Corrosion rate continued monitoring

Rowlands and Bentley have provided an account of the possibilities for continuously monitoring corrosion rates by polarisation resistance measurements, and they also describe the development of a commercial instrument, which uses low-frequency square-wave current to polarise the test specimens. 

Polarization resistance (Rp) techniques can be used to continuously monitor the instantaneous corrosion rate of a metal. Mansfeld provided a thorough review of the use of the polarization resistance technique for the measurement of corrosion currents. R is defined as ... 

There are many other techniques for measuring corrosion. Some of these may rely on chemical solution analysis or physical measurement of metal loss. One widely used additional technique is provided by ASTM G 96, Practice for On-Line Monitoring of Corrosion in Plant Equipment (Electrical and Electrochemical Methods). This guide covers two basic procedures. In one, a sample of the material, usually in the form of a continuous wire, is immersed into the environment of interest (liquid, air, solid, or multiphase). The electrical resistance through the wire is determined. As corrosion consumes the cross section of the wire, the resistance increases proportionately. The second procedure uses the polarization resistance technique described in ASTM G 59 to determine the corrosion rate in the environment. 

TLA was developed in the United Kingdom [20,27]. The surface of a sample (which can be a coupon or a section or tube or pipe) is irradiated to make it slightly radioactive. Two samples are irradiated. One sample is protected from corrosion the second is exposed to the corrosive fluid, in this case the water system. The radioactivity of each of two samples is monitored 2is a function of time. There will be a radioactive decay curve characteristic of the protected sample and a second decay curve for the exposed sample. The difference between the two curves indicates metal loss, which can be converted to corrosion rate. This technique has not been broadly used, but some field use has been reported [27,22]. It shows promise of being an automated coupon which provides a continuous record of corrosion. Figure 4 shows a pipe specimen with an activated area, and Fig. 5 shows data generated in an inhibited cooling tower system. 

This recently reported [26] technique is similar to the Synthetic Crevice approach. In this method, two parallel flow channels are operated at different water flow rates. The low flow channel becomes an active anode, which is connected through a zero resistance ammeter to the high flow channel, which acts as a cathode. The current flow then indicates the corrosion rate of the anodic surface. This technique shows promise as a continuous on-line instrumental monitor that will show changes in corrosion rate under deposits. 

Table 3 represents an example of a comparative study of weight loss and LPR corrosion rates in inhibited hydrochloric acid at 200°F [44]. The corrosion rate was continuously monitored and plotted as a function of time. Subsequently, the area under the curve was integrated, converted to weight loss, and compared with the actual weight loss. In this manner a ratio, R, of actual weight loss versus LPR... 

Continuous, on-line corrosion monitoring can be used to operate FGD systems at optimal efficiency while avoiding high corrosion rates. Conventional corrosion monitors are designed for use in bulk liquids and do not function satisfactorily in the thin condensate films that form on the surfaces of FGD ducts. Recent EPRI research has made use of an advanced electrochemical method of corrosion surveillance developed by the Corrosion and Protection Centre Industrial Services (CAPCIS) in England [30]. This method allows on-line monitoring of corrosion activity in either thin condensate films or bulk liquids. EPRI introduced the CAPCIS system to the U.S. electric utility industry in 1985, sponsoring several field demonstrations. 

Thin layer activation coupons have been used to continuously monitor corrosion rates in continuous digesters, and to verify the effectiveness of anodic protection systems [180], The surface of a thin layer activation coupon is irradiated to a shallow depth and monitoring is performed with a Geiger counter 6x>m outside the digester wall. Subtracting for effects of half-life decay, the corrosion rate can be estimated from the decreased activity of the coupon. 

Atmospheric corrosion involves a series of processes with periods of high corrosion rates interrupted by periods of negligible corrosion rate. For a deeper understanding of atmospheric corrosion, there is a need for more sophisticated techniques for measuring instant corrosion rates coupled with monitoring the deposition of the most important corrosion stimulants. Activities in these and other areas are presently being carried out, and it is anticipated that atmospheric corrosion will continue to develop into a multidisciplinary scientific field. 

In critical situations, where high corrosion rates are anticipated over a small area, solid coupled probes may be welded directly onto the pipe or vessel at the suspect locations in order to permit continuous monitoring of wall thickness. The proposed welding procedures should be submitted for approval prior to the probe attachment. 

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