If they are less than 2 years old and because it doesn't appear to be caused by cutting or scratching the plastic coating, they would be covered.
Their are 2 types of impeller magnets, rare earth (neodymium) and ferrite. Ferrite is the common grey magnet material most pumps have, it is ceramic and cannot corrode. Rare earth magnets are used for higher torque or greater energy efficiency. The magnets have more than twice the power of ferrite magnets so the electromagnet can be much weaker (less electricity) and produce the same power or you can use the same power as with ferrite but have nearly double the torque for high head pumps. Because these magnets themselves are very expensive, they are not often used for impellers but the bigger Eheim pumps and many other brands do use them. They have to be plastic coated because they do corrode and if anything cuts or breaches the coating, they corrode very quickly.
BTW, the top image is of a 9410 impeller and the bottom image is a 9420.
