Online corrosion monitors replace conventional laboratory-based analysis and allow real-time corrosion monitoring as part of everyday process control. The two-

wire, 4 to 20 mA transmitters with integrated Hart protocol and built-in functionality detect general and localized corrosion. The industry-standard casing contains technologies that put plant operators in the position to react to corrosion before it can lead to far-reaching consequences. This technology represents an effective means for cost reductions in power plants or closed-circuit cooling systems.

While these devices are manufactured in both electrical resistance and linear polarization types, this article will focus on linear polarization corrosion monitoring technology.

Monitoring General and Pitting Corrosion

The term corrosion describes a condition that nearly everyone has seen on a rusty car or fouled electric contacts that do not function. Objects made of steel often remain fully intact even as they corrode: because the metal under the paint has turned into a brown, powdery material, the body of an old car will eventually collapse completely. Yet this is only one type of corrosion.

According to the German Industry Standard DIN 50900 part I, corrosion describes the reaction of metal to its environment, resulting in measurable changes in the material. Corrosion can change the characteristics of a metallic object or the complete system it is part of, thereby degrading its function. In most cases, an electrochemical reaction leads to the corrosion, but it also can be the result of a chemical or metal physical reaction. Clearly, corrosion means far more than simply rust.

In some industrial processes such as in power plants, damages caused by corrosion represent a tremendous cost factor. For example, steam generators are used in virtually any industry. Just about any manufacturing plant operates at least one generator of this type. Yet, all steam generators are subject to the damaging and thereby costly effects of corrosion. It is estimated that corrosion is responsible for annual costs of more than $273 billion. This includes areas such as public utility companies ($47.5 billion) as well as the paper and cellulose industry ($6 billion).

Steam Made from Pure Water

Industrial plants generate electricity for their own use, and public power plants produce electricity for public use. Both use power generators of differing dimensions. However, all of them need the same type of pure water.

Pure water is made from raw water that is passed through a number of filtering and demineralization stages. This continuous flow of purified water is fed into the steam generator, which produces the steam that drives the turbine used to generate electricity. Part of the steam is used to preheat the pure water before it enters the steam generator. After passing through the generator, the steam turns into condensate, which then is collected and fed back into the cycle together with newly purified water. The overall construction of such a power plant depends on the thermodynamic characteristics typical of the respective plant.

Energy generation requires high investments into technology. System maintenance is vitally important to keep profitability, safety and lifecycle at a high level. Pure water has corrosive effects when it comes in contact with the steam generator and the condensing system. In addition, leaks may allow chemicals to enter the system, thereby increasing the corrosive effect. For example, a leaking condenser might lead to untreated cooling water mixing with the purified water. Leaking seals at the turbines and pumps might allow oxygen to enter the feeding water and increase its aggressive potential. In order to prevent such risks, special corrosion inhibitors usually are added to the feeding water.

To protect the capital investment of the power plant, effective chemical-physical treatment of the feeding water is necessary, especially during maintenance work at the boiler and condensing systems. Poor quality water fed into the boiler may lead to severe problems such as tindering, pitting or corrosion of the components inside the boiler and condensing systems. The danger of corrosion can be limited by adding special chemical additives to the purified feeding water. Most plant operators measure the water quality by means of conductivity sensors or pH probes. However, while such measurements are suitable to sufficiently analyze the water quality, they provide no information as to the condition of the piping of the condensing systems.

Practical Solution

Using the 4 to 20 mA real-time signal of a multivariable corrosion monitoring transmitter, plant operators are able to compare historical corrosion data with up-to-date measurements. In this way, they can quickly determine:

•   Whether the water quality has changed.

•   Whether there are changes in the chemical setup of the water.

•   Whether the corrosion inhibitors perform correctly.

All these conditions affect piping corrosion and can be detected and monitored efficiently with help of a multivariable corrosion monitor.

Following the principles of proactive maintenance, the plant operator is able to prepare and schedule the exchange of components affected by corrosion before the effects of corrosion lead to damage. The performance of multivariable corrosion monitors assists the operator in monitoring both general and local corrosion. In particular, local corrosion can lead to severe damage if it is not detected at an early stage. This type of corrosion is able to actually puncture a pipe within a short time. Yet, it can be counteracted effectively if corrective measures are taken before it is too late. Multivariable corrosion monitors make it possible to monitor and control corrosion just like any other process parameter.

Real-Time Monitoring of the Corrosion Rate

At the core of online corrosion monitors are modern algorithms and data analysis techniques to provide exact measurement of corrosion rate and local corrosion (pitting). To measure the general corrosion rate, the system determines the linear polarization resistance (LPR) using a generally accepted industry standard. This method is optimized further by a harmonic distortion analysis (HDA). During the measuring cycle, the corrosion sensor also performs an electrochemical noise (ECN) measurement, which allows a dependable determination of pitting. At the end of each measuring cycle, the respective corrosion rate and the pitting value are calculated and provided via Hart or as a 4 to 20 mA signal.

The linear polarization resistance method is the generally accepted industry standard used for general corrosion monitoring. It is based upon the Stern-Geary relationship, which describes the general relationship between the polarization resistance and the corrosion current. Provided the corrosion current is known, calculation of the corrosion rate is a simple process using the Stern-Geary B value constant. The polarization resistance can be determined on the basis of the measured current and the stimulation potential of the measuring electrodes (provided their potential is sufficiently low). Harmonic distortion analysis allows calculation of the general corrosion rate without the need to know an exact B value, which is an elementary part of the Stearn-Geary relationship. In this way, feeding a low-frequency sine-wave voltage to the measuring electrodes and analyzing the distortions allows determination of the general corrosion rate.

This corrosion monitoring technology combines both procedures in order to achieve quick, reliable measuring results. This includes determination of the B value. In order to further increase precision, conductivity measurements also are included into the calculation. The resulting value provides additional information concerning the state of the electrodes. Additionally, the electrochemical noise method is used to measure the intensity of local corrosion. Electrochemical noise describes the measurement of spontaneous potential fluctuations that are generated randomly at the corroding interface between metal and solution. Statistic analysis of the measured current allows the determination of a pitting factor, which is an indicator of the speed and intensity of local corrosion.

The standard probes used for corrosion detection consist of three electrodes. One of them induces a low-power signal while the others measure the resulting potential and current. In order to gain precise measuring results, these electrodes need to be made of the same material as the piping or container to be monitored. The electrodes are placed immediately within the flow of the corrosive media and are induced with a weak signal. Within only a few minutes, this signal is monitored and analyzed by the transmitter to determine the rate of corrosion. As a result, service technicians are provided with the needed information to schedule repair and service work according to the actual need, putting them into a position to react before corrosion has gone too far and degrades the ongoing process.

 In this way, online corrosion monitoring technology contributes to saving time and costs. It also is the basis for proactive maintenance and makes corrosion monitoring part of the daily routine. 

Related: Monitoring Corrosion in Aqueous and Non-Aqueous Liquid Streams