Dye penetration test, also known as liquid penetration is NDT technique widely used to locate the surface breaking flaws such as laps, cracks, porosity and seams, and other surface discontinuities. The method is also useful for testing on-site as it is portable, and when certain techniques are applied such as visible color contrast there is no need for additional recourses like water and electricity. Dye penetration may vary as per the potential system, component size, discontinuity type, but some common steps are described below –
Pre cleaning and surface penetration
Although, it is a basic step it is an important one. In this step, the surface is cleaned from grease, water, oil, or any other contaminants. The penetrant should be able to freely enter the discontinuities. Cleaning methods may be solvent,
alkaline cleaning steps, etc. Sometimes, etching may be required for making the defects open to the surface, dry and free from contamination.
Penetrant Application and removal of access pentrant
In the next step, the liquid penetrant is applied on the surface and allowed to soak into the flaws for its dwell time. Dwell time may vary according to the size, test material as smaller flaw sizes require longer penetration time. After the application, an extra pentrant is removed according to the penetrant type.
This process is also known as bleed out. Various types of developers are available like a non-aqueous wet developer, dry powder, and water-soluble. Based on the compatibility of the penetrant, the developer is selected. The developer draws back the dye from defects to the surface so those can be visible.
Inspection and post-cleaning
Inspection is performed in the next step using adequate light. Inspection is done by using visible lights for visible dye penetrant and ultraviolet radiations of adequate intensity for fluorescent penetrate examinations. In the post-cleaning step, the applied developer is thoroughly removed.