chapter 3.pptx - Corrosion engineering R.Venkatesh...

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Corrosion engineering R.Venkatesh, Assistant Professor, Thermal, Fluids and Energy Science- Stream, Department of Mechanical Engineering, PSG College of Technology, Coimbatore. [email protected] 9843387887
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Corrosion basics We have all seen corrosion and know that the process produces a new and less desirable material from the original metal and can result in a loss of function of the component or system. The corrosion product we see most commonly is the rust which forms on the surface of steel and somehow Steel → Rust
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TWO REACTIONS of corrosion For this to happen the major component of steel, iron (Fe) at the surface of a component undergoes a number of simple changes. Firstly, Fe → Fe n+ + n electrons the iron atom can lose some electrons and become a positively charged ion. This allows it to bond to other groups of atoms that are negatively charged.
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TWO REACTIONS of corrosion We know that wet steel rusts to give a variant of iron oxide so the other half of the reaction must involve water (H 2 O) and oxygen (O 2 ) something like this O 2 + 2H 2 O + 4e - → 4OH - This makes sense as we have a negatively charged material that can combine with the iron and electrons, which are produced in the first reaction are used up.
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TWO REACTIONS of corrosion We can, for clarity, ignore the electrons and write 2Fe + O 2 + 2H 2 O → 2Fe(OH) 2 Iron + Water with oxygen → Iron Hydroxide dissolved in it
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TWO REACTIONS of corrosion Oxygen dissolves quite readily in water and because there is usually an excess of it, reacts with the iron hydroxide. 4Fe(OH) 2 + O 2 → 2H 2 O + 2Fe 2 O 3 .H 2 O Iron hydroxide + oxygen → water + Hydrated iron oxide (brown rust)
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Steps of corrosion This series of steps tells us a lot about the corrosion process. (1) Ions are involved and need a medium to move in (usually water) (2) Oxygen is involved and needs to be supplied (3) The metal has to be willing to give up electrons to start the process
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Steps of corrosion (4) A new material is formed and this may react again or could be protective of the original metal (5) A series of simple steps are involved and a driving force is needed to achieve them The most important fact is that interfering with the steps allows the corrosion reaction to be stopped or slowed to a manageable rate.
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UNIFORM CORROSION
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UNIFORM CORROSION – 30% of failures Uniform corrosion, as the name suggests, occurs over the majority of the surface of a metal at a steady and often predictable rate. Although it is unsightly, its predictability facilitates easy control, the most basic method being to make the material thick enough to function for the lifetime of the component.
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UNIFORM CORROSION – 30% of failures Uniform corrosion can be slowed or stopped by using the five basic facts; (1) Slow down or stop the movement of electrons
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