This black coating is on the ring for two important reasons. The coating
is manganese phosphate. The reason rings are coated is twofold (1) rust
prevention and (2) oil retention for protection against scuffing in
early engine operation.
Rust prevention is necessary for the period of time the ring is in the
set box sitting on a shelf sometimes in very moist, or humid areas of
Scuff protection is most critical during early engine operation. Even
when an engine has been pressure pre-lubricated the ring belt area of
the piston receives little if any oil during the pre-lube process and
receives oil only after the engine is running and oil is being spun from
the crankshaft. The piston rings then, must depend on the oil which was
applied to them before they were installed in the cylinders for a brief
period after the engine is initially started.
Manganese phosphate coating has excellent properties to accomplish its
task of scuff protection because it is porous and quite soft. It can be
likened to a sponge in that it soaks up and retains oil for the period
before the ring is receiving off from the running engine.
Manganese phosphate coatings have been used on piston for many years. It
is in some instances used on pistons and cylinder walls, and has many
other applications such as knife blades, gun barrels, and machine parts.
Many trade names are used for this coating such as Graphitox, Granoseal, Parco Lubrite, Phos Dip, etc.
–Tech Tip courtesy of Hastings Manufacturing