Cause And Effect Of Chemical Reaction : Corrosion

 
  Corrosion is the deterioration of a metal as a result of chemical reactions between it and the surrounding environment. Both the type of metal and the environmental conditions, particularly gasses that are in contact with the metal, determine the form and rate of deterioration.

Do All Metals Corrode?

   All metals can corrode. Some, like pure iron, corrode quickly. Stainless steel, however, which combines iron and other alloys, is slower to corrode and is therefore used more frequently.

All small group of metals, called the Noble Metals, are much less reactive than others. As a result, they corrode rarely. They are, in fact, the only metals that can be found in nature in their pure form. The Noble Metals, not surprisingly, are often very valuable. They include copper, palladium, silver, platinum, and gold.

Types of Corrosion

There are many different reasons for metal corrosion. Some can be avoided by adding alloys to a pure metal. Others can be prevented by a careful combination of metals or management of the metal's environment. Some of the most common types of corrosion are described below.

    1. General Attack Corrosion:

This very common form of corrosion attacks the entire surface of a metal structure. It is caused by chemical or electrochemical reactions. While general attack corrosion can cause a metal to fail, it is also a known and predictable issue. As a result, it is possible to plan for and manage general attack corrosion.

   2. Localized Corrosion:

Localized corrosion attacks only portions of a metal structure. There are three types of localized corrosion:

• Pitting -- the creation of small holes in the surface of a metal.
• Crevice corrosion -- corrosion that occurs in stagnant locations such as those found under gaskets.
• Filiform corrosion: corrosion that occurs when water gets under a coating such as paint. weakness.

   3. Galvanic Corrosion:

Galvanic corrosion can occur when two different metals are located together in a liquid electrolyte such as salt water. In essence, one metal's molecules are drawn toward the other metal, leading to corrosion in only one of the two metals.

   4. Environmental Cracking:

When environmental conditions are stressful enough, some metal can begin to crack, fatigue, or become brittle and weakened. 

Cause of  The Corrosion

    Corrosion is an electrochemical reaction that appears in several forms, such as chemical corrosion and atmospheric corrosion, the latter of which is the most common form. When acidic substances (including water) come in contact with metals, such as iron and/or steel, rust begins to form. Rust is the result of corroding steel after the iron (Fe) particles have been exposed to oxygen and moisture (e.g., humidity, vapor, immersion). When steel is exposed to water, the iron particles are lost to the water’s acidic electrolytes. The iron particles then become oxidized, which results in the formation of Fe⁺⁺. When Fe⁺⁺ is formed, two electrons are released and flow through the steel to another area of the steel known as the cathodic area.

Oxygen causes these electrons to rise up and form hydroxyl ions (OH). The hydroxyl ions react with the FE⁺⁺ to form hydrous iron oxide (FeOH), better known as rust. Where the affected iron particles were, has now become a corrosion pit, and where they are now, is called the corrosion product (rust). 

Effects Of Corrosion

     The chemistry corrosion that helps to understand the effect of corrosion over metal surface. In general, metals have tendency to loss electrons and convert into metal cations through metal oxidation reaction.

M→M++e−

Some metals loss one electron like sodium and some more than one like Fe, Cu etc. The loss of electrons from metal is a common anodic reaction.

Fe→Fe2++2e−

The free electrons produce from metal atoms at move to cathodic site for further reaction. At cathode, electrons react with atmospheric oxygen and moisture (water) to for hydroxyl ions.

O2+4e−+4H2O→4OH−

These hydroxyl ions quickly react with metal ions to form metallic hydroxides. Overall metal deteriorate to metal oxides. Some of the noble and non-reactive metals like gold, platinum do not react with atmospheric oxygen and do not corrode.

Corrosion Prevention

    The World Corrosion Organization estimates the global cost of corrosion to be about US$ 2.2 trillion annually, and that a large portion of this - as much as 25% - could be eliminated by applying simple, well-understood prevention techniques. Corrosion prevention should not, however, be considered solely a financial issue, but also one of health and safety. Corroded bridges, buildings, ships, and other metal structures can and do cause injury and death.

An effective prevention system begins in the design stage with a proper understanding of the environmental conditions and metal properties. Engineers work with metallurgical experts to select the proper metal or alloy for each situation. They must also be aware of possible chemical interactions between metals used for surfaces, fittings, and fastenings.