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What is the Effect of pH on General Corrosion Rate

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Effect of pH on General Corrosion Rate

 

The effect on Corrosion of the pH of water to which iron or steel is exposed is influenced in the following manner.

 

The potential of hydrogen or symbol (pH) is defined as the negative logarithm of the hydrogen concentration, represented as [H+] in moles/liter.

 

pH = -log [H+]

 

The pH value is used to represent the acidity of a solution.

 

First, consider the exposure of iron to aerated water at room temperature (aerated water will contain dissolved oxygen).

 

The corrosion rate for iron as a function of pH is illustrated in Figure 1.

 

effect oh pH on Iron Corrosion Rate

Fig 1: Effect of pH on Iron Corrosion rate in water

 

 

 

In the range of pH 4 to pH 10, the corrosion rate of iron is relatively independent of the pH of the solution.

 

In this pH range, the corrosion rate is governed largely by the rate at which oxygen reacts with absorbed atomic hydrogen, thereby depolarizing the surface and allowing the reduction reaction to continue.

 

For pH values below 4.0, ferrous oxide (FeO) is soluble. Thus, the oxide dissolves as it is formed rather than depositing on the metal surface to form a film.

 

In the absence of the protective oxide film, the metal surface is in direct contact with the acid solution, and the corrosion reaction proceeds at a greater rate than it does at higher pH values.

 

It is also observed that hydrogen is produced in acid solutions below a pH of 4, indicating that the corrosion rate no longer depends entirely on depolarization by oxygen, but on a combination of the two factors (hydrogen evolution and depolarization).

 

For pH values above about pH 10, the corrosion rate is observed to fall as pH is increased.

 

This is believed to be due to an increase in the rate of the reaction of oxygen with Fe(OH)2 (hydrated FeO) in the oxide layer to form the more protective Fe2O3 (note that this effect is not observed in deaerated water at high temperatures).

 

this article is taken from: DOE FUNDAMENTALS HANDBOOK CHEMISTRY Volume 1 of 2 - DOE-HDBK-1015/1-93 JANUARY 1993

 

see also

What is Corrosion?

What is Rust?

Corrosion Types or Corrosion Form

What is pH?

Metal Rust Remover

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