Re: What makes stainless steel stainless?

Hi Tomasz,

Several elements can be alloyed with iron to make stainless steel, however, 
chromium is the most significant component in stainless steel.  Iron and 
chrome both react with oxygen to form oxides at the metal surface.  The 
chrome oxide forms a protective layer on the metal surface which slows the 
rate of oxidation.  Iron oxide is more permeable and the base metal 
continues to react and oxidize.  Aluminum is a very reactive metal.  The 
thin layer of oxide on the surface protects the base metal from reacting.  
If you place liquid mercury on aluminum metal, the mercury forms an amalgam 
that is more permeable and destroys the protective layer.  On a humid day 
the aluminum will burn in air!

Your second question is more difficult to answer.  Low thermal expansion 
alloys generally have high nickel content (40 to 50%).  The curie point of 
these alloys is much lower than for carbon steels.  The curie point is the 
temperature at which a ferromagnetic material changes to become 
parramagnetic.  Magnetostriction is a change in dimension which occurs as 
the magnetization of the crystal structure is changed.  In a low thermal 
expansion alloy, the composition is carefully controlled so that the 
effects of thermal expansion are offset by  magnetostriction. 

Low thermal expansion steels with high nickel content are much more 
expensive than plain carbon steels used in applications such as rails.  I 
would be interested in the composition of a low thermal expansion steel 
used for rail applications.

Bob Novak
Specialist, Process R&D
Carpenter Technology