The electrochemical emission spectroscopy (EES) technique is a newly developed on-line corrosion monitoring technique, which is capable of detecting localized corrosion as well as measuring uniform corrosion. The main difference between this technique and the traditional electrochemical noise technique is the use of an inert microelectrode to sense the current signal from a working electrode instead of using two identical working electrodes to generate the current signal. In this paper, the ability of the EES technique is evaluated for pitting corrosion monitoring. Pitting corrosion is generated on three systems: stainless steel types 304 and 316 in aerated 3% NaCl solution at 50C and stainless steel type 304 in 6% FeClq solution at room temperature. In all cases, the on-set of pitting corrosion is clearly indicated in both potential and current spectrums. A parameter called the corrosion admittance, which is defined in the EES technique, is capable of indicating instantaneous localized corrosion activities.
Pitting corrosion is extremely localized and can result in deep holes in the metal. The total metal loss due to pitting corrosion maybe negligible, but a small hole, for instance, on the bottom of a liquid storage vessel makes the vessel completely useless. This is why pitting corrosion is one of the most destructive and dangerous types of corrosion. It should be closely monitored whenever possible so that the problem can be solved in its earliest stage. So far, very few on-line corrosion monitoring techniques are commercially available to detect the on-set of pitting corrosion reliably. Nevertheless, the electrochemical noise (EN) technique, which is being developed rapidly in recent years, has shed some light on this aspect.
The EN technique is often employed to investigate the mechanism of pitting corrosion. Since it is easier to generate pits in the potential controlled condition than in the freely corroding condition, many EN measurements reported in the literature concerning pitting corrosion are carried out in a potentiostatic model-9. However, France and Greene pointed out that controlled potential tests and conventional corrosion tests under a freely corroding condition might not yield similar rates or types of localized attacks under identical environmental conditions. Therefore, it is fundamental to investigate the mechanism of pitting corrosion under a freely corroding condition as it is in reality.
Under freely corroding conditions, the potential noise can be easily measured. In some cases, the potential noise along generates enough information concerning the on-set of pitting corrosion. For instance, during the pitting corrosion test of iron in a neutral aerated sodium chloride solution, a special potential noise pattern - a sudden potential drop followed by a slow recovery ?]-?5-can be observed indicating the initiation of pitting corrosion. It is generally accepted that such a unique potential transient is caused by the breakdown of passive film and the reactivation afterwards, although there are different models137iG?17to explain the origin of such potential transients. However, in most cases, there are no unambiguous potential noise patterns to distinguish localized corrosion from uniform corrosion. The chance of detecting pitting corrosion may be enhanced if the current noise is also evaluated besides the potential noise. To obtain the current noise as well as the potential noise, two identical working electrodes are used. The potential noise is measured from one of the working electrodes with respect to a reference electrode, and the current noise is measured between two working electrodes with a zero resistance ammeter. However,
