The apparatus 2 wherein said protective coating is selected from {HEAD ~ the group} consisting of cobalt alloys, {TAIL ~ nickel alloys} and tungsten carbide alloys 
consisting of
The apparatus 2 wherein said protective coating is selected from { HEAD ~ the group } consisting of cobalt alloys , { TAIL ~ nickel alloys } and tungsten carbide alloys
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH