Dye Penetrant Testing (PT) is capable of detecting surface-connecting discontinuities in ferrous and nonferrous alloys, as well as nonmetal objects. PT can be used to examine weld joint surfaces, completed welds, and perform intermediate checks of individual weld passes. This technique is commonly employed on austenitic stainless steels where MT is not possible. The examiner should recognize that many specifications limit contaminants in the penetrant materials that could adversely affect the weld or parent materials. Most penetrant manufacturers provide material certifications on the amounts of contaminants (e.g. chlorine, fluorine, sulfur, and halogens).

Capillary action (a force resulting from adhesion, cohesion, and surface tension in liquids that are in contact with solids, as in a capillary tube) is the basis for the penetrant to be drawn into a material. Reverse capillary action is the principle behind the visualization of indications after the application of developer.

A limitation of PT is that standard penetrant systems are restricted to a maximum of 125 °F (52 °C), so the weld or material to be inspected must be below this temperature, which significantly slows down the welding operation. Additionally, this method requires extensive clean-up so as not to contaminate the weld surface. High-temperature penetrant systems can be qualified to extend the temperature envelope.

During PT, the test surface is generally solvent cleaned and coated with a penetrating liquid that seeks surface connected discontinuities. The length of time the penetrant remains on the surface, also known as dwell time, is based upon several factors, primarily the type of imperfection that can be expected to be present. After the excess surface liquid penetrant is carefully removed, a solvent-based powder suspension (developer) is normally applied by spraying. The liquid in any discontinuity bleeds out to stain the powder coating. An indication of depth is possible if the inspector observes and compares the indication bleed-out to the opening size visible at the surface. The greater the bleed-out to surface-opening ratio, the greater the volume of the discontinuity.

 Liquid Penetrant Techniques

The two general penetrant techniques approved for use include the color-contrast penetrant technique (normally red in color to contrast with a white background) and the fluorescent penetrant technique, which uses a fluorescing dye that is visible using ultraviolet light, as shown in Figure 1. Fluorescent penetrant techniques are generally used to detect fine linear-type indications. The examination is performed in a darkened area using a filtered ultraviolet light source, commonly referred to as a blacklight.

Three penetrant systems are available for use with both techniques. They are:
a) solvent removable,
b) water washable, and
c) post emulsifiable.

Figure 1—Fluorescent Penetrant Technique

Compatibility with base metals, surface profile (welds), and process material should be considered before penetrants are used, since they can be difficult to completely remove.

ASME Section V, Article 6, Paragraph T-620, lists general requirements for PT. Codes and specifications may list compliance with these requirements as mandatory. API Standard 650, ASME B31.3, and ASME Section VIII, Division 1, require PT to be performed in accordance with ASME Section V, Article 6. Some requirements listed in this article include the following:

a) inspection to be performed in accordance with a procedure (as specified by the referencing code section);
b) type of penetrant materials to be used;
c) details for pre-examination cleaning including minimum drying time;
d) dwell time for the penetrant;
e) details for removing excess penetrant, applying the developer, and time before interpretation;
f) evaluation of indications in terms of the acceptance standards of the referencing code;
g) postexamination cleaning requirements;