I have just bought a new set of wheel caps for my CLK. The present OE articles have developed white (corrosion?) marks around the outer silver ring.

Is this a common issue with these caps?

 

Sounds like it could be galvanic corrosion fairly common with aluminum parts touching another metal...
http://en.wikipedia.org/wiki/Galvanic_corrosion

Are you using one of those acidic wheel cleaning products? That could be the culprit as well.

 

The problem was with the older style center caps that had a spring steel retainer that would react with the aluminum wheel. The newer ones are just plastic, so no worries.

 

So it was galvanic corrosion

 

My caps are plastic only. I have posted on the w204 forum as well due to sample size there & it seems common. I don't expect it on a garage queen like mine with a mere 14,000 miles on the clock.

I can't vouch for the first 9000 miles on my car so maybe aggressive alkali wheel cleaners had been used. The white spots are almost like a delamination of the coating.

 

I had my set replaced (under warranty) last year for my ML.I spoke to the parts man and he said about every 2 weeks he has a set he has to replace for someone.

 

Odd, I had the plastic ones on my W163 ML for about 6 years and 100K miles with no problems - and I did a lot of off-road driving and used a pressure washer to clean the wheels.