Corrosion is one of the main concerns for both fastener manufacturers and buyers. Corrosion leads to deterioration of fastener’s structural integrity, and may also damage the item that is being held together. Hence, it is of utmost importance that people check for apt corrosion resistance before opting for a fastening solution. Fasteners made from robust materials like stainless steel, or fasteners that come equipped with a corrosion resistive coating or plating have a longer life span, and they also lead to lower costs over time.
Fasteners Corrosion v/s Rusting:
Trying to differentiate between Fasteners corrosion and rusting is one of the common mistakes people make. Corrosion is defined as the oxidation of the upper surface of metals (copper, aluminium, gold, silver, etc.). This oxide sticks to the surface of metals and prevents further oxidation. It’s not the case with iron , here the oxide doesn’t stick on but it chips off from the surface thus exposing the inner layers and this is termed as rusting.
Types and Causes of Fasteners Corrosion:
Following are the different types of Fasteners corrosion, their common place of occurrence, causes and prevention:
1. Cavitation Corrosion
Occurrence: Pump impellers, propellers, hydraulic turbines, etc.
Cause: Caused due to liquids flowing with high velocity with constant change in pressure.
Prevention: Reduction in turbulence and velocity of flow. Using inhibitors and better raw materials for fasteners.
2. Crevice Corrosion
Occurrence: Metals or alloy components with holes and joined by fasteners or rivets with or without gaskets.
Cause: Caused due to collection of a salt solution in impurities such as dust, sand or corrosive substances.
Prevention: Seal the crevices by continuous welding or sealing. Ensuring proper drainage and avoiding collection of materials.
3. Fatigue Corrosion
Occurrence: Common in heavy machinery. An axle or a shaft in a machine can be subjected to this form of corrosion.
Cause: A metal fractures under repeated cycles of stress. Metals under this form of fasteners corrosion leads to large smooth areas and small rough surfaces.
Prevention: Reduction in fatigue caused by vibration. Using better materials for fasteners. Using coating and inhibitors. Avoiding pressure fluctuations.
4. Fretting Corrosion
Occurrence: Engine components, bolted parts of a machine subject to excessive vibrations and automobile parts.
Cause: Caused mainly due to vibration. The contact between two vibrating parts of a machine lead to this form of corrosion. It is a form of atmospheric corrosion.
Prevention: Reducing vibration using dampeners. Using adequate foundation.
5. Galvanic Corrosion/ Bimetallic Corrosion
Occurrence: Corrosion of anodized steel sheet or tin coated steel containers.
Cause: When two metals with two dissimilar electrode potentials come into contact with an electrolyte, one metal gets corroded faster.
Prevention: Choosing the right type of metals and alloys. Opting for proper insulation of materials. Applying suitable coating and keeping it intact.
6. Inter-granular Corrosion
Occurrence: Austenitic stainless steel prone to chromium depletion at the granular interfaces.
Cause: Austenitic stainless steel is made from iron, carbon and chromium. During welding, the chromium and carbon react to form chromium carbide which dissolves.
Prevention: To prevent this from of granular corrosion, carbon content is reduced to less than 0.05%. Other option is to use other metals like titanium
7. Pitting Corrosion
Occurrence: Aluminium alloys and stainless steels are the most affected by this type of fasteners corrosion. The corrosion results in the formation of pits.
Cause: These metals and alloys have oxide layer, which is liable to attack by agents like acids, alkalies, chloride.
Prevention: Using anodic protection and using sustainable materials. Eliminating contact with acidic, alkaline and chloride salts.
Occurrence: Any component of a machinery. Irrespective of the metals or alloys used.
Cause: any component of a machinery in constant stress and facing a corrosive material at the same time.
Prevention: Controlling stress, pre-determining chemicals likely to come into contact and choosing suitable materials of construction. Controlling operating temperature may also prevent stress corrosion.
Occurrence: This is a form of corrosion which occurs in a uniform manner on the surface of cast iron, or mild steel structures. Progression of this form of corrosion is also uniform.
Cause: Oxidation of all ferrous materials that are exposed to the environment. This is the most common form of corrosion, which happens due to environmental factors.
Prevention: Using thicker materials. Using corrosion inhibitors. Applying apt coatings. Using cathodic protection.
10.Microbes aided Corrosion
Occurrence: Chemical industries, reaction vessels, under-ground pipes, power generating industries.
Cause: This form of corrosion if aided by micro organisms – Aerobic (under presence of oxygen), or Anaerobic (absence of oxygen). Some microbes target manganese or other metals in steel alloys.
Prevention: Chemical treatment with microbicides, bactericides. Mechanical cleaning and reduced contact with water.