corrosion and painting

CORROSION AND PAINTING

BY HARBEER SINGH(08AU615)

CORROSION• Corrosion is the gradual destruction of material, usually metal,

by chemical reaction with its environment. This means electrochemical oxidation of metals in reaction with an oxidant such as oxygen.

• Many structural alloys corrode merely from exposure to moisture in the air, but the process can be strongly affected by exposure to certain substances.

• Chemicals that inhibit corrosion include salts in hard water, chromates, phosphates, conducting polymers and long-chain organic molecules with ionic end groups.

• Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area more or less uniformly corroding the surface. It occurs on exposed surfaces.

Methods of protection from corrosion

1. Surface treatments:

I. Applied coatings:Plating, painting, and the application of enamel are the most common anti-corrosion treatments. They work by providing a barrier of corrosion-resistant material between the damaging environment and the structural material.

Flexible polyurethane coatings, like Durabak-M26 (have excellent abrasion and chemical resistance), can provide an anti-corrosive seal with a highly durable slip resistant membrane


II. Reactive coatings

Corrosion inhibitors are added in a controlled environment. These form an electrically insulating or chemically impermeable coating on exposed metal surfaces, to suppress electrochemical reactions.


III. Anodization

Anodizing is a process used to increase the thickness of the natural oxide layer on the surface of metal parts. The process is called "anodizing" because the part to be treated forms the anode electrode of an electrical circuit. Anodizing increases corrosion resistance and wear resistance, and provides better adhesion for paint primers and glues than does bare metal.

Process:Alloys are cleaned in either a hot soak cleaner or in a solvent bath and may be etched in sodium hydroxide, ammonium bifluoride or brightened in a mix of acids.


IV. Biofilm coatings

A new form of protection has been developed by applying certain species of bacterial films to the surface of metals in highly corrosive environments. This process increases the corrosion resistance substantially.


2. Cathodic protection

Cathodic protection (CP) is a technique to control the corrosion of a metal surface by making that surface the cathode of an electrochemical cell. Cathodic protection systems are most commonly used to protect steel, water, and fuel pipelines and tanks; steel pier piles, ships, and offshore oil platforms.

PAINTING OF AUTOMOBILE• The corrosive nature of a metal used in a vehicle body

construction, necessitate the application of an anti-corrosion coating. For this reason the painting should be done.

Function of Painting

1. It reduces the corrosion and protects the vehicle Body.2. It is used for cleanliness.3. It is used for esthetic appearance.4. It is used to reduce the friction due to the aerodynamic drag and allow the vehicle in stream lining.5. It is used for identification purpose.

Requirements of Good Painting

1. It should be anticorrosive nature.2. It should have esthetic appearance.3. It should be smooth finish for stream lining.4. It must reduce the aerodynamic drag.

Main Constituents of paints

1. Pigments2. Drying oil3. Thinners4. Dry Extenders5. Plasticizers6. Resins.

Selection of type of painting• roller or brush for tight spaces• spray for larger coating areas

Spray Painting procedure1. First of all the old paint should be scrapped with the help of scrappers2. If there is nail or any obstruction they will be removed with the help of pincers3. The removal of old paint must be by application of caustic soda solution and spirit solutions.4. In some portions heating is necessary with the help of blow lamp for removal of old paint.5. The surface should be cleaned from dirt, dust, rust, grease etc.6. The surface should maintain perfect level from any kind of bends.7. Then once again clean with red oxide.8. After wards duco paints or delux paints or required colour may be sprayed on the vehicle as the primary coating.9. After first coating is over we must see the variations of the surface clearly and apply second coating.10. Finally polish the vehicle thoroughly.