T. 0. 1-.A.9
TABLE 2-20. CONTROL OF PICKLING, DE-SCALING OR DISSOLVING ACTION OF NITRICHYDROFLUORIC ACID
Pickling or dissolving action is more severe
Pickling or dissolving action is less severe
1. When the nitric acid content is decreased.
1. When the nitric acid content is increased.
When the hydrofluoric acid content is increased.
When the hydrofluoric acid content is decreased.
2. When the temperature is increased.
2. When the temperature is decreased. /1
3. When immersion time is increased.
3. When immersion time is decreased.
1. Holding bath below 120°F reduces the rapidity of intergranular attack in sensitized areas, such
as would prevail in localized weld zones that has not been removed by overall annealing.
Descaling the Straight-Chromium (400 series) Grades. Scale removal from these grades is more difficult than
removal of scale from nickel chromium grades. All of the straight chromium grades can be descaled in the nitric-
hydrofluoric solutions. However, tests as previously described are required in addition to use of abrasives. Lightly
adhering scales may be pre-cleaned by initially sand-blasting or vapor-blasting with silica sand (clean) and then following
by normal nitric-hydrofluoric acid pickling treatment as cited in paragraph 2-195a.
Immediately upon removal of parts from acid de -scaling solutions, thoroughly rinse in clean
hot water (140° 180°F) and then completely dry with clean air blast, (free of oil, etc.,) or other
approved method before starting/ the welding operation.
2-196. The mechanical methods (grinding, wire brushing, abrasive blasting/sanding) of cleaning are recommended
where chemical removal is impractical because of rinsing difficulties or structural complexities.
Protect faying surfaces (lap joints), etc., against entry of dust particles, filing and other
contaminants which may result in corrosion damage after parts are put into use. Do not use
common steel wool, wire or brushes on corrosion resisting steel.
2-197. Light scale/oxide may be removed by abrading weld area/zone with a stainless steel wire brush (0.005 inch
diameter wire) No. 240 are finer emery cloth (polish if required to specified requirements) and stainless steel wool
2-198. Heavy scale in some instances can be removed by sand, bead or vapor blasting, however, it shall be
accomplished as specified by approved data or process engineer. Normally, sand blasting shall be followed by chemical
cleaning unless otherwise specified to remove embedded particles of sand/scale.
Severe abrasive blasting/cleaning shall not be accomplished on materials thinner than 0.0625,
especially if the material is to be used for skin surface of an aircraft or missile or other areas
subject to high stress loads.
After oxide removal, welding shall be accomplished immediately or within 36 hours.
2-199. WELD FORMATION/PRACTICES. Table 2-21 and Table 2-22 are being provided as a guide in selecting
machine settings for spot and seam welding corrosion steels. The values cited will vary with different welding machines,
materials, tips, surface conditions, etc. Minimum electrode pressures are required to prevent cracks or porosity in the
weld. The dwell time shall be long enough to obtain complete fusion in the weld area. Generally, if the weld penetration
is less than 20%, it is below minimum requirements and if it is above 80% it exceeds maximum allowed. Welds with
approximately 50% penetration though each sheet are the most satisfactory.