The visual appearance of a compression ring is a matte black color characteristic of the coating used to protect the cast iron. 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 the country.
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.