Please enable JavaScript.
Coggle requires JavaScript to display documents.
FORMS OF CORROSION (INTERGRANULAR CORROSION (Localized attack along and…
FORMS OF CORROSION
INTERGRANULAR CORROSION
Localized attack along and adjacent to grain boundaries, while the bulk of the grains remain largely unaffected. Caused by:
- Impurities at the grain boundaries
- Enrichment of alloying element
- Depletion of alloying element
In aluminium, segregation of irons in the grain boundaries caused IGC while in stainless steels, IGC is caused by the depletion of chromium in the grain boundaries.
Prevention
ICG
- Use post-weld heat treatment
- Decrease the amount of carbon content (below 0.03%)
- Use stabilized grades alloyed with titanium or niobium
- Environment with lower acidity and less oxidizing condition
- Alloy is heated above 815 degree celsius to redissolve chromium carbide followed by rapid cooling
KLA
- Heat above 815 degree celsius to dissolve chromium carbide and formed Nb or Ti carbide
Knife Line Attack (KLA)
- Occurs only a few grain diameters immediately adjacent to weld bead in stabilized austenitic stainless steels (321 and 347).
- No formation of chromium carbide since carbon is reacted with Nb or Ti.
- When the material is heated more than 1230 degree celsius followed by rapid cooling, carbon adjacent to weld bead will remain in solid solution.
- If the material is reheated to critical temperature, localized sensitization and IGC occured in the narrow sensitized region.
Mechanism
When the metal is heated between 425-815 degree celsius, chromium in the stainless steel will reacts with carbon and formed chromium carbide at grain boundaries.
-
IGC in various alloys
- Ferritic stainless steels: Solubility of carbon and nitrogen is significant in ferrite. Ferrite stainless steels sensitize rapidly at low temperature. IGC in ferrite stainless steels only can prevented by soaking at 800 degree celsius or slow cooled at 700-900 degree celsius to replenish chromium-depleted grain boundaries.
- Duplex stainless steels: Chromium diffuses faster from ferrite than from austenite. Carbides are preferentially at the ferrite-austenite boundaries.
- High strength aluminium alloys: Intermetallic compounds active in aluminium matrix and corrode preferentially at the grain boundaries.
GALVANIC CORROSION
Prevention
- Electrically insulate the two metals from each other
- Ensure there is no contact with electrolyte
- Choose metals with similar electrode potentials
- Cathodic protection by using sacrificial anodes
- Add inhibitors to decrease aggressiveness of environment
-
Factors
- Electrode potential
- Area effect - Corrosion is likely to happen at the area ratio consists of large cathode and small anode (high current density)
- Corrosion = current density (I/A)
- Environment effect - Condensate near seashore contains salt thus more corrosive
CREVICE CORROSION
Localized attack on a metal surface caused by the deposition of dirt, dust or adjoining surface caused by gaps and voids.
Mechanisms
- Anodic reaction: Fe → Fe2+ + 2e-
- Cathodic reaction: O2 + 2H2O + 4e → 4OH-
- After a period of time, oxygen within the crevice is depleted.
- The crevice has excess positive charged due to continuous dissolution of metal.
- This induce the migration of chloride ions into crevice, resulted in hydrolysis of ferrous ions
Fe+2Cl- + 2H2O → Fe(OH)2 + 2H+Cl-
- HCL increased the acidity inside the crevice which causes further acceleration of corrosion.
-
Prevention
- Use welded butt joints instead of riveted or bolted joints
- Use solid, non-absorbent gaskets such as Telfon
- Eliminate crevices in lap joints by continuous welding or soldering
- Use higher alloys for increased crevice corrosion resistance
- Avoid stagnant conditions and ensure complete drainage
PITTING CORROSION
Localized corrosion that leads to the creation of small holes in the metal caused by:
- Localized chemical or mechanical damage to the protective oxide film
- Localized damage to the protective coating
- Presence of halogen ions (Cl-, Br-, I-)
- Stagnancy of electrolyte
-
Mechanism
Autocatalytic Process
- Anodic area- Inside the pit due to low diffusion of oxygen.
Cathodic area- Outside the pit which is covered by passive film.
- Excess positive charges inside the pit attract the migration of aggressive ions (Cl-).
- Metal ions diffused outward, hydrolyzed and deposited in front of pit.
- Hydrogen ions diffused inward causing acidity level in the pit increased.
-
- Localized adsorption of aggressive ions
- Passive film breaks down (corrosion potential > electrode potential of pitting)
- Pitt growth if repassivation cannot occur. Rate of corrosion is high inside the pit due to autocatalytic process
-
Prevention
- Proper selection of materials with resistance to service environment
- Control pH, chloride concentration and temperature
- Cathodic protection
- Use higher alloy to increase corrosion resistance
-
