Faraday’s Law for Cathodic Protection

Quick Definition

A simplified Faraday’s Law relationship for mass is:

In symbols:

Where:

• m is mass consumed or deposited.
• I is current.
• t is time.
• EW is equivalent weight.
• F is Faraday constant.

For learner calculations on this page, use supplied values only. Do not select material-specific equivalent weights or efficiencies unless they are provided by a controlled source or problem statement.

When Faraday’s Law Is Used in CP Work

Faraday’s Law may be used to understand:

• why anode consumption is related to current;
• why time matters in electrochemical reactions;
• how current and charge relate to mass change;
• why anode life is connected to current output.

This page does not calculate a guaranteed anode life.

This page does not prescribe material-specific design values.

This page does not prove CP protection.

Formula Reference

Conceptual form:

Variable and Unit Table

Learner unit notes:

• 1 A = 1 coulomb per second
• Q = I × t
• if current is in amps and time is in seconds, charge is in coulombs;
• use the supplied equivalent weight and constant in practice problems;
• do not invent material-specific values.

For simplified learner examples in this page, use:

• F = 96,485 C/equivalent

How to Use the Formula

Use Faraday’s Law carefully.

1. Use current in amps.

2. Use time in seconds.

3. Use equivalent weight from the problem or a controlled source.

4. Use the Faraday constant from the problem or controlled source.

5. Calculate mass.

6. Treat the result as a formula calculation, not a field guarantee.

If time is given in hours, convert hours to seconds first:

If time is given in days, convert days to seconds first:

Worked Example 1 — Charge from Current and Time

A current of 2 A flows for 1 hour. What charge passes?

Known values:

• I = 2 A
• t = 1 hour
• Q = ?

Convert time:

Use:

Substitute:

Calculate:

Answer:

The charge is 7200 C.

Field meaning:

Charge increases when current or time increases.

Worked Example 2 — Mass from Current and Time

A simplified Faraday’s Law exercise uses:

• I = 1 A
• t = 3600 s
• EW = 12 g/equivalent
• F = 96,485 C/equivalent

What mass is calculated?

Use:

Substitute:

Calculate:

Answer:

The calculated mass is approximately 0.448 g.

Field meaning:

This is a formula-mechanics example using supplied values. It is not a design guarantee.

Worked Example 3 — More Time, More Mass

Using the same supplied values as Example 2, compare 1 hour and 2 hours at 1 A.

For 1 hour:

For 2 hours:

Answer:

At 1 hour, the calculated mass is approximately 0.448 g. At 2 hours, the calculated mass is approximately 0.895 g.

Field meaning:

With the same current and equivalent weight, doubling time doubles the calculated mass.

Worked Example 4 — Solve for Time

A simplified exercise uses:

• m = 0.448 g
• F = 96,485 C/equivalent
• I = 1 A
• EW = 12 g/equivalent

What time does that represent?

Use:

Substitute:

Calculate:

Answer:

The time is approximately 3600 s, or about 1 hour.

Field meaning:

Small rounding differences come from rounded mass values.

Common Mistakes

Common Faraday’s Law mistakes include:

1. Using hours when the formula expects seconds.
   Convert time before calculating.

1. Inventing material-specific values.
   Equivalent weight and efficiency values must come from a controlled source or problem statement.

1. Treating a formula result as a field guarantee.
   Real anode consumption depends on field conditions and assumptions.

1. Confusing current with charge.
   Current is flow rate. Charge is current multiplied by time.

1. Forgetting that more current or more time increases mass.
   Mass is proportional to current and time.

1. Mixing units without converting.
   Use consistent units.

1. Using too many decimals.
   Formula examples often use rounded values.

1. Assuming Faraday’s Law proves CP protection.
   It explains electrochemical quantity relationships. Protection requires proper CP measurements and criteria.

Practice Problems

Problem 1

A current of 3 A flows for 10 seconds. What charge passes?

Problem 2

A current of 2 A flows for 2 hours. How many seconds is that, and what charge passes?

Problem 3

A simplified exercise uses I = 1 A, t = 3600 s, EW = 10 g/equivalent, and F = 96,485 C/equivalent. What mass is calculated?

Problem 4

Using the same values as Problem 3, what mass is calculated if time doubles to 7200 s?

Problem 5

If current doubles and time stays the same, what happens to calculated mass?

Problem 6

What is the formula for charge when current and time are known?

Problem 7

Should learners invent equivalent weight or efficiency values when they are not supplied?

Problem 8

Does a Faraday’s Law calculation by itself prove CP protection?

Practice Problem Answer Key

Related Learning

Use these pages to continue studying related concepts:

• Anode Life
• Anode Output
• Current Requirement
• Current Density
• Ohm’s Law

Related formulas:

• Anode Life
• Anode Output
• Current Requirement
• Current Density