Automatic recording of Rp versus time

Files:
General corrosion (Rp).EXP
General corrosion (Rp) 002_15.CRV
General corrosion (Rp) 22.CRV

The Rp polarisation resistance is an important parameter to evaluate the anti-corroding strength of a corrosion inhibitor or to study a uniform corrosion process at a metal surface. The polarisation resistance variations versus time are automatically recorded. Each experimental point (polarisation resistance and potential versus time) is obtained from one individual voltammetry which is automatically processed with a special algorithm [1]. These individual voltammetries are saved. That makes it possible to process them by means of the 2nd Stern method for instance. Between two successive voltammetries, the system is left at rest (the circuit is open) during a waiting time set by the user.

Solution: CaCl2 0.01M in tap water
WORK: Copper disc ( diameter=2mm)
REF: Calomel
AUX: Platinum
Cell: CP06 (not thermostated)

Solution CaCl2 0.01 M in tap water at room temperature. Areas for WORK equals 1 cm². Current densities are assimilated to currents in these conditions. The potential is scanned 25 mV (Overvoltage) from the OCP of the system at a Scan rate of 5 mV/s (start at OCP, potential scanning toward OCP - 25 mV and potential scanning toward OCP + 25 mV). The circuit is open during 5 minutes between two successive individual voltammetries. This waiting time enables the WORK to stabilise its zero-current potential. At the end of the experiment, the system is left at the free potential (Open circuit at end = Yes). The 10 individual voltammetries are saved as well as the polarisation potential and corrosion potential (open circuit potential) versus time.

1) With the file General corrosion (Rp) 22.crv
Display: Type = Normal X = Time Y1 = Resistance Y2 = Potential

This file comprises 9 experimental points. Each point corresponds to an automatic calculation of the polarisation resistance according to a specific algorithm dedicated to inhibitors studies [1] The values for point number 2 (which correspond to the third point) are given with the cursor.

Polarisation resistance = Rp = 150 kohm.cm²
Open circuit potential = Potential = -50.7 mV

These results can be compared with the results obtained with Rp determination in post run processing obtained from a linear regression or Stern equation.

2) With the file General corrosion (Rp) 002_15.CRV

Traject: Path=Forward Cycle=1
Display: Type = Normal X = Potential Y1 = Current Y2 = No

This file is the third file out of the 9 individual cyclic voltammetries recorded. Each voltammetry i = f(E) can be processed manually with the linear regression or the Second Stern calculations to evaluate the corrosion potential and the polarisation resistance. It must be underlined that the experimental conditions are not valid to perform Tafel calculation since the overvoltage applied versus the open circuit potential is smaller (25 mV) than the overvoltage required for Tafel calculation (200 mV). Calculations are performed on the traject which correspond to the anodic scan. Use the Traject post processing tool to select this part of the curve prior to the calculations.

2) a) Calculate the polarisation resistance according to a linear regression

Traject: Path=Forward Cycle=1

Display: Type=Normal X=Current Y1=Potential Y2=No

Linear regression----------------------- 17-09-1999, 13:32:23

X min. : -104.999
X max. : 200.001
Mode : y=f(x)
Result : y(mV) = 0.179*x(nA/cm²) -58.898
x(y=0) = 330.086
Coefficient : 0.990803

Polarisation resistance = Slope = 179 kohm.cm²

Zero current potential = -58.8 mV

2) b) Calculate the polarisation resistance according to the 2nd Stern equation
Traject: Path=Forward Cycle=1
Display: Type=Normal X=Potential Y1=Current Y2=No

Calculate the polarisation resistance according to the 2nd Stern equation 17-09-1999, 13:37:24
Smoothing : 9
Segment : 25 mV
E(i=0) : -60.4 mV
Rp : 206.90 kohm.cm²
Coefficient : 0.993

Polarisation resistance = Rp = 206.9 kohm.cm²
Zero current potential = E(i=0) = -60.4 mV

The difference between the polarisation resistance determined automatically and values determined by linear regression or Stern analysis is due to the differences of the algorithms used for the calculations. The difference between the "Zero current potential = E(i=0) and the "open circuit potential" is due to the fact that it is not the same potential. The Open Circuit Potential corresponds to the potential of the WORK at rest, prior to the individual voltammetry. The zero current potential E(i=0) corresponds to the potential at which the measured current is close to zero during the voltammetry when the potential is scanned in anodic direction.

[1] Calculations are performed according to the GFC-L-109-A-90 Standard
Consult the Help in General corrosion (Rp): How are determined Rp values?