Unit6: SSC, H, CP

Unit6: SSC, H, CP

1. Stress Corrosion triangle

   • Susceptible alloy
   • Corrosive environment
   • Tensile stress

2. Types of corrosion

   • Filiform corrosion
     • Active head and inactive tail with insoluble corrosion products
   • Erosion-corrosion
     • Self-acceleration defect loop
     • Prevention
       • Designing smooth flow paths
       • Filtration
       • Protective coatings
         • Elastomer/Rubber
         • Ceramic
       • CP
       • Strategic alloy selection
         • Duplex SS
         • Titanium alloys
   • Cavitation corrosion
     • Bernoulli's Principle
       • P<Pvapour
   • Corrosion fatigue
     • Progresive cracking of a material caused by fluctuating or cycling stresses
     • Non-specific (unlike SCC)
     • disttinct zones
       • Initiation site
       • Progressive growth region
       • Final overload region
     • Lower f=longer t=more damage
     • Regime II=Paris Lav
       • a/c>1 high stress concentration
       • a/c<1 lower local stress concentration

3. H embrittlement

   • Silent killer
   • Fracture far bellow the nominal yield
   • Cathodic reduction at surface
     • Neutral/alkaline:
     • Acidic media:
     • Recombination-harmless
     • Absorption
   • Mechanism
     • Hydride formation
     • HIC-internal stress for gas inside
     • HEDE and HELP
   • Crack-Tip chemistry
     • Occluded geometry
     • Anodic Dissolution & Hydrolysis
     • free H+ dropping the local pH inside the crack
     • Hydrogen generation
     • Stress concentration
   • How fast hydrogen moves
     • The Devanathan-Stachurski Cell
       • palladium layer
       • oxidation current=rate of hydrogen permeation
   • Hydrogen recombination poisons
     • H2S-forcing a much higher fraction of atomic H to absorb into the metal
   • From
     • Excessive CP
     • Sour Service Environments (H2S)

4. CP

   • Electrochemical & Galvanic series
   • Pourbaix Diagrams
   • Cannot protect dry atmospheric steel surface