Control of the Compound Zone or White Layer

The compound zone is more commonly known as the white layer, simply because when the nitrided sample is sectioned through the case, and then polished and etched with a standard solution of nital (2 to 5% nitric acid and alcohol), the immediate surface etches out white in appearance above the nitrided case. This chapter focuses on the methods to control the compound zone or white layer. It first provides information on a test to determine the presence of the white layer, and discusses the processes involved in the reduction of the compound zone by the two-stage process. Next, it describes the methods for controlling compound zone formation, and, finally, reviews the factors related to the determination of case depth nitriding.