Location: Sydney, Australia. Found it during bushcare.
The brass barb fitting and the powdery filling suggest some sort of kiln burner to me, but the dark green paint on the outside of the tube looks rather ordinary and not like it has been through high temperatures.
The soft, powdery cemetitious filling has a copper-green tint. Only one end has a hole.
If it were not for the brass barb and coppery fill colour I would assume this is just a bit of structural steel from someone's carport (or similar) that has filled with cement and now been cut to pieces for disposal. But a carport with a barb fitting? WTH?
We find all sorts of garbage in this bushland because it's sandwiched in suburbia. Traditionally it was a dumping ground (mattresses, furniture, asbestos, whole cars) and today still people use it illegally as a dump (mainly building materials and soil). Lots of random materials get deposited by or uncovered by stormwater runoff & floods too. There is no limit to the craziness of what you find here.
I was going to reply with "you can't use barbed fittings at high pressures", but I looked it up and found some claiming 150psi (10 atmospheres). Huh. Perhaps this did start life as a hydraulic cylinder that has had some parts lopped off.
I don't think it's built up corrosion. The pipe is steel and corrodes to red/brown iron oxide, as visible around the circumference at the end. The green colour in the filling is not an iron oxide. It might be a copper oxide, or some dye in the white material.
That type of fitting is definitely low pressure because it is barbed.