Page 330 - Advanced Mine Ventilation

Gas Transport in Underground Coal Mines 307

are unstable and tend to revert to a more stable chemical combination such as the orig-
inal ore, that is, an oxide, sulfide, or carbonate. At the corroding area, direct current
electricity flows from the metal into the surrounding electrolyte. The metal from which
the current leaves is known as the anode. The metal that receives current from the elec-
trolyte is known as the cathode. A loss of metal results at the anode, while the corroded
metal particles deposit on the cathode. The amount of metal loss varies with different
metals and current flow; for example, a Ampere current flow for 1 year will cause los-
ses of 20 pounds of steel, 74 pounds of lead, 45 pounds of copper, 23 pounds of zinc,
and 6.4 pounds of aluminum [2].
Different metals demonstrate different electropotentials, and a current will flow be-
tween two dissimilar metals when immersed in an electrolyte and connected electri-
cally. The galvanic series of metals indicate the electropotential of each metal, and
current will always flow from the metal higher in the series to any metal lower in
the series when contact is made between the two metals. A similar relationship of
anode to cathode exists with iron and steel as shown by Lewicki [3].

1. New steel pipe.
2. Old steel pipe.
3. New wrought iron pipe.
4. Old wrought iron pipe.
5. New cast iron pipe.
6. Old cast iron pipe.

As in the case of the galvanic series, current will flow between a metal connected to
another metal lower in the listing. From this listing, it is apparent that new steel will
corrode faster than old steel to which it is connected; or old steel pipe will corrode
when connected to a new cast iron pipe.
Corrosion currents will be produced on any metal on which conditions create an
anode and cathode immersed in an electrolyte. Some conditions which create anodic
and cathodic areas are stresses in the pipe, chipped or scratched surfaces, dissimilar
metals connected together, dissimilar metal surfaces on the pipe such as caused by ac-
cumulations of mill scale, differences in the surrounding electrolyte stray electric
currents.

18.5.1 Protection Against Corrosion

Corrosion of the pipeline can be alleviated by preventing the flow of current between
anode and cathode. This can be done by using a dielectric coating or by reversing the
current by the use of cathodic protection so that the current flows to the pipeline from a
sacrificial anode. In the case of stray electric currents in coal mines, corrosion by this
source can be prevented by stopping the flow of stray currents into the pipeline or by
giving the stray currents a free path from the pipe back to the source (ground) so that
the current does not leave the pipe at anodic points.
Before protective measures are undertaken, however, it is important that the condi-
tions to be guarded against are clearly defined. Often it is desirable to obtain measure-
ments of soil resistivity, thus providing an estimate of the strength of electrolyte which