TASK 70-24-02-110-035
1 . General.
CAUTION:
THIS SOLUTION WILL ATTACK THERMAL SPRAYED SILVER COATINGS AND BRAZE ALLOYS. REACTION WITH THESE MATERIALS IS VERY RAPID. BASE MATERIAL MUST BE COMPATIBLE WITH THIS SOLUTION.
This procedure is useful for dip etching Inconel 718 as a preliminary step before fluorescent-penetrant inspection. It may be used as directed by specific Shop Manual procedures. This method must not come in contact with thermal sprayed silver coatings and braze alloys which may be attached to part being etched.
2 . Materials.
Subtask 70-24-02-110-351
A.Refer to TASK 70-24-99-990-005, Consumable Materials - Etching Solutions.
(1)Storage: Class G etchants may be stored in glass bottles, if desired. Caps on bottles of Class G etchant shall be vented. Store away from heat, and keep from freezing.
(2)Shelf Life: The shelf life of Class G etchant is six months.
(3)Handling: Each container of stock solution should be labelled with the Class identification of the contents, and the expiration data of the shelf-life period.
3 . Procedure.
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B.Unless otherwise specified in the Shop Manual, etch specimens as follows:
(1)All surfaces to be etched must be free from oil, grease, scale, or other extraneous material.
NOTE:
Prior to dip etching hardware, dip test panel of the same material to determine etching rate and effectivity.
WARNING:
THE ACTIVE INGREDIENTS OF ETCHANTS ARE TOXIC AND CORROSIVE. USE IN A WELL VENTILATED AREA, WEAR PROTECTIVE CLOTHING, GLOVES AND FACE SHIELD. AVOID PROLONGED BREATHING OF VAPORS AND CONTACT WITH THE SKIN. IF ETCHING SOLUTION GETS INTO EYES, FLUSH THOROUGHLY WITH COOL WATER UNDER EYELIDS, AND OBTAIN MEDICAL ATTENTION AT ONCE.
CAUTION:
DO NOT PUT DISSIMILAR METALS INTO ETCHANT WITHOUT FIRST CHECKING POSSIBLE ELECTROCHEMICAL ATTACK.
(2)Immerse cleaned parts for 3-4 minutes in the etchant at room temperature 60°-100°F (16°-38°C) or for a maximum of 7 minutes. The test panel of similar material should be trial dipped, up to a maximum of 7 minutes, to determine the time required to a visual macroetch.
NOTE:
If macroetch is not observed within 7 minutes, check solution per paragraph 4. Control of Solution.
(3)Remove part from acid and thoroughly rinse with potable water.
(4)Remove smut formed on the etched surface with and air/water spray. Use wet cheese cloth or clean wet paper towels, if necessary, to remove any remaining residues.
(5)Dry part with clean shop air.
4 . Control of Solution.
Subtask 70-24-02-110-353
C.Maintain chemical concentration controls by checking with test panels and/or chemical analysis.
(1)Test Panel Control: Prior to dip etching hardware use a panel of similar material to determine etching rate and effectivity as follows:
(a)Dip the panel into the etching solution for3-4 minutes.
(b)Withdraw panel and remove smut with water pressure spray and/or wet cheese cloth.
(c)If no visual macroetch is apparent, put panel back into etchant again for 2-3 minutes.
(d)Remove panel and clean off smut with water pressure spray and/or wet cheese cloth.
CAUTION:
BE SURE TO CHECK ENVIRONMENT REGULATIONS BEFORE DISCARDING SOLUTIONS.
(e)If no satisfactory macroetch is visible, dump tank and recharge or chemically analyze for tank additions.
(2)Chemical Analysis and Control
(a)When sludge from etching builds up in bottom of tank, remove sludge as follows:
1Let tank settle for 8 hours.
2Drain off clear acid into another container. Remove sludge from tank and rinse tank clean with water.
3Pump tank dry and refill with acid previously pumped out.
4Chemically analyze, as below, to determine chemical additions required. See control limits below and in paragraph 4.A.(2)(b).
5Make additions and bring up level to required height with potable water.
(b)Maintain the solution concentrations between the following limits:
250-300 grams per liter of Ferric Chloride 100-120 grams per liter of Hydrochloric Acid (Muriatic).