chrome pitting

[James <nyb@xxxxxxx>]

Title:

The why's associated with pitting are not exactly straightforward, but here are the basics:

A pit results with the development of a local anode where the metal is oxidised (loses electrons).  Electrons from the now oxidised metal atoms travel through the metal to a cathodic area where water is reduced (gains electrons).  The metal ions formed in the process can either be simply washed away, or they may be carried by water to the cathodic site where they can join up with the products of water reduction to form an oxide.  In the case of iron this is why you find orange deposits off to the side of the pit. 

What causes a site in the metal surface to become anodic has a lot to do with the metallurgy of the metal in question.  Pits often form at stress points, areas containing impurities (in stainless steels sulfur is a frequent culprit), and because alloys are never truly homogeneous some parts will simply oxidise more easily than others.  Pits in chromed bits and pieces may also start in areas where there are flaws in the chrome plating, including microscopic cracks, that can expose more easily oxidised metals underneath.  Chromium itself is pretty corrosion resistant due to the formation of a thin oxide layer that acts as a barrier, the same principle goes for aluminium.  However, scratches and other damages (including rocks that hit headlights) that result with time can both expose what lies beneath, and perhaps more significantly, create areas where water can collect.  Even if your car doesn't get wet too often, or the pitted part is inside the car, don't discount the presence of water vapour in the air. 

As for prevention, as stated earlier, a lot depends on the manufacture of the metal.  However, keeping the car dry, clean (acids in "dirt" can acclerate the process), and trying not to damage the metal will help.  Polish as necessary, but keep in mind that every time you do that you add more little scratches.  You also remove the aforementioned oxide layer, making the metal form a new one, reducing the amount of metal in the metallic state, although this is a minor point.

James

Captden30@xxxxxxx wrote:

i was polishing the headlite rings from my 53 imperial, which has some pitting. i became curious as to why these pieces "pit" and what exactly is pitting. we all know what it looks like, but what is really the root cause. i would assume that age is a factor, but i also think it is more than that. the piece is made out of pot metal, and what is that made of and is that a contributing factor to pitting? i suppose the why's dont matter that much, but perhaps we can better prevernt our cars from getting future pitting if we know the answers. i know that there are many people on this site who can shed light on this subject.  thanks,  dennis  53 imperial[for sale]