What Causes Cooling System Deposits?

[ConfederateTyrant]

I understand that the use of non-distilled, oil contamination, and waiting too long to flush the coolant can cause the cooling system to gather deposits. However, I just got done flushing my wife's coolant and the radiator and water outlet had some brown residue which was easily wiped up with a gentle, finger swipe. This car is a 2004 Nissan Sentra 1.8L, I assume the coolant may have been flushed at Carmax, but unsure, either way, is there any real reason that deposits would have formed that fast? Or is that normal?

I plan to use RMI-25 to remove these deposits, but that will be later.

 

[phenomic]

I would assume that contaminants in the fluid pile up in places. Also internals in the system break down and/or oxidize which can cause deposits.

 

[Big Jim]

May have been stop leak. Many cars come from the factory with a light dose of stop leak that looks like what you are describing.

 

[ConfederateTyrant]

Big Jim,
I did not even think of the stop leak, and since this is an engine known to have head gasket problems, it's possible that the previous owner had the head gasket replaced and Nissan added stop leak. So far the first UOA are confirming no coolant contamination, and the second one is about to be sent off as well.