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# Direct Current (DC) Tests

We can use Faraday's Law to find the Mass Loss during an electrochemical experiment.

Faraday's Law shows that the Mass Loss (
Δm) is equal to the Current (I) times the Time (t) times the Atomic Mass (a) divided by the Valence Change (n) and Faraday's Constant (F).

Δm = Ita/nF

In the equation below, the Mass Loss (Δm) per Unit Area (A) per Unit Time (t) is equal to the Corrosion Rate (CR).

CR = Δm/At

If we substitute Faraday's Law for Mass Loss (Δm), we obtain the equation below:

CR = Ita/nFAt

Time (t) on the top and bottom of the above equation cancel each other out. As a result, the equation is reduced to:

CR = Ia/nFA

The Current (I) divided by the Area (A) is equal to the Current Density (i). When we replace I/A (in the above equation) with i, the new equation becomes:

CR = ia/nF

The Current Density (i) in the above equation is the same as the icorr that is found during an electrochemical experiment - so, the final equation is:

CR = icorra/nF

We already know the Atomic Mass (a), the Valence Change (n), and Faraday's Constant (F). If the Corrosion Current Density (icorr) can be defined during a DC electrochemical experiment - such as the Schematic of the Tafel Extrapolation Test shown below - the Corrosion Rate can then be calculated. A Penetration Rate (such as mmpy or mpy) can be found by dividing the above equation with ρ (density). The new equation then becomes:

CR = icorra/nFρ

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