T.O. 33B-1-12-26Chemical removal or etching of deformed or disturbed surface metal is necessary if flaws are to be detected bypenetrant inspection. There are a number of mechanical processes that deform the surface of a part (see paragraph2.3.6.1). The deformation is a thin layer, surface metal flow that seals or reduces the opening of discontinuities (seeparagraph 2.3.6.2). The smeared metal over the surface opening prevents or severely restricts the penetrant entry intoany discontinuities. Chemical etching to remove the smeared metal allows penetrant inspection to be performed. Theetching is done using a mixture of appropriate acids or alkalis plus inhibitors. The type of etching solution depends onthe part material and condition. Chemical etching requires very close control of the etching solution composition,process procedures, and time of contact. Minor deviations in processing parameters will result in a number of adverseeffects, such as:a. Excessive metal removal.b. Selective etching of critical surfaces.c. An increase in the parts susceptibility to stress corrosion.d. Reduction of residual surface stress with a corresponding reduction in fatigue life.2.3.5.14 RemovalofCleaningProcessResidues.Cleaning process residues are removed by rinsing with fresh water. The use of warm water and agitation followed byrepeated immersions in fresh water assist in complete removal. In some cases, residues of strong alkalis and acids aresubjected to a rinse with a weak neutralizing solution followed by fresh water rinses.2.3.5.15 SummaryofCleaningProcesses.Table 2-2 is a summary of cleaning processes. Not all of the processes listed may be available at each operatinglocation.Table 2-2. Non-Mechanical Cleaning Processes that may be Used Prior to PenetrantInspection.SOLVENT MATERIALS POLAR OR IONIZING MATERIALSImmersion – Para. 2.3.5.6 Alkaline Cleaning – Para. 2.3.5.2Spray – Para. 2.3.7 Steam – Para. 2.3.5.3Ultrasonic – Para. 2.3.5.8 Detergent – Para. 2.3.5.4Emulsion – Para. 2.3.5.5Paint Removal – Para. 2.3.5.9Carbon Removal – Para. 2.3.5.10Salt Bath – Para. 2.3.5.11Acid Cleaning – Para. 2.3.5.12Chemical Etching – Para. 2.3.5.132.3.6 MechanicalWorkingProcesses.2.3.6.1 Description.Mechanical working processes involve displacement or removal of metal on the part surface. Intense mechanicalworking processes (sand blast, grit blast, and wire brushing) are used to remove heavy or tenacious contaminates, suchas scale or rust. The less severe mechanical working processes (tumbling, liquid honing, vapor blasting) are frequentlyused to remove light oxides and residual combustion products. Classification and methods of mechanical workingprocesses are listed in Table 2-3. Metal removal methods are listed by the type of cutting action, while abrasive blastmethods are categorized by the type of abrasive media.
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