11/11/23 ANNOUNCEMENTS • Remember to Sign-In! • Reading: > Chapter 17, all sections except 17.4, 17.10 • Week 8 Recitations: No Quiz ... Midterm 2, Tomorrow, 11/14/23, 8:00-8:50 am > Students with accommodations take exam in ODR Test Proctoring Center > Chapter 17 H/W Problems: 17.2, 17.5, 17.7, 17.12 Recitation Problems: As announced in recitation Chapter 17 -32 32 ANNOUNCEMENTS - 2 (Final Exam) • Final Exam: >Tuesday, December 12, 10:30-12:30: Ø Main Auditorium for most of the class. Ø ODR Accommodations: ODR Test Proctoring Center ... need to register again in ClockWork. Ø Coverage: Cumulative, with an emphasis on material covered since Midterm 2. Ø Part of 1 Q will be a "Bonus" question ... so >100% possible. Ø No early exams for personal convenience ... term is not over until the end of Finals Week ... · More than 3 Exams on same day? Ø See Provost's Office policy ... Ø https://drexel.edu/registrar/scheduling/exams/exam-policy/ Ø Request for relief must be made to the Provost's Office at least three (3) days before the examination date Ø Fall & Spring, courses with higher numbers give early exams Chapter 17 - 33 33 1
11/11/23 RECOGNIZING the FORMS of CORROSION . So far, we have mainly assumed that: Ø Electrochemical corrosion is the only deterioration mechanism occurring Ø Anodic (oxidation) and cathodic (reduction) reactions take place all over the electrode surfaces with the locations of these reactions constantly changing places on the surface Ø There are no significant macroscopic differences in the electrolyte concentration over the metal surface and the metal is fairly homogeneous ... · These assumptions lead us to one type of corrosion: Ø Generalized Attack or Uniform Corrosion Chapter 17 -34 34 RECOGNIZING the FORMS of CORROSION . Other forms of corrosion depend on deviations from these assumptions, which may arise due to: Ø Design differences ... i.e., the macro or micro-geometry of the metal surface, such as surface roughness Ø The specific combination of the metal and its environment Ø The state of the surface ... especially its cleanliness and/or roughness Ø Other deterioration mechanisms ... Chapter 17 -35 35 2
11/11/23 FORMS OF CORROSION . Stress Corrosion Stress & Corrosion working together at crack tips · Uniform Attack Oxidation & Reduction occur uniformly over surface (general "rusting") · Selective Leaching Preferential corrosion of solid sol'ns - 1 element/constituent (e.g., Zn from brass (Cu-Zn)) Forms of Corrosion · Intergranular Corrosion along grain boundaries, often where special phases (Cr23C6) exist • Galvanic Dissimilar metals physically joined. The more anodic one will corrode. (Table 17.2) Zn & Mg very anodic g.b. prec. attacked zones Based on Fig. 17.18, Callister & Rethwisch, 10e. 36 · Erosion-Corrosion Mech. abrasion of passivating layer + chemical attack (e.g., at pipe elbows/bends) · Pitting Downward propagation of small pits and holes Fig. 17.17, Callister & Rethwisch, 9e. (Fig. 17.17 from M.G. Fontana, Corrosion Engineering, 3" ed, McGraw-Hill Book Company, 1986.) · Crevice Conc. diffs. betw. 2 regions of the same metal Rivet Holes Fig. 17.15, Callister