What materials should be used for the main pipeline of liquid ammonia loading arms to ensure corrosion resistance?
Liquid ammonia (anhydrous ammonia) is relatively chemically stable, but it can form alkaline solutions (ammonia water) in the presence of moisture, causing corrosion to some metals. Additionally, trace impurities in liquid ammonia (e.g., hydrogen sulfide, oxygen) may accelerate corrosion. Thus, the main pipeline materials must resist corrosion from liquid ammonia itself and potential corrosive byproducts. Suitable materials include:
Stainless Steel
304 Stainless Steel: Suitable for liquid ammonia with trace moisture, as its chromium-nickel alloy resists weak alkaline corrosion. Risk of intergranular corrosion in high-concentration ammonia water should be noted.
316 Stainless Steel: Superior to 304 due to molybdenum content, offering better resistance to pitting and crevice corrosion, especially in liquid ammonia containing sulfide impurities.
Low-Alloy Steel
e.g., Q345R (formerly 16MnR): Has similar corrosion resistance to carbon steel in dry liquid ammonia but with higher mechanical strength, suitable for high-pressure pipelines.
Aluminum and Aluminum Alloys
Pure aluminum or rust-proof aluminum (e.g., 3A21): Stable in dry liquid ammonia with low corrosion rates. Lightweight and cost-effective but limited to medium-low pressure due to lower strength. Avoid contact with moisture to prevent aluminum hydroxide corrosion.
Non-Metallic Materials
PTFE (Polytetrafluoroethylene): Highly corrosion-resistant, used as liners or coatings to protect metal pipes from wet ammonia or impurities, requiring a metal framework for structural support.
FRP (Fiberglass-Reinforced Plastic): Resistant to alkalinity, lightweight, and anti-aging, suitable for low-pressure pipelines but not for high temperatures or severe impacts.
