Which material is explicitly not allowed for CNG service components?

The correct answer highlights that aluminum fittings are explicitly not allowed for CNG service components due to their susceptibility to corrosion and fatigue under stress. CNG typically operates at high pressures, and aluminum materials can fracture or degrade over time when exposed to these conditions combined with the chemical properties of natural gas.

In contrast, the other materials listed have been generally accepted for use in CNG systems under appropriate conditions. Malleable iron castings are often utilized for their strength and durability. Fiberglass pipes offer excellent resistance to chemical corrosion and can be beneficial in specific applications. High-density polyethylene tubing is designed to withstand the pressures required in CNG services and is resistant to the corrosive effects of natural gas.

Thus, the elimination of aluminum fittings from CNG service components is rooted in safety and reliability considerations, ensuring that materials used can handle the demands of the application without compromising system integrity.