Physical implications of an effective activation energy for arc erosion on oxidised cathodes

Abstract

An effective energy of activation for arc erosion has been deduced from mass loss rates observed on oxidised copper cathode surfaces exposed to arcs of long duration rotating in atmospheric pressure air. An analysis of rate processes expected to be associated with the observed low value for the effective activation energy excludes the involvement of ion migration or the filamentation of pure copper across the oxide layer in the cathode erosion process in an oxidising medium. Independent confirmation is obtained from a comparison of mass loss rates on oxidised cathodes differing in metallic composition, but subject to common operating conditions for pulsed stationary arcs in air. A causative correlation is indicated between erosion rates and properties of the pure metal in the cathodes, rather than with properties of their corresponding oxides. This trend further implies that oxide contributions to electrode erosion occur on timescales which preclude commonly accepted averaging procedures over the lifetime of the arc.

A theoretical and experimental investigation of copper electrode erosion in electric arc heaters: I. The thermophysical model

“…Detailed experimental investigations of electrode erosion and its relation to different physical-chemical phenomena occurring on the surface of copper cathodes were carried out by Guile et al [9][10][11][12][13][14][15]. These authors came to the conclusion that the main mechanism influencing the erosion, for steady state operating conditions, was associated with the properties of the pure metal substrate, rather than with those of the oxide overlayers.…”

“…Guile et al concluded that a correlation can be made between the logarithm of the mass erosion rate and the inverse electrode temperature for pure electrode materials. They proposed the use of an Arrhenius-type relationship for the calculation of the mass erosion rate [15] dm/dτ = A exp −( G e /kNT ) (1) where dm/dτ is the mass erosion rate, A is the 'rate factor', G e is the erosion reaction activation energy, k is Boltzmann's constant, N is Avogadro's constant and T is the cathode surface temperature. G e , in the formula above, was found by Guile et al to be rather close to the activation energy for the interatomic bond 'loosening' (fusion) of the substrate material.…”

Investigation of the parameters of a nonstationary arc spot on a copper cathode by the thermospectroscopic method. I. Current density

“…To understand this we analyzed the possible influence of the oxide layers on spectroscopic recording of the copper line, too. As has been shown in [25], the oxides CuO and Cu 2 O are always present on the copper surface in air at atmospheric pressure. The stability of Cu 2 O decreases at temperatures higher than 250 o C. Above this temperature, we have decomposition of the Cu 2 O oxide to form a more stable CuO and to release atomic copper, i.e., Cu 2 O → CuO + Cu.…”

“…Table 1 (composed according to the data of [25]) gives the activation energies of elementary processes that can be involved in the erosion of the copper cathode. It is seen that the total expenditure of energy in fusing and subsequently evaporating copper (which refers to the basic cathode material) are very close to the expenditure in decomposing the oxide Cu 2 O → CuO + Cu on the surface.…”

Erosion of the Cold Electrode as an Ablation Process in Unsteady Arc Spotsa

“…Essentially, unsteady thermal effects occur on the surface of the electrode within the fast moving arc spot. Guile et al 2 were the first to recognize that the fusion of the electrode material plays the dominant role in the erosion of cold electrodes. It has also been shown 3 that the dynamics of the motion of the arc spot over the electrode surface (either in a continuous or in a jumping‐like motion) is an important factor in determining the erosion behavior.…”