Cause of corrosion in steel and also between dissimilar metals

Corrosion is a natural process, which converts a refined metal to a more chemically-stable form, such as its oxide, hydroxide, or sulfide. It is the gradual destruction of materials (usually metals) by chemical and/or electrochemical reaction with their environment.

It requires a combination of moisture and oxygen. Once those two elements come together, and in contact with steel, A multi-stage process begins:

First, the iron (Fe) atoms that comprise steel lose some electrons and become positively charged. Positively charged ions attract negatively charged ions.
Second, water (H2O) and oxygen (O), mix together and become even more negatively charged, thus attracting themselves to the positively-charged iron atoms mentioned above. The result is a chemical called iron hydroxide (4Fe(OH)2).
Iron hydroxide continues to react with oxygen, yielding 2Fe2O3.H2O – also known as hydrated iron oxide OR brown rust.

BIMETALLIC CORROSION

The second type of corrosion that affects steel members is called bimetallic corrosion. This type of erosion occurs when a chemical reaction is caused by two metals coming in contact. Bimetallic corrosion occurs most frequently in steel structures that are submerged or buried.

Corrosion Protection.

• Cathodic protection
• Corrosion inhibitors.

1. Cathodic protection – The principle of this method is to alter the electrode potential of the metallic structure so that they can lie in the immunity regionTwo methods are used to apply the cathodic protection to a metal structure.
   1. Impressed Current – This method is used for the protection of pipelines and the hulls of ships in sea water. In this method, an electric current is applied to the metal surface by use of DC electrical circuit. The negative and positive terminal of the current source is connected to the metal requiring protection and an auxiliary anode respectively. The flow of electric current charges the structure with electrons and changes the electrode potential in the negative direction. This process continues until it reaches the immunity region. The current flows from anode to cathode. Thus it protects the metal surface from corrosion.
   2. Sacrificial Anode – This is especially used for ships, offshore oil and gas production platform etc. In this technique, the more reactive metal is used to alter the electrode potential and get the immunity region. Zinc is generally used as a sacrificial anode. It generates the anodic dissolution current with more negative potential. The cathodic curve intersection is now at a more negative potential which is the immunity region. At this region, the corrosion rate of steel is negligible.
2. Corrosion Inhibitors – According to surface chemistry, the presence of foreign molecules affect the surface reactions.
   Corrosion processes are also a type of surface reactions. These can be controlled by foreign compounds which are known as inhibitors. 
   The inhibitors get adsorbed on the reacting metal surface. It attaches directly to the surface or adsorbs up to one molecular layer of the metal surface. This is a well-known method for controlling corrosion.
   The inhibitors can work in different ways; it may block the active sites of corrosion and restrict the rate of the anodic or cathodic process, or it may increase the electrode potential etc. 
   Hexylamine or sodium benzoate are used as inhibitors for anodic reactions. 
   Similarly, oxidising agents like nitrite, chromate, red lead, amines, thio-urea etc are also used as corrosion inhibitors.