The Structure and Hardness of Electrolytic Chromium

Abstract

Although the structure of electrodeposits of copper, nickel, and iron is well known, little has been published on the structure of chromium. Investigation of the chromium structure has been greatly complicated due to the difficulty of finding a suitable etching agent for metallographic study and due to the anomalous behavior of chromium and the widely different appearances of the structure. The chromium crystals are generally so fine that they are not always visible under the microscope, even at very high magnifications. Furthermore, the deposit contains numerous inclusions which will frequently conceal the true crystal structure. On the basis of the three hundred samples examined in this investigation, it is concluded that a certain analogy exists between the well‐known structure of other electrolytically deposited metals and the structure of chromium deposited from chromic acid baths, in spite of the frequent presence of many inclusions which are in general vertically oriented to the base. It was observed that every change in bath composition or in the conditions of electrolysis had a distinct effect on the structure and the hardness of the chromium deposit, a change in structure corresponding to a change in hardness, ranging from 400 H to 1,200 H on the Brinell scale.