b) For new plant, it is wise to consult with a corrosion or materials engineer who can advise on optimal probe locations. Whilst comprehensive monitoring is a useful thing to have, it is important to consider cost-efficiency and ensure that the corrosion measurement in itself will deliver value rather than represent a cost sink.
3. How do I measure the corrosion ? Hopefully, going through the above exercises will have drawn you to a short-list of candidate techniques and sensors. The following descriptions are necessarily brief, but provide an introduction to the types of monitoring methods that are available and hopefully will assist in an initial review of appropriate techniques for your own corrosion needs.
• “Cumulative Loss" techniques, including: Weight Loss Coupon, Electrical Resistance (ER), Thin Layer Activation (TLA), Field Signature Method (FSM), Ultrasonic Thickness (UT) measurement and other non-destructive examination methods (e.g. radiography).
• “Corrosion Rate" techniques, including: Linear Polarization Resistance (LPR), Harmonic Distortion Analysis (HDA), Electrochemical Noise (ECN), Zero Resistance Ammetry (ZRA), Electrochemical Impedance Spectroscopy (EIS), Inter Modulation Distortion (IMD), Potentiodynamic Polarization
Cumulative Loss: The Cumulative Loss techniques will only show some signs of change when sufficient corrosion has been sustained to cause a change in the 'bulk' material properties. As such, most are used off-line and do not provide real-time data. The most popularly employed of these measurements for field corrosion evaluation are as follows:
The Weight Loss Coupon is the most widely used method of corrosion evaluation. A metal coupon of known metallurgy, size, shape and weight is exposed to an environment for a scheduled period of time (typically three months). An example of coupons and coupon holders is shown in Figure 1 (photo courtesy of Metal Samples). At the end of the exposure period, the coupon is subjected to: visual and optical examination, removal and analysis of corrosion products, measurement of weight loss due to corrosion, further visual and optical examination. The accuracy of this technique is typically limited by the ability to weigh the coupon accurately and reproducibly to a lower limit of some 0.1mg.
Electrical Resistance (ER) measurement can be considered almost as an electronic coupon. As the probe 'element' corrodes, so its electrical resistance will change and this can be converted to a cumulative metal loss value (mils or mm). One of the major benefits of electrical resistance is that the technique is applicable to measurement in most environments, i.e. conductive and non-conductive. However, the measurement is affected by temperature and so a temperature compensation must be included. This also renders ER incompatible with monitoring of corrosion at high temperatures. Electrical resistance probes are available in a variety of configurations including wire loop, tube loop, flush-mount, and cylindrical element. Figure 2 (photo courtesy of Metal Samples) shows a selection of ER probes.
For conventional ER the sensitivity of measurement is typically 0.1% of the element thickness (similar limitations apply to TLA, FSM and UT techniques). Such methods are most appropriate to use where the corrosion rate is relatively stationary, i.e. the rate does not suffer from large excursions. As accounting tools they are useful in providing an indication of the cumulative wastage of a resource, and hence the likely useful remaining lifetime. Overall, the techniques are relatively insensitive to localized corrosion.
Corrosion Rate: The Corrosion Rate techniques have a much higher resolution and have been developed to provide a fast appreciation of the electrochemical rate processes taking place at the metal/environment interface. Measurements using these techniques may take only a few minutes.
The corrosion current is a consequence of the corrosion process and its value is directly related to the rate of the metal loss. The electrochemical monitoring methods have been developed specifically to estimate the corrosion current. If the corrosion process is relatively stationary i.e. at a “steady-state” then the relationship between the polarization resistance and the corrosion current is given by the Stern-Geary relationship;
where Rp is the polarization resistance, B is the Stern_Geary constant and Icorr is the corrosion current (which is directly related to the rate of metal loss or corrosion).
(Note that the Stern-Geary constant is a system, not a universal constant)
This approximation is the basis for the use of LPR and EIS for the estimation of general corrosion rates. In the case of the LPR measurement the polarization resistance is a simple function of the applied voltage and the current reponse at low frequency. By comparison, the EIS technique is used to determine the characteristic impedance of the corroding metal, typically over the frequency range 1 millihertz to 100 kilohertz.
Harmonic Distortion Analysis (HDA) and Inter-Modulation Distortion (IMD) techniques also rely on a steady state approximation, but require more mathematical treatment than the simple LPR.
For the HDA the current response to a low frequency voltage sine wave is analyzed in terms of the fundamental response and the higher harmonics, to provide values for the corrosion current, the characteristic anodic and cathodic coefficients, and hence a value for the Stern-Geary constant.
The IMD technique is slightly different in that a composite signal comprised of two superimposed sine waves is used to polarize the system. The current response is analysed for the inter-modulation distortion products of the of the two sine waves. This leads directly to the estimation of the corrosion current and the characteristic anodic and cathodic coefficients, and again a value for the Stern-Geary constant.