T.O. 33B-1-12-25Ultrasonic cleaning has limitations: its efficiency is affected by part size, configuration, and the cleaning solution actionon the soil to be removed.2.3.5.9 PaintRemovalNOTEMany paint removal operations leave a thin film of dissolved or softened paint andremover chemicals on the part surface or in discontinuities. This often occurs whenlocal or spot paint removal is performed. Care must be taken to ensure the area tobe inspected is free of paint and remover residues since they interfere with thepenetrant inspection process.There are a large variety of paint coatings and finish systems in use on aircraft parts and surfaces. Some conventionalcoatings are readily removed using standard methods. However, advances in paint technology have resulted in finishesthat can only be removed with unique materials and techniques. For conventional coatings, there are three generaltypes of removers: solvent, bond release, and disintegrating. Some proprietary removers contain multiple types, such assolvent or disintegrating compound in combination with a bond release material.2.3.5.10 CarbonRemoval.Carbon, varnish and other tightly held soils can present special problems in removal. The soils may have been baked atelevated temperatures to form a vitreous or glass-like coating. There are special solvent and alkaline cleaners for bakedsoil removal. Many of the paint removal materials and processes are used in removing carbon, varnish and othertightly held soils that are not baked.2.3.5.11 SaltBath.Molten salt baths are used for removing heavy, tightly-held scale and oxide from low alloy steels, nickel and cobalt basealloys, and some types of stainless steel. Salt baths cannot be used on aluminum, magnesium, or titanium alloys. Theprocess involves immersing the parts in molten caustic soda at about 700F. The difference in thermal expansionbetween scale and base metal separates some scale and causes the remainder to crack. The molten caustic soda alsochemically reacts with the scale, reducing it to lesser oxides and metals. When the part is removed from the moltensalt, it is plunged into water creating a thermal shock, plus steam at the part surface which scours or blasts anyremaining scale from the part.2.3.5.12 AcidCleaning.NOTEAcid cleaning requires very careful control of procedures and solutions to preventdamage to the parts.Solutions of acids or their salts are often used to remove rust, scale, corrosion products, and dry shop soils. The type ofacid and its concentration depends on the part material and contaminant to be removed. Acid cleaners are notgenerally effective on oily soils. Oils and greases must first be removed by some other cleaning method so the acid canreact with the scale, oxides, or other tightly held soil.2.3.5.13 ChemicalEtching.CAUTIONChemical etching SHALL be done only with engineering approval and writtendetailed process and application instructions.
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