The Viability of Sol–Gel Coatings for the Corrosion Protection of NdFeB Permanent Magnets

NdFeB permanent magnets are essential components in electric vehicles, wind-turbine generators, high-performance motors, and other energy technologies because of their exceptional magnetic performance. However, their multiphase microstructure and highly reactive Nd-rich grain-boundary phase make them particularly susceptible to galvanic and intergranular corrosion, which can cause grain detachment, pulverization, loss of coercivity, and premature failure. This review critically examines the corrosion mechanisms and recent advances in protective coating technologies for NdFeB magnets, including chemical conversion, vacuum deposition, electrodeposition, electroless plating, physical vapor deposition, organic/inorganic coatings, and sol–gel-derived systems. Reported quantitative results demonstrate the substantial potential of advanced coatings: phosphate layers with thicknesses of 10–18 μm improved corrosion protection while preserving magnetic properties; gaseous phosphating increased neutral salt-spray resistance from 4 to 12 days; amorphous Al–Mn coatings reduced the degradation current density by approximately three orders of magnitude; plasma-assisted Al coatings reduced corrosion current density to 22.05 × 10−2 μA cm−2, representing a two-order-of-magnitude improvement over uncoated magnets; and sol–gel alumina coatings reduced corrosion current density by approximately one order of magnitude. More advanced hybrid sol–gel systems provide even greater protection, with PEI-functionalized graphene oxide/silane coatings achieving a corrosion current density of 8.365 × 10−8 A cm−2 and a water contact angle of 148.5°. Lanthanum-doped silane coatings delivered a protection efficiency of 92.5%, while self-healing B–PDMS/SiOx coatings reduced the corrosion current density from 3.52 × 10−6 to 6.39 × 10−10 A cm−2. Overall, sol–gel and organic–inorganic hybrid coatings emerge as promising, adaptable, and potentially sustainable alternatives, although crack formation, adhesion, defect control, long-term stability, and industrial scalability remain important challenges.