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Form of Corrosions (Crevice Corrosion (What is crevice corrosion?…
Form of Corrosions
Crevice Corrosion
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Prevention
- Close crevices in existing lap joints by continuous welding, caulking, or soldering.
- Use welded butt joints instead of riveted or bolted joints in new equipment. Sound welds and complete penetration are necessary to avoid porosity and crevices on the inside (if welded only from one side)
- Avoiding crevices during fabrication
- Alloy and material selection to avoid crevice
corrosion
Type of Crevices Corrosion 
Intergranular Corrosion
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Prevention
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Reduce Carbon content
- Reduce to below 0.03%
- Insufficient carbon for carbon formation
- Low Carbon usually attractive to avoid sensitization
- Disadvantage, Low carbon give lower strength at high temperature
Welding Technique
- Low Carbon Welding and Electric Arc Welding are recommended
Galvanic Corrosion
Mechanism
When two metals come in contact in the presence of an electrolyte, the more reactive metal acts as an anode whereas the less reactive metal acts as a cathode.
The electrolyte offers a channel for the movement of particles, which in turn leads to the rapid eroding of the anodic metal.
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Introduction
- Corrosion damage induced when two dissimilar materials are coupled in a corrosion electrolyte.
- occurs when two or dissimilar metal are brought into electrical contact under water.
- one metal become anode(corrode faster), another become cathode(corrode slower).
- rate of corrosion depends on the standard electrode potential, upper more cathodic(reduction), lower more anodic(oxidation).
Prevention
- Choose metals that are not dissimilar or as close together as possible in the galvanic series.
The more closely together the metals in the galvanic series, the lesser the potential difference and hence the lesser the galvanic current.
- Application of a protective metallic coating
- sacrificial coating
- provide galvanic protection to the base metal when the coating is measurably more anodic than the base metal.
- Galvanic corrosion will take place with the anodic material when the base material is exposed.
- Ensure no contact with an electrolyte
using water-repellent compounds such as greases, or by coating the metals with an impermeable protective layer, such as a suitable paint, varnish, or plastic
- Keep small anodes from contacting large cathodes
- The rate of corrosion depends on the surface area of the anode with respect to the cathode.
- The smaller the surface area of the anode relative to the surface area of the cathode, the more concentrated the flow of electrons at the anode and the faster the rate of corrosion.
Pitting Corrosion
What is Pitting
- A form of very localized corrosion attack in which cavities or holes form
- Ordinarily penetrate from the top of a horizontal surface downward in a nearly vertical direction
- One of the most damaging forms of corrosion
- Found on passive metals and alloys such aluminium alloys, stainless steels and stainless alloys
Types of Pitting Corrosion
Mechanism
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- The reaction taking place will be : Fe = Fe2+ + 2e- (dissolution of iron)--anodic
1/2O2 + H2O + 2e- = 2(OH-)—cathodic
*In presence of chloride ions pits are growing by autocatalytic mechanism. *The electrons given up by the anode flow to the cathode (passivated surface)*These results the electrolyte enclosed in the pit gains positive electrical charge.*The positively charged pit attracts negative ions of chlorine Cl- increasing acidity of the electrolyte according to the reaction:
FeCl2 + 2H2O = Fe(OH)2 + 2HCl*PH of the electrolyte inside the pit decreases (acidity increases) which causes further corrosion process.
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Prevention
- Proper selection of material with high pitting resistance
- Control pH, chloride concentration and temperature
- Cathodic protection and anodic protection
- Use high alloy for increased resistance to pitting corrosion
- Agitation of stagnant fluid
- Good surface finish required
- Controlling oxygen level