There are a couple of concepts that need to be explained first.  The first is the fact that a mackinawite lattice has Fe-Fe parameters that are nearly the same as alpha iron.  This means that the mackinawite can form and grow on the original iron atomic lattice.  The second feature of mackinawite is the fact that it is a metallic conductor of electrons.  This means that the cathodic half of the electrochemical reaction can occur on the surface of the mackinawite with electron transfer occurring at the solid-electrolyte boundary.

So, the process is simply one where an H2S or HS- grouping approaches the surface of the solid.  The S atom is then attracted to the valley that exists between the four Fe atoms of the Fe crystal at the surface of the metal/mackinawite.  You now have H2S (or HS-) adsorbed on the solid surface.  The next step is separation of the H+ ions from the S and then electron transfer between the sold and the H+ to form H0.

As for the Fe ions, they can either move from the metal surface to the solid/electrolyte through features such as cracks or dislocations or by diffusion along the large planar gaps that exist in mackinawite.  At higher temperatures, the Fe ions can even migrate through the FeS lattice.

Does that address your question?

Dr. Stephen N Smith:

 Thank you for your answer. The nuclear, growth and tansiformation of Fe-S corrosion products is my research subject. I am glad to read your answer, and It is a completed process about the nuclear of products. However,  the whole reaction formula still puzzled me, especially the solid state reaction formula. Do you have some paper which discussed the reaction formula? 

Also to be noticed is that hydrogen sulphide (H2O) gas becomes considerably more corrosive for iron if the gas is humid.

Can anyone tell me for removal of 89mg/L of Hydrogen sulfide to produce FeS how many mg/L iron is require?