材料科学与工程导论双语课件教学课件 ppt 作者 陈克正 王玮 刘春延 等编Chapter 7.pptVIP

College of Materials Science Engineering QUST INTRODUCTION TO MATERIALS SCIENCE AND ENGINEERING Qingdao University of Science Technology Kezheng Chen Wei Wang Chunting Liu 国家级双语教学示范课程 CHAPTER 7 CORROSION AND DEGRADATION OF MATERIALS ISSUES TO ADDRESS… What is corrosion and why is it important? How does corrosion occur? What are the forms of corrosion? How can we suppress or prevent corrosion? WHAT IS CORROSION? ● Corrosion: the environmental degradation of materials Al Capone’s ship, Sapona, off the coast of Bimini. PROCESS Chemical reaction involving the transfer of electrons at the metal surface e.g. Zn in HCl in simplified form (oxidation reaction, anode) (reduction reaction, cathode) Which can be conveniently divided into 2 reactions: Reduction (cathode) Oxidation (anode) DEFINITIONS IN ELECTROCHEMISTRY Loss of electrons is oxidation (leo) Gain of electrons is reduction (ger) Anode: electrode whose atoms lose e- to the external circuit, becoming + ions; (oxidation) Cathode: electrode whose ions gain e- from the external circuit; (oxidation) Electrolyte: solution capable of conducting electric current via the movement of positive or negative ions. Note: anode and cathode could be on the same piece of metal! Standard Electrode Potential STANDARD HYDROGEN (EMF) TEST Metal is corroded Metal is deposited EMF-electromotive force, 电动势 Metal is the anode (-) Metal sample mass Metal is the cathode (+) Metal sample mass EMF-electromotive force, 电动势 The Standard EMF Series EMF-electromotive force, 电动势 STANDARD EMF SERIES ● EMF Series ● Metal with smaller corrodes. ● Ex: Cd-Ni cell GALVANIC CORROSION Requirements: ● 2 different metals ● Electrically connected ● In same electrolyte Carbon steel pipe soldered to copper pipe-steel is heavily attacked More anodic metal corrodes faster. Less anodic metal corrodes slower (is protected). Galvanic Series predicts which metal will corrode faster or slower (e.g. in seawater). steel copper GALVANIC SERIES Ordered list of rela