Specific contaminants And Their Effects

T.O. 33B-1-1 2-21 b. Contaminants within the discontinuity, that do not interfere with the penetrating mechanism, still take up physical space and reduce the volume of penetrant dye inside the discontinuity. The amount of penetrant that can enter may be insufficient to produce a noticeable indication. c. Contaminants that are strongly acidic or alkaline, such as residues from cleaning processes, can attack the dyes. This attack can result in a loss of color or fluorescence, resulting in a less noticeable indication. 2.3.4.4 Specific contaminants And Their Effects. 2.3.4.4.1 Light Oils and Soft Films. a. Examples of light oils and soil films are: hydraulic oils; lubricating oils; machining and cutting fluids; thin greases, such as petroleum jelly; and film corrosion preventive compounds. When present as thin films, these contaminants are easily removed by solvents. However, when they contain solid particles, such as metal chips, sand, or dirt removal is more difficult. The oily phase is readily removed, leaving the solid particles adhering to the surface. Removal of the solid particles may require a mild mechanical action, such as hand wiping, pressure spray, solution agitation or ultrasonic vibration. b. Light oils and soft films have several adverse effects on the penetrant inspection process. They readily enter surface openings, thus reducing or preventing penetrant entrapment. Oily materials on the part surface interfere with the mechanisms causing the penetrant to enter and exit from discontinuities. Also, many oils and greases fluoresce under black light. When on a part surface, this fluorescence could obscure a discontinuity indication or produce a false indication. 2.3.4.4.2 Heavy Oils and Solid films. a. Examples of heavy oils and solid films are viscous oils; thick greases; hard film corrosion preventative compounds; and particulate lubricants, such as graphite and molybdenum disulfide. These contaminants or soils are more difficult to remove than light oils. They require solvent or chemical action plus considerable mechanical action. Mechanical action can be solution agitation, manual scrubbing or pressure spraying. Cleaning for penetrant inspection presents special problems. The heavy oils and greases are viscous and flow very slowly; many of them have excellent penetrating ability and readily enter surface discontinuities. Removal of heavy oils requires considerable mechanical action where the forces are concentrated at the surface. Use of excessive mechanical forces to remove heavy oils and films may further aggravate problems by smearing metal over narrow discontinuities. b. Heavy oils and solid films have the same adverse effects on penetrant inspection as light oils and soft films. They enter or bridge surface discontinuities and either prevent or reduce the amount of penetrant entrapment. Many heavy oils and semi-solid films fluoresce under black light. This fluorescence can obscure valid indications and produce false indications. Heavy oils and films on the surface of a part, even in trace amounts, interfere with the entry and exit of penetrant discontinuities. 2.3.4.4.3 Carbon, Varnish and other Tightly Held Soils. a. Examples of origins of carbon, varnish and other tightly held soils are: partially burned petroleum and other combustion products; residues from evaporated fuel and oils; and dry film lubricants. These types of soil are very adherent and are difficult to remove. They require special cleaning compounds and processes to dissolve and loosen the soils. Strong mechanical action, such as scrubbing, pressure spray, or solution agitation is also required. Care must be used, since many of the cleaning compounds will attack some metals and alloys. b. Tightly held soils, such as carbon, engine varnish, and other dry soils, can seriously interfere with the penetrant inspection process. They can bridge over or partially fill the discontinuity, blocking or