Equivalent Weight

Definition - What does Equivalent Weight mean?

The equivalent weight of any substance is the mass of the substance in grams that combines with or is chemically equivalent to eight grams of oxygen or one gram of hydrogen. In other words, it is equal to the mass of the substance in grams that would react with or replace one gram of hydrogen. Equivalent weight is expressed as the ratio of atomic weight or molecular weight to the valence.

Corrosionpedia explains Equivalent Weight

Equivalent weight (EW) is an important parameter to understand the corrosion rate (also called the mass loss rate) of a metal when it undergoes the corrosion process.

For pure elements, the equivalent weight is calculated as:

EW = W/n


W = atomic weight of the element

n = the number of electrons required to oxidize an atom of the element in the corrosion process, i.e., the valence of the element

The corrosion rate (CR) or mass loss rate is determined using Faraday’s Law. It is expressed in terms of equivalent weight and is directly proportional to equivalent weight. Corrosion rate is expressed as:

CR = (Icorr x K x EW) (d x A)


Icorr = Corrosion current in amperes

K = Corrosion rate constant

EW = Equivalent weight

d = Density in gram per cubic cm

A = Area taken as a sample for determining corrosion rate in sq. cm

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