Lecture 10 - Corrosion

Question 1

Uniform Attack (Corrosion)

Answer 1

• 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

Question 2

Galvanic Corrosion

Answer 2

• “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

Question 3

Interstitial Fluid

Answer 3

Thin layer of fluid surrounding body cells

Question 4

Serum

Answer 4

• 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

Question 5

Corrosion

Answer 5

• Chemical process
• Reaction and/or dissolution in presence of water (metals - reaction dominates, ceramics/polymers - dissolution dominates)
• Any chemical attack on solids

Question 6

Ionization

Answer 6

• General corrosion
• Direct formation of cations under acidic or reducing conditions
• Oxidized: lost electron
• Losing parts of metal

Question 7

Oxidation

Answer 7

• 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)

Question 8

Types of Metal Reactions

Answer 8

• 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

Question 9

Hydroxylation

Answer 9

• 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

Question 10

Reaction

Answer 10

• Combination of metal or metallic ions with other cations and anions
• Complex formation
• Dominant process of metals

Question 11

Effects of Corrosion

Answer 11

• Take into account parent metallic component

- Take into account formation of reaction products of corrosion

Question 12

Pourbaix Diagram

Answer 12

• Classifies all possible reactions between metallic element and water by pH and potential
• Regions: Immunity, Passivation, Corrosion

Question 13

Pourbaix Diagram: Immunity Region

Answer 13

• Dominant reaction of ionization (still have some corrosion but very small amount)
• Low equilibrium concentration of ions (< 10^-6 M)

Question 14

Pourbaix Diagram: Passivation Region

Answer 14

• 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

Question 15

Pourbaix Diagram: Corrosion Region

Answer 15

• Variety of processes attack metallic chromium

- Equilibrium concentration in solution > 10^-6 M

Question 16

Crevice Corrosion

Answer 16

• 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)

Question 17

Pitting Corrosion

Answer 17

• 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)

Question 18

Intergranular Corrosion

Answer 18

• 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)

Question 19

Leaching (Corrosion)

Answer 19

• 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

Question 20

Erosion Corrosion

Answer 20

• 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)

Question 21

Stress and Fatigue Corrosion (“Mechanically Induced”

Answer 21

• 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)