Factors Influencing Removability

Figure 2-14. Comparison of Adequate Dwell versus Insufficient Dwell on a Thermally Cracked Aluminum Block.

Removal of Water Washable Penetrant, Method A

TM-1-1500-335-23 Nondestructive Inspection Methods Manual
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T.O. 33B-1-1 2-47 sometimes used interchangeably in commercial application; however, the materials specification and this manual will use "washability" only in the case of water washable penetrants. 2.5.3 Factors Influencing Removability. 2.5.3.1 Part Surface Condition. The surface condition of the part has a direct effect on removability. Smooth, polished surfaces such as chromium plated panels can be easily processed by any of the removal methods with no residual background. As the surfaces become rougher, such as chemically etched or sand blasted parts, the removal of surface penetrant becomes more difficult. Rough surfaces reduce removability in two ways: the roughness restricts the mechanical force of the spray rinse in the indentations or low points and prevents the emulsifier from evenly combining with the surface penetrant. It is not always possible to produce a background-free surface on rough parts. The wash or emulsification time required for a completely clean surface may result in removal of some of the penetrant entrapped in flaws. In this case, the wash or emulsification time must be shortened, leaving some residual background. The amount of residual background must be limited to allow any flaw indications to be visible through the background. 2.5.3.2 Part Shape or Geometry. The part shape and geometry may indirectly affect removability by causing a thicker layer of penetrant to accumulate during the dwell period and restricting accessibility to the test surface by the spray rinse. One of the factors involved in removing excess surface penetrant is the mechanical action or force of the spray rinse. When parts contain surfaces where the spray cannot directly strike the surface, such as concave or recessed areas, holes, and screw threads, the removal time is increased in these local areas. Also, the thickness of the penetrant layer in these inaccessible areas is usually greater than that on the adjacent surfaces. This is due to the tendency of the penetrant to drain and collect in these areas. For example, during the dwell period the penetrant will drain from the top or crown of a thread and will flow into the thread root area. The increased layer thickness in the thread root requires a longer removal time than the thin layer at the thread crown. The inaccessible surfaces usually have thicker layers of penetrant and require additional removal time. Care must be exercised to prevent over-removal on the accessible surfaces with thinner penetrant layers, while trying to adequately clean the thicker penetrant layer from an adjacent inaccessible surface. 2.5.3.3 Flaw Size and Shape. Flaw size and shape complicate the removal process. Narrow, deep flaws, while requiring long penetrant dwell times, provide a relatively large reservoir to hold entrapped penetrant. The narrow surface opening reduces both the diffusion rate of emulsifier into the flaw and the mechanical force of the spray rinse on the entrapped penetrant. The result is that narrow, deep flaws produce highly visible indications with a minimum of removal problems. 2.5.3.3.1 The removal process becomes slightly more critical when narrow, shallow flaws are present. Narrow, shallow flaws do not have a large reservoir to hold entrapped penetrant. The visibility of an indication depends on the amount of penetrant that exits from the flaw. If the flaw is shallow, only a small amount of penetrant is available, and the indication may be faint. Over-removal of any entrapped penetrant will reduce the visibility of an already faint indication. In addition, a small amount of residual background (insufficient removal) will obscure faint indications. 2.5.3.3.2 Broad, shallow flaws are defined as those with the surface opening equal to or greater than the depth. They present the most critical case for penetrant removal. The opening does not reduce the force of the spray rinse, nor does it restrict the emulsification rate, and entrapped penetrant is easily removed. Extreme care must be used during penetrant removal if broad shallow flaws are likely to be present. 2.5.3.4 Type Of Penetrant. Penetrant materials vary widely in their ease of removal. There are differences in removability between the various penetrant types, classes, and sensitivity levels. Also, similar penetrants provided by different manufacturers vary in removability. One penetrant characteristic affecting removability is the viscosity. High viscosity (thick) penetrants are more difficult or more slowly removed than low viscosity (thin) penetrants. The penetrant system sensitivity level also affects removability. Higher system sensitivity level penetrants contain more dye per unit volume, and trace quantities

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