pH range than most other materials. 

The minimum rate of boiler steel corrosion (i.e., the maximum development of dense, adherent, and protective magnetite) occurs at pH of 11 to 12. However, at significantly lower pH values (acid conditions) hydrogen ions are discharged (depolarization), the magnetite layer becomes porous, and corrosion rates increase. 

HP IR cells need to exhibit high mechanical strength and resistance to corrosion by solvents and reagents. They are often fabricated from austenitic steels (e. g. type 316) which are satisfactory for relatively mild temperatures and pressures but can be corroded by acid or form [Fe(CO)5] and [Ni(CO)4] by reaction with CO. Alternative materials for construction include some titanium alloys (which can be vulnerable to primary alcohols at high temperature) and nickel-molybdenum-chromium alloys (e. g. Hastelloy C-276, Hastelloy B2) which are highly resistant to reducing, oxidising and acidic conditions. 

When operated under these conditions for a week, no scale was evident, either by a decrease in heat transfer coefficients or by examination at the end of the run. Thus, it is apparent that scale can be prevented by this mechanism when the equilibrium pH is exceeded by something more than 0.3 to 0.4 and less than 1.5 pH units. All heat transfer tests were made with use of acid for scale prevention. In these tests, the pH was kept below the equilibrium pH in order to be on the safe side, and no scaling was ever detected. As evident from the above data, the pH required is not so low as to involve operation under acidic conditions, where accelerated corrosion might be expected. 

Corrosion of the zinc anode is a significant side reaction under acidic conditions because zinc reacts with H+(aq) to give Zn2+(aq) and H2(y). Under basic conditions, however, the cell has a longer life because zinc corrodes more slowly. The alkaline cell also produces higher power and more stable current and voltage because of more efficient ion transport in the alkaline electrolyte. 

Less information is available about the stability of ceramic membranes. It is generally thought that ceramic membranes have excellent solvent stability. Acid conditions may be more problematic it was shown [57] that an alumina nanofiltra-tion membrane was very sensitive to corrosion effects in dynamic experiments, whereas the performance of a similar titania membrane was stable in the pH range from 1.5 to 13. 

The stability of buried metals largely depends on a combination of pH and redox (Edwards 1996). Under high redox values (oxidizing conditions) most metals will easily corrode, whereas under low redox values (reducing conditions) they will tend to remain as uncorroded metal. In addition, acidic conditions (low pH) will assist corrosion, whereas alkaline conditions will tend result in the formation of a stable corrosion matrix in most metals. Thus, in a well-drained, acidic sand or gravel site, all metals except the most inert (e.g., gold) will corrode rapidly and extensively. However, under most other burial conditions, most metal will be capable of recovery, albeit in a corroded state even after many centuries. 

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