Alloys potential measurements

Fig. 21.2 Galvanic series showing ranges of potentials of metals and alloys in flowing hot domestic water at 71°C (Long Island, N.Y.). Potentials measured weekly for three months and then monthly for a period of ten months. (After Butler, G. and Ison, H. C. K., Corrosion and its Prevention in Water, Leonard Hill, London (1966))...

Although potential measurements are used primarily to determine activities of electrolytes, such measurements can also be used to obtain information on activities of nonelectrolytes. In particular, the activities of components of alloys, which are solid solutions, can be calculated from the potentials of cells such as the following for lead amalgam ... 

Figure 7.5 Effect of antimony on the residual potential (measured after first and fifth cycles) of Sb-Se alloy.

At the interface alloy/electrolyte the concentration of the alloy components must be the same as in the bulk of the alloy. In aqueous electrolytes or in molten salts the less noble component might dissolve in the electrolyte and thus lead to concentration failures. This could be avoided by keeping up a small cathodic current preventing dissolution. The current may not distort the potential measurement. 

For these reasons, potential measurements of alloys are frequently made at higher temperatures with solid electrolytes or by using molten salt electrolytes. Measurements in aqueous electrolytes and at room temperatures were mostly restricted to mercury alloy (amalgam) electrodes. 

Figure 3.8 (a) Ceramic cup for potential measurements in the alloy system Ag u, . (1) Ag plate with connecting Pt wire (8), (2) Ag electrode, (3) glass film melted on the Ag electrode (thickness ca. 0.1 mm), (4) Ag Ati alloy electrode with platinum contact wire (8) in a ceramic cup, (5) pressed on the electrolyte layer by the cover of the ceramic cup (6), and an alumina tube (7). (b) Apparatus used for potential measurements in the alloy system Ag u, with the ceramic cup (5) shown in detail in Figure 3.8a. (7) alumina tube, (8) platinum wire connections to the electrodes, (9) wire to hold the ceramic cup, and (10) thermocouple. ... 

Figure 3.23 Potential measurements of CuZn alloys at room temperatures. The bracketed Une was calculated with the data of Olander. ...

The current density of various materials was determined as a function of the potential difference between the anodic and cathodic branches of the current potential curves in 0.9% NaCl with a stable redox system Fe (CN)6" 7Fe (CN) [1]. The saline solution containing this redox system had a resting potential closely resembling that of a tissue culture fluid which has a redox potential of 400 mV. Ti and its alloys Ta and Nb exhibit a better resistance than the stainless steel AISI 316L and a wrought CoNiCr alloy. The same ranking can be observed during the measurement of the polarization resistance of the different materials [1]. Breakdown potential measurements of various implant materials in... 

Two types of solutions were examined for Ag-Sn alloy electrodeposition [18] sulfate solution containing thiourea as a complexing agent for Ag ions and pyrophosphate and iodide solution which form a stable complex with both Ag and Sn ions. In sulfate solution, Sn was a normal metal, while Ag was intermediate one due to formation of complexes with thiourea and iodide ions. In pyrophosphate solution, both metals belonged to intermediate ones due to formation of complexes with pyrophosphate and iodide ions. The polarization curves for alloy electrodeposition measured by the potential sweep method (v = 0.5 mV s ) in sulfate and pyrophosphate-iodide solutions are shown in Fig. 7.15. A current density rapidly increased at about —0.07 V versus NHE with the current density plateau up to about —0.27 V versus NHE corresponding to the pure Ag electrodeposition in the sulfate solution. Additional current density increase and plateau at more negative potentials correspond to the alloy electrodeposition ( ). Similar behavior is detected for pyrophosphate-iodide solution ((D). In both electrolytes, electrodeposition of both metals was suppressed due to complexes formation. The content of Ag in both cases abruptly decreased with the increase of electrodeposition current density. 

Due to its simplicity, open circuit corrosion potential measurements (see Chapter 20 of this manual) have been used in MIC studies for many years. Corrosion potential measurements as a function of time have been used to obtain information on MIC of steel, aluminum alloys, stainless steels, and other passive alloys. By itself, the corrosion potential of plain carbon and low alloy steels indicates very little because these steels can corrode at a wide range of potentials. Rapid changes in the corrosion potential, however, can be used to indicate cathodic depolarization, or an enhancement of the anodic reaction, or to the formation of a semi-protective film. 

Base metals and alloys used in medical and dental devices are corrosion-resistant due to the presence of an oxide film on the surface that is protective [49]. These materials are not corrosion-resistant initially as is evident from their positions in the electromotive force series. The galvanic series, a listing of electrode potentials measured in seawater, indicates the changes in the noble and active tendencies of these materials in practical use for this given environment. Passivity is dependent on environmental factors such as solution pH, temperature, ions, oxygen, etc. Some ways of minimizing corrosion of these materials follow. Others are given in the discussion of the types of corrosion that can occur. 

In the 2XXX system, corrosion potential measurements were conducted on high-purity aluminum and various binary Al-Cu alloys up to and beyond the limit of solid solubility of 5.65% copper. Also, it was possible to produce large particles of the stoichiometric precipitates CuAla and CuMgAla so that their corrosion potentials could be measured. Figure 19.2 is a plot of the corrosion potential of the various materials as a function of copper content, showing that significant potential differences of as much as... 

The galvanic series is an arrangement of metals and alloys in order of their corrosion potentials in the environment. The potentials of the metals and alloys are measured in the desired environments, with the most noble (positive) at the top and the most active at the bottom. Table 2.5 shows a galvanic series of some commercial metals and alloys in seawater. The potentials are measured in seawater by means of a saturated calomel electrode and all potentials... 

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