A coolant tank is a container for holding a liquid called coolant, which is used to cool down or regulate temperature in machines or engines. Full definition
Yours is not the only mark \ model that has this type of issue: jag x type are known for leaking coolant tanks, again made of plastic. (mechanics.stackexchange.com)
After driving some time, is it normal that the plastic spillover coolant tank, usually attached to the fan housing, that takes boiling overflow, get so hot that you get bubbles boiling out of it even though the engine itself, according to the temperature gauge on the dashboard, does not get overheated? (mechanics.stackexchange.com)