A metallurgical bond can be
achieved without any visible transition zone if the coating is applied
correctly to a clean substrate. This is an example of hard
chromium plating onto steel.There is usually a very thin copper coating electroplated onto the cleaned iron surface, followed by nickel. These transition layers improve the bond between the iron and the hard chrome. Antique tools often lack the transition copper layer and consequently exhibit gross peeling of their nickel plating as soon as a little rusting of the steel begins. Chromium wasn't added on top of the nickel plating in early tools. The chromium on this example is quite thick.
This is a pump component of tempered martensite shown at 500X with a Nital etch. The unetched chromium at the top has a hardness of Rockwell C64 due to its extremely fine grain size as well as to the strains locked into the grains during the room temperature deposition process. This is a good microstructure with normal porosity and adherence. In this example the chromium serves both for corosion protection and also as a wear resistant layer over the tempered martensite, which thereby can be tempered for toughness rather than for wear resistance.
Specimen 10
shows another kind of chromium coating on
steel.