11/13/23 HOW to CONTROL CORROSION? · Use self-protecting metals! > Metal ions combine with Oxygen to form a thin, adherent oxide layer that slows corrosion Metal oxide layer Metal (e.g., Al, Stainless Steel) . Reduce T (slows kinetics of oxidation and reduction) · Use Inhibitors or Barriers: > Slow oxidation/reduction reactions by removing reactants (e.g., remove dissolved O2 gas by reacting it with an inhibitor) > Slow oxidation reaction by attaching other species to the surface (e.g., paint or coat it!). · Cathodic (or sacrificial anode) protection: >Attach a more anodic material to the one to be protected ... let that be the one to corrode ... sacrifice one to protect the other Example: Zinc-coated nail Zn2+ Zinc Zinc 2e 2e Steel Adapted from Fig. 17.24, Callister & Rethwisch, 10e. Steel pipe Example: Mg Anode - Cu wire Adapted from Fig. 17.23(a), Callister & Rethwisch, 10e. Mg > Mg2+ Abode Earth Chapter 17 -56 56 CATHODIC PROTECTION · Very effective against ~ all forms of corrosion: > Supply, from external source, e to metal being protected, making it the cathode, forcing the normal oxidation reaction, i.e. M ® Mn+ + ne- in the reverse (reduction) direction · Galvanic Couple: Ø Electrically connect item we wish to protect to a more reactive (anodic) metal Ø Latter will then be oxidized (corroded) ... a sacrificial anode - Commonly Zn and Mg ... from anodic end of EMF/Galvanic Series - Galvanizing ... dip component into molten Zn o Examples? - The Zn does corrode, but very slowly due to high anode-to-cathode surface area ratio - Passive technique Chapter 17 -57 57 1
11/13/23 CATHODIC PROTECTION · Active Cathodic Protection also possible: Ø Supply e-, from an external DC power source o -ve connected to the structure to be protected o +ve connected to inert anode, e.g., a piece of graphite, buried in the ground Ø Current path between cathode & anode completed via the soil Tank Current Rectifier 58 Ø Impressed Current Cathodic (ICC) protection: - Underground storage tanks - Buried metal pipes - Marine structures Gravel Current --- Backfill Adapted from Fig. 17.23(b), Callister & Rethwisch, 10e. Chapter 17 -58 CATHODIC PROTECTION 59 Poire Chapter 17 -59 Currorc 2
11/13/23 If the voltage generated by a non-standard electrochemical cell, as calculated using the Nernst equation, is negative then ... 1. The calculation contains an error of some kind ? 2. The assumed overall reaction direction should simply be reversed ? 3. The corrosion simply proceeds more slowly than if the voltage were positive ? Chapter 17 Slide # 29 Chapter 17 - 60 60 DEGRADATION of POLYMERS · Polymers: Ø Also interact with their environment ... Ø Known as "degradation" rather than corrosion Ø Different from metals ... not an electrochemical process Ø Physiochemical in nature for polymers Ø Wider range of possible behaviors: - Swelling and dissolution - Covalent bond rupture ... heat, chemical, radiation o Loss of mechanical integrity - More complex ... o Example: Polyethylene, when heated in an