Liquid metals corrosion product deposition

Presence of crevices, dead-ends, etc. Effective protection by inhibitors relies on the continued access of inhibitor to all parts of the metal surface Aeration and Movement of the Liquid). It frequently happens that this condition is difficult to achieve due to the presence of crevices at joints, deadends in pipes, gas pockets, deposits of corrosion products, etc. Corrosion will then occur at these sites even though the rest of the system remains adequately protected. 

Chloride ions are easily washed away from the metallic surface by precipitation, while sulfate ions concentration remains more constant. It means that corrosion takes place under very variable concentrations of chloride ions and in the presence of less variable concentration of sulfate ions. Changes in the amount of chloride ions concentration should depend on the particular rain regime of the place. A notable difference has been found between chloride deposition rate determined by collectors protected from liquid precipitations and chloride content in corrosion products. 

Erosion corrosion is caused by the conjoint action of corrosion and mechanical abrasion by a moving fluid or suspended material in the fluid. Turbulent flow or jets of liquid on a metal surface may lead to erosion corrosion. The mechanical action of the fluid removes the protective corrosion deposit, thus exposing fresh metal to the corrosive. As corrosion products build up, they are removed and so the process continues. The surface of a piece of metal exposed to this type of corrosion has a characteristic structure (Fig. 8). 

Deposition of corrosion products in a circulating AT liquid metal system is important for three reasons. First, the degradation of heat transfer perfonmance of heat exchangers must be predicted. Second, radiation exposure limits for maintenance in certain areas of nuclear reactor systems that transport and deposit radioactive species must be controlled. Third, the tendency for all deposits to become detached by thermal shock or flow perturbations must be known since there is concern that these types of debris could block critical coolant channels. It is therefore valuable, when possible, to monitor reactions involving deposition as well as dissolution. 

Corrosion reactions can occur by a simple dissolution mechanism, whereby the containment material dissolves in the melt without any impurity effects. Material dissolved in a hot zone may be redeposited in a colder area, possibly compounding the corrosion problem by additional plugging and blockages where deposition has taken place. Dissolution damage may be of a localized nature, for example, by selective dealloying. The second corrosion mechanism is one of reactions involving interstitial (or impurity) elements such as carbon or oxygen in the melt or containment material. Two further subforms are corrosion product formation and elemental transfer. In the former the liquid metal is directly involved in corrosion product formation. In the latter the liquid metal does not react directly with the containment alloy rather, interstitial elements are transferred to, from, or across the liquid. 

General Description. Cavitation is a form of erosion-corrosion that is caused by the formation and collapse of vapor bubbles in a liquid against a metal surface. Cavitation occurs in hydraulic turbines, on pump impellers, on ship propellers, and on many surfaces in contact with high-velocity liquids subject to changes in pressure. The appearance of cavitation is similar to pitting except that surfaces in the pits are usually much rougher. The affected region is free of deposits and accumulated corrosion products if cavitation has been recent. 

When corrosion products such as hydroxides are deposited on a metal surface, a reduction in oxygen supply occurs, since the oxygen has to diffuse through deposits. Since the rate of metal dissolution is equal to the rate of oxygen reduction, a Hmited supply and limited reduction rate of oxygen will also reduce the corrosion rate. In this case the corrosion is said to be under cathodic control. In other cases corrosion products form a dense and continuous surface film of oxide closely related to the crystalline structure of metal. Films of this type prevent primarily the conduction of metal ions from metal-oxide interface to the oxide-liquid interface, resulting in a corrosion reaction that is under anodic control. When this happens, passivation occurs and metal is referred as a passivated metal. Passivation is typical for stainless steels and aluminum. 

Study of liquid and membrane extraction processes is a matter of primary importance for intensive development of extraction, separation, and concentration methods of different nature substrates, especially such valuable ones as rare and scattered metals. They are used in metalluigy for production of special cast iron grades, steel, and nonferrous metals alloys. Rare earth metals (REM) additives increase quality of metalluigical products improve such properties as shock resistance, viscosity, and corrosion resistance. Such materials are used particularly in aerospace industry. A conventional extraction process of rare earth metals from such solutions comes down to chemical deposition with oxalic acid or ammonium carbonate, formation of oxalates or metals carbonates and further washing, filtration, and calcination to oxides that then are dissolved in hydrochloric acid and separated. - Researches that were carried out during the last years have demonstrated that liquid and membrane extraction processes are the most promising methods of extraction, concentration, and separation of rare and scattered metals ions." ... 

Control over the residue content (required by Specification D 1835) is of considerable importance in end-use applications. In liquid feed systems residues may lead to troublesome deposits and, in vapor offtake systems, residues that are carried over can foul regulating equipment. Those that remain will accumulate, can be corrosive, and will contaminate following product. Water, particularly if alkaline, can cause failure of r ulating equipment and corrosion of metals. 

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