ENGR_220_Ch17_ppt_Callister8e_RK_1 - 11/2/11 PASSIVITY Some...

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11/2/11 1 Chapter 17 - 2 2 PASSIVITY O Some metals lose chemical reactivity and become very inert: Cr , Fe , Ni , Ti + many of their alloys Why? Formation of highly adherent, very thin oxide film on metal surface Protects against further corrosion e.g. S-Steel ~11% Cr , minimizes rusting, forms passive surface film in oxidizing environments Caution change the environment and ??? Al also passivates film reforms very rapidly if damaged Chapter 17 - 2 3 O Uniform Attack Oxidation & reduction occur uniformly over surface (general rusting) O Selective Leaching Preferred corrosion of solid solns - 1 element/constituent (e.g., Zn from brass (Cu-Zn)) O Stress Corrosion Stress & corrosion work together at crack tips O Galvanic Dissimilar metals are physically joined. The more anodic one corrodes. (See Table 17.2) Zn & Mg very anodic O Erosion-Corrosion Mech. abrasion of passivating± layer + chemical attack (e.g. at pipe elbows) FORMS OF CORROSION Forms of Corrosion O Crevice Conc. diffs. bet. 2 pieces of the same metal Fig. 17.15, . (Fig. 17.15 is courtesy LaQue Center for Corrosion Technology, Inc.) Rivet holes O Pitting Downward propagation of small pits & holes Fig. 17.17, Callister & Rethwisch 8e . (Fig. 17.17 from M.G. Fontana, Corrosion Engineering , 3 rd ed., McGraw-Hill Book Company, 1986.) Fig. 17.18, Callister & Rethwisch 8e . attacked zones g.b. prec. O Intergranular Corrosion along grain boundaries, often where special phases (Cr 23 C 6 ) exist
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11/2/11 2 Chapter 17 - 2 4 ± Self-protecting metals! Metal ions combine with O
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ENGR_220_Ch17_ppt_Callister8e_RK_1 - 11/2/11 PASSIVITY Some...

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