To test the corrosion behavior (or resistance) of an engineering material in an aqueous medium one can carry out immersion tests for specific periods of time across which the mass loss and corrosion rates can be calculated. This method, although of its simplicity, gives a good prediction of the material behavior in the field and allows relating the corrosion rates to the corrosion products properties. So why do we carry out the potentiodynamic polarization? Yes, it is an electrochemical tool that separates the cathodic reactions from the anodic reactions and gives the corrosion rates in (A/cm2), but let’s consider the many scan rates and ranges this method can be carried out with and its reproducibility as a method, How close is it in predicting the behavior in a real field situation? How important is it in describing the significance of the developing corrosion products -- which do not develop at open-circuit potentials, but by a catalyzing anodic scan?
The purpose of potentiodynamic polarization experiments (and similar electrochemical experiments) is primarily to enable identification and understanding the reactions, rate determining processes, and the stability of the passive film. Not to provide accurate estimates of long-term corrosion damage. Even if the technique could produce very accurate rate measurements, it would only be good for the pristine surface immersed into the solution for a relatively short time and the precise solution chemistry, temperature, aeration, etc. at the moment of measurement. From my experience, mass loss experiments very rarely exhibit constant rates over time. So, why would you expect a short term experiment that does not contain any of the factors that alter corrosion rates with time of exposure to be able to represent long term behavior? This is equivalent to using a straight line through the first two points in a mass loss time series to represent the entire series. It is very rarely, if ever, accurate. Assuming that what we all would like is an experimental technique that in one afternoon can predict 20 years of corrosion behavior, what we need is very accurate tools for measuring fundamental reaction kinetics, which electrochemistry promises to provide, and mathematical tools (models) that enable representing the time dependent changes that occur for different types of exposure conditions, mechanistic scenarios, and the stochastic nature of these processes. For more discussion of my thoughts on this, see "On Using Laboratory Measurements to Predict Corrosion Service Lives for Engineering Applications," J. ASTM Intl., 5 (7), 10 pgs (2008) doi: 10.1520/JAI101350 and "Can Corrosion Testing Make the Transition from Comparison to Prediction?" JOM, 47 (9) 32-35 (1995).
The purpose of potentiodynamic polarization experiments (and similar electrochemical experiments) is primarily to enable identification and understanding the reactions, rate determining processes, and the stability of the passive film. Not to provide accurate estimates of long-term corrosion damage. Even if the technique could produce very accurate rate measurements, it would only be good for the pristine surface immersed into the solution for a relatively short time and the precise solution chemistry, temperature, aeration, etc. at the moment of measurement. From my experience, mass loss experiments very rarely exhibit constant rates over time. So, why would you expect a short term experiment that does not contain any of the factors that alter corrosion rates with time of exposure to be able to represent long term behavior? This is equivalent to using a straight line through the first two points in a mass loss time series to represent the entire series. It is very rarely, if ever, accurate. Assuming that what we all would like is an experimental technique that in one afternoon can predict 20 years of corrosion behavior, what we need is very accurate tools for measuring fundamental reaction kinetics, which electrochemistry promises to provide, and mathematical tools (models) that enable representing the time dependent changes that occur for different types of exposure conditions, mechanistic scenarios, and the stochastic nature of these processes. For more discussion of my thoughts on this, see "On Using Laboratory Measurements to Predict Corrosion Service Lives for Engineering Applications," J. ASTM Intl., 5 (7), 10 pgs (2008) doi: 10.1520/JAI101350 and "Can Corrosion Testing Make the Transition from Comparison to Prediction?" JOM, 47 (9) 32-35 (1995).
As in my experience in a lot of cases, it helps to choose the right material for your application much faster than immersion tests.
However, the combination of both appears to be the best solution.
As Dr. Ricker points out, potentiodynamic polarization experiments are run to obtain a qualitative understanding of the corrosion process. For corrosion rate determination, Polarization Resistance or Tafel Plots are used. These electrochemical experiments are popular because they are fast compared to weight-loss measurements. A Polarization Resistance measurement gives you a "snapshot" of the corrosion rate, while a weight-loss measurement "integrates" the corrosion rate over the time of the immersion. So the corrosion rates from these two techniques may exhibit significant differences.
it is difficult to say that the electrochemical and even immersion tests can really predict the long term behaviour of the material in real field situation . The lab tests as rightly pointed out by Dr. Ricker are done on the fresh surface while the field system to be analysed might be running for years. Even though the other NDE techniques like UT scanning is used to identify the thickness loss due to internal corrosion only; unless we know the loss in property/ies due to the corrosion it will be difficult to predict the behaviour of already corroded section in the same /different or continuously changing atmosphere.
Potentiodynamic polarization gives a decent estimate of the corrosion potential under the controlled conditions of laboratory testing. The corrosion currents are measured using tafel extrapolation and can be quite off compared to real life situations especially when a corrosion product persisits on the surface. For real life situations, analysis of the corrosion products at regular intervals from immersed samples gives an idea of the progression of corrosion in terms of rates as well as in terms of changes in surface characteristics and rate controlling reactions.
Both mass loss measurements and potentiodynamic polarization have their limitations. Mass loss measurements may be inaccurate and/or not applicable for passive materials that have a risk for localized corrosion types (e.g., Stainless steel), and potentiodynamic polarization may be too far from the real case. Especially for the determination of any risk for pitting corrosion and/or for the screening of different materials (including passive ones) potentiodynamic polarization will give more relevant information.
1. As mass loss does not exibit (or very rarely) constant rate over time, for this experiment (exposure to real environment) you need one year, according to ISO 9223. So, you can never extrapolate the mass loss of 3 or 4 months (or days) to 1 year.
2. However, for metals which present localised corrosion (pitting corrosion and not general one), the mass loss value (even after 1 year of test) can give you a significant error, because the real area of the samples is very different from their geometric one.
3. As Dr. P. Peterson, Dr. R. Richer, Dr. M. Schumura commented, the electrochemical tests help to enable identification and understanding of the anodic and cathodic reactions, active and passive states, stability of passive film (current and potential range of the passive state, if you need to apply, for example anodic protection or cathodic protection). You also can compare electrochemical behavior of diferent metals. But correlate the Icorr (as corrosion rate) to mass loss of metal exposed to real environment, this is not possible.
4. For electrochemical test, I prefer to register the open circuit potential of the metal, exposed to electrolyte of interest for a long period of time. In real conditions the metals are not polarized (except the case of cathodic and anodic protections). After that you try to characterise the corrosion process using EN, FTIR, PSD, for example.
All good points - What I usually like to tell students is that we cannot measure rates at open circuit. So, we have to perturb the system and measure its response from which we make estimates of the corrosion rate. All the different electrochemical techniques are just different methods of creating these perturbations. However, since these perturbations can influence the steady state conditions, the trend over time has been to reduce the magnitude or severity of the perturbations and minimize the deviations from the natural steady state conditions by using better instrumentation and higher resolution characterization. So, the large amplitude perturbations of potentiostatic polarization became faster (potentiodynamic, voltammetry), then smaller amplitude (polarization resistance), then smaller still at varying rates (AC impedance or EIS), and then we started trying to use the natural oscillations of the system (electrochemical noise).
A similar problem is induced with mass loss measurements using environments that "accelerate" corrosion. That is, these "accelerating environments" use "accelerating factors" that are perturbations from the expected service conditions and result in inducing uncertainties. This is especially true when looking at new materials where the influence of the "accelerating factors" on corrosion are not really known. As a result, there is also a trend in mass change measurements toward the use of higher resolution instrumentation and better characterization, but for a variety of reasons this trend is not as evident as it is for electrochemical measurements.
As all academic staffs (experts) explained above the electrochemical methods are OK for short time corrosion estimation but the results will be reliable if we use different methods to evaluate the agreement. However the electrochemical techniques will be very useful for the studies to do comparison about the effect of some parameters and conditions on the corrosion of the same simple.
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