Environmental changes and corrosion inhibition

Changing the environment

Changing the corrosive environment is a common method for reducing and inhibiting corrosion. Changing the environment is influenced by the following parameters:

1. Temperature change
2. Change in oxygen concentration
3. Change in rate
4. Change in concentration
5. Change in composition
6. Change in moisture content

Temperature change

In acids, the corrosion rate decreases with decreasing temperature. However, at pH=7, decreasing temperature increases corrosion (corrosion in boiling water is much slower than in cold water).

Changing the oxygen concentration:

In metals and alloys that are corroding, increasing oxygen decreases the corrosion rate, and decreasing oxygen increases the corrosion rate.

Changing the velocity:

One of the methods often used to control corrosion is to reduce the velocity of the fluid. Increasing the velocity usually increases the corrosion rate. Although there are important exceptions to this. Metals and alloys that corrode, such as stainless steels, perform better in moving environments than in static solutions.

Composition change:

The presence of some substances such as chlorine, ammonia, hydrogen sulfide, sulfur dioxide, inorganic acids such as hydrochloric acid, sulfuric acid, and nitric acid, and organic acids such as acetic and formic acid in a corrosive environment is important. In general, inorganic substances are more corrosive than organic substances.

Humidity change:

Since corrosion occurs under the influence of an electrochemical process, an electrolyte is needed to transfer the charge. Water containing ions is an excellent electrolyte. The amount and percentage of humidity are important factors in atmospheric corrosion.

One of the important factors in which humidity plays a role is the dew point, which has a significant effect on corrosion. The dew point is the temperature at which the moisture in the air reaches a saturated state and, after condensation, turns into dew.

For atmospheric corrosion to occur, a critical relative humidity is required, below which the presence of humidity has no effect on atmospheric corrosion. When the relative humidity of the environment reaches a critical value, the electrolyte necessary for metal corrosion is provided, and with an increase in the relative humidity, we will also see an increase in the corrosion rate of the metal. The relative humidity depends on the type of metal and other pollutants. At higher humidity levels, the corrosion rate is affected by factors such as gases and solids that cause air pollution.

Therefore, in many cases, by reducing the temperature, removing oxidizing or oxygen-generating agents, and finally by changing the electrolyte concentration, corrosion can be controlled to a significant extent. Of course, the effect of the aforementioned factors depends on the specific conditions of the metal and the environment, but usually reducing the fluid velocity, especially in abrasive corrosion, has a reducing effect.

However, in the case of active-inactive metals, reducing the velocity has the opposite effect. On the other hand, by various methods (such as using neutral gas, passing water, steam boilers over the steel, using a vacuum), oxygen-generating agents can be removed. Finally, by reducing the electrolyte concentration, corrosion can also be controlled to some extent.