Photo captions c139ZincCrystals.jpg. Large single crystals of zinc are easily seen using the naked eye. c139ZincPhasDiag.jpg. Phase diagram for zinc and iron. c139ZincIronLayers.jpg. The layers formed in hot-dipping of iron with zinc. Keith Holmes, of Corstorphine, asks :-
I notice that metal hand rails (galvanized steel?) on our stepped pathways have taken on a bright but dark burnished polish wherever the human hand has touched and not elsewhere. It seems to be resistant to rust and is quite an attractive finish to the metal. What is the chemistry going on here? Has it been exploited in any way?
Milo Kral, a metallurgist at the University of Canterbury, responded.
Most likely the zinc is reacting with oil /sweat from our hands and the environment to create the pattern, as shown in a typical pattern of etched zinc crystals.
In hot dip galvanizing, iron and zinc react to form layers of intermetallic compounds (see the phase diagram). The outermost layer is essentially pure zinc (the Eta layer with a hardness of 70 DPN), the innermost is pure steel (iron with a hardness of 159 DPN). In between, there are 3 Fe-Zn compounds. The 2nd topmost layer (the Zeta layer with a composition 94% zinc and 6% iron with a hardness of 179 DPN). Next is the Delta layer (90% zinc and 10% iron with a hardness of 244 DPN). Below that is the Gamma layer (79% zinc and 25% iron of hardness 250 DPN.)
So it could be that over time the soft zinc layer is abraded away leaving the harder Zeta layer exposed. Only a scanning electron microscope examination would allow me to tell which it is.
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