Oxidation and Corrosion: Types, Causes, and Protection Methods
Oxidation and Corrosion
Oxidation
If a substance combines with oxygen, the process is called oxidation, and the resulting combination is called rust.
Examples: film, forge (Fe2O2), carbon dioxide (CO2), aluminum oxide (Al2O3).
Oxidation also releases heat. The more rapid the oxidation, the higher the temperatures reached.
A very rapid oxidation, with a strong development of light and heat, is called combustion.
When oxygen is removed from an oxide, it is called reduction.
Examples: mercury oxide decomposition, electrolysis of water, obtaining most metals.
Many metal oxides occur in nature as minerals. They are important because they are used to extract metals. For example, iron is obtained by removing oxygen from iron oxide (iron ore) using carbon and heat in a blast furnace.
The formation of rust on iron exposed to moist air is a common example of oxidation. This rust has a spongy texture, allowing oxidation to proceed through the pores until all the iron has been affected. This process is called corrosion.
In some metals, such as lead, zinc, and aluminum, oxidation occurs but not corrosion. These metals form a thin layer of oxide that protects the underlying metal from further oxidation.
Metals like chromium and nickel are resistant to spontaneous oxidation, while others, such as gold, silver, and platinum, do not combine with oxygen at all. These are called noble metals.
Corrosion
Corrosion is the chemical, electrochemical, or physicochemical reaction of a metallic material with its environment, which leads to a change in its properties.
In general, materials are influenced by the atmosphere. The air contains oxygen, water vapor, smoke, sulfur and phosphorus compounds, combustion gases such as carbon dioxide or sulfur dioxide, and dilute acids such as carbonic acid, sulfuric acid, and nitric acid.
Most metals combine with oxygen, water, sulfur, phosphorus, or carbon in the form of minerals. Once extracted, metals tend to return to their initial state. This is the cause of the destruction (corrosion) that appears in many metals.
Types of Corrosion
- Uniform Corrosion: Erosion occurs almost uniformly over the entire surface, e.g., oxide formation.
- Pitting Corrosion: Occurs only at specific points on the surface.
- Crevice Corrosion: Occurs in cracks and crevices where moisture accumulates. It is due to the different concentrations of oxygen in the water.
- Galvanic Corrosion: Occurs when two metals are joined in the galvanic series through an electrolyte. The less noble metal corrodes.
- Intergranular Corrosion: Occurs along grain boundaries and results in reduced strength.
- Transgranular Corrosion: Usually occurs under tensile loading and runs parallel to the direction of deformation within the grain.
Corrosion with Mechanical Stress
- Stress Corrosion Cracking: Caused by tensile stress and the action of corrosive media. Cracking occurs intergranularly or transgranularly.
- Corrosion Fatigue: A type of fatigue caused by alternating stress in a corrosive environment.
Protection Against Corrosion (Non-Metallic Coatings)
Corrosion protection involves separating the metallic material from the corrosive environment by applying coatings to prevent or reduce corrosion.
- Oiling and Greasing: Used for parts that need to be kept bright (e.g., calipers). Grease and oil should not contain acids.
- Painting (Brush or Spray Gun): Red lead primer forms a protective layer on the metal. The appropriate topcoat (oil, varnish, etc.) is applied according to the application.
- Enameling: Powdered enamels are applied by dusting or spraying and then baked at 800°C or 1000°C. The coating is resistant to chemicals and heat. The enamel is composed of powdered glass, a mixture of quartz, feldspar, clay, and pigments.
- Plastic Coatings: Obtained by immersion in liquid plastic or by coating. Synthetic resin varnishes, turpentine, and chlorinated rubber are replacing traditional oil paints. There are also lacquers that dry in an oven at 120°C or 150°C and provide good corrosion protection.
- Phosphating (Bonderizing): An aqueous solution of manganese phosphate or zinc phosphate (phosphoric acid salts) is applied to the previously cleaned and degreased metal surface by spraying or dipping. This forms a protective layer of iron phosphate, which usually serves as a base for other coatings.
- Electrolytic Surface Treatment (Anodizing): An artificial oxidation process that enhances the natural oxide layer of aluminum alloys. Surface defects, cracks, and scratches become visible after treatment. The protective layer is hard and durable.
- Anodizing in Sulfuric Acid: A lead plate (negative pole) and the aluminum piece (positive pole) are placed in a sulfuric acid bath (electrolyte). When a direct current is passed through, an oxide layer forms on the aluminum part due to oxygen release. This is called anodizing (aluminum oxidized at the anode).