Ultrasonic cleaning has limitations: its efficiency is affected by part size, configuration, and the cleaning solution action
on the soil to be removed.
Many paint removal operations leave a thin film of dissolved or softened paint and
remover chemicals on the part surface or in discontinuities. This often occurs when
local or spot paint removal is performed. Care must be taken to ensure the area to
be inspected is free of paint and remover residues since they interfere with the
penetrant inspection process.
There are a large variety of paint coatings and finish systems in use on aircraft parts and surfaces. Some conventional
coatings are readily removed using standard methods. However, advances in paint technology have resulted in finishes
that can only be removed with unique materials and techniques. For conventional coatings, there are three general
types of removers: solvent, bond release, and disintegrating. Some proprietary removers contain multiple types, such as
solvent or disintegrating compound in combination with a bond release material.
Carbon, varnish and other tightly held soils can present special problems in removal. The soils may have been baked at
elevated temperatures to form a vitreous or glass-like coating. There are special solvent and alkaline cleaners for baked
soil removal. Many of the paint removal materials and processes are used in removing carbon, varnish and other
tightly held soils that are not baked.
Molten salt baths are used for removing heavy, tightly-held scale and oxide from low alloy steels, nickel and cobalt base
alloys, and some types of stainless steel. Salt baths cannot be used on aluminum, magnesium, or titanium alloys. The
process involves immersing the parts in molten caustic soda at about 700F. The difference in thermal expansion
between scale and base metal separates some scale and causes the remainder to crack. The molten caustic soda also
chemically reacts with the scale, reducing it to lesser oxides and metals. When the part is removed from the molten
salt, it is plunged into water creating a thermal shock, plus steam at the part surface which scours or blasts any
remaining scale from the part.
Acid cleaning requires very careful control of procedures and solutions to prevent
damage to the parts.
Solutions of acids or their salts are often used to remove rust, scale, corrosion products, and dry shop soils. The type of
acid and its concentration depends on the part material and contaminant to be removed. Acid cleaners are not
generally effective on oily soils. Oils and greases must first be removed by some other cleaning method so the acid can
react with the scale, oxides, or other tightly held soil.
Chemical etching SHALL be done only with engineering approval and written
detailed process and application instructions.