By definition, penetrant testing (PT) is the resurgence of a liquid (or a gas) from a discontinuity in which it had previously been introduced during a penetration operation. A logical extension of the visual examination, this method makes it possible to detect discontinuities opened to the surface of the controlled part in the form of colored or fluorescent indications, respectively observed against a white background or on a black background.

 PT comes in four phases, the first of which is to clean the part to be controlled. This is a much important step. Badly performed, it leads to unusable results. A colored and / or fluorescent liquid, called "penetrant", is then applied to the surface to be controlled during the second phase. It penetrates by capillarity inside the discontinuities (corrosion cracks, porosities, pits, sinkholes, cold drops, lines, folds, gaps, rectification cracks, heat treatment taps, micro-incipient fractures, etc.), requiring a penetration time before the next step.

During the third phase, excess surface penetrant is removed in such a way as to keep the entrapped penetrant untouched. The surface can then be covered with a thin layer of "developer", which adsorbs the penetrant contained in the discontinuities.

During this fourth phase, the developer helps in making easily visible the indications of discontinuities that appear on the surface.

In this paper, we examine the 5L Gr steel API to detect surface defects and the external situation of the weld seam by experience, pursuit and then conclusion.