Dye-Penetrant Examination Surface Preparation
Surface preparation is very important. The surface must be both clean and dry to enable the liquid penetrant to wet the surface completely, and to be drawn into flaws. This condition is generally achieved by washing the surface with a solvent to remove oil, grease, sludge deposits, etc.
Light grinding to remove surface irregularities can be beneficial. Innocuous surface blemishes can make it difficult to distinguish between these harmless artifacts and flaws that have developed during service. Heavy grinding should be avoided, because surface deformation can cover over flaws and make it difficult for the penetrant to enter.
Corrosion scales can block entry of the penetrant into the flaw and must be removed. Wire brushing or light abrading may be adequate for relatively thin scales, but abrasive blasting may be required for heavy scales. Corrosion scales can also fill the cavity and greatly reduce the quantity of liquid penetrant that can be drawn into the opening, which can make surface cracks very difficult to detect with PT.
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