Black coolant tank walls

[AzMar]

so for the past 2 years  black spots keep appearing on the coolant tank walls,  the coolant itself is crystal clear there is no oily residue or anything and to make sure i took it to the dealership and they checked the hoses and the whole coolant circuit and they told me it was clean, the spots aren't on the outside either so i'm assuming the plastic itself is burning? idk if anyone can check the pics and any help is appreciated.🙂
ps: 2015 fiesta titanium 1.25 4 Cyl

 

[Neb_engineer]

i have seen this on other vehicles, it happens because the plastic starts to degrade on the surface from the coolant. it is completely harmless it just means that eventually you will have to clean the inside when it gets too cloudy to see the level. An example of a vehicle i have seen this on is a 2003 mk3 nissan micra which has done thousands of miles, it still has the original reservou we just use a piece of cardboard with the levels marked as a dipstick to see the level of coolant.

The other thing that happens is after a while the antifreeze comes out of solution and stains the walls of the reservou, this is completely normal.

[JONATHAN_11_80]

Looks like oil emulsion to me. Never seem this on any ford.  However my old Citroen had a head gasket issue and the header tank looked similar in the early stages.

[AzMar]

thanks for the responses but as Neb_engineer said the walls are stained, i checked the coolant and it's clean also the oil level is the same for months now so i don't think there is any leak from one to the other fortunately.

[isetta]

My first thought on looking at the pics was mould.

[ThaiFiesta]

These tanks are prone to failure due to thermal stress. They’re cheap enough that it’s probably worth swapping yours out as a major failure of the tank will lead to catestrophic engine loss. Ford swapped mine under warranty.