The Deposition and Corrosion of Metals

Nickel generally forms smooth deposits in sulfate or chloride solutions even in the absence of colloidal addition agents since the deposition of nickel is always accompanied by the simultaneous discharge of hydrogen 

Electrolyte.—The nature of the anion often has a very important influence on the physical form of the deposited metal for example, lead from lead nitrate solution is rough, but smooth deposits are obtained from silicofluoride and borofluoride solutions. The valence state of the metal may affect the nature of the deposit thus, from plumbic solutions lead is deposited in a spongy form whereas relatively large crystals are formed in plumbous solutions. In an analogous manner, smooth deposits of tin are obtained from stannate baths, but from stannite solutions the deposits are of poor quality. The difference in the behavior of different electrolytes is sometimes due to the possibility of the formation of colloidal matter which serves to give a fine-grained deposit this may be the case in the deposition of lead from silicofluoride and borofluoride solutions where a certain amount of colloidal hydrous silica or boron trioxide may be formed by hydrolysis. 

The best electrodeposits of chromium are obtained from solutions of chromic acid containing small amounts of certain anions, particularly the sulfate ion. There is little doubt that the nature of the deposit is influenced by the formation of a complex chromic chromate diaphragm on the cathode. The function of the anion appears to be to affect the nature of this diaphragm in such a manner so that it does not block the cathode completely it thus does not prevent access of ions, yet it is suflftciently adherent to interfere with the growth of crystal nuclei. 

Basis Metal.—Although the external form of a deposited metal is rarely affected by the basis metal used as the cathode, there is ample evidence that the latter has some effect on the crystal growth. It is apparent that in many instances the general orientation of the crystals, at least in the first layers of the deposit, is a continuation of that in the basis metal. 

If the conductance of the electrolyte is low, the current lines will tend to concentrate on the parts of the cathode nearest the anode and the throwing power will be bad. With a solution of good conductance, however, there will be no particular preference, as far as this factor is concerned, for one portion of the cathode over any other high conductance will thus improve throwing power. The conductance of an electrolytic bath is generally so good, however, that there is no noticeable influence on the throwing power. 

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Acid producers. Corrosion usually is moderate and localized. Almost all significant attack is associated with anaerobic bacteria (facultative and obhgate), as aerobic acid-producing varieties usually reside near the top of deposits and corrosion products contacting oxygenated waters. Thus, the direct effect on corrosion at metal surfaces is limited. Additionally, although acidic products may be expected to increase corrosion rates, acidity cannot be pronounced in deposits to put it simply, the deposits and corrosion products would dissolve at sufficiently acidic pH. 

In many STM studies little effort has been made to control the atmosphere within the electrochemical cell. Yet oxygen is known to exert a major role in the chemistry and corrosion of many transition metals. For example, several STM studies have used the copper/copper ion reference electrode, yet the electrode is known to be polarized from its reversible condition by oxygen, leading to significant dissolution [154]. These effects become particularly significant in the smdy of metal deposition and dissolu-... 

The previous section discussed the structure at the junction of two phases, the one a solid electron conductor, the other an ionic solution. Why is this important Knowledge of the structure of the interface, the distribution of particles in this region, and the variation of the electric potential in the double layer, permits one to control reactions occurring in this region. Control of these reactions is important because they are the foundation stones of important mechanisms linked to the understanding of industrial processes and problems, such as deposition and dissolution of metals, corrosion, electrocatalysis, film formation, and electro-organic synthesis. 

On the other hand, electroinactive but surface active substances (SAS) adsorbed at the electrochemical interface affect the rate of electrode reactions and clear examples are organic additives used in metal deposition and inhibitors of metal corrosion. In a few cases, these substances accelerate the rate of electrode reactions [118, 119]. 

Radiochemical methods are applied for the study of a wide range of electrochemical surface processes. The most important areas are as follows - adsorption and -> electrosorption occurring on the surface of electrodes the role of electrosorption in -> electrocatalysis -> deposition and dissolution of metals - corrosion processes the formation of surface layers, films on electrodes (e.g., polymer films), and investigation of migration processes in these films study of the dynamics of - electrosorption and - electrode processes under steady-state and equilibrium conditions (exchange and mobility of surface species) electroanalytical methods (e.g., radiopolarog-raphy). 

On the other hand, if the protective layer is of a less noble metal, the base material becomes cathodic in any region where there is a crack or fault, favouring metal deposition. This constitutes what has become known as cathodic protection and is employed extensively in the protection from corrosion of metals in marine use. 

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. 

A pyrrole-based silane has been reported to promote polypyrrole (Ppy) adhesion on insulating substrates such as n-type silicon photoanodes [10], p-type silicon wafers [11] and glass fibres [12]. To the authors knowledge, this approach has not been used with reactive metals. If both ECPs and silane compounds are combined in a single coating, it should be possible to obtain a composite film with improved corrosion protection, chemical/mechanical stability and adhesion. In the end, simplification of the deposition and processing of ECPs would be feasible. 

RW. Lipfert, M. Benarie, and M.L. Daum, Metallic corrosion damage functions for use in environmental assessments, in Proceedings of the Symposia on Corrosion Effects of Acid Deposition and Corrosion of Electronic Materials, Las Vegas, NV, 1985, R Mansfeld, S.R Haagenrud, V. Kucera, RH. Haynie, and J.D. Sinclair (eds.). The Electrochemical Society, Rennington, NJ, vol. 86(6), p. 108 (1986). 

Calcium carbonate (calcite) scale formation in hard water can be prevented by the addition of a small amount of soluble polyphosphate in a process known as threshold treatment. The polyphosphate sorbs to the face of the calcite nuclei and further growth is blocked. Polyphosphates can also inhibit the corrosion of metals by the sorption of the phosphate onto a thin calcite film that deposits onto the metal surface. When the polyphosphate is present, a protective anodic polarization results. 

Porous metal oxide deposits also permit the development of high boiler water concentrations. Water flows into the deposit and heat appHed to the tube causes the water to evaporate, leaving a concentrated solution. Again, corrosion may occur. Caustic attack creates irregular patterns, often referred to as gouges. Deposition may or may not be found in the affected area. 

With the increasing emphasis on energy conservation and environmental considerations, additives for fuels that can correct combustion-related problems have aroused considerable interest. Many commercial fuel additives are combinations of organometaHics, dispersants, emulsifiers, and carrier solvents. The organometaHic, often a metal soap, acts as a combustion catalyst, increasing efficiency with reduction of smoke, deposits, and corrosion. 

Stainless steels attacked by sulfate reducers show well-defined pits containing relatively little deposit and corrosion product. On freshly corroded surfaces, however, black metal sulfides are present within pits. Rust stains may surround pits or form streaks running in the direction of gravity or flow from attack sites. Carbon steel pits are usually capped with voluminous, brown friable rust mounds, sometimes containing black iron sulfide plugs fFig. 6.10). 

The classic signature of erosion-corrosion is the formation of horseshoeshaped depressions, comet tads, grooves, or sand dunelike surface contours oriented along the direction of fluid flow (Figs. 11.1,11.2,11.3,11.5, and 11.8). Occasionally, erosion-corrosion will produce smooth, almost featureless, surface contours (Fig. 11.15), although even in this case oriented metal loss often exists around the perimeter of the affected region. If erosion-corrosion has been recently active, affected surfaces will be free of accumulated deposits and corrosion products. 

Removal of deposits and corrosion products from internal surfaces revealed irregular metal loss. Additionally, surfaces in wasted areas showed patches of elemental copper (later confirmed by energy-dispersive spectroscopy) (Fig. 13.12). These denickelified areas were confined to regions showing metal loss. Microscopic analysis confirmed that dealloying, not just redeposition of copper onto the cupronickel from the acid bath used during deposit removal, had occurred. 

Several of the welded junctions were removed from the system for metallographic examination (Fig. 15.20). As can be seen from Fig. 15.20, the internal surface was covered with reddish and tan deposits and corrosion products. The metal surface itself retained a bright, metallic luster. 

Metal loss in these areas had produced a smooth surface, free of deposits and corrosion products. The rest of the internal surface was covered by a thin, uniform layer of soft, black corrosion product. The graphitically corroded surfaces of the pump casing provided soft, friable corrosion products that were relatively easily dislodged by the abrasive effects of high-velocity or turbulent water (erosion-corrosion). 

In addition to the formation of scale or corrosion of metal within boilers, auxiliary equipment is also susceptible to similar damage. Attempts to prevent scale formation within a boiler can lead to makeup line deposits if the treatment chemicals are improperly ehosen. Thus, the addition of normal phosphates to an unsoftened feed water ean eause a dangerous eondition by elogging the makeup line with preeipitated calcium phosphate. Deposits in the form of calcium or magnesium stearate deposits, otherwise known as "bathtub ring" can be readily seen, and are caused by the eombination of ealcium or magnesium with negative ions of soap stearates. 

The carbon dioxide produced can contribute to the corrosion of metal. The deposits of ferric hydroxide that precipitate on the metal surface may produce oxygen concentration cells, causing corrosion under the deposits. Gallionalla and Crenothrix are two examples of iron-oxidizing bacteria. 

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