From: Chris W (Avatar) 01/04/2001 19:29:10
Subject: Stainless Steel post id: 269923

What is added to iron to make stainless steel? How do the other components act to inhibit rust?
Who was the clever clogs that work it out?


From: Dr Paul {:~)} (Avatar) 01/04/2001 21:46:27
Subject: re: Stainless Steel post id: 269998
Hi Chris W.,

my son and I have just been researching this very question for a Metalwork assignment (Hi Adam, sitting behind me munching on a rollup)

Iron is mixed with scrap metal (sorted and analysed for other metal contents to control the alloy) iron ore (to get more oxygen in there) and limestone.

This is all heated above the melting point for iron, the oxygen in the furnace oxidises the carbon out of the iron and the limestone fuses with any impurities such as silicates, sulfur, phosphates (etc) and floats as slag above the mix.

is this OK?

Paul


From: Di (Avatar) 01/04/2001 21:53:33
Subject: re: Stainless Steel post id: 270009
Hmmm... Stainless steel has a high proportion of Chromium which reacts before the iron will. This pacifies the surface by forming a chromium oxide layer, protecting it from further oxidation.

From: Chris JW 02/04/2001 0:44:19
Subject: re: Stainless Steel post id: 270100
And yet, it is an annoyance that stainless steel "Termimesh", a product that stops termites in buildings, decays very fast on the top of wooden house stumps that we have in the country.

From: bob s 02/04/2001 7:58:59
Subject: re: Stainless Steel post id: 270136
When iron rusts the volume of the rust is greater than the original metal so that the rust does not adhere to the underlying iron properly to form a coating impermiable to more water and oxygen so the rusting continues. On the other hand aluminium oxide has almost the same volume as the metal and therefore forms an impermiable vey thin layer of oxide on the metal. The thickness of thes oxide layer can be increased by a process called anodising.
To make stainless steel the iron is alloyed with chromium and nickel. The oxide layer which is then produced naturally by exposure to air has a volume close to that of the underlying metal so that it does not become partly detached and so prevents further access of oxygen to the metal.
The oxide layer is vulnerable to attack by some acids or salts such as chlorides.


From: Alan™ ® 07/05/2001 7:13:56
Subject: re: Stainless Steel post id: 294610
A few Jelly Beans to be spread around. Di got the basic ingrediant, Boris provided a good link, and Bob S was doing well till he mentioned aluminium. Dr Paul, unfortunately your son is off the track, he's describing the process involved in reducing iron ore, using it would seem an electric arc furnace and not the production of stainless steel.

00000000
It was in interesting thread Kelvin. It's probably something for the Not FAQ Metals Manufacture

What is added to iron to make stainless steel? How do the other components act to inhibit rust? Who was the clever clogs that work it out?

Answering the last question first. Many things in the field of metallurgy are gradual progressions. It has been known for a long time that additions of chromium into steel improved the corrosion resistance, as was its effect on the crystal structure or phase present, the effect of nickel was also known. However, somebody at some stage started adding relatively huge additions of these alloying and noticed that there were even improved corrosion resistance. But metallurgically speaking, there is no difference structurally between chromium steels and chromium-nickel steels and their higher concentration stainless steels, somebody drew an arbitrary line and classed everything with higher concentrations of chromium and nickel were classed as stainless steels.

From above you can basically guess that stainless steels contain chromium and often nickel (and obviously iron). Other alloying elements are carbon, manganese, silicon, and then several alloying elements involved with precipitation hardening, such as molybdenum, nitrogen, aluminium, copper, niobium and tantalum

Chromium is a strong ferrite producer (BCC crystal structure), on it's own you will get properties associated with ferrite, such as (ferro)magnetism. However the most important effect of the chromium is to produce a non porous, well adhered oxide layer, that protects the base material below, by a process known as passivation. More on this below.

Nickel is a strong austenite producer (FCC crystal structure) and counteracts the ferrite producing of the chromium. It also assists. Nickel in itself is good at avoiding the oxidation and to some extent, its inclusion substitutes the presence iron, so usually the austenitic stainless steels have better corrosion resistance, but as nickel is expensive, these stainless steels are more expensive. The formation of austenite has 2 effects, it forms a non (ferro)magnetic form of stainless (very useful) and also permits the formation of martensitic stainless steels, which have are harder and more wear resistant.

Carbon in stainless steels, is generally considered a bad thing, however because carbon is an integral part of the general steel making process, completely removing carbon is difficult. Carbon forms carbides with metals, unfortunately chromium has a high affinity for forming carbides, which means that the chromium because locked away from providing the oxide protection.

Manganese is added to improve the hardenability of the stainless steel. By this I mean that manganese will alloy the formation of martensite, at slower cooling rates hence deeper into the stainless steel, but also reduce some of the problems of forming martensite, such as cracking, due to the rapid change in volume.

Silicon is also a hang over from the steel making process.


From: Alan™ ® 07/05/2001 7:14:19
Subject: re: Stainless Steel post id: 294611

The precipitation hardening alloying elements, are they to help harden the materials by preventing the motion of dislocations (voids in the crystal structure). It's the motion of these dislocations that give metals the property of ductility and ultimately leads to their failure under load, and many other properties. By blocking their motion, metals achieve higher strengths. The precipitates formed use some of the carbon in the steel, so generally the alloying elements used have a stronger affinity for carbon than chromium or they form precipitates based upon nitrogen, hence it's inclusion.

The two groups not mentioned in Boris's link were the precipitation hardened stainless steels, but also duplex stainless steels which are becoming rapidly more significant. These are based upon having both the ferrite and austenite phase present. The combination of both phases improves the strength and ductility, gives comparable corrosion resistance of the austenite stainless steels, improved weldability etc.

Going back to the protection mechanism. Chromium has a stronger affinity for oxygen than iron does. But unlike iron the oxide of chromium will adhere will to the base metal. The lack of adhesion involved with iron, means it's more porous allowing oxygen (and water) access to the base metal, causing more oxidation. It's this chromium oxide film which results in the film which I said to be passive.

In corrosion you talk about 3 regions, active, where corrosion occurs, inert where no corrosion occurs and passive, where corrosion has occurred, but the corrosion provides protection, by preventing further corrosion occurring. Change the environment, i.e. the pH or add chloride ions to an aqueous solution, and you might cause a change in the region, from passive too active, hence the reason why stainless steels are not necessarily effective in salt water or sulfur based environments

Because of the transport phenomena involved, oxygen, chromium and iron cannot migrate easily in either direction across the film. In ordinary steels some forms of iron oxides, forming well adhered oxides ( FeO) , iron or oxygen can migrate across the film, but at slower rates than to cause the formation of the standard form of rust (Fe2O3), this is really important for high temperature alloys, as it's this mechanism which is used to protect the base material, by letting it corrode at known rates.

It should be said, by someone that if you damage a chromium oxide layer, the chromium in the base metal will migrate to the region of damage and will then oxidise, repairing the damaged area. The film is fairly rapid in forming, so the iron does not get involved. However if the oxide layer is regularly being damaged, it leads to chromium depleting, which allows the iron to oxidise. The migration effect takes is relatively short range and also does take time.


For Bob S

The volume expansion of aluminium is probably greater than steel, roughly something in the order of 10 - 20 times the loss of base material. It's just the bonding between aluminium and its oxide (or really complex hydroxides) is so much better. If you want to get a better understanding of the anodising process at a theoretical level, I suggest chasing up papers by Wood, G.C., Thompson, G.E., O'Sullivan, J.P., Furneaux, R.C., et al, during 60's - 80's, all out of a electrochemical research centre in Manchester UK. Their work is still well advanced of anything else down in the anodising industry, and their work formed a large part of literature survey, for my thesis in a related area of aluminium anodising.



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