- A Technique for measuring both Localised Corrosion and General Corrosion (*patents pending)

*LCM™ is a new technique developed in partnership between ACM Instruments and Baker Petrolite. It is revolutionary for electrochemical testing for localised corrosion monitoring and goes about solving the many pitfalls of electrochemical current & voltage noise tests and the electrode scanning techniques. Below is a good explanation of the technique, but for questions and further discussion you should contact us.

Abstract
This technique enables the user to monitor localised corrosion such as that caused by pitting as well as general corrosion rates with respect to time. The technique primarily monitors Potential and then periodically uses a special LPR type polarisation. Data from the polarisation is used to calibrate naturally occurring transients in the Potential data into Current data. A simple case uses a Ohms Law relationship. A more elaborate method of calibration uses a V/I graph that takes into account the non linearity of Electrochemistry, especially where large transients are involved. It is then a matter of simple mathematics to turn the number of Coulombs passed by a Potential transient into metal loss and pit magnitude.  The technique is known as LCM™, or Localised Corrosion Monitoring, a development between Baker Petrolite and ourselves.

Theory
Potential Monitoring by itself does give information as to localised activity on the electrodes surface. It is possible to detect Potential Transients in the data relating to a localised event. However the technique is blind in that there is no current information. A bit like trying to measure the speed of a car from the rev counter alone, without knowing which gear the car is in. It is necessary to have current information as well.

Consider a metal electrode with a single pit site. When this pit goes active, the corrosion activity throws electrons into the rest of the electrode from the Pit. This reduces the potential of the electrode which has a Cathodic Protection type effect, helping to repassivate an active pit and generally stopping other pit sites from going active. The electrons are used up by other areas of the electrode and the potential slowly climbs back up again.

Now consider an area of the electrode that is not a pit site.  During a pitting event, this area experiences a drop in potential caused by an excess of electrons that increases the Cathodic Reaction. The passive area of the electrode can not possibly know where the electrons came from. It does not know the electrons came from a pit. As far as the passive area is concerned, the electrons could just as easily have come from a Potentiostat, it will respond in the same way.

Thus, we can use a Potentiostat to Polarise an electrode using the same Potential Transient signal as produced by the naturally occurring pit. In this way we can work out as exact as possible the number of coulombs of current produced by the pit. Such a method takes into account fully the non linearity of Electrochemistry, no matter if the system is diffusion controlled or activation controlled. It also takes into account charge and discharge characteristics of the surface as the same polarisation with the same sweep rates is used as the naturally occurring transient. There is a problem with this method however, in that it assumes that no naturally occurring pits are going to activate during the calibration phase. This is largely the case during the initial negative part of the polarisation, as the Polarisation will tend to protect the Electrode by Cathodic Protection from Pitting events. If a natural pit is about to fire off, then this will typically be put on hold by the negative going polarisation. As the polarisation is returning towards the initial Rest Potential, pits are more likely to fire off causing errors in the calibration data.
To compensate for these errors, only the negative going side of the calibration sweep is applied to the electrode. The electrode is then isolated at the end of the polarisation and left to float whilst potential measurements are again taken.

To calibrate the Potential Transients using this method we can either assume a linear calibration value of Rp, or produce a calibration V - I table that takes into account non linearity of electrochemistry. If the V - I table is used, then the magnitude of the polarisation should be of the same magnitude as the previous maximum amplitude transient recorded since the last calibration phase. It will be a matter of choice as to which calibration method is used. A simple Rp value does not need to polarise the electrode by much, which has traditional appeal. Personally I like to use the full calibration table, even if the polarisation is as much as 100mV. A polarisation at 100mV may give a polarisation current that is 10 times more than a simple Rp value would suggest obtained by a polarisation of just 5mV. In this case a simple Rp method would seriously underestimate the magnitude of the current caused by the pitting event.

Having got a method to monitor and measure the magnitude of individual localised events, it is relatively easy to separate the transients using a Histogram, showing frequency and magnitude.  Further presentation of the data leads onto distance, speed and acceleration, or total pit depth, pitting rate and rate of change of pitting rate. I have made some assumptions here, as to the profile of the pits and the number of pits. However the method, especially with knowledge of the system under test will be able to give useful pitting information together with the general corrosion rate obtained from the calibration sweeps.