Alloy deposition surface concentrations

Hessami and Tobias [70] extended the mechanisms of Bockris et al. and Matulis etal. of the deposition of single metals (Ni, Fe) to the mathematical modeling of codeposition of Ni—Fe alloys. This mathematical model for the anomalous alloy deposition describes the electrode processes using the calculated interfacial concentrations. The inhibition (reduction) of nickel partial current density during alloy deposition and the anomalous deposition are explained on the basis of the relative concentrations of metal-hydroxide ions, [MOH]+. Calculations show that the [FeOH]+ concentrations are higher than [NiOH]+ because it has a much smaller dissociation constant ([MOH]+ = (M2+)(OH )/A surface sites, and the result of this competition is inhibition (decrease) in Ni deposition in the presence of [FeOH]+ ions. Figure 30... 

Plating of alloys of Mo was studied intensively in recent years by Landolt and his co-workers. It was shown that in fonnation of Ni-Mo alloys, the rate of deposition of Mo (i.e., the partial current density for deposition of this metal) is controlled by the concentration of Ni in solution. This is consistent, of course, with the idea that the precursor for deposition of the alloy is a mixed-metal complex, as proposed for Ni-W alloys by Gileadi et al. It is also expected in view of the similarity of the chemistry of W and Mo ions in aqueous solutions. However, the mixed metal complex for Ni-Mo alloy deposition was assumed to be [NiCit(Mo02)]ads The most important difference between the assumed mixed-metal complexes are that in the case of W the complex is in solution, while in the case of Mo it is assumed to be adsorbed on the surface. More-... 

It is worth noting that the catalytic effect of the ad-atom-modified electrodes can be different from that found for an alloy of these metals, since even in the case of solid solutions in the whole composition range, the surface structure is not the same. In the alloy, there is a microscopic mixture of the two species participating in the alloy, whereas in the ad-atom-modified electrodes, the adatoms are deposited on top of the surface. Moreover, the surface concentration of the different metallic atoms in the alloy can be completely different from that obtained in the bulk, making the interpretation of the results obtained with the alloy electrodes more difficult. In this respect, much more research is needed in order to fully understand the different behavior of the alloys. 

Several electrodeposits were formed at potentials ranging from 0 to -O.IV. EDS examination of the as-deposited surface indicated the presence of aluminum and niobium in all of the electrodeposits examined. Chlorine was not detected in any of the samples indicating that the deposits contained no entrained electrolyte. Figure 7 is a plot of alloy composition as a function of deposition potential. The highest niobium concentration detected was 13.5% (atomic fraction). This was observed at a deposition potential of O.OV. As the deposition potential is made more negative, the niobium concentration is dramatically reduced. This implies that the kinetics for aluminum deposition are much faster than that of niobium, or that the niobium reduction is simply mass transport limited in the potential range examined. The fact that pure niobium deposits are apparently not achievable at potentials more positive of the aluminum deposition potential may be an indication that the codeposition of niobium and aluminum at negative potentials follows an mduced codeposition mechanism i.e., niobium deposition is only possible when aluminum is codeposited. 

It is natural to start with different concentrations of copper or tin ions in the electrolyte solution to modify the foam structure, because the content of metal ions affects the rate of alloy deposition. Figure 9 shows the morphology of copper electro-deposits as a function of the content of copper sulfate. 3-D foam structure was evident at 0.1 to 1.0 M of copper sulfate. The pore size at the surface of the deposits increased with copper sulfate content while the... 

Calcium carbonate has normal pH and inverse temperature solubilities. Hence, such deposits readily form as pH and water temperature rise. Copper carbonate can form beneath deposit accumulations, producing a friable bluish-white corrosion product (Fig. 4.17). Beneath the carbonate, sparkling, ruby-red cuprous oxide crystals will often be found on copper alloys (Fig. 4.18). The cuprous oxide is friable, as these crystals are small and do not readily cling to one another or other surfaces (Fig. 4.19). If chloride concentrations are high, a white copper chloride corrosion product may be present beneath the cuprous oxide layer. However, experience shows that copper chloride accumulation is usually slight relative to other corrosion product masses in most natural waters. 

Substituting one alloy for another may be the only viable solution to a specific corrosion problem. However, caution should be exercised this is especially true in a cooling water environment containing deposits. Concentration cell corrosion is insidious. Corrosion-resistant materials in oxidizing environments such as stainless steels can be severely pitted when surfaces are shielded by deposits. Each deposit is unique, and nature can be perverse. Thus, replacement materials ideally should be tested in the specific service environment before substitution is accepted. 

Most cases of crevice corrosion take place in near-neutral solutions in which dissolved oxygen is the cathode reactant, but in the case of copper and copper alloys crevice corrosion can occur owing to differences in the concentration of Cu ions however, in the latter the mechanism appears to be different, since attack takes place at the exposed surface close to the crevice and not within the crevice in fact, the inside of the crevice may actually be cathodic and copper deposition is sometimes observed, particularly in the Cu-Ni alloys. Similar considerations apply in acid solutions in which the hydrogen ion is the cathode reactant, and again attack occurs at the exposed surface close to the crevice. 

General corrosion occurs in the weld metal and HAZ of welded Zr-2.25 Nb alloys in an environment of H2SO4 at temperatures greater than 343 K, the rate increasing with concentration. Above 70< oH2S04 both general corrosion and IGA occur, whilst above 80% hydrogen embrittlement was found also. Sulphides were found to be deposited on the metal surface . 

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