Anodic dissolution alloys

Nickel. Most nickel is also refined by electrolysis. Both copper and nickel dissolve at the potential required for anodic dissolution. To prevent plating of the dissolved copper at the cathode, a diaphragm cell is used, and the anolyte is circulated through a purification circuit before entering the cathodic compartment (see Nickel and nickel alloys). 

Ores are mined and are then refined in an energy intensive process to produce pure metals, which in turn are combined to make alloys (see Metallurgy Mineral RECOVERY and processing). Corrosion occurs because of the tendency of these refined materials to return to a more thermodynamically stable state (1—4). The key reaction in corrosion is the oxidation or anodic dissolution of the metal to produce metal ions and electrons... 

It is also of interest to note that Wranglen considers that the decrease in the corrosion rate of steel in the atmosphere and the pitting rate in acid and neutral solution brought about by small alloying additions of copper is due to the formation of CU2S, which reduces the activity of the HS and Scions to a very low value so that they do not catalyse anodic dissolution, and a similar mechanism was put forward by Fyfe etal. to explain the corrosion resistance of copper-containing steels when exposed to industrial atmospheres. 

Wilde, B. E. and Teterin, G. A., Anodic Dissolution of Copper-Zinc Alloys in Alkaline Solutions , Brit. Corrosion J., 2, 125 (1967)... 

Pickering, H. W. and Byrne, P. J., Partial Currents During Anodic Dissolution of Cu-Zn Alloys at Constant Potential , J. Electrochem. Soc., 116, 1492 (1968)... 

Langenegger, E. E. and Robinson, F. P. A., Effect of the Polarisation Technique on Dezincihcation Rates and Physical Structure of Dezincihed Zones , Corrosion, 24, 411 (1968) Brooks, W. B., Discussion of the De-alloying Phenomenon , Corrosion, 24, 171 (1968) Pickering, H. W., Volume Diffusion During Anodic Dissolution of a Binary Alloy , J. Electrochem. Soc., 115, 143 (1968)... 

Pickering, H. W., Preferential Anodic Dissolution in Binary Alloys , Proc. Conf. Fundam. 

Frankenthal, R. P., The Effect of Surface Preparation on Pitting and Anodic Dissolution of Iron-Chromium Alloys , J. Electrochem. Soc., 114, 201c (1967)... 

Hodge, F. G. and Wilde, B. E., Effect of Chloride Ion on the Anodic Dissolution Kinetics of Cr-Ni Binary Alloys in Dilute H2SO4 , Corrosion, 26, 146 (1970)... 

This work has been carried out by Marcus and his co-workersand deals with the influence of sulphur on the passivation of Ni-Fe alloys. For sulphur-containing Ni-Fe alloys, sulphur segregates on the surface during anodic dissolution. Above a critical sulphur content a non-protective thin sulphide film is formed on the surface instead of the passive oxide film. 

The alloying elements molybdenum and copper do not, by themselves, enhance passivity of nickel in acid solutions, but instead ennoble the metal. This means that, in practice, these alloying elements confer benefit in precisely those circumstances where chromium does not, viz. hydrogen-evolving acidic solutions, by reducing the rate of anodic dissolution. In more oxidising media the anodic activity increases, and, since binary Ni-Mo and Ni-Cu alloys do not passivate in acidic solutions, they are generally unsuitable in such media. 

Another indication of the influence of precipitated phases on anodic behaviour may be seen in the curve for Alloy C in Fig. 4.28, where the small peak in the middle of the passive range is probably attributable to anodic dissolution of an intermetallic phase (n) and MjC carbide . ... 

Pitting of nickel and nickel alloys, as of other metals and alloys, occurs when passivity breaks down at local points on the surface exposed to the corrosive environment, at which points anodic dissolution then proceeds whilst the... 

Fig. 19.15 Schematic representation of range of corrosion potentials expected from various chemical tests for sensitisation in relation to the anodic dissolution kinetics of the matrix (Fe-l8Cr-IONi stainless steel) and grain boundary alloy (assumed to be Fe-lOCr-lONi) owing to depletion of Cr by precipitation of Cr carbides of a sensitised steel in a hot reducing acid (after Cowan and Tedmon )...

The electrochemical machining (ECM) of metals rests on the selective local anodic dissolution of metal. It is used to give metal parts the required shape and size, to drill holes, create hollows, cut shaped slots, and fashion parts of a complex pattern (e.g., the blades of gas turbines). It is an advantage of this method that it can also be used for hard metals (high-alloy steels and other alloys, metals in the quenched state, etc.). 

In the late 1960s it was discovered (Entina, 1968 Binder et al., 1972) that a strong synergy effect exists in the platinum-ruthenium system. Alloys of these two metals are two to three orders of magnitude more active catalytically for the anodic oxidation of methanol than pure platinum, whereas pure ruthenium is altogether inactive for this reaction. Prolonged exploitation of such anodes is attended by a gradual decrease in catalytic activity of the alloys because of slow anodic dissolution of ruthenium from the surface layer. A similar simation is seen for platinum-tin alloys, which... 

TLC is used to determine copper in aluminum alloys. The process involves the sampling of the investigated material by anodic dissolution, development of TLC plate with acetone -f HCl -f HjO (70 15 15), and the identification of analyte by 1-(2-pyridylazo)-2-naphthol [70]. A TLC system comprising silica gel as stationary... 

Anodic passivation can be observed easily and clearly with iron group metals and alloys as shown in Fig. 11-10. In principal, anodic passivation occurs with most metals. For instance, even with noble metals such as platinum, which is resistant to anodic dissolution in sulfuric acid solutions, a bare metal surface is realized in the active state and a superficial thin oxide film is formed in the passive state. For less noble metals of which the affinity for the oxide formation is high, the active state is not observed because the metal surface is alwa covered with an oxide film. 

Therefore many attempts have been made to use Li alloys which are solid at high temperatures. The compound LiAl melts at 718 , i.e. it is solid at the operating temperature of the SB, which is usually below 450°. Experiments have been carried out on the anodic dissolution of this alloy it shows a voltage between -1.95 to -2.0V at a Li content between 14.7 and 48 atom % (77) and SB cells of more than 120 Wh/kg ED, intended for off-peak energy storage and for car propulsion, are under development. 

In a series of pubhcations, the results of anodic dissolution and passivation of well-defined single-crystal surfaces of Ni (and other metals) and its alloys were presented by Marcus et al. [19-24]. Their approach aimed at a better understanding... 

Dissolution of gold and silver from Au/Ag alloys in aerated cyanide solutions has been investigated using rotating disc electrodes [551]. Dissolution was partially controlled by transport of either oxygen or cyanide. Kinetics of anodic dissolution of gold in cyanide solutions containing different metal ions has been extensively... 

Results. The presence of Pt reduces the corrosion rate of Ti by shifting the free corrosion potential to more noble values (Fig. 6) where the Ti dissolution rate is slower. This shift is produced by the catalytic effect of Pt on hydrogen recombination which alters the cathodic reactions at the alloy surface. At the corrosion potential, the cathodic and anodic currents are equal. Although the shift in corrosion potential reduces the anodic current, anodic dissolution of Ti still occurs. The long-term corrosion rate of a surface alloy depends upon what happens to the Pt as the Ti is being dissolved. If Pt is removed from the surface, the corrosion rate will increase as the implanted volume of the alloy is dissolved. If Pt builds up on the surface, the corrosion rate should remain low. 

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