Adhering deposits

Lead is not generally attacked rapidly by salt solutions (especially the salts of the acids to which it is resistant). The action of nitrates and salts such as potassium and sodium chloride may be rapid. In potassium chloride the corrosion rate increases with concentration to a maximum in 0.05m solution, decreases with a higher concentration, and increases again in 2m solution. Only loosely adherent deposits are formed. In potassium bromide adherent deposits are formed, and the corrosion rate increases with concentration. The attack in potassium iodide is slow in concentrations up to 0.1m but in concentrated solutions rapid attack occurs, probably owing to the formation of soluble KPblj. In dilute potassium nitrate solutions (0.001 m and below) the corrosion product is yellow and is probably a mixture of Pb(OH)2 and PbO, which is poorly adherent. At higher concentrations the corrosion product is more adherent and corrosion is somewhat reduced Details of the corrosion behaviour of lead in various solutions of salts are given in Figure 4.16. 

On ferrous metals immersion deposition in the copper sulphate bath produces non-adherent deposits, and a cyanide copper undercoat is therefore normally used. Where the use of a cyanide strike cannot be tolerated, an electroplated or immersion nickel deposit has been used . Additions of surface-active agents, often preceded by a sulphuric acid pickle containing the same compound, form the basis of recent methods for plating from a copper sulphate bath directly on to steel ". 

Use and care of electrodes. Electrodes must be free from grease, otherwise an adherent deposit may not be obtained. For this reason an electrode should never be touched on the deposition surface with the fingers it should always be handled by the platinum connecting wire attached to the electrode. Platinum electrodes are easily rendered grease-free by heating them to redness in a flame. 

In areas of the system where the heat gradient is less severe, calcium carbonate precipitates in both crystalline and amorphous forms. It may precipitate as a calcite or aragonite sludge, but more usually an aragonite scale is produced. Aragonite is hard and adherent, depositing in FW lines and various boiler surface components such as boiler tubes. 

Modification of SS to form adherent deposits of intractable sludges... 

Electrogravimetry, which is the oldest electroanalytical technique, involves the plating of a metal onto one electrode of an electrolysis cell and weighing the deposit. Conditions are controlled so as to produce a uniformly smooth and adherent deposit in as short a time as possible. In practice, solutions are usually stirred and heated and the metal is often complexed to improve the quality of the deposit. The simplest and most rapid procedures are those in which a fixed applied potential or a constant cell current is employed, but in both cases selectivity is poor and they are generally used when there are... 

Existing deposits Gums and adherent deposits can be removed from fuel storage and distribution system components... 

A hard, tightly adherent deposit on the metal surfaces of automobile engines resulting from resinous oxidation products of gasoline and lubricating oils. 

Scales from individual salts vary in their heat-transfer insulating effects, but most adherent deposits found on heat-transfer surfaces consist of one or two predominant salts (such as calcium carbonate or calcium phosphate) intimately combined with a variety of lesser amounts and types of scales, together with some corrosion debris and other fouling matter. 

In this as in most other electroplating operations, it is desired to obtain a firm adherent deposit of metal, which can thereafter be burnished or polished. To secure such a deposit, careful regulation of acidity, temperature, current density, concentration of electrolyte, and rate of deposition must be maintained throughout the entire electrolysis. Failure to control one or more of these variables usually results in flaky or crumbly nonadherent deposits. 

Abbott et al. studied the deposition of zinc from a 1 2 choline chloride (ChCl) ZnCl2 ionic liquid [109] at 60°C and found deposits with a similar morphology to that shown by Sun. The optimum current density was found to be between 2 and 5 Am 2 and higher current densities led to powdery, non-adherent deposits. This is due primarily to the high viscosity and low conductivity of the choline-based liquids. The current plating efficiency in this liquid was found to be effectively 100% and the deposition process was shown to be almost totally reversible, with only the UPD material remaining on the surface. 

Chemical bonds, covalent or ionic as shown in Figure 6c and d, at the metal oxide/deposit surface are potentially strong with theoretical values over 10 N m. it is however, impossible to estimate the number of sites and the size of contact areas at the interface where the chemical bonds may be effective. In any case, the cohesive strength of the deposit matrix is the limiting factor since it is lower than that of chemical bonds by several orders of magnitude. In practice, this means that when a strongly adhering deposit is subjected to a destructive force, e.g. sootblower jet, failure occurs within the deposit matrix and there remains a residual layer of ash material firmly bonded to the tube surface. 

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