Linear Polarization Resistance LPR and the SternGeary Equation

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• Linear Polarization Resistance (LPR) and the
  Stern-Geary Equation
• With this widely used technique in corrosion
  monitoring, the polarization resistance of a
  material is defined as the slope of the
  potential-current density (DE/Di) curve at the
  free corrosion potential, yielding the
  polarization resistance Rp that can be related
  (for reactions under activation control) to the
  corrosion current by the Stern-Geary equation
•  
• where
• Rp is the polarization resistance
• icorr the corrosion current
• The proportionality constant , B, for a
  particular system can he determined empirically
  (calibrated from separate weight loss
  measurements) or, as shown by Stern and Geary,
  can be calculated from ba and bc, the slopes of
  the anodic and cathodic Tafel slopes, i.e.

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• Corrosion Monitoring
• Linear Polarization Resistance (LPR)
• Polarization resistance is particularly useful as
  a method to rapidly identify corrosion upsets and
  initiate remedial action, thereby prolonging
  plant life and minimizing unscheduled downtime.
  The technique is utilized to maximum effect, when
  installed as a continuous monitoring system. This
  technique has been used successfully for over
  thirty years, in almost all types of water-based,
  corrosive environments. Some of the more common
  applications are
• Cooling water systems
• Secondary recovery system
• Potable water treatment and distribution systems
• Amine sweetening
• Waste water treatment systems
• Pickling and mineral extraction processes
• Pulp and paper manufacturing
• Hydrocarbon production with free water
• The measurement of polarization resistance has
  very similar requirements to the measurement of
  full polarization curves. There are essentially
  four different methods of making the measurement
  according to whether the current or the potential
  is controlled and whether the current (or
  potential) is swept smoothly from one value to
  another, or simply switched between two values.
  In addition the measurement may be made between
  two nominally identical electrodes (a
  two-electrode system), or a conventional
  three-electrode system (working, reference and
  counter) may be used

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Electrochemical impedance spectroscopy (EIS) EIS
has been successfully applied to the study of
corrosion systems for thirty years and been
proven to be a powerful and accurate method for
measuring corrosion rates. But in order to access
the charge transfer resistance or polarization
resistance  that is proportional to the corrosion
rate at the monitored interface, EIS results have
to be interpreted with the help of a model of the
interface. An important advantage of EIS over
other laboratory techniques is the possibility of
using very small amplitude signals without
significantly disturbing the properties being
measured. To make an EIS measurement, a small
amplitude signal, usually a voltage between 5 to
50 mV, is applied to a specimen over a range of
frequencies of 0.001 Hz to 100,000 Hz. See how
our Sponsor can help you with these test
methods The EIS instrument records the real
(resistance) and imaginary (capacitance)
components of the impedance response of the
system. Depending upon the shape of the EIS
spectrum, a circuit model or circuit description
code and initial circuit parameters are assumed
and input by the operator. The program then fits
the best frequency response of the given EIS
spectrum, to obtain in fitting parameters. The
quality of the fitting is judged by how well the
fitting curve overlaps the original spectrum. By
fitting the EIS data it is possible obtain a set
of parameters which can be correlated with the
coating condition and the corrosion of the steel
substrate. Amongst the numerous equivalent
circuits that have been proposed to describe
electrochemical interfaces only the following
apply in the context of a freely corroding
interface at or close to kinetic equilibrium
Simplest RC representation of an electrochemical
interface One relaxation time constant with
extended diffusion Two relaxation time constants
Impedance of pitting processes of Al based
materials.
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Model for a corroding paint on steel substrate

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• Electrochemical NoiseElectrochemical Noise,
  commonly abbreviated ECN, is an electrochemical
  technique in which the potential and/or current
  signals that arise directly from the
  electrochemical reactions taking place on an
  electrode surface are measured and interpreted.
  ECN is of intense ongoing interest because of the
  totally non-invasive nature of the measurement
  when performed with a Zero Resistance Ammeter
  (ZRA).  The electrochemical instrument is not
  applying any signal to the sample that might
  improperly influence the result.  This
  perturbation may be of concern in every
  electrochemical experiment other than noise.
• ECN can also be measured with the
  electrochemical instrument configured as a
  potentiostat or galvanostat.  The number of
  actual applications of this sort is quite small. 
  Most ECN experiments are carried out with ZRA's,
  so that no signal at all is applied to the sample
  by the instrument.ECN has been used to monitor
  localized corrosion (pitting), uniform corrosion
  through measurement of the Noise Resistance, and
  the deterioration of paints on metal substrates. 
  ECN does not yet enjoy the level of acceptance of
  DC techniques and Electrochemical Impedance
  Spectroscopy. 

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