When I bought my first Crest, a 575D;
I wasn't sure if it was working as it didn't do a whole lot, so someone on Subguns.com said to drop a piece of aluminum foil in it and check it after 5 or 10 minutes, I did and pulled out a piece of dulled foil, upon closer inspection holding it up to the light I could see tiny pin holes in the foil, put it back in for 10 minutes and the pin holes were being eaten away at and you could see the edges of the foil were breaking down and almost looked tattered.
I upgraded to a 690HT when it was posted on this board by Shane in NE, and sold the 575D.
I use simple green 50/50 with water in mine so I don't know which to blame but I have removed the blue from a Walther P22 Zinc Slide and a Bushmaster M16 Aluminum upper receiver.
I use it to clean all the internals of the MP5's, M16's and 1919, it removes even the most stubborn carbon and crud on the MP5SD, I place the whole MP5 receivers in the Crest and it cleans them totally inside and out, blow them out with compressed air, oil and reassemble.
I put the receiver of the 1919 in it (about 7/8 submerged) and it kinda discolored the park on it, but not too awful bad.
Scott