The test surface is cleaned to remove any dirt, paint, oil, grease or any loose scale that could either keep penetrate out of a defect or cause irrelevant or false indications. Cleaning methods may include solvents, alkaline cleaning steps, vapor De-greasing or media blasting. The end goal of this step is a clean surface where any defects present are open to the surface, dry, and free of contamination. Note that if media blasting is used, it may "work over" small discontinuities in the part, and an etching bath is recommended as a post-bath treatment.
Applications of Penetrate:
The penetrate is then applied to the surface of the item being tested. The penetrate is allowed time to soak into any flaws (generally 5 to 30 minutes). The dwell time mainly depends upon the penetrate being used, material being testing and the size of flaws sought. As expected, smaller flaws require a longer penetration time. Due to their incompatible nature one must be careful not to apply solvent-based penetrate to a surface which is to be inspected with a water-washable penetrate.
Excess Penetrate Removal:
The excess penetrate is then removed from the surface. Removal method is controlled by the type of penetrate used. Water-washable, solvent-removable, lipophilic post-emulsifier or hydrophilic post-emulsifier are the common choices. Emulsifiers represent the highest sensitivity level, and chemically interact with the oily penetrate to make it removable with a water spray. When using solvent remover and lint-free cloth it is important to not spray the solvent on the test surface directly, because this can the remove the penetrate from the flaws. This process must be performed under controlled conditions so that all penetrate on the surface is removed (background noise), but penetrate trapped in real defects remains in place.
The inspector will use visible light with adequate intensity (100 foot-candles is typical) for visible dye penetrate. Ultraviolet (UV-A) radiation of adequate intensity (1,000 micro-watts per centimeter squared is common), along with low ambient light levels (less than 2 foot-candles) for fluorescent penetrate examinations. Inspection of the test surface should take place after a 10 minute development time. This time delay allows the blotting action to occur. The inspector may observe the sample for indication formation when using visible dye. Also of concern, if one waits too long after development the indications may "bleed out" such that interpretation is hindered.
The test surface is often cleaned after inspection and recording of defects, especially if post-inspection coating processes are scheduled.