1. Principle
Corrosion monitoring resistance method, that is, to measure the resistance change of the pressure vessel shell and the metal of the pressure component. If the corrosion is substantially uniform, the change in resistance is proportional to the increase in corrosion. From each reading, we can calculate the total amount of corrosion over time. This can thus also calculate the corrosion rate. If we choose components with sufficient sensitivity, we allow relatively fast changes in corrosion rate. If the corrosion rate change parameter is very important, we can also choose other more suitable methods.
Pressure Vessel Corrosion
The resistance method has been well-verified in practical applications. Its use and interpretation of results are generally straightforward. Commercial instruments can use one or more corrosion probes for periodic or continuous measurements. Therefore, it is possible to relate corrosion to process parameters. This method is one of the basic methods for corrosion monitoring during the operation of pressure vessels. And people often use it for corrosion control and also as a diagnostic tool. It allows for estimating the impact of process variations on corrosion. For example, the effect of evaluating corrosion inhibitors.
Advantage
The advantage of this method is that it can measure corrosion in liquid or gas phases. Also, unlike using the polarization resistance method, the liquid phase must be an electrolyte. The downside: Unless the corrosion is uniform, it is not easy to interpret the measurements. Therefore, this method is not suitable for pitting, stress corrosion cracking, or other localized corrosion damage. Furthermore, it shares the same weaknesses as those technologies in which test elements are typically inserted into production setups as probes. Probe behavior is unlikely to be identical to the behavior of the production unit itself.
2. Probe design
Test elements are usually in the form of filaments, tubes, or sheets. Because of this, a close dependence between the resistance of such elements and temperature can be expected. So set up a “comparison” element. Temperature compensation can then be built into the Corrosometer probe. The comparison element must be corrosion-resistant. and placed close to the corroded components, thus subject to the same temperature conditions. The comparison element is in the probe body. And it can be protected by epoxy resin or ceramics filled in the Corrosion monitoring probe. When making measurements, it acts as the second arm of the bridge. A second “comparison electrode” is usually within the probe body. It is a “check” measurement of the integrity of the fill system and the probe’s internal circuitry. For access to aggressive media, exposed test elements are usually mechanically protected with narrow-channel or similarly constructed sleeves.
Principles of Choice
Purchases of commercially available probes can be considered based on the range of alloys and types and thicknesses of the various components. When selecting a component, there is a trade-off between operating life and sensitivity. For measurements where thickness variations are small, probes with thin elements in the form of tubes or sheets work for maximum sensitivity. But the useful life of thin elements is much shorter than those of thicker filamentary elements. Filamentary elements, on the other hand, are less sensitive.
Most sensitive probes use thin metal foils as elements. This element can work in atmospheric corrosion experiments. Tubular elements often have the best overall performance for field applications. Because this element combines appropriate sensitivity with strength. It can protect the machine against mechanical damage. And can avoid flow velocity influence. This element has other advantages as well. For example, it tends to be more representative of the metallurgical condition of the pressure vessel than a sheet or filamentary elements (when pitting occurs, the effect on the cross-sectional area is relatively small). And it has a longer life than filamentary elements.
Special probes have various purposes. A particular probe was designed because a particular alloy was being tested. Or, the environment of use makes the usual probe cavity or sealing material unsuitable. Or due to the need to adopt a special geometric shape.
3. Installation and use
Many chemical and petrochemical production plants and environments can use the electrical resistance method. It can be expected that it will become a standard method and be extended to processes and other industries. But this technique also has its limitations. To obtain useful information, both probe selection and placement are critical.
a. The type of use environment.
It can measure corrosion in the liquid or gas phases. Also, the liquid phase does not have to be a good electrolyte. This technique has also worked in “solid phase” environments. For example, simulate the corrosion of steel bars in concrete. The main limitations in the application of this technique are those imposed by probe design and materials. Also, the corrosion must be substantially uniform;
b. Selection of probe type and size.
We should use sensitive thin components when the etch rate is low or when the etch rate varies frequently in relation to other parameters. Thicker elements will have a longer service life if the corrosion rate is high. The best compromise between sensitivity and lifetime depends on the environment.
c. Probe position.
Probes should in positions that simulate the required test conditions. Important parameters are temperature and flow rate. Avoid high-velocity fluids directly scouring the probe unless fretting and corrosion effects are the object of the study. Where fluid velocity may be important, more accurate results may be obtained with flush-mounted probes mounted flush against the vessel surface.
When using it, it is necessary to judge whether the probe is in a place where the corrosion rate may be the highest. Also, it is in a location that simulates the “average” corrosion behavior of the pressure vessel. This judgment is related to the reason for the monitoring performed. Multiple probes can illustrate the distribution of corrosion in various parts of the vessel;
d. Stability of gasket and packing pad.
For a corrosion monitoring probe of a given configuration, its effective working limit is usually limited by the stability of the gasket and packing material in the monitoring environment. The damage is usually in the form of leakage at the metal-nonmetal interface rather than overall deterioration. Probes filled with epoxy resin should not work in media containing chlorinated hydrocarbons, organic acids, or amines. Probes with ceramic fillers should not work in systems with a pH greater than 9 or in media where F- is present. Even this probe is work in some less corrosive environments. Its effective temperature limit should also be lower than the nominal value throughout its effective use period.
4. Sources of error
The raw data obtained by the resistance method is the resistance change of the metal element. Of course, such measurements are prone to error. However, incorrect or uninterpretable conclusions were from these data. The main reason for this is the assumption that the change in resistance of the element is proportional to the corrosion of the container. Then consider the resolution limit of the instrument probe combination and take into account that this technique cannot properly follow rapid changes in corrosion rate.
Then, the main possible sources of error are as follows:
a. Metallurgical condition of the probe element.
Differences in metallurgical conditions between probe elements and production units are usually unimportant. But sometimes it’s worth noting. Working at high temperatures, component stress relaxation can also produce errors.
b. Probe installation.
With Corrosion monitoring probes of conventional construction, the flow rate conditions experienced by the test element may be significantly different from the pressure vessel surface. The corrosion behavior of components and container surfaces will also be different. In fact, this difference in flow rate conditions is more likely to be a significant factor than metallurgical differences. But in most cases, this effect is not very great.
c. Temperature changes.
In materials with rapidly fluctuating temperatures, the accuracy of individual measurements is reduced. This requires averaging the corrosion results. Because exposed components tend to respond faster to temperature changes than compared to components that are protected. So, this happens. For example, on a distillation overhead line, if you encounter “unvaporized droplets” of vapor or liquid. then it is impossible to obtain meaningful measurements at this point
d. Local corrosion.
Corrosion Monitoring resistance probes do not readily measure localized corrosion. Such as pitting corrosion, stress corrosion cracking, etc. But we can’t make meaningful measurements of these phenomena either. However, localized damage can cause a generalized corrosion rate to be higher. If these types of corrosion occur on exposed components, the effect on resistance will not be significant at first. This effect is greater when the probe is near the end of its service life. Therefore, an apparent increase in apparent corrosion rate is suspect if localized damage phenomena are likely to occur.
e. Corrosion products.
Generally, deposits or corrosion products on components do not affect the resistance measurements. Because metals are generally much more conductive than these substances. But when the measurement is carried out at high temperatures, the situation may be different for components with some kind of scale, such as sulfide.
A new Corrosion Monitoring probe inserted into an already-used pressure vessel. The surface condition of its elements differs from the surface of the container. This is a challenge that can arise with various monitoring techniques that employ probes for measurement. The initial corrosion rate may not be the same as the corrosion rate on the vessel surface. But that changes when a film similar to that on the surface of the container forms.
f. Electrolyte resistance.
In a highly conductive electrolyte, it is possible for a certain current to flow from the electrolyte in parallel with the current flowing through the probe element. In most cases, this error is not important. But we have to take this into account when it requires precise measurements.
The resistance method is to be a good method under a wide range of conditions of use. Now it is a standard method in many industries. The sources of error we have discussed are often not obvious. However, like many monitoring techniques, it is only reliable if applied within the range of its use.
