Chemical removal or etching of deformed or disturbed surface metal is necessary if flaws are to be detected by
penetrant inspection. There are a number of mechanical processes that deform the surface of a part (see paragraph
188.8.131.52). The deformation is a thin layer, surface metal flow that seals or reduces the opening of discontinuities (see
paragraph 184.108.40.206). The smeared metal over the surface opening prevents or severely restricts the penetrant entry into
any discontinuities. Chemical etching to remove the smeared metal allows penetrant inspection to be performed. The
etching is done using a mixture of appropriate acids or alkalis plus inhibitors. The type of etching solution depends on
the 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 adverse
effects, 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.
Removal of Cleaning Process Residues.
Cleaning process residues are removed by rinsing with fresh water. The use of warm water and agitation followed by
repeated immersions in fresh water assist in complete removal. In some cases, residues of strong alkalis and acids are
subjected to a rinse with a weak neutralizing solution followed by fresh water rinses.
Summary of Cleaning Processes.
Table 2-2 is a summary of cleaning processes. Not all of the processes listed may be available at each operating
Table 2-2. Non-Mechanical Cleaning Processes that may be Used Prior to Penetrant
POLAR OR IONIZING MATERIALS
Immersion Para. 220.127.116.11
Alkaline Cleaning Para. 18.104.22.168
Spray Para. 2.3.7
Steam Para. 22.214.171.124
Ultrasonic Para. 126.96.36.199
Detergent Para. 188.8.131.52
Emulsion Para. 184.108.40.206
Paint Removal Para. 220.127.116.11
Carbon Removal Para. 18.104.22.168
Salt Bath Para. 22.214.171.124
Acid Cleaning Para. 126.96.36.199
Chemical Etching Para. 188.8.131.52
Mechanical Working Processes.
Mechanical working processes involve displacement or removal of metal on the part surface. Intense mechanical
working processes (sand blast, grit blast, and wire brushing) are used to remove heavy or tenacious contaminates, such
as scale or rust. The less severe mechanical working processes (tumbling, liquid honing, vapor blasting) are frequently
used to remove light oxides and residual combustion products. Classification and methods of mechanical working
processes are listed in Table 2-3. Metal removal methods are listed by the type of cutting action, while abrasive blast
methods are categorized by the type of abrasive media.