Decorative Chrome Plating of Steel and Aluminum

Chrome refers to the element chromium, atomic weight 52 on the Periodic Table of the elements. It is generally electroplated over steel, although sometimes aluminum or some other base metal is used. Chrome is plated sometimes thousandths of an inch thick, for strength (so-called "hard chrome"), in the manufacture of internal automotive engine parts. Sometimes chrome is plated only millionths of an inch thick, for decorative purposes (e.g. chrome bumpers or other trim and household items).

If Aluminum - If Steel

If the base metal is steel, nickel can be applied directly to the steel. However, in order to avoid adhesion failure, nickel to be applied to aluminum must not be applied directly; rather, a zincate step¹ must be included that lays down an extremely thin layer of zinc. The zinc adheres to the aluminum and the nickel (or first, copper from an alkaline copper bath²) adheres to the zinc.

Nickel In-Between: Semi-Bright First

Whether it initially receives a coating of copper or not, a steel bumper that is to receive a shiny chrome surface, is best electroplated with two coats of nickel. Why two coats? It is because two different types of nickel are electroplated from two chemically different nickel plating baths. The first coating of nickel comes from a "semi-bright" nickel bath, useful for producing adhesion to the steel and excellent corrosion resistance, but also for leveling-for increasing surface smoothness. A desirable formulation for semi-bright nickel might be sulfur-free and ductile, to allow bending without cracking. Following that process, nickel from a "bright" nickel bath is applied, primarily to decrease corrosion of the underlying nickel layer, so that it doesn't reach to the base metal.

Nickel In-Between: Bright Second

Bright nickel follows the semi-bright nickel layer. It may be a sulfamate formulated bath, which supplies abrasion and wear resistance. In addition, as Finishing Dot Com says, "Bright nickel is anodic to the semi-bright nickel, and sacrificially protects it, spreading the corrosion forces laterally instead of allowing them to penetrate through to the steel."

At Last - Chrome!

Finally, it is time to electroplate chrome. Chrome is an extremely hard metal, providing the attractive tinge of blue to which enthusiasts are accustomed. It also affords a mirror-like shine and is very scratch resistant. It is plated to mere millionths of an inch in thickness. If the reader has ever been to an automotive show where automobiles are modified from the original, he may be duly impressed by the abundant use of exposed, chromium metal-plated parts.

¹ Zincate is short for sodium zincate. This chemical reacts by the equation sodium zincate plus aluminum produces sodium aluminate plus zinc. In other words, the topmost layer of atoms of aluminum are replaced by zinc, making the copper or nickel bath "think" it is a zinc piece that is being plated, rather than aluminum.
² Alkaline cyanide copper was historically used, though there are now available alternatives.

References and Resources:

Finishing Dot Com - Chrome Plating FAQ

Atotech - Satin, Bright, and Semi-bright Nickel