August, 2021
There are different methods of measuring the corrosion rate. No measurement method is universal. Each of them has its advantages and disadvantages.
All methods of measuring the corrosion rate are divided into two categories:
- The first category includes methods that are based on the assessment of changes in dimensions and mass of a metal sample;
- The second category includes methods based on the control of electrochemical reactions occurring on the surface of the corroding metal.
The methods of the first category are simpler. They can be used in any environment because it does not depend on its electrical conductivity. The list of these methods includes first of all samples-witnesses of corrosion or corrosion coupons.
The method of witness samples, or the gravimetric method, consists in the installation inside the pipeline of metal samples that are present in the circuit. These samples have known dimensions and mass and are kept in the coolant flow inside the pipeline for a certain time interval. The average corrosion rate is calculated based on the sample mass loss during this time. Based on the appearance of the sample, the nature of corrosion is also assessed and deposits are analyzed.
Samples-witnesses of corrosion – the most a commonly used monitoring technique.
Witness samples are relatively cheap, very useful for evaluating the inhibition of the coolant, as well as in those places where the use of electrical devices is impossible for some reason.
In most systems, technological conditions are not constant. Pressure, temperature, fluid flow regime can vary within the same system in wide ranges. Accordingly, the aggressiveness of the liquid will not be the same for all places in the circuit. In any system, there will necessarily be the most dangerous places, which are characterized by the highest aggressiveness, which means the highest rate of corrosion.
Determining the most corrosive and dangerous places of the circuit as monitoring places is one of the most important elements of building a workable monitoring system. It is obvious that measuring the corrosion rate in the wrong place can give incorrect or even false information. Reliability can be increased by increasing the number of points at which measurements are made.
The material of witness samples and corrosion rate sensors installed in certain monitoring locations may have some differences from the material of which the pipeline is made. In addition, the hydrodynamic conditions in which the witness sample is located during the exposure period in the pipeline can also be significantly different from the conditions in which the inner surface of the pipeline wall operates.
Together, these differences lead to differences in the corrosion mechanism of the pipeline and the reference sample. Because of this, the corrosion rate measured by witness samples is not necessarily exactly equal to the corrosion rate of the pipeline wall itself. However, the use of samples is necessary, because their behavior during exposure in the coolant shows the main trends in the development of corrosion damage. On the basis of these data, it is possible to evaluate the effectiveness of inhibition and make predictions.
In order for discrepancies in estimates and forecasts to be minimal, special attention should be paid to the execution of witness samples. For this, it is necessary that their constructive implementation and access to the flow ensure being in the most aggressive phase and similar hydrodynamic conditions.
