Survivors learn to adapt – Zufa Cenva Survivors learn to adapt – Zufa Cenva

Corrosion Most corrosion is electrochemical Galvanic cell is necessary No corrosion if one of these is missing: anode cathode electrolyte electrical connection No corrosion if one of these is missing: anode cathode electrolyte electrical connection anode cathode electrolyte electrical connection

Corrosion Most corrosion is electrochemical Galvanic cell is necessary Corrosion at anode Me = Me +n + ne – (metal dissolves) Corrosion at anode Me = Me +n + ne – (metal dissolves) anode cathode electrolyte electrical connection No dissolution at cathode

Corrosion What constitutes an anode/cathode pair? Zn Cu electrolyte electrical connection Dissimilar metals Ex: Cu & Zn (Zn corrodes) Ex: Cu & Zn (Zn corrodes) How do we know? Many combos tried experimentally How do we know? Many combos tried experimentally

Corrosion Result: the galvanic series Zn Cu electrolyte electrical connection anode cathode electrolyte electrical connection more cathodic more anodic

Corrosion What constitutes an anode/cathode pair? Dissimilar metals Different electrolytes 1 2 Ex: 1 contains little dissolved oxygen 2 contains much dissolved oxygen Ex: 1 contains little dissolved oxygen 2 contains much dissolved oxygen

Corrosion What constitutes an anode/cathode pair? Dissimilar metals Different electrolytes 1 2 Practical example: crevice corrosion Practical example: crevice corrosion

Corrosion What constitutes an anode/cathode pair? Dissimilar metals Different electrolytes 1 2 Deformation ––> non-homogeneous residual stress Deformation ––> non-homogeneous residual stress

Corrosion What constitutes an anode/cathode pair? Dissimilar metals Different electrolytes 1 2 Deformation ––> non-homogeneous residual stress Deformation ––> non-homogeneous residual stress Non-passivated vs. passivated metal or alloy

Corrosion What constitutes an anode/cathode pair? In summary– Anything that makes two half-cells different gives anode & cathode ––> corrosion In summary– Anything that makes two half-cells different gives anode & cathode ––> corrosion We can call these types macro cells

Corrosion Then what about micro cells? anode & cathode within single material operating on a micro scale Then what about micro cells? anode & cathode within single material operating on a micro scale Grain boundary - high energy region…anodic wrt bulk of grain Differently oriented grains - some planes of atoms dissolve faster than others Segregation (coring)

Corrosion Then what about micro cells? anode & cathode within single material operating on a micro scale Then what about micro cells? anode & cathode within single material operating on a micro scale Segregation (coring)

Assignment: Review today's classnotes a. The Galvanic cell b. Types of macro cells c. Types of microcells Read: pp Assignment: Review today's classnotes a. The Galvanic cell b. Types of macro cells c. Types of microcells Read: pp