What are the forms of corrosion?
uniform corrosion
galvanic corrosion
pitting corrosion
crevice corrosion
intergranular corrosion
enviromental assisted cracking
flow affected corrosion
hydrogen damage
What are characteristics of uniform corrosion?
corrosion over all surfaces
What is a good what is bad uniform corrosion rate?
Good < 500µm/year
bad > 5000µm/year
Which type oof corrosion do you see here?
Galvanic corrosion
What are the characteristics of Galvanic Corrosion?
The corroded species is in contact with a more noble material.
The materials work as a galvanic cell in which the corrosion of the less noble material is increased and the corrosion of the noble material is halted.
Over whcih distances does galvanic corrosion work?
Depends on the electrolyte
In sea water over meters, in distilled water over milimeters
How can galvanic corrosion be prevented?
seperating coupled materials (paints, flanges)
different material choice
cathodic protection
What type of corrosion do you see here?
Pitting corrosion
Which Ions most commonly break down passivity?
Cl-, Br-, I-
Where does passivity breakdown occur preferentially?
chemical discontinuities
dislocations
grain boundary
What are the steps in pitting?
Why is there a vacancy space growing below passivated films?
Why can CL- migrate through oxides?
The atom radii are similar
What are the three mechanisms that initiate pitting?
Film penetration
Film thinning
film rupture
How does the film thinning mechanism work?
adsorbtion of ions -> complex building and subsequent dissolution
Why do formed pits propagate?
In the case of Aluminum the formed Aluminum ions (Al3+) attract CL- ions electrically and form H+ ions and further decrease the ph level.
What is the effect of CL- ions on the
What is the meaning of the pitting potential?
Potential value as measure for the resistance to pitting. It is the potential value of passivity breakdown.
What is the meaning of the repassivation potential?
The value under which passivation occurs again, if pits have been created.
What kind of corrosion can you see here?
Crevice corrosion
What is characteristic for the appearance of crevice corrosion?
It appears in small geometric clearances.
Why does Crevice corrosion appear?
because of a hard accessible cavity, the oxigen cannot diffuse as fast to the surface in the cavity as to the regular surface. By that a potential difference is created.
What is the reason for crevice propagation?
The Ions react with water to form acidic solutions, which in turn attract Chloride ions and increase corrosion rate
What is the effect of temperature on crevice and pitting corrosion?
Crevice and pitting corrosion have a minimum activation temperature.
What would be a strategy to prevent crevice corrosion in the following piece:
Installing a butt weld
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