Monel Alloy 400 UNS N04400 is a ductile nickel-copper alloy with resistance to a variety of corrosive conditions. The alloy is most frequently specified in environments ranging from mildly oxidizing through neutral and in moderately reducing conditions. An additional application area of the material is in marine environments and other nonoxidizing chloride solutions.
The alloy has a long history of use as a corrosion resistant material, dating back to the early 20th century when it was developed as an attempt to use a high copper content nickel ore. The nickel and copper contents of the ore were in the approximate ratio which is now formally specified for the alloy.
As with commercially pure nickel, Alloy 400 is low in strength in the annealing condition. For this reason, a variety of tempers are used which have the effect of increasing the strength level of the material.
Alloy 400 is more corrosion resistant than commercially pure nickel (UNS N02200) under reducing conditions, and more resistant than refined copper alloys under oxidizing conditions.
In moderately reducing acids, neutral or alkaline solutions, Alloy 400 may be considered for use. The alloy is resistant to most alkalies, salts, organic substances, and atmospheric conditions.
Alloy 400 is a consideration for cooler alkaline caustic conditions, although high temperature, high stress, and high concentrations of caustic have produced caustic stress corrosion cracking in the material. The alloy is used in reducing acids like sulfuric and hydrochloric, especially in the absence of aeration and oxidizing species.
Alloy 400 is exceptionally resistant to chloride stress corrosion cracking.
Application in waters, including seawater and brackish water, is a major use of the material.
Alloy 400 is attacked in sulfur-bearing gases above approximately 700°F (371°C), and molten sulfur attacks the alloy at temperatures over approximately 500°F (260°C).
