Diffusion flux in Phase Diagrams and Phase Transformations Formulas

No, the Diffusion flux, measured in Mass Flux cannot be negative.

Diffusion flux is usually measured using the Kilogram per Second per Square Meter[kg/s/m²] for Mass Flux. Kilogram per Hour per Square Meter[kg/s/m²], Kilogram per Hour per Square Foot[kg/s/m²], Pound per Hour per Square Foot[kg/s/m²] are the few other units in which Diffusion flux can be measured.