2. Essential physicochemical factors involved

2.1 Penetrant

The penetrant must have two essential characteristics:

• good wettability: the penetrant must spread over the entire surface to be inspected in the form of a thin, uniform and continuous film, whatever the metallic material (or not, sometimes), whatever the surface condition (machined, as-cast, as-forged, etc.);

• good penetrating power, so that it can penetrate all open-to-surface surface discontinuities by capillary action. In other words, the penetrant must...