ATI 400™ alloy (UNS N04400) is a ductile nickel-copper alloy with resistance to a variety of corrosive conditions. The alloy is most frequently applied in a range of environments ranging from mildly oxidizing through neutral and to 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 twentieth 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, the ATI 400™ alloy is low in strength in the annealed condition. For this reason, a variety of tempers are used which have the effect of increasing the strength level of the material.