Anodizing (Or the beauty of corrosion)
Bill describes how metals like aluminum and titanium are made resistant to corrosion by growing an oxide layer into the metals. These is the same process used on many Apple products.
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Anodizing Aluminum The Secret of Apple's Durable Laptops & iPods EngineerGuy Series #4
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I love the "unibody" design Apple uses in their laptops.
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They're made from aluminum or titanium that's processed
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to give a polished and refined look with a tough surface.
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They use a similar process on their iPods as well.
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The colorful aluminum cases look painted, but the surface is actually a layer
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of aluminum oxide grown into the aluminum with a dye locked into it.
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What's really bizarre is that these layers come from
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carefully controlled corrosion, in other words, rust.
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Every piece of aluminum develops an air-tight oxide coating on its surface
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almost immediately when exposed to air.
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Now, while we think of corrosion as a force of destruction,
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when used creatively by engineers it can yield incredible utility,
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as seen in Apple's products.
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To create the coating used on these products,
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engineers enhance the growth of that oxide layer electrochemically.
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Let me show you with a piece of titanium.
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I've placed a strip of titanium in this solution,
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and exaggerate the scale
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we can see that each color corresponds
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to an oxide layer of a different depth.
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The color comes from the interference of light rays
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that bounce off the titanium surface at the bottom
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of the transparent oxide and those that reflect from its surface.
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The thickness of the layer determines how those two rays interfere.
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For each thickness of the oxide layer two waves of a specific color
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will be exactly a half wavelength out of phase and
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so when they recombine at the surface they cancel each other.
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The color observed, then, if light source is white
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will be the complement of that color.
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While titanium is fascinating, anodizing is of most importance for aluminum,
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because the coating can be made thicker, more robust and protective.
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To get vivid colors, we dye the aluminum
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and seal the surface to lock in the color.
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Anodizing aluminium starts out much like titanium.
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Using aluminum as the positive electrode, engineers first pass
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enough current to grow a thin "barrier" layer
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similar to that which forms naturally.
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Then, as the anodizing proceeds, the current pushes this
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barrier deep down into the aluminum converting
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the aluminum above into a very porous oxide layer.
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It isn't a layer being put on top, but instead the reaction
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consumes and converts the aluminum; this is one of the reasons
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it's so effective at preventing corrosion.
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The pores in this layer give the aluminum a unique characteristic
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important for a consumer device
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The ability to be colored.
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The pores formed on the surface have a honeycomb pattern.
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Inside these layers one can place dye of any color.
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Once the pores are filled engineers seal the layer
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by boiling the aluminum in hot water.
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This closes the pores, locking the color in forever,
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you cannot scrape it off without removing the aluminum.
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The toughness comes from the oxide being structurally
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similar to tough gemstones.
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Sapphire is an aluminum oxide
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with trace amounts of iron and titanium to give it a blue color
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it's also the basis of ruby, the same crystal structure
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with chromium that absorbs yellow-green.
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The natural occurrence of corrosion made useful for everyday objects.
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I'm Bill Hammack, the engineer guy.
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This video is based on a chapter
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in the book Eight Amazing Engineering Stories.
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The chapters features more information about this subject.
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This video is Creative Commons https://creativecommons.org/licenses/... and it is from https://goo.gl/tIuRCA
