Development of pulse-plating technology for the preparation of coatings with varying composition along their thickness: a historical overview

Electrodeposition has long been used for the preparation of coatings consisting either of a single metal or a combination of two or more metals in the form of an alloy. At a certain level of technological development, the demand arose to obtain coatings which can combine the advantageous properties of different metals none of which alone nor any of their mutual alloy combinations can offer the desired properties. This need was the driving force behind the development of electrodeposition methods to produce layered coatings consisting of alternating constituent layers with widely different behaviour. These so-called ‘compositionally graded coatings’ can be compositionally modulated alloys or multilayers. In the present contribution, a historical overview of the development of various pulse-plating methods for the preparation of compositionally graded coatings from a single bath is given. The most common procedure is the use of a repetition of a pulse sequence. This sequence consists of two or more consecutive deposition pulses of different lengths, current densities and/or deposition potentials. As a particular case, the historical development of corrosion-protective electrodeposited multilayers based on the Zn–Fe system and the state of the art of the system is described.