Corrosion rate measurements

The corrosion rate is probably the nearest the engineer will get with currently available technology to measuring the rate of deterioration. There are various ways of measuring the rate of corrosion, including AC Impedance and electrochemical noise (Dawson, 1983). However, these techniques are not suitable for use in the field for application to the corrosion of steel in concrete so this section will concentrate on linear polarization, also known as polarization resistance or LPR, and will discuss various macrocell or galvanic current measurement techniques. 

It is possible, with varying degrees of accuracy, to measure the amount of steel dissolving and forming oxide (rust). This is done direcdy as a measurement of the electric current generated by the anodic reaction  

The linear polarization technique (also known as polarization resistance or conflated to LPR) requires us to polarize the steel with an electric current and monitor its effect on the reference electrode potential. It is carried out 

Rp gives the technique its alternative name of polarization resistance . The change in potential must be kept to less than 20 mV or so for the equation to be valid and remain linear (hence the name linear polarization ). Also, there is an iR drop in the circuit. This is the voltage that exists because a current is flowing through concrete which has a finite electrical resistance. This is also referred to as the solution resistance . This can be measured by switching off the current at some point during the measurement process so that the potential without the iR drop is measured. The iR drop is discussed further in the section on cathodic protection. 

The measurement is made in one of two ways. Either steadily fixed levels of current are applied and the potential monitored (galvanostatic), or the current increased to achieve one or more target potentials (potentiostatic). In both cases allowances for the iR drop (solution resistance) may be made. A plot of change in current vs. change in potential gives a gradient of the polarization resistance Rp (equation 4.2) to calculate the steel section loss rate. 

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Linear polarization re.slstance probe.s. LPR probes are more recent in origin, and are steadily gaining in use. These probes work on a principle outlined in an ASTM guide on making polarization resistance measurements, providing instantaneous corrosion rate measurements (G59, Standard Practice for Conducting Potentiodynamic Polarization Resistance Measurements ). 

Use of Corrosion Probes The major use of corrosion monitoring probes is to measure the corrosion rate in the plant or the field. In addition to corrosion-rate measurements, corrosion probes can be used to detect process upsets that may change the corrosion resistance of the equipment of interest. This is usually equally as important a measurement as corrosion rate since a change in the process conditions can lead to dramatic changes in the corrosion rate. 

Corrosion Rate Measurements Determining a corrosion rate from measured parameters (such as mass loss, current, or electrical potential) depends on converting the measurements into a corrosion rate by use of relationships such as Faradays law. 

Information on the process reaction conditions may be impoi tant to prolonging the lifetime of process equipment. Techniques such as EIS and potentiodynamic polarization can provide just such information without being tied to a specific corrosion-rate measurement. 

Analysis of biological activity does not automatically lead directly to a corrosion-rate measurement. However, with detection and correlation with process conditions, such information may also lead to improvements in the corrosion lifetime of the process equipment. 

The corrosion process is observed as a series of events which all contribute to the overall corrosion rate. Measurement of rest potential fluctuations between two identical electrodes of potential fluctuations with respect to a fixed reference can be carried out. The electrochemical noise output spectrum is analysed using digitised data. The interpretation requires electrochemical expertise, and the method is therefore usually provided as a specialised service. 

Raleigh DO, White JT, Ogden CA (1979) Anodic corrosion rate measurements in LiCl-KQ eutectic. 1. Electrochemical considerations. J Electrochem Soc 126 1087-1093... 

Corrosion monitoring, including corrosion coupons and possibly more specialist techniques, such as portable or permanently installed corrosion rate measuring equipment. 

Most of the reference electrodes embedded in concrete are used for control of cathodic protection (CP) systems. Potential stability is then less important, compared to corrosion state monitoring. Control of CP systems requires only short-term stability, maximum 24 hours. Corrosion rate measurement, like linear polarisation resistance (LPR) measurements, also requires short-term reference electrode stability. However, regardless of application, a reference electrode which is to be permanently embedded in the test solution, e.g. concrete, must have a long life when exposed to this environment. 

D. Drazic and V. Vassic, J. Electroanal. Chem. 155 229 (1985). Theoretical analysis of the electrochemical corrosion rate measurement. 

Figure 1.69 Schematic diagram showing the variation of cathodic potential with current density for steel in seawater, and the correlation of corrosion rate measured by weight loss. (Reproduced from Corrosion for Science and Engineering, Tretheway and Chamberlain, Copyright Pearson Education Ltd)...

Corrosion rate measurements of mild steel specimens showed contradictory results with the conventional trend of corrosion reduction as surface roughness increased. The specimens with diamond polished surfaces had the highest corrosion rate compared with 600 and 200 grit surface finished specimens. 

Abstract Quality control of corrosion test results implies the validation of the corrosion test method and estimation of the uncertainty of corrosion rate measurement. The corrosion test in an artificial atmosphere of the salt spray mist needs evaluation of corrosivity of the test cabinet by reference specimens. Such calibration of corrosion environment raises very strict requirements for the method description and details of all procedures and used specimens. Reliable corrosion measurements by spray tests require validation of the experimental device together with the experimental procedure and determination of corrosivity uncertainty of the test cabinet environment. 

Systems able to supply in situ monitoring of the corrosion rate were developed as well in the recent years [9]. Such systems make use of different methodologies, and consist of a probe inserted in the plant component, and of an instrument which converts the probe signal into a corrosion rate measurement. 

It turns out that the reliability of the corrosion rate measurements depends on the probe and instrument performance, on a good understanding of the phenomenon (as far as materials and chemistry are concerned) and on a good knowledge of the plant operating conditions. 

In order to simplify and improve the use of the corrosimeter, an expert system has been installed on the corrosimeter itself, for controlling the corrosion rates measured by any single probe, and for verifying that the values, the trends and the transients of such corrosion rates are not anomalous. This expert system is called ERICE (Expert Reasoning Instrument for Corrosive Environments). 

Electrochemical Corrosion-Rate Measurement Methods and the Uniform-Corrosion Consideration... 

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