信息技术应用于英语课堂教学的反思A.pptVIP

Why Study Corrosion 1 Sheldon W. Dean 12/7/02 What is Corrosion? What does it look like? How do you know when you have it? What is Corrosion? Metals made by smelting ( reduction of ore) Metals in air want to return to their oxidized state Corrosion is a natural process! Is Corrosion Important? National issue? International issue? What industries are affected? Does it affect us personally? Importance to USA CC Technologies study – March 2002 In 2000 about 3.1% of GDP spent on corrosion repair and control Total cost $276Billion! Of industries analyzed, the cost was $137.9 Billion Examples of Corrosion Damage Automotive problems - body rust Home examples- - Plumbing fixtures - Mail box - Exterior fixtures and fasteners Concrete cracking Corrosion Affects Our Safety! Unexpected failures of equipment and devices Need to replace damaged items Corrosion weakens structures - Need to evaluate fitness for service when corrosion occurs What Does Corrosion Look Like? Rust and tarnish, red blue or gray scale 8 types of damage – Fontana Several others also. Most Common Forms of Corrosion General attack (uniform wastage) Pitting/ Crevice attack Cracking (embrittlement) Galvanic corrosion Corrosion is Electrochemical Two different reactions occur - oxidation and reduction Electron transfer occurs Potential (voltage) driving force required Oxidation occurs at anode Reduction occurs at cathode Corrosion Reactions Oxidation - e.g. Fe ? Fe++ + 2e Reduction – e.g. O2 + 2H2O ? 4OH- - 4e Secondary - e.g. 4 Fe++ + O2 ? Fe2O3 + 8H+ Conclusions Corrosion rate depends upon the corrosion products that form Solubility of corrosion products determines the rate If solubility is high, the rate is high! If solubility is low, the rate is low Potential/pH (Pourbaix) Diagram Very useful way to present data Developed by Marcel Pourbaix (1966) Available f