Just to add my perspective on an interesting discussion...

Chrome plating or any other layer of foriegn material to enhance the surface condition or performance in service is a common practice, and typically can achieve the intended or desired results. You have to be pretty sure about what the intended results are and make sure you are careful in the processing steps. It is a route "fraught with peril" to quote an unforgettable, and likely inadvertant mentor of my past. Coatings, including plating of Chromium or Nickel, is subject to a variety of conditions that will deposit atoms of the intended material along with unintended materials. In addition, they can cause absorbtion of gasses, like Hydrogen, which, as noted earlier, are prone to alter the mechanical properties of high strength steels (Hydrogen embrittlement weakens grain boundaries, leading to fractures at relatively low, steady state stress levels). In addition, anything sticking to the surface of the base material does nothing to improve the base material properties at the interface, and subjects the interface "bond" to the same mechanical stresses/loads that the base material sees. Often this results in failures of the coating bond, which leads to more complex failures.

Nitiriding, and shot peening for that matter, put the outside layer of the base material in compression while increasing hardness, kind of like preloading the surface by either mechanically working it (shot peening) or chemically jamming Nitrogen molecules into the grains/grain boundaries of the base material. With the surface layer in compression, the fatigue performance, or endurance limit , is improved in the application in question. Kind of like putting a fastener in a condition of preload so that the alternating loads of the application do not physically excercise the fastener.

So, on a production basis the Nitriding approach is far superior to the chrome plating, however, if all you want is a harder, more wear resistant surface because the base material is fine for the application as it is, other than hardness, carefully chrome plating should work ok.

In some cases I would question whether or not chrome plating actually helped - the condition of the original material might have been adequate to begin with and chrome plating only made things that were ok, a little better. In my past experience though, a good quality chrome plating job was so rare, I would tend to avoid them altogether.

Hope this helps, Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)