The etching is used to determine the soundness of a weld and also make visible the boundary between the base metal and the weld metal.

Concept and Purpose
(i) Macro etch examination:
After preparing the specimen by polishing and etching, it is examined either by the naked eye or by low power magnification up to X15. Macro examination gives a broad picture of the specimen by studying relatively large sectioned areas.
 
Macro examination reveals in welded specimen
(i) Cracks,
(ii) Slag inclusion,
(iii) Blowholes,
(iv) Shrinkage porosity,
(v) Penetration of the weld,
(vi)The boundary between the weld metal and the base metal, etc

(ii) Micro etch examination: After preparing the specimen by polishing and etching, it is examined under a microscope at magnifications from X20 to X2000.
Micro etch examination involves areas much smaller than those considered in macro etch examination and brings out information that can never be revealed by macro examination.

Micro examination determines in a welded specimen
(i) Cracks and inclusions of microscopic size.
(ii) Grain boundaries and solidification structures of weld metal, heat affected zone and the base metal.
(iii) Distribution of micro constituents in the weld metal.
(iv) The quality of heat treatment, etc.
Preparation of Test Specimen
(i) The specimen shall be the full thickness of the material at the welded joint and the weld reinforcement and penetration bead shall be left intact. The specimen shall contain a length of the joint of at least 10 mm and shall extend on each side of the weld for a distance that includes the heat affected zone and some base metal portion.
(ii) Specimen after being cut from the plate is filed or ground to obtain flat surface on the specimen.
(iii) Intermediate and fine grinding is carried out using emery papers of progressively finer grades, i.e., of grades, 200 grit, 320 grit, 400 grit and 600 grit (from coarse to fine).
(iv) Rough and fine polishing of the specimen* is carried out on a rotating polishing wheel. Fine polishing removes the scratches and very thin distorted layer remaining on the specimen from the rough polishing stage. (v) Etching. The specimen is then etched in order to make visible the grain boundaries, heat affected zone, the boundary between the weld metal and parent metal, etc. Etching imparts unlike appearances to the metal constituents and thus makes metal structure apparent under the microscope.
Etching is done either by
(a) immersing the polished surface (of the specimen) in the etching reagent or by
(b) rubbing the polished surface gently with a cotton swab wetted with the etching reagent.
Given below are a few etching reagents:
1. Hydrochloric Acid. The reagent contains equal parts by volume of concentrated HCI and water. Specimen is immersed in this reagent at or near the boiling point. This will usually enlarge gas pockets and dissolve slag inclusions, enlarging the resulting cavities.
2. Nitric Acid. One part of concentrated nitric acid is added to three parts of water by volume. The reagent may be applied to the surface of the weld either with a glass stirring rod at room temperature or the weld be immersed in boiling reagent provided the room is well ventilated. This reagent is used on polished surfaces only to show the weld metal zone as well as the refined zone. Nital contains 2CC HNO3 Conc. + 98CC absolute methyl alcohol.
3. Ammonium persulphate. Mix one part of ammonium persulphate (solid) to nine parts of water by weight. The reagent thus prepared is rubbed vigorously on the surface of the weld with cotton saturated with this reagent.
4. Iodine and potassium iodide. One part of powdered iodine (solid) is mixed with twelve parts of a solution of potassium iodide by weight. The latter solution should consist of one part of potassium iodide to five parts of water by weight. The reagent is brushed at room temperature on the surface of the weld.