Calculating Pitting Corrosion

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Localized Corrosion

OLI Software Corrosion Analyzer can predict the tendency or propensity of an alloy to localized corrosion.

The relationship between the corrosion potential (Ecorr) and the repassivation potential (Erp) indicates the likelihood of localized corrosion to occur.

The Ecorr is the potential at which the anodic and cathodic rates are the same under a determined environment; and the Erp is the potential below which localized corrosion, such as pitting and crevice corrosion, does not occur.

Whenever Ecorr exceeds Erp, localized corrosion is predicted. Now the larger the difference between these two potentials, the greater the propensity to localized corrosion, as is depicted in the Figure below.

 Corrosion and Repassivation potential illustration.
Figure 1. Corrosion and Repassivation potential illustration.

Predicting Pitting/Crevice Corrosion

Predicting Corrosion and Repassivation Potentials

In OLI Software you can simulate a simple scenario like NaCl in water at 95oC for different alloys such as 304 and 316.

It is important to keep in mind that for simulating pitting corrosion you need to have enough oxidizing species in your simulation to raise the corrosion potential above Erp. If Ecorr is determined just by the reduction of water on a passive surface, then it is usually to low to provide a driving force for localized corrosion. If we include oxygen, then we get localized corrosion depending on the chloride concentration.

In the example below, two simulations were run for alloy 316 as a function of NaCl concentration – one without oxygen and one with an oxygen amount that roughly corresponds to oxygen content in air (0.2 O2 and 0.8 N2):

 Predicted Corrosion and Repassivation potential of alloy 316 as a function of NaCl.
Figure 2. Corrosion and Repassivation potential of alloy 316 as a function of NaCl.

Erp (greenish line) does not depend on oxygen but Ecorr (brown lines) certainly does. If we have no oxygen (i.e., the only reduction reaction on the passive surface is the reduction of water), then Ecorr is always below Erp. If we have air, then Ecorr exceeds Erp at NaCl concentrations a little below 0.1 m.

Polarization curves

The predicted polarization curve of the above case reflects pitting by showing both the hypothetical polarization curve in the absence of pitting (red curve in the plot below) and the predicted reverse portion of the polarization scan (labelled as peak current density – see the greenish curve below. Of course, the forward portion of the polarization curve is not predicted because we do not predict the pitting potential, just the repassivation potential.

 Polarization curve of alloy 316 in a 5M NaCl solution.
Figure 3. Polarization curve of alloy 316 in a 5M NaCl solution.

Editor: Diana Miller Author: Andre Anderko