I am so embarrassed of my public school education. I should know this stuff. It is really base, simple information.
I have been told that Aluminum (or, A-Loo-Min-Ee-Um, to the Dopers across The Pond) is the most abundant element in the Earth’s crust. This got me thinking on a long tangent. Please correct me where I am wrong.
Steel is actually one of the few metals that oxidizes, or rusts. Metals that do not oxidize are:
[ul]
[li]Aluminum[/li][li]Copper[/li][li]Lead[/li][li]Tin[/li][li]Iron[/li][li]Mercury[/li][/ul]
There are probably more, but I cannot think of them right now. So, the only benefit with Stainless Steel over the above listed metals is that it is stronger than all of them.
So, what is oxidation, exactly? If the surface of Steel has a chemical reaction when it encounters Oxygen and Hydrogen, then I assume that this is a sign of impurity, correct?
But, what does Steel consist of? If it is an alloy, (if I’m using this word correctly) what metals go into steel?
And, how do you make Aluminum or Copper? I know that they are elements, but you cannot just pull a hunk of Aluminum or Copper out of the ground. So what is the (smelting?) process to create pure Aluminum and Copper?
Aluminum is not the most abundant element on the Earth´s crust, Silicon is: Besides it does oxidize, pretty darn fast actually, a layer of aluminum oxide forms quickly on any exposed surface, but that same layer prevents further oxidation so it doesn´t rot away as iron or steel.
The other elements you listed do rust (exept AFAIK mercury, what happens to mercury anyway???) If you polish a piece of those metals after some time the shiny surface will turn dull, that´s oxidation right there. Whish is, as you should have guessed the combination of the original metal with oxigen, which (IMNA chemist neither) binds quite strongly to other elements because… OK, I´ll let someone with better credentials take on this one. I´m a bit fuzzy about the details right now.
Regarding steel, it´s an alloy mostly composed of iron and carbon, basically; the carbon increases the strenght of the alloy; other elements can be incorporated to obtain certain properties: cromium for stainless steel (it produces a passive film as aluminum that prevents further oxidation), molydenum (hardness I think), sulfur, nitrogen, copper, etc…
As for Aluminum it´s extracted from an ore named Bauxite, trhough an electrochemical process, wich I don´t remember in detail right now…
I’m not a metallurgist either, but mercury definitely oxidises, and it’s poisonous to boot. Copper goes green (copper carbonate) or blue (copper sulphate).
All metals oxidize - just depends on the environment. Oxidation potentials are a better measure of a metal’s inertness. Aluminum will oxidize if u have alkali present - try putting some lye (draino) on anything aluminum. Copper will oxidize in presense of acids - try some lemon juice on anything copper and u’ll see verdigris/malachite (green deposit). Similarly u have Lead Sulfide , … BTW Stainless Steel is an alloy
Oxidation is the process when the metal goes to a higher oxidation state. Fe -->Fe2+ is oxidation for example. This means iron reacting with sulfuric acid is also oxidation.
The stainless property of Stainless Steel is due to Chromium. Chromium forms a chromium oxide film on the surface which cuts off any oxygen getting inside. whenever this film is destroyed (scratch, stain, etc.) chromium migrates from the bulk to the surface, gets oxidized and forms a fresh resistant film. this stainless property lasts only till there is more than 9% (IIRC) chromium. If for whatever reason chromium falls below this level, the alloy loses its strength drastically. Stainless Steel has this problem in Chloride environments (known as pitting corrosion). So u’ll never find stainless Steel being used in saltwater.
Steel as such is an alloy of Iron and Carbon. Stainless steel has carbon, nitrogen, Chromium, Nickel, vanadium, etc. added to get desired properties for different environments.
Aluminum is melt by electrolyzing Bauxite (Aluminum Oxide) in molten cryolite (sodium aluminum flouride) at hight temperature using graphite/carbon electrodes. Copper is made by smelting as you rightly point out. Copper occurs in many ores however - the most common is copper pyrites (copper sulfide). Which is smelted to make Sulfuric acid out of the sulfur dioxide and the pyrite is reduced to copper. It used to be refined by green logs in olden times where the hydrocarbons purified the smelted copper.
You mean the aluminum smelting process?, that one wasn´t figured out until the second half of the 19th century.
As for the other metals, it´s not really that complicated to extract the metal from the ore, you heat it the metal melts and the other stuff floats over it.
Steel is nominally defined as iron with 0.2-1.6% or thereabouts carbon. The carbon molecules makes different crystalline structures giving more hardness as carbon goes up. When carbon is above 2% it’s called cast iron. When carbon is below 0.2% it’s called wrought iron. When steel has more of another metal than its carbon content it’s usually referred to as an alloy steel. Depending on how strictly you define your terms straight carbon steel isn’t an alloy.
As others have said all those metals oxidize. Aluminun has the interesting property of its oxide being much, much harder than the pure metal. In fact pure, crystalline aluminum oxide is second in hardness only to diamond. You may be more familiar with it as sapphire and ruby where specific impurities give it color. Makes a hell of a scratch proof crystal in high end watches. Less pretty forms are used for sandpaper.
The hard oxide property is specifically used in a type of sheet metal for aircraft call alclad. I’m not sure how it’s manufactured but a thin layer on the surfaces is dead soft aluminum that will readily oxidize. After it’s formed into a part the surface layeroxidizes, protecting the part from further corrosion and abrasion.
Wait. Aluminum Oxide in crystalline form is Sapphire and Ruby? Fascinating.
Why do iron and steel rot away?
What about other metals such as Gold, Nickel, and Platinum? I’m assuming that these oxidize as well, but as said before about Aluminum, the oxidation is simply a thin layer on the surface. Is this correct?
Aluminum was made in the early part of the 19th century by heating Aluminum Chloride with Sodium/Potassium metals. It was very expensive back then, even more expensive than Silver - and it was used as a royal metal for “silverware”.
Its not so simple-very few metals if at all can be obtained by just heating the ore. Usually, the ore is heated with Charcoal (Coal) where the Carbon (and or CO) reduces the ore to the metal at higher temperatures.
There is plenty of information about ancient smelting techniques available on the web for anyone interested.
As someone who has participated in several pit firings for ceramics, I can personally attest that getting the high heat required for these sorts of processes is not something that can be done casually and the person overseeing the project definitely needs skill and experience to pull it off. Knowledge of the process alone doesn’t cut it.
In other words, just because a technology is simple, doesn’t mean it’s easy.
-Um, you sure about that? The local shops carry piles of stainless fasteners, chain, cable and plate, to help support the local commerical fishing fleets.
I used to help maintain a 42’ twin-inboard-diesel sternpicker, and except for the engines themselves, we used stainless nuts, bolts and washers everywhere. Even galvanized wouldn’t last nearly as long as stainless. We used stainless steel woodscrews in the head fixtures, stainless steel machine screws to hold the railings together, stainless steel bolts to hold the net reels together, stainless bolts as hull plugs, the sacrificial anodes were bolted to the skegs with stainless, the net cutters and prop shafts were stainless steel… Props were Naval Bronze though.
Yes, stainless is tough to cast, as I recall the mold has to be evacuated or you get atmospheric contamination. Or some such.
In any case, I understand that Naval Bronze is even better in saltwater than stainless steel. You hear of the ship’s bells being brought up, from centuries-old shipwrecks, and having very little corrosion or damage.
Stainless is resistant, but it’s nowhere near as impervious as Naval Bronze.
Yeah, I’m sure of that. Look into the blades of the propellor or any wetted parts and see if it is stainless steel. It will be monel metal (Cupro Nickel) or Titanium if very good. Stainless steel just looks good - but does’nt have much strength in cloride environments.
Well, yes; but it isn´t as if the folks back in the Iron Age had plasma furnaces, the fuel was either wood or charcoal (plenty of carbon both) anyway.
In any case I should have been more precise about that.
I´m a bit fuzzy about the details right now.
