How To Prevent And Control Ship Corrosion|
Corrosion : Gradual wearing away of materials by oxidation.
Erosion : Degradation of material surface due to mechanical action.
Corrosion
The corrosion of metals may be considered as the material returning to its original form as a metal oxide. If conditions are correct for corrosion moisture, acids, salts, etc. The tendency is for the material to revert back to an oxide of iron by combination with oxygen. (An oxide is an element combined with oxygen, hence oxygen must be present for the transformation to take place). For example, chromium, which is used in the alloy stainless steel, can form a microscopic film of chromium oxide upon the surface of the steel which prevents further corrosion. Furthermore, aluminium, which corrodes very rapidly, is quickly rendered non-corrosive owing to the passive oxide film which forms.
There are many types of corrosion:
1. Corrosion of Metals in Sea Water (Galvanic Corrosion)
Galvanic corrosion is caused when two dissimilar metals come into contact with each other while immersed in an electrolyte like salt water.
With dissimilar metals in sea water the strength and direction of the action depends upon where the metals are in the galvanic series table. If the metals are close together then there is very little interaction between the two metals. If they are far apart in the table then there will be much more interaction happening. In this process it is the more anodic metal that corrodes.
Sacrificial anodes are sometimes used deliberately to give cathodic protection to more expensive material ; a well-known example is the zinc anodes, which are used as cathodic protection for propellers. The anodes are placed on the hull of the vessel during dry dock and they corrode away instead of the propeller. The anodes are then replaced at the next dry dock.
2. Graphitisation of Cast Iron
This phenomenon occurs in non-alloy cast iron exposed to medium acid or soft water.
When this type of corrosion occurs, a layer of iron oxide containing graphite forms on the surface. This layer preserves the shape of the part while corrosion is taking place and, therefore, this type of corrosion cannot be detected by means of a visual inspection.
Effect of speed of ship on graphitisation
If the velocity of the sea water relative to the material increases (due to the ship moving through the water) the corrosion rate also increases. The reason for this is two fold :
(1) increased supply of oxygen
(2) erosion of protective oxide films formed by corrosion.
3. Stress Corrosion
Stress-corrosion occurs when a material exists in a relatively inert environment but corrodes due to an applied stress. The stress may be externally applied or residual. This form of corrosion is particularly dangerous because it may not occur under a particular set of conditions until there is an applied stress.
4. Dezincification
This mainly occurs in alloys like brass which is a combination of zinc and copper.
Dezincification is the leaching of zinc from copper alloys in an aqueous solution. It is an example of dealloying in which one of the constituents of an alloy is preferentially removed by corrosion. Dezincification is similar to graphitization in that one constituent of the alloy (zinc) is selectively removed, leaving the other (copper) behind.
Dezincified brass retains the original shape and dimensions of the metal component before corrosion, but the residue is porous and has very little strength.
5. Fretting corrosion
Fretting corrosion can occur where two surfaces in contact with each other undergo slight oscillatory motion, of a microscopic nature, relative to one another. Components to which this may occur are those which have been shrunk, hydraulically pressed or mechanically tightened one to the other. The small relative motion causes removal of metal and metal oxide films. The removed metal may combine with oxygen to form a metal oxide powder that will, in the case of ferrous metal, be harder than the metal itself thus increasing the wear. Removed metal oxide film would be repeatedly replaced increasing the damage.
6. Pitting corrosion
Pitting corrosion is often caused by exposure to aggressive environments such as saltwater, acidic or alkaline solutions, and high-chloride environments. It can also be caused by localised mechanical damage to the surface of the metal, which can create small areas of exposed metal that are more susceptible to corrosion.
The corrosion process in pitting corrosion typically begins with the breakdown of the passive film that normally protects the metal surface from corrosion. This can occur due to the presence of aggressive ions or localised mechanical damage to the surface. Once the passive film is breached, an electrochemical reaction occurs at the site of the pit, leading to the formation of a small cavity or pit.
As the corrosion process continues, the pit can grow deeper and wider, eventually leading to perforation of the metal surface. Pitting corrosion can be particularly dangerous because it can occur even in metals that are typically resistant to corrosion, such as stainless steel.
7. Corrosion fatigue
Corrosion fatigue occurs when a metal is repeatedly subjected to cyclic loading, such as stress or strain, in a corrosive environment. The repeated loading causes microcracks to form on the metal surface, which can then grow and propagate over time due to the corrosive environment. These microcracks can eventually lead to macroscopic cracks, which can cause the metal to fail.
The corrosive environment can accelerate the crack growth process by reducing the strength and ductility of the metal, as well as by introducing new crack initiation sites. The corrosive environment can also lead to the formation of corrosion products, which can act as stress raisers and further accelerate crack growth
How to Avoid Corrosion and Erosion on Ships
To avoid corrosion and erosion, ships need to implement preventive measures. Here are some ways to avoid corrosion and erosion on ships:
- Regular Inspections: Regular inspections can help identify areas that are prone to corrosion and erosion. These areas can be monitored and repaired before they become a bigger issue.
- Protective Coatings: Applying protective coatings to metal surfaces can help prevent corrosion and erosion. These coatings act as a barrier between the metal and the environment, preventing the metal from coming into contact with water or other corrosive materials.
- Cathodic Protection: Cathodic protection is a method of preventing corrosion by placing a sacrificial metal near the metal that needs to be protected. The sacrificial metal will corrode instead of the protected metal.
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