Contaminants and Soils

T.O. 33B-1-1 2-20 be attacked by cleaning compounds containing halogen and sulfur compounds if they have residual cleaning compounds present and are exposed to high temperatures. c. Part surface condition. Rough surfaces tend to hold soil, making it harder to remove. d. Part surface accessibility and geometry. Complex shapes make it difficult to clean all of the surfaces, and soils lodged in restricted areas may escape the effects of cleaning. e. Required degree of cleanliness as dictated by the surface treatment that will follow or what service conditions will be encountered by the cleaned part. f. Availability and adequacy of cleaning facilities. For example, a large part cannot be placed in a small alkaline or ultrasonic cleaning tank. 2.3.4 Contaminants and Soils. 2.3.4.1 Definition of Contaminants and Soils. In this section, the terms “contaminants” and “soils” are used interchangeably and refer to matter on a part or component that may affect the penetrant testing process. Contaminants may be intentionally applied, such as paint or corrosion prevention compounds; may result from prior processes, such as machining, heat treating, or cleaning; or may be the consequence of service, such as corrosion, carbon deposits, lubricating fluids, or dirt particles. The effects of contaminants on the penetrant inspection process depend on the type of soil and whether it is on the part surface or entrapped in a discontinuity. 2.3.4.2 Effects Of Surface Contaminants. Contaminants that cover or bridge the surface opening of a discontinuity will prevent or reduce the formation of a penetrant indication. An indication can form only when the penetrant enters and exits the discontinuity. Any closure or reduction of the surface opening will restrict the formation of an indication. 2.3.4.2.1 Contaminants that do not bridge a discontinuity but are adjacent to the opening will also reduce the effectiveness of penetrant inspection. A common example of this condition is a painted part with a crack extending through the paint layer. While penetrant may enter and exit the defect to form an indication, the retention of penetrant in the adjacent contaminant can produce a residual background that masks or reduces the contrast of any discontinuity indication. These contaminants can also result in the formation of false indications. 2.3.4.2.2 One of the requirements for a penetrant to function is that it forms a continuous, even layer in intimate contact with the surface of the area to be inspected. This action is called surface wetting and is one of the mechanisms of penetration. Some types of contaminants prevent surface wetting thus prohibiting penetrant entry and the forming of an indication. 2.3.4.3 Effects Of Contaminants Trapped in Discontinuities. Foreign materials that fill discontinuities will block penetrant entry and indications will not form. A common occurrence of filled discontinuities is the tire bead seat area on aircraft wheels. When cracks occur in this area, they are frequently sealed by fine rubber particles from the tire. Eddy current or visual inspection will show cracks not indicated by penetrant inspection. 2.3.4.3.1 Contaminants within, but not completely filling discontinuities, can have several types of adverse effects: a. Some contaminants interfere with the penetrating mechanism and either prevents or reduces penetrant entry and exit from the discontinuity.