PT (Paint penetration inspection)
Procedure
The principle of the procedure is that due to its low surface tension and high capillarity, the penetration agent penetrates micro-fine cracks.
Surface faults can be proven with all metallic materials, many synthetics, glass and ceramics. A penetration check can only be carried out on non-porous materials. Furthermore, it has to be made sure that the surfaces to be checked is free of any coating or impurity to enable the penetration agent to penetrate probably existing surface faults.
The penetration procedure can be applied in daylight with the paint penetration procedure or with fluorescing penetration media under UV-light. The inspection can be carried out directly at the component or with “penetration equipment”, e.g.for standard parts.
The procedure comprise the following steps:
Pre-cleaining
For the pre-cleaning the surface has to be carefully cleaned of any coating or contamination.
Penetration Procedure
The agent can either be brushed on, sprayed on, or applied electrostatically or in a soaking bath. The penetration agent should cover the complete surface to be inspected and have 30 minutes’ time to react at a temperature between 5 and 50 degrees C.
Intermediate Cleaning
During the intermediate cleaning* the excess penetration agent is removed from the test surface, this is done with a special system related purifier or with water. For the intermediate cleaning it is especially important to take care that potentially existing bad spots are not washed out.
The procedure of the intermediate cleaning is carried out in three steps:
- Step 1: Pre-cleaning with water
- Step 2: Application of the emulsifying agent (makes the penetration agent water soluble)
- Step 3: Rewashing
Drying
Drying can be done by exposure to air or in a suitable drying oven.
Development
Also the developer can be applied in different ways ( e.g. as wet developer, as powder, or electrostatic ). Here it is to be ensured that the developer layer is applied evenly and thinly. After the developing time which is as a rule equivalent to the penetration time, the inspection of the test surface begins.
With the penetration agent the indications are in colour (mostly red) on a white background. With the fluorescent paint penetration media the fault indicators are bright yellow-green on dark-violet background.
To avoid the danger of excess washing, often re-emulgating penetration agents are used. This is a non- water-soluble penetration agent.
*For the fluorescent inspection both the intermediate cleaning as well as the assessment is done under UV-light.
