What is the component that prevents stainless steel from rusting?
Chromium
What is the major reason for Stainless steel corrosion failure?
MnS inclusions
What is the mechanism of stainless steel corrosion?
MnS dissolutes in chloride solution and elemental sulfur is deposited on the inclusion
steel matrix is dissolved by the Cl and S combination
trenches form around the inclusion, where the pH value drops due to hydrolisis
low pH value and potential cause further corrosion
How does the Molybdenum content effect pitting resistance?
Increase in Molybdenum increases Pitting potential <=> pitting is becoming more difficult
Why does Molybdenum increase pitting resistance?
molybdenum inhibits the corrosion products and educts
What is the CPT?
The Critical Pitting temperature <=> the lowest temperature at which pitting is possible
What is the PREN and how does it calculate?
Pitting Resistance Equivalent Number which indicates the susceptibility to pitting
How do Duplex steels differ from other stainless steels?
lower Nickel content, but higher Cromium
What PREN value is needed for Sea water application
>40
What is Sensitization?
The corrosion on chromium depleted grain boundaries
What is the structure of the Aluminium oxide layer?
Two layers:
outer porous (permeable) and hydrated layer
inner compact smorphous layer
Is the outer or inner layer of aluminium oxide more protective?
The inner layer
Into which products does Al corrode into acid and neutral/alkaline envroments?
What is the white component in Aluminium rust?
Bayerite
What are the rough pH values of the passivation spectrum of Aluminium?
4-9 pH
What is the isoelectric point?
The pH value for which there is no charge on the surface. (See ceramics)
What is the isoelectric point of Alumina? And what happens if the pH value is beneath or above?
9,5
positive charge if below
negative charge if above
What is the major form of corrosion in alloyed Alluminium?
Pitting corrosion
What is the correlation between yield strength and corrosion resistance?
They are antiproportional
Which elements increase Aluminium corrosion potential, which elements decrease the corrosion potential?
Increase: Mn, Cu, Si
Decrease: Mg, Zn
What is difference in corrosion between a anodic and acthodic (compared to the bulk material) inclusion?
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