Best tips for determining the corrosion rate.
What are the corrosion specialist doesn't tell you , their secrets in calculating corrosion rates .
Why most researchers got confused when using potentiodynamic polarization technique for determining the corrosion rate ?
Author : Shaimaa Ali Abou El Ela .(material science engineer)
Author e.mail : shaimaaaliabouelela@gmail.com
Faculty of petroleum and mining engineer , metallurgy and material science department .
Corrosion is happening every where . I found most of metallurgical engineers doesn't care about learning corrosion and how to protect against corrosion . Usually most of them care about learning about alloys manufacturing . So I decided to put an explanation about the most confusing points about corrosion .
For example corrosion rate , how to determine it precisely , how to change units from mpy to mmpy or vice versa . Of course I will explain some tips of how to calculate the corrosion rate from different techniques .
First of all most people think that the best way of calculating corrosion rates by using potentiodynamic polarization technique but NOW I am telling you please don't use this technique if you want to get the accurate corrosion rate , because both cathodic an anodic lines can get interrupted by for example resistance , concentration or activation polarization or the tested material is having a passivation behavior . These factors will really interrupt your Tafel slopes and the tangents will intersect in a wrong point which will show a wrong corrosion current . So to avoid this using linear polarization technique will help you to draw your tangent precisely . Figure (1) show that the cathodic and anodic tangents in potentiodynamic curves (V-log I) gave wrong deduction in calculating the corrosion rate . But in figure (2) the same experiment was redrawn using V-I chart (linear) and the corrosion rate was calculated precisely and the results was accurate .
But note that you can't use the same sample for both techniques . The sample which is used in potentiodynamic polarization technique experiment shouldn't be used again when holding an another experiment with a linear polarization technique because the surface was highly accelerated to get corroded so the obtained corrosion rate from the second experiment will be so high . So please avoid this mistake .
The most important step you have to make before starting your experiment is waiting for about 15 minutes . Let your sample after connecting it in your corrosion cell to reach equilibrium this will give you the most accurate corrosion rate .
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| figure (1) |
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| Figure (2) |
