Mechanism Of Dry Corrosion

A ppt on dry corrosion

1. DRY CORROSION

2. C orrosion 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 what gases that are in contact with the metal, determine the form and rate of deterioration

3. Types of corrosion 1. Wet corrosion – It is also known as electrochemical corrosion . Such type of corrosion is due to the flow of electron from metal surface anodic area towards cathodic area through a conducting solution. 2. Dry corrosion – It is also known as chemical corrosion . Such type of corrosion takes place due to direct chemical action of atmospheric gases and anhydrous liquids on the metal surface.

4. Dry corrosion It takes place due to direct chemical action of atmospheric gases like CO2 , SO2 . It is of three types 1 ) Oxidation corrosion :- It takes place by direct action of oxygen on metal. It occurs in absence of moisture . It mostly occur at ordinary temperature.

5. Mechanism : When a metal is exposed to air it gets oxidized by loosing its valence electrons & reduction of oxygen take place M+n /4 (o^2) => M^n + + O^2- At point of contact of m^n + & o^2- metallic oxide will form & that metallic oxide scale forms a barrier to restrict further oxidation of inside metal . Since size of cation ( M^n +) is smaller than anion . Hence cation will diffuse much faster than anion through the scale for continuation of oxidation , it can be possible if the metallic oxide barrier is sufficiently porous . The nature of oxide film plays very important role in oxidation corrosion. a. When oxide film is stable and tightly adhering , it will act as protective coating and corrosion is prevented. b. When oxide film is unstable and has tendency to decompose back to metal and oxygen , it does not go into oxidation corrosion.

6. c . When film is volatile then metal surface again come into contact with air and oxidation take place d . If film is sufficiently porous then continuous oxidation take place. 2 . Liquid metal corrosion - When a liquid metal is allowed to flow over solid metal at high temperature is called liquid metal corrosion. Due to this solid metal gets weak . Example - In nuclear reactor sodium metal is used as coolant & it leads to corrosion of cadmium .

7. 3. Corrosion by other gases :- Some gases react with certain metal and forms a protective or non-protective layer on metallic surface . Due to chemically combination of metals with gases metals undergo corrosion . The extend of corrosion depends upon – A- Nature of environment . B- Nature of oxide film formed .

8. Prevention Corrosion requires three conditions: moisture, a metallic surface, and an electron acceptor. If you remove any of these three criteria, corrosion is inhibited. Corrosion can be prevented by giving the metal to be protected a negative charge. One way to protect metals from corrosion is by sacrificing a metal with a more active reduction potential through sacrificial coating or sacrificial anodizing. Developing a thick layer of corrosion in a process known as anodizing protects underlying metals from corrosion.