Monel 400 is a nickel-copper, solid solution alloy with lesser components of copper and manganese. Only cold-working will harden it, making it suitable for subzero temperature applications. Additionally, it is applicable over a wide range of temperatures and environments. Like many nickel alloys, it is also highly resistant to corrosion.This metal is used in acidic and alkaline environments due to its resistance to substances such as sea water and other caustic solutions. It has wide applications in marine industries, as well as heat-exchangers. However, Monel 400 often required strengthening because of its low-annealed strength.Tempering is often required to improve hardness and elasticity. Monel 400 is additionally poor in the area of oxidation resistance; however, it is excellent under stress and corrosion in a variety of salt water, fresh water, brackish water and hydrochloric environments. It is also all but immune to chlorine. Monel 400 offers the same corrosion resistance as nickel, but often is chosen because of its reduced cost and much higher ability to withstand pressures and temperatures.