Ferrite magnets are extremely popular due to their characteristics. Being made from oxides of iron, they simply do not corrode when placed in water. They are therefore regarded as corrosion free. For long term performance they are superb; if looked after they are capable of exceeding most products lifecycles.
Ferrite / Ceramic Magnets resist chemical attacks from many of the alkaline solutions, dilute acids, organic solvents, and some hydroxides. They are fine for use in salt water conditions (no corrosion issues at all). It can be etched with acids (e.g. hydrochloric acid).
They do not need a coating but have been known to be coated only in specialist applications where any ferrite surface dust is not wanted (the coating locks in any dust that may remain from the production processes – an issue which does not affect over 99% of all applications). An example where coating may be required is in medical applications (e.g. painting, nickel plating, etc).
So, for most applications, ferrite magnets (ceramic magnets) do not require any coating and can be used as is.
For note, ferrite magnets are naturally a darker grey colour and are often referred to as having a “pencil lead” colour.