Lecture 10 - Corrosion Flashcards
1
Q
Uniform Attack (Corrosion)
A
- Most common form of corrosion
- Occurs in all metal in the corrosion (passivation) region with either local microenviroment shifts out of passivative zone or movement out of passivative region into non-passivative region
- No large impact on material’s properties, more concerned with biological response of surrounding areas
2
Q
Galvanic Corrosion
A
- “Coupled Corrosion”
- Two different metals in contact and immersed in ionic conducting fluid (serum or interstitial)
- Metals have different electrochemical potential and surface of less noble metal experiences attack
- Concerned with uneven corrosion within part
3
Q
Interstitial Fluid
A
Thin layer of fluid surrounding body cells
4
Q
Serum
A
- Fluid and solute component of blood not playing role in clotting
- Includes proteins such as electrolytes, antibodies, antigens, hormones, exogenous substances
- Does not contain white/red blood cells, platelets, or clotting factors
5
Q
Corrosion
A
- Chemical process
- Reaction and/or dissolution in presence of water (metals - reaction dominates, ceramics/polymers - dissolution dominates)
- Any chemical attack on solids
6
Q
Ionization
A
- General corrosion
- Direct formation of cations under acidic or reducing conditions
- Oxidized: lost electron
- Losing parts of metal
7
Q
Oxidation
A
- Direct reaction of metal with gaseous or dissolved oxygen without participation of water
- Oxide layer corrodes less than metal itself
- Oxide layer must be well adhered/attached to surface below (don’t want leaves of metal oxide peeling off into body)
8
Q
Types of Metal Reactions
A
- Ionization
- Oxidation
- Hydroxylation
- Reaction
- Each decreases amount of pure metal (parent material) present which could add areas of weakness to implant over time
- Each produces metal baring ions and compounds which could combine with proteins/haptens and illicit immune response
9
Q
Hydroxylation
A
- Reaction of metal with water under alkaline or oxidizing conditions to yield hydrated oxide or hydroxide (not very soluble in aqueous solution - stable)
- Good passivating film because good adhesion
- Areas where stable/unstable depending on environment
10
Q
Reaction
A
- Combination of metal or metallic ions with other cations and anions
- Complex formation
- Dominant process of metals
11
Q
Effects of Corrosion
A
- Take into account parent metallic component
- Take into account formation of reaction products of corrosion
12
Q
Pourbaix Diagram
A
- Classifies all possible reactions between metallic element and water by pH and potential
- Regions: Immunity, Passivation, Corrosion
13
Q
Pourbaix Diagram: Immunity Region
A
- Dominant reaction of ionization (still have some corrosion but very small amount)
- Low equilibrium concentration of ions (< 10^-6 M)
14
Q
Pourbaix Diagram: Passivation Region
A
- Dominant reactions lead to formation of oxides and hydroxides (boundaries identify when breakdown of passivating layer which reveals metal)
- Solubility of oxides/hydroxides is low, metal ion concentration < 10^-6M
15
Q
Pourbaix Diagram: Corrosion Region
A
- Variety of processes attack metallic chromium
- Equilibrium concentration in solution > 10^-6 M
16
Q
Crevice Corrosion
A
- Requires presence of narrow, deep crack in material (interface between parts of device, defect - i.e. incomplete fatigue crack that doesn’t progress with cycles)
- Oxygen depletion in crevice (anodic metallic corrosion along crevice faces, cathodic protection of metal at mouth of crevice)
- If corroding at face, deeper crack (increases stress concentration of crack which could help propagate into full fracture)
- If corroding at mouth, wider crack
- Common in multipart devices (mating surface)
17
Q
Pitting Corrosion
A
- Special case of crevice corrosion
- Initiated by inclusions (to chemistry), scratches, or handling damage
- Similar process to crevice corrosion (directed corrosion along imperfection)
- Pits often occur in large numbers (grow in direction of gravity
- Act as sites of stress concentration (act as site of fatigue crack initiation which could lead to eventual failure)
18
Q
Intergranular Corrosion
A
- Not necessarily different phases/structure, but slightly different chemistry
- Common in cast materials (multiple grains with impurities at gb)
- Chemistry of adjacent grains may be different (intergranular attack) because of different cooling rates
- More common in alloys (could corrode out individual grains which changes surface texture and puts abrasive particles into surroundings)
- Can occur in stainless steel if not properly heat treated to even out chemistry (relative depletion of Cr from gb)
- Can occur at weld lines (local melting and re-solidification)
19
Q
Leaching (Corrosion)
A
- Leaching out certain atoms of grains, not whole grains
- Large difference in rate of loss of alloy components (weakly bound)
- Induced by solution attacking on component preferentially (electrochemical potential determines preference)
- Induced by presence of more than one phase in alloy (individual grains with different compositions, preferential corrosion of one chemistry)
- Example: ASTM F562 Co-Ni-Cr-Mo
20
Q
Erosion Corrosion
A
- Accelerated attack of metal because of relative (continuous) movement between surrounding fluid and metal
- Serves to increase rate of attack by other processes (corrosion process self limiting, flow sweeps away corrosion product and provides new reactants like O2 and Cl-)
- Continually replenish with components helping to move corrosion forward
- Appearance similar to pitting with pits elongated in direction of flow (circular/equiaxed - indicates pitting, elongated/teardrop - indicates erosion)
21
Q
Stress and Fatigue Corrosion (“Mechanically Induced”
A
- Tensile stress increases chemical activity of metals
- Flexed metal where one part under tension and one part under compression
- Difference in electrochemical potential of side under tension vs. compression (convex surface - anode, concave surface - cathode)
- Corrosion attacks convex surface (increased corrosion rate)
- Both still undergo corrosion but convex side corrodes much faster (uneven corrosion results in poor distribution of mechanical forces across implant which could lead to fracture)
