Direct current potential drop

The Direct Current Potential Drop Method for Testing of Powder Metallurgical Parts. 

The Direct Current Potential Drop method (DCPD) has been evaluated for non destructive testing of uniaxially produced Powder Metallurgical (P/M) parts. The aim is to adapt DCPD to be functional as an ndt tool during production of parts. Defects can occur at different stages in the production cycle which means that DCPD has to be performed on components in different states and searching for different defects. 

The insertion is monopolar when the electrode functions exclusively as either the anode or the cathode and in this case it is connected with one of the poles of the electric source as shown in Figure 6.22 (a-f). Since the voltage for a single cell is very small (of the order of one to a few volts) several cells are usually connected in series and parallel combinations such that the overall potential drop corresponds to the available direct current power source. With bipolar insertions, however, there are a number of electrodes in each cell which function as anodes on one side and as cathodes on the other apart from the end electrodes, these are not directly connected to the electric source. 

Potentiostatic current sources, which allow application of a controlled overpotential to the working electrode, are used widely by electrochemists in surface kinetic studies and find increasing use in limiting-current measurements. A decrease in the reactant concentration at the electrode is directly related to the concentration overpotential, rj0 (Eq. 6), which, in principle, can be established directly by means of a potentiostat. However, the controlled overpotential is made up of several contributions, as indicated in Section III,C, and hence, the concentration overpotential is by no means defined when a given overpotential is applied its fraction of the total overpotential varies with the current in a complicated way. Only if the surface overpotential and ohmic potential drop are known to be negligible at the limiting current density can one assume that the reactant concentration at the electrode is controlled by the applied potential according to Eq. (6). 

As illustrated in Figure 26, which is a varied presentation for a single pore from the scheme shown in Figure 19, there are five possible phases in the current path in which significant potential drops may occur. The distribution of the applied potential in the different phases of the current path depends on doping type and concentration, HF concentration, current density, potential, illumination intensity and direction. The phases in the current path... 

If one looks along the strip in the direction of the current, with the magnetic lield directed downward, then, with si rips of antimony, cohall, zinc, or iron, the electric potential drop is toward the right and the effect is said to he positive. With gold, silver, platinum, nickel, bismuth, copper, and aluminum, it is toward the left, and Ihe effect is called negative. The transverse electric potential gradient per anil magnetic lield intensity per unit current density is called the Hall coefficient" for the metal in question Thus, the Hall coeflicienL is delined us... 

A potentiostatic, three-electrode circuit allows the separation of both functions physically for the reference potential, a non-polarisable electrode is used (a calomel or AglAgCl reference electrode), while the electrical-current conducting electrode is an inert metal electrode. With electrochemical, direct-current methods, the effect of this modification is limited to a reduction of the so-called IR-drop (or ohmic-drop), which is caused by... 

Concentration overpotential — The concentration overpotential of an electrode reaction at a given electrode current density is basically the difference in equilibrium potentials across the diffusion layer. More precisely, it is the potential of a reference electrode (of the same -> electrode reaction as the -> working electrode) with the interfacial concentrations which establish themselves under direct current flow, relative to the potential of an identical -> reference electrode with the concentrations of the bulk solution. From such a measured potential difference, with flowing direct current, one has to subtract the ohmic potential drop prevailing between the two electrodes. 

Polarogram — Figure. Potential program and the respective (a) direct current (DC) (staircase ramp), (b) normal pulse (NP) and (c), differential pulse (DP) polarograms of 0.1 mM Cd(NC>3)2. All measurements were in water with 0.1 M KC1, E is versus a SCE, scan rate = 2 mVs-1 and drop time = 2 s. Differential pulse height = 10 mV... 

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