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Form of Corrosion (Pitting Corrosion (Mechanism ((Localized adsorption of…
Form of Corrosion
Galvanic Corrosion
- Occurs when dissimilar metallic materials are brought into contact in the presence of electrolyte.
- Occur between metal and other conducting materials.
- Attack at the junction between two dissimilar metals or alloys.
Factors
- Electrode potential based on Galvanic series position.
- Area effect is ratio of anodic to cathodic areas.
- Corrosion = current density (I/A)
- Environmental affect (moisture, soil etc)
Properties
- Significant if difference corrosion potential of metals/alloys more than 250mV.
- Corrosion potentials for different two metals needed to be close as possible for metals to be in contact.
Control
- Combinations of metals as close as possible.
- Avoid the unfavourable area effect of a small anode and large cathode.
- Insulate dissimilar metals wherever practicable.
- Add inhibitors to decrease the aggressiveness of the environment.
- Install third metal that is anodic to both metals in the galvanic contact.
Pitting Corrosion
- Form of extremely localized attack in environment that contains aggressive ions resulting in holes in metal.
- Diameter of pits maybe the same or less than the depth
Pitting factor:
P/d
where p: deepest pit length
d: penetration thickness ave. of uniform
corrosion
Mechanism
- Localized adsorption of aggressive ions
- Passive film breakdown (due to corrosion potential increases ≥ Epitt)
- Pitt growth (if repassivation cannot occur) rate of corrosion inside pits is significantly high due to autocatalytic process.
Type of Pitting
- Narrow & deep
- Elliptical
- Wide & shallow
- Subsurface
- Undercutting
- Determined by microstructural observation
Autocatalytic process
- Initially a small pit is formed
- Dissolved Oxygen is difficult to diffuse into the pit - leading to the formation of very small anodic area inside the pit and very large cathodic area outide the pit which still covered by the passive film.
- Excess positive charge due to metal ion accumulation inside the pit t is
necessarily balanced by the migration of aggressive ions into the pit.
Control
- Adding inhibitor including increase pH
- Cathodic protection
Crevice Corrosion
- A localized form of corrosion caused by the deposition of dirt, dust, mud and deposits on a metallic surface or by the existence voids, gaps and cavities between adjoining surfaces.
- Associated with small volumes of stagnant solutions.
- Must be wide enough to permit liquid entry, sufficiently narrow to maintain a stagnant zone
Mechanism
- Anodic & cathodic reaction occur uniformly over the entire surface of metals.
- After a short interval, the oxygen within the crevice is depleted due to convection restriction.
- Dissolution of metal inside crevice continues, excess positive charge is produced inside the crevice.
- This results in migration of chloride ions into crevice (to balance charge). Hydrolysis of ferrous ions is taking place (except for alkali metals).
- Both chloride and hydrogen ions accelerate the dissolution rates.
- Both chloride and hydrogen ions accelerate the dissolution rates and corrosion of steel inside crevice .
- Thus, protect external surface and attack localized within shielded areas.
Control
- Use welded butt joints instead of riveted or bolted joints.
- Close crevices in existing lap joints by continuous welding or soldering.
- Design vessels for complete drainage, avoid sharp corners and stagnant
areas.
- Inspect equipment and remove deposit frequently.
- Remove solid in suspension early in the process of plant flow sheet.
- Use solid nanoabsorbent gaskets such as Teflon.
Intergranular Corrosion
Localized attack at and adjacent to grain boundaries, with relative little corrosion at the grains.Caused by
- Impurities at the grain boundaries
- Enrichment of one of the alloying elements
- Depletion of elements in the grain boundaries areas
Knife line attack (KLA)
- Occurs for only a few grain diameters
- Carbon reacted with Nb/Ti to form stable carbide
- Heated > 1230 oC, followed by rapid cooling
- Reheated to critical temperature - Failure can occur
Preventing IGC :
- Lower acidity and less oxidizing conditions
- Solution annealing
- Heating the alloy above 815 oC, followed by rapid cooling.
- Decreases carbon content to ≤0.03%
- Alteration of environment
- Use stabilized stainless steel
Preventing KLA :
- Heating above 815 oC - dissolve chromium carbide, to form Nb/Ti carbide
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