What's this inside my radiator?

[xtreme_exploder]

I have this black coating inside my radiator (Fluidyne) and it's not oily or anything like that. In fact it won't wipe off or anything, it's like black anodizing. The only thing that takes it off is acid mag wheel cleaner. I cleaned it once, put it back in and less than a year later it looks like this again. The rest of the cooling system, including the aluminum inside the heads, etc. is perfectly clean. I've searched the interwebs for this issue and came across nothing. My only guess is Fluidyne used crappy material that reacts to water or coolant. Any knowledge out there on this?

 

[SnakeBitten07]

I happen to have first hand knowledge of this. The coating is aluminum oxide - caused by the chemical interaction of the coolant with the aluminum of the radiator. One or two elements in the aluminum alloy is high enough to cause the reaction (I cannot recall what elements for the life of me, I have the information at work) - this is why you don't see it in the heads or anywhere else. Is the water pump body clean of the oxide as well? What coolant are you using, and have you run any additives in the system?

Rest assured, the oxide is harmless.

 

[xtreme_exploder]

Awesome, I figured it was something along those lines but I wasn't sure. Not sure about the water pump, I only looked in the heads where the crossover comes out, and the inlet adapter on the block. I was running regular green antifreeze and distilled water, with redline water wetter at one point. I got rid of the water wetter after I saw little "floaties" in the coolant, and I didn't notice any difference in cooling.

 

[SlowSVT]

Aluminum oxide (corrosion) is usually a dull gray and is a good thing to have as it protects the substrate from further corrosion. Not sure what the back coating is perhaps Fluidyne chem films (iridite) their radiators (probably to keep it shiny until it gets exposed to heat and anti-freeze) and this is how it may reacts. If it's no where else in the system it's something Fluidyne coats the rad with. I wouldn't worry about it.