MPI vs DPI - Which Inspection Method Do You Need?

Magnetic particle inspection (MPI) and dye penetrant inspection (DPI) are two of the most widely used surface inspection methods in non-destructive testing. Both detect surface-breaking defects such as cracks, laps and seams — but they work very differently, and choosing the right one matters.

How MPI works

Magnetic particle inspection works by magnetising a ferromagnetic component and applying magnetic particles to the surface. Where a defect interrupts the magnetic field, the particles gather and reveal the flaw. It's fast, reliable and can also detect some near-surface defects that sit just below the surface.

The limitation: MPI only works on ferromagnetic materials — carbon steel and most iron-based alloys. It cannot be used on stainless steel, aluminium or other non-magnetic materials.

A centre-line crack found using MPI

How DPI works

Dye penetrant inspection involves applying a coloured or fluorescent penetrant liquid to a clean surface. The penetrant seeps into surface-breaking defects, excess is removed, and a developer draws the trapped penetrant back out — making defects clearly visible.

DPI works on almost any non-porous material, including stainless steel, aluminium and non-ferrous alloys. The trade-off is that it only finds defects open to the surface, and it requires thorough cleaning before and after inspection.

DPI on stainless steel flange welds

Which do you need?

As a general rule: if the component is carbon steel, MPI is usually the quicker and more sensitive choice. If the material is non-magnetic — stainless steel, aluminium, brass — DPI is the answer. The application also matters: weld inspections, castings, forgings and lifting equipment each have their own considerations.

If you're unsure which method your components require, our PCN certified inspectors can advise on the right approach for your material, application and code requirements, wherever you are in the Hull, Grimsby or wider UK area.

Need surface inspection?

Testa provides MPI and DPI inspection services across Hull, Grimsby and nationwide, including mobile fluorescent MPI for on-site work. Contact us today to discuss your requirements.