BS/EN 4707-2014 pdf download.Aerospace series – Acid pickling of aluminum and aluminum alloy without hexavalent chromium.
— Areas to be processed;
— Treatment.
5 Condition of parts prior to processing
Fabrication of the parts shall have been completed before pickling.
Toolings:
Toolings. fixturings, and metal toohng masking must be protected from the corrosion and/or free from corrosion or another deterioration prejudicial to the treatment during their use.
Design of the toolings and racking must be achieved in order to:
— avoid any retention of air or solution on the parts,
— allow the neutralisation and the suppression of the solutions during rinsing operations,
— avoid any accidental contact during treatment between parts and tank equipments, and between various parts.
Tanks and rackings:
Materials used must be resistant to temperature and chemically compatible with the products involved in the
sequence of operations.
6 Process schedule
The pickling process shall be performed according to Table 1.
2) A second alkaline degreasing shall be performed after pre-degreasing, if necessary.
The alkaline degreasing shall be:
— corrosion inhibited;
— preferably of a non-silicated type (especially for dye penetrant inspection).
Generally alkaline solutions with pH between 9 and 11 are considered as a degreasing-etching solutions, whereas those with pH between 7 and 9, are considered only as a degreasing solution.
For the parts to be bonded, a non-silicated alkaline solution is mandatory.
3) After alkaline degreasing/cleaning. the parts shall be thoroughly water rinsed and inspected for water
break. Cleaning shall be repeated if break occurs.
8 Treatment
Immediately after pre-treatments, the parts shall be pickled in one of the solutions specified below.
To ensure a well pickled surface, excessive bath loading and shielding of parts by each other shall be avoided:
Type of substances that can be used in pickling:
— Nitric or sulfonitric;
— Phosphosulfonitric;
— Sulfonitro-fernc fluoride free;
— Sulfonitro-ferric with fluoride.
9 Rinsing
After each treatment bath, water rinsing is mandatory. This operation has to be composed by at least one recycled rinsing. Recycled rinsing is generally completed by non-recycled or low-flow rate rinsing. Several quality of rinsing water can be used but the water used before surface treatment (see Table 1) rinsing shall have a conductivity not more than 70 pSlcm.
Rinsing shall be carried out consecutively to the pickling step in running water with a temperature not exceeding 40 C.
11.1.1 Inspection before treatment
By visual inspection. make sure that the parts to be protecteditreated are free from defects such as:
scratches, corrosion pits, cracks … prejudicial to the aspect o the effectiveness of protection. Adherent chips and metal particles must be removed beforehand using a clean cloth.
11.1.2 Cleanliness
During the degreasing phases, each rinsing operation must be followed by a water break test in order to evaluate the wettability (and the cleanliness) of the surfaces.
This test consists of covering the parts with a uniform water film. This water film must cover the parts for at least 30 s without any discontinuity in the film. Otherwise, degreasing phases have to be repeated.
11.1.3 Inspection after treatment
When subjected to visual inspection, the pickled surfaces shall be of a homogeneous appearance and free from pitting, stains and contamination.
11.2 Process
11.2.1 Air used for drying or other operations shall be substantially dry and free from oil.
11.2.2 Chemical analysis of the pickling solutions (and different baths involved in the sequence of operations) shall be carried out at regular Intervals to determine the content of heavy metal, chloride, iron copper etc., when requested.
11.2.3 The bath shall be discarded when pitting of parts occurs or when solid material starts to precipitate on cooling the solution to ambient temperature. The process must be managed by monitoring the significant parameters as aluminium and polluant contents.
11.2.4 The purity of the rinsing water shall be determined by conductivitylresistivity measurements.
11.2.5 When required for adhesive bonding, the test to the relevant bonding standard shall be carried out to ensure that the process has been correctly applied.
11.2.6 Each pickling process must be characterized by Its specific etching rate range. All pickling process must not lead to localized attack.
11.2.7 The etching rate of the complete sequence of operations must be periodically measured to ensure the control of the process.
NOTE The etching rate and macimum dipping time cannot be generally specified but has to be specified according to final customer requirements.BS/EN 4707-2014 pdf download.