Chemical Engineering @ChemengFiles.Com

The bites are one of the most destructive forms of corrosion as a potential cause of failure of equipment due to perforation and / or penetration. These usually occur in areas protected by metal oxide coatings such as stainless steel, aluminum, etc.. Typically in boiler water systems and supply, increased risk of pitting corrosion increases dramatically with increasing content of oxygen in the fluid.

Stings can occur in any metal surface.

Corrosion Mechanism

In the figure below, the oxygen abundance in the liquid comes into contact with the metal surface thus becomes a cathode (top). At the bottom, the low oxygen level that party becomes an anode. This will form a complete circuit where the metal is ionized to release an electron and to form the ion Fierro (FE2 +), this electron will move to the top to react with oxygen (O2) (and water, H2O) to form ions hydroxide form (OH-). Iron ion (Fe2 +) react with hydroxide ions (OH-) to form Iron Oxide (Fe2O3) that is typically a brown rust.

Preventive Measures

There are various measures to avoid corrosion bite. These include:

* Selecting appropriate material, such as the molybdenum SS316 has higher resistance to pitting compared to the SS304.

* Use high alloys (ASTM G48) to increase resistance to pitting.

* Control the amount of oxygen scavengers by injecting the same into the process water.

* Proper control of pH, chlorine concentration and temperature.

* Cathodic protection and / or anode.

* Monitoring of oxygen and chlorine levels.

* Agitation stagnant fluid.