A small amount of neodymium added into pure iron will reduce the annihilation rate of valence electrons and increase the core annihilation. Nd ions may be negatively charged relative to Fe ions

so a positron is attracted by rare earth impurity atoms in iron. The electrical properties of proton and positron are similar. If the difference in their mass doesn't change the essence of their interaction with impurity

we can easily believe that neodymium is a hydrogen trap in steel. It can attract proton

restrict the migration and enrichment of hydrogen in steel and thereby delay the hydrogen induced delayed fracture.