Of the four available methods to remove surface penetrant during the fluorescent penetrant inspection process, the hydrophilic post emulsifiable penetrant process (Method D) is used when tighter control of the penetrant process is desired. In the water washable penetrant process (Method A), it is critical to control the penetrant rinse time, since water washable penetrant has the potential to be over washed. Decreased sensitivity and decreased brightness of penetrant indications may result from over removal of water washable penetrant. The Method D hydrophilic post emulsifiable penetrant process offers tighter control of surface penetrant removal because the post emulsifiable penetrant is not removable with water alone. Hydrophilic emulsifier is necessary to remove excess penetrant from the test surface. The Method D penetrant process allows for tighter control of the penetrant removal step and improved sensitivity of the fluorescent penetrant process.
Method D fluorescent penetrant inspection utilizes hydrophilic emulsifier to remove the surface penetrant. Hydrophilic emulsifier is supplied as a concentrate by the manufacturer, and the hydrophilic emulsifier is diluted with water prior to use. The steps in the Method D penetrant process are penetrant application, penetrant dwell, water spray pre-rinse, immersion in the diluted hydrophilic emulsifier solution and then a water spray post rinse, followed by hot air drying, developer application, and inspection under UV light. The hydrophilic emulsifier is used at the concentration recommended by the manufacturer (typically 20% by volume for immersion applications).