COSHH SHEET 37

SURFACE FINISHES:

ACID TREATMENTS –

PICKLING AND SAMPLE ETCHING

Brand Names

Applicable to: corrosion study; microscopic examination; scale removal.

Uses/Processes

The corrosion resistance of iron and steel can be improved by treatment with sulphuric and phosphoric acids. The Footner process uses immersion in 5% (about 0.5 M) sulphuric acid at 60 C for 15 minutes, two rinses in hot water and then five minutes in 2% (about 0.2 M) phosphoric acid.

When beating copper or gilding metal, it is heated to anneal it from time to time. The copper oxide can be removed by immersing the copper in 0.5 M sulphuric acid for a few minutes.

If specimens are being prepared for examination under a metallurgical microscope, the polished surface is often etched with particular solutions to reveal the crystal structure.

Assessment

Some etchants contain toxic constituents but at the concentrations used this is not a problem. The solution used for etching lead is harmful.

Sodium hydroxide solution for etching aluminium and Fry’s reagent for etching iron are corrosive. The undiluted acids are corrosive.

Most of these solutions are irritant to the eyes, skin and respiratory system.

Nital solution (which contains ethanol) for etching is a highly flammable liquid.

The diluted acids used for pickling and cleaning copper present minimal risks so long as the metal is first cooled in water.

Controls

When staff are diluting the acids they must wear eye protection and add the acid slowly to the water. The sulphuric acid will need cooling between additions.

When staff are preparing, and students are using, the etching solutions, they must wear eye protection.

When adding nitric acid to ethanol, there should be no naked flames in the room.

Details of Etching Solutions

Aluminium etching: Use 2 M sodium hydroxide solution (corrosive) at 60 to 70 C (ie 20 g solid sodium hydroxide in 250 ml water).

Copper etching: Dissolve 20 g hydrated iron (III) chloride (ferric chloride) in 80 ml water and add 20 ml concentrated hydrochloric acid. Label the solution irritant.

Iron or steel etching (Fry’s Reagent): Dissolve 59 g hydrated copper (II) chloride in 40 ml water and add 60 ml concentrated hydrochloric acid. Label the solution corrosive.

Iron or steel etching (Nital solution): Add no more than 2 ml concentrated nitric acid slowly to 98 ml ethanol (industrial methylated spirit). Label the solution highly flammable but do not store longer than a week.

Lead etching: Just before use, mix together equal volumes of 5 M nitric acid and 15% ammonium molybdate solution (ie 1.5 g in 10 ml water). Wearing gloves, apply with a swab for 30 seconds and rinse. Label the solution corrosive and harmful.

Cleaning cast silver: Oxide and flux residues can be removed with 2 M nitric acid. Label the solution corrosive.

Storage

In polypropylene bottles, in a cool, dry, well ventilated place away from ignition sources. Small volumes of nital can be stored temporarily in a highly flammable liquids cupboard. All bottles should be labelled with a name and hazard warning symbol.

Disposal

Small quantities of these solutions can be diluted in a large volume of water and flushed into the foul water drain. Large quantities (more than 500 ml) require an authorised waste disposal contractor.

Immediate Remedial Measures

If swallowed: DO NOT INDUCE VOMITING. Keep casualty at rest and obtain medical attention.

If fumes are inhaled: Remove casualty to fresh air. Keep warm and at rest. If breathing is irregular or has stopped, administer artificial resuscitation. Obtain medical attention.If sodium hydroxide is in the eyes: Irrigate immediately with water, holding eyelids apart. Send for an ambulance and continue irrigation all the way to hospital and until attention is received.

If acid is in the eyes: Irrigate immediately with water for at least ten minutes, holding eyelids apart. If discomfort continues, obtain medical attention.

If solutions splash onto the skin: Remove contaminated clothing. Wash skin thoroughly with soap and water or use a proprietary skin cleanser.

If spilt in the workshop: If Nital is spilt, remove sources of ignition. Ventilate area. Contain and collect the spill with non-flammable absorbent material, e.g. sand or earth. Store safely pending disposal. Do not allow to enter drains.