《researchpotentiostatmanual》由会员分享，可在线阅读，更多相关《researchpotentiostatmanual（12页珍藏版）》请在金锄头文库上搜索。
1、Localised Corrosion Monitoring (LCM)This analysis has the ability to supply information relating to Localised Corrosion Monitoring in the Initiation phase, both in terms of magnitude, an indication of the total depth of penetration and the General Corrosion Rate. Even in situations where localised c
2、orrosion is beyond the initiation stage, higher than expected values from the general corrosion rate will tell the user that something is not acceptable.The LCM technique looks for negative going transients in the potential data caused by localised events such as a pitting initiation process. These
3、are converted into a quantity of Coulombs via a calibrating polarisation and a V/I transient calibration curve. From the charge passed we can calculate the quantity of metal lost by a localised corrosion process necessary to produce the same quantity of charge by an Anodic reaction. For example Fe =
4、 Fe2+ + 2e-. We assume all pits look like typical tin cans and the user can enter the ratio of width to depth. Thus it follows that if we know the charge passed by a single transient, we can calculate the penetration or depth of pit produced by such a transient. The calibrating polarisation is also
5、used to measure the general corrosion rate via the LPR method.Interestingly, although all pits do not look like tin cans in shape, the difference in penetration for a hemispherical pit to a tin can shaped pit with equal aspect ratio, is only about 10% deeper, so in many cases it is good enough to as
6、sume all pits look like tin cans.The job of this analysis software is to convert the raw data into something that is useful for the user. This is primarily done with a series of graphs.The first of these graphs presents the raw potential data with respect to time. Blocks beneath transients detected
7、in the data, indicate the magnitude and duration of the transients detected. The second Graph Type presents the data in a form of a Histogram. The Histogram presents frequency on the Y axis, or the number of times a transient occurred, against one of two scales on the X axis. The X axis scales are e
8、ither the quantity of charge passed by each transient in decade ranges, for instance x10-5 to x10-6, or allowing for the metal type and pit geometry, penetration range per transient. Interestingly the depth or width of a pit is proportional to the cube root of the charge passed. So a tenfold increas
9、e in charge passed increases the depth of a pit by a factor that is 2.15 x bigger or near enough double the depth or width.The third main graph type introduces the element of time into the data and present the data in terms of Total Penetration, Penetration Rate, and Penetration Acceleration. Most u
10、sers will be familiar with the concept of Penetration Rate v Time as this is the same type of graph as Corrosion Rate v Time. Indeed General Corrosion Rate is presented on the same graph as Penetration Rate v Time. Each graphic line that presents localised corrosion, takes its data from an individua
11、l bar in the above histogram. Mechanism of a TransientIt is worthwhile to consider the mechanism of a localised event such as pitting initiation and consider its interrelation with the general corrosion rate. They are often seen as unrelated, however we intend to show you how they are very much rela
12、ted.Consider a localised corrosion site. As the site starts to loose metal ions to solution, in some form of mass breakout, electrons are dissipated into the rest of the electrode. These electrons dont hang around, they rapidly spread about all over the surface of the electrode. This general excess
13、of electrons leads to a dropping of the potential of the electrode. If you consider yourself as part of the electrodes surface that is not corroding locally, you will experience a flow of electrons into your little patch. This extra supply of electrons will persuade you to up the rate of production
14、of the Cathodic manufacturing process, where electrons and ions are combined. As the supply of electrons dwindles, this will encourage you to decrease the rate of production. As an external observers with our voltmeter between a stable reference electrode and the electrode that is experiencing these
15、 localised events, we will see the localised event as a decrease in potential due to the excess of electrons, followed by an increase in potential as the area undergoing the localised event and the supply of electrons dries up. This effect is known as a transient. Transients can be quite short, perh
16、aps just a few seconds, or last several hours.Consider now applying a polarisation to the electrode via a Potentiostat. The potential polarisation is the same magnitude and time duration as that monitored during the natural pitting event. In this case electrons are supplied by a wire connected to the test electrode as opposed to a naturally occurring pit.If as before, you consider yourself as part of the electrodes surfac
