Cast iron has a unique natural process of corrosion resistance due to the chemical composition and reaction to atmospheric exposure, which is called Patination. The patina effect is actually a thin layer of oxides that acts to slow down further corrosion. Unlike steel, cast iron is inherently resistant to continued rapid corrosion. Steel oxidizes into flakes, which lift off and exposes fresh surfaces.
On the cast iron (Ductile/Grey) the patina is iron oxide (to the metallurgist it’s iron oxide and to the rest it’s just rust). Cast iron rusts very quickly when the humidity exceeds 60%, however, the rust is not invasive but will act as coating to prevent deep rusting.
The patina effect is a typical trait in cast iron covers and grates which goes through predictable set of stages. Other metals are known for their patinas, but exhibit many range of colours; from greens (Bronze) to yellows and even clear.
The appearance of cast iron covers/grates can change with the local conditions and the amount of traffic the units are exposed to; example high trafficable area, the appearance in most cases comes up with a silver shine. The choice of cast iron access cover and grates are redominately chosen by design engineers because of the corrosion resistance, strength, durability and in some cases appearance (rusty look).