Corrosion Monitoring Techniques

A wide range of corrosion monitoring techniques is now available, allowing determination of total corrosion, corrosion rate, corrosion state, analytical determination of corrosion product or active pieces, detection of defects, or changes in physical parameters. Associated costs can be small where simple instrumentation and a few measurements are appropriate, but in some cases may be extremely costly and require expert skiUs. 

Much of the progress made in the past few years has been due to advances in electronics that have allowed multi-probe measurement and recording at a tolerable cost. Instantaneous feedback of corrosion information can be obtained, from various parts of the plant, that can be fed to the plant control room and/or plant computer to permit control of the necessary process variable to provide corrosion control. Table 8.1 indicates corrosion monitoring techniques available, some of which are described in more detail. 

---

The failure of plant by corrosion can be gradual or catastrophic. Gradual failure has few implications for safety providing it is monitored. Direct corrosion-monitoring techniques are described in Section 53.8. Indirectly, the correct interpretation of records relating to metal contamination of products or the loss of efficiency of heat exchangers, etc. can provide useful information. 

A number of corrosion-monitoring techniques based on electrochemical principles are available. These give an indication of the instantaneous corrosion rate, which is of use when changing process conditions create a variety of corrosion effects at different times in a plant. Some... 

Asher, J., Conlon, T. W., Tofield, B. C. and Wilkins, N. J. M., Thin Layer Activation-A New Plant Corrosion Monitoring Technique , Proc. Conf. On-Line Surveillance and Monitoring , entitled On-Line Monitoring of Continuous Plants, Ellis Horwood, London (1983)... 

Some of the corrosion monitoring techniques along with the advantages and disadvantages are given in Table 4.49. 

TABLE 4.49 Corrosion Monitoring Techniques with Advantages and Disadvantages... 

NACE Publication ID 177 [7] describes corrosion monitoring techniques for drilling operations. Some components of drUUng equipment may be fabricated from corrosion resistant alloy (CRA). CRA selection may involve laboratory testing, such as that described below for producing weUs. 

The particular corrosion monitoring techniques selected depend upon their applicability to the system and the information being sought. Some techniques provide information that is effectively instantaneous. Other techniques provide... 

Most corrosion monitoring techniques are best suited to situations where corrosion is of a general nature, but some techniques provide at least some information on localized attack, such as pitting. 

An overview of the various corrosion monitoring techniques used in cooling water systems is shown in Tables 1-3. The essential information is listed in Table 1, whereas Table 2 lists some of the characteristics of the monitoring techniques. Table 3 is a summary chart of the various monitoring techniques overall. 

Numerous corrosion monitoring techniques and associated sensors are available. All of these techniques have certain advantages and disadvantages, which are discussed in detail in Sec. 6.4.4. There are many pitfalls in selecting suitable techniques, and the advice of a corrosion monitoring expert is usually required. An algorithm, described by Cooper, for evaluating the suitability of two commonly utilized techniques, LPR (one of the electrochemical techniques) and ER (electrical resistance), is shown in Fig. 6.14. 

Figure 6.14 Algorithm for suitability of ER and LPR corrosion monitoring techniques. (Adapted from Cooper.

To the uninitiated engineer, the plethora of available corrosion monitoring techniques can be overwhelming in the absence of a categorization scheme. The first classification can be to separate direct from indirect techniques. Direct techniques measure parameters that are directly associated with corrosion processes. Indirect techniques measure parameters that are only indirectly related to corrosion damage. For example, measurements of potentials and current flow directly associated with corrosion reactions in the linear polarization resistance technique represent a direct corrosion rate measurement. The measurement of the corrosion potential only is an indirect method, as there is at best an indirect relationship between this potential and the severity of corrosion damage. 

TABLE 6.7 Advantages and Disadvantages of Corrosion Monitoring Techniques Continued) o ... 

Indirect techniques. A plethora of indirect corrosion monitoring techniques for different modes and submodes of corrosion damage is available. A multidisciplinary science and engineering team approach is often required for implementation. For example, the indirect monitoring of microbiologically induced corrosion in water systems typically requires expertise and laboratory infrastructure from the fields of corrosion engineering, microbiology, and chemistiy. 

The first requirement can be met with real-time corrosion monitoring systems, provided that the monitoring techniques selected are sufficiently sensitive to respond rapidly to changes in the process conditions. Corrosion monitoring techniques (such as coupons) that yield only retrospective, cumulative corrosion damage data are not suitable for this purpose. 

Rebar potential measurements. The simplest electrochemical rebar corrosion monitoring technique is measurement of the corrosion potential. A measurement procedure and data interpretation procedure are described in the ASTM C876 standard. The basis of this technique is that the corrosion potential of the rebar will shift in the negative direction if the surface changes from the passive to the actively corroding state. A simplified interpretation of the potential readings is presented in Table 6.8. 

A corrosion monitoring technique rarely gives wrong information, unless the equipment used is faulty. Nonsense results arise because the information is correct, but irrelevant in the corrosion sense. The polarization resistance method, for example, measures the combined rate of any electrochemical reactions at the surface of the test sample. If the main reactions are corrosion, the rate measured is the corrosion rate. If however, other reactions are possible at rates that are comparable or greater, the measured rate includes the other reactions. 

Table 8.2 Characteristics of corrosion monitoring techniques Technique Time for individual measurement... 

---