Factors Affecting Corrosion

Introduction:

When materials react with the surrounding environment, electrochemical and chemical reactions may occur. These reactions cause corrosion of the material. The term corrosion is used multiple in the case of metals. Rusting iron is the best example of metal corrosion. Corrosion gradually destroys the material. Corrosion is an area of major concern because it weakens the material and eventually destroys it. We will discuss factors affecting corrosion and some common methods developed to prevent corrosion..

The causes of corrosion in materials are as follows: 

1. Nature of the metal

• Purity of metals
• The physical state of metal 
• Position in Galvanic series 
• Position in electrochemical  series
• Nature of the oxide film

2. Nature of environment in which the material is kept

• pH 
• temperature
• pollutants in atmosphere
• moisture and dew
• humidity

1. Nature of metal: Concentration of corrosive effect on material when electrode potential of the material is high. Gold, silver, platinum noble metals have least corrosion rate, whereas other metals like magnesium, zinc, aluminium with low electrode potential undergo corrosion faster. The high difference in electrode potential between two metals that are in contact with each other have a greater chance of corrosion. For example when iron is in contact with copper, iron corrodes rapidly. Taking this into consideration the same metals should be used and usage of dissimilar metals shouldn’t be taken into consideration.

• • Purity of metals: The content in molecular level of a  metal is very important in the case of Corrosion. Pure metal have less chance of Corrosion. But in the case of impure metals due to the presence of minute electrochemical cells, there are high chances of Corrosion.
  • The physical state of metal: Physical state of metal plays a major role in faster Corrosion rate. Metal with less stress corrodes at a slower rate. But in the case where there is excess stress in the metal the rate of Corrosion will be high.
  • Position in Galvanic series: Metals that are placed at a higher position in the Galvanic series show greater oxidation power thus have more tendency to become anodic and thus exhibit more Corrosion rate.
  •  Position in electrochemical series: Metals with a higher reaction rate will show more Corrosion rate, on the other hand, less reactive metals exhibit less corrosion rate. The electrochemical series is used to find the reactivity of the metals.
• Nature of the oxide film: Specific volume ratio is the term used to find the amount of metal oxide to the metal. If the Specific volume ratio is more in metal there are less chance of oxidation and therefore less chance for Corrosion

2. Nature of corroding environment

• Moisture and dew: It is one of the most important factors which causes corrosion of Metals. When Metals are exposed to areas that contain high amounts of moisture and dew it results in the formation of rust or corrosion rapidly. Dry and non-humid conditions are the ideal setup for metals, especially non-alloy metals. It is impossible to use metals only in dry and non-humid conditions. We will have to use metals other than dry and non-humid conditions as well. Painting and using alloy metals can be useful for preventing corrosion or rusting. Now days specially built anti-rust paints are available that prevent metal decay due to corrosion to a certain extent.

• Temperature: Temperature also plays an important role in the corrosion of Metals. High temperature is a significant issue in the industries like automotive, pulp and paper, mineral and metallurgical processing, refining and petrochemical, aerospace, etc.. 

Any component is potentially at risk when it is in a non-inert environment and exposed to higher temperatures. At higher temperatures, most of the alloys and metals corrode and oxidize. The acceleration of the corrosion when exposed to higher temperatures is because of the rise in the temperature results in the increase of the active centre of corrosion in material or metal. Thus protecting equipment and metal products from higher temperatures are necessary. Coating metals with the high-temperature structural coating will help to prevent corrosion due to higher temperatures to an extent.

• Pollutants in the atmosphere: Metals that are near to sea and industries are seen corroding faster. This is due to pollutants present in the atmosphere like carbon dioxide, hydrogen sulphide, sulphur dioxide and HCl which comes from the industries. On the other hand NaCl present in the atmospheric air of coastal areas comes in contact with the surface layer and increases the liquid presence on the surface of metals. Due to these problems oxidation and rate of corrosion increase.

• Humidity: The Corrosion rate is seen more in the places with more humidity. This is because moisture content will increase the rate of oxidation and Corrosion rate of metals.

Prevention of  Corrosion

Corrosion is an undesirable event that happens to metals and other materials. The phase of Corrosion may be slow but it will affect the core of the metal and decrease its stability. There are different methods to overcome corrosion some of them are: 

• Painting
• Enameling
• Tarring
• Galvanizing 
• Electroplating
• Tin plating

CONCLUSION: 

It is clear that when materials react with the surrounding environment or some medium, electrochemical and chemical reactions may happen; these reactions cause corrosion of the material. Most of the time the term corrosion is used in the case of metals. Corrosion is an undesirable event that happens to metals and other materials. The phase of Corrosion may be slow but it will affect the core of the metal and decrease its stability. Different factors affect and boost the rate of Corrosion. Taking proper preventive measures is the most effective aid to prevent Corrosion.