Corrosion I and II Flashcards
(15 cards)
1
Q
corrosion types
A
- often overlap with each other
- uniform, galvanic, crevice, pitting, intergranular, selective, leaching, erosion, stress
2
Q
corrosion in stents
A
- any gap, crevice, or hole in a stent coat will create a galvanic potential … local concentrations of released ions results in crevice corrosion
- uniform corrosion can occur as well, if there are no crevices
- corrosion can cause premature device failure
- by-products may be toxic and cause adverse biological/tissue responses
3
Q
galvanic corrosion
A
- ‘dissimilar metal corrosion’
- electrochemical reaction where two dissimilar metals are in electrical contact
- an anode and cathode form due to electrical potential difference (anode corrodes preferentially)
- examples: stents with radiopaque markers and overlapping stents of different materials
4
Q
galvanic series
A
- compares nobility of metals, use to determine which will corrode
- the ‘least noble’ metal becomes the anode and will corrode more rapidly
5
Q
microgalvanic corrosion
A
- occurs on the microscale, leading to self-corrosion of the material
- formation of galvanic couples between microstructures within a heterogenous alloy surface
6
Q
bioresorbable stents
A
- designed to degrade over time to reduce occurrence of thrombosis at long-term follow up
7
Q
analyzing corrosion rate
A
- incorporate fretting corrosion into fatigue/durability testing
- methods to analyze rate: hydrogen evolution testing, electrochemical testing, cyclic electrochemical testing, mass loss measuring, and morphology analysis
8
Q
hydrogen evolution testing
A
- immerse sample in a corrosion medium, then collect the H2 gas produced
- H2 is collected on the basis that the primary cathodic reaction indicates the rate of anodic dissolution
- need to couple with mass loss measurements!
9
Q
electrochemical testing
A
- electron flow occurs in current generated by two dissimilar metals
- anodes lose electrons, cathodes gain
- set up reaction to glean the relationship between the current and anode/cathode potential
- Tafel extrapolations!
- can use to find the current density and likewise the corrosion rate
10
Q
cyclic potentiodynamic testing
A
- shows behavior of metals with varying voltage
- passivity: region where certain metals (chromium, iron, nickel, titanium) lose their reactivity and become very inert… protective barrier of an oxide film
- active, passive, and transpassive regions
- potential is increased then decreased until repassivation to observe behavior
11
Q
Er, Eb, Ep
A
- Er: rest potential, measured current is close to zero as anodic and cathodic reaction rates sum to zero
- Eb: breakaway/critical pitting potential… corrosion will initiate and propagate (higher Eb = higher corrosion resistance)
- Ep: protection potential, where reverse intersects forward scan… only occurs if there is a breakdown (higher Ep = higher tendency to repassivate)
- with greater hysteresis between Eb and Ep, the severity of crevice corrosion is increased
12
Q
Ef and Ev
A
- not all materials exhibit protection potential
- in that case, Ef is a preset potential at which a scan is stopped
- Ev: vertex potential where scan direction is reversed… oxygen evolution has occurred at the surface
13
Q
Eb values
A
- Eb represents the point at which the passive layer breaks down
- Eb > 300 mV is generally acceptable
- Eb >= 600 mV indicates excellent corrosion resistance
14
Q
mass loss
A
- corrosion causes material breakdown, so devices lose mass over time
- localized/pitting processes cause penetration, so make it more difficult to estimate corrosion from simply mass loss
15
Q
morphology analysis
A
- thickness of corrosion product layer can be measured
- energy dispersive x-ray spectroscopy (EDX) and x-ray microtomography (micro-CT) for uniform and localized corrosions
- to evaluate pitting, can use a standard rating chart and generate a pitting factor
- extent of pitting and intensity of corrosion
- pitting factor = deepest penetration / average penetration (factor of 1 indicates uniform corrosion)
