Corrosion parameters using Tafel calculation

The Tafel calculation gives access to the corrosion current regularly present at the surface of a metal which is in contact with a corroding solution. The corrosion is expressed in loss of metal thickness per year. The Thermodynamical Ba and Bc parameters of the Butler Volmer equation are also determined.
A voltammetric curve i = f(Vg) having a zero current potential can be processed according to the Tafel method. The Tafel method gives also access to other corrosion parameters such as the zero current potential (corrosion potential), the corrosion current and the polarisation resistance of the sample under study. The Tafel calculation is a complete tool to study the corrosion process at a metal surface.

This voltammetric curve was obtained from a Pot. Linear V method carried out on a Inox 10/18 [AISI430] (demonstration curve: Polarisation curve Inox18 10_025.CRV).

In blue: log (i) = f(E) Voltammetric experimental curve.
In red: Cathodic and anodic tangents to the curve, straight line of equation x = E (i = 0) (corrosion potential). These three lines intercept at a point which co-ordinates are: (corrosion potential, corrosion current).

The Tafel calculation gives:

• the zero current potential (E (i=0)) as the corrosion current,
  
  
• the Rp polarisation resistance (a parabolic regression is performed on the curve around the zero current potential, Rp is determined as the slope of the tangent to this parabola at the zero current potential).
  
  
• the i corr. corrosion current,
  
  
• the Ba slope of the tangent to the anodic branch of the curve,
  
  
• the Bc slope of the tangent to the cathodic branch of the curve,
  
  
• the Corrosion rate calculated as a number of mm of iron thickness corroded per year (the Atomic mass, Valence, Density and Working electrode area entered in the Initial data are used to calculate the corrosion rate). For this iron species: Corrosion rate = 2.455 mm/year
  
  
• the correlation coefficient displayed (Coef.: 0.9997) indicates the quality level of the parabolic regression. This coefficient is comprised between 0 and 1. Closer to 1 this coefficient is, better is the regression.

The Tafel calculation is an ideal tool to determine the corrosion rate at an iron sample surface. When the linearity range of the log (i) = f(E) curve covers more than one current decade of the cathodic branch and around one decade of the anodic one. In such conditions, the corrosion rates of iron species can be determined accurately.

Note (polarisation resistance): The Rp polarisation resistance can also be determined by using the 2^nd Stern method (see Examine your experiment: Corrosion parameters using 2^nd Stern calculation).