Call 01527 557740 for the SIFCO guide
to aluminium etching

Sifco Applied Surface Concepts (UK) Ltd
Units 12-14 Aston Fields Trading Estate
Aston Road
Bromsgrove
B60 3EX
Tel: 01527 557740
Fax: 01527 832856

website www.sifcoasc.com ...email m.smith@sifco.co.uk

Alumininium propeller inspection.

Inspection of aluminium propellers and blades.

Carefully inspect aluminium propellers and blades for cracks and other injuries. A transverse (chordwise) crack or flaw of any size is cause for rejection. Refer any unusual condition or appearance revealed by these inspections to the manufacturer. Acceptable inspection methods are the acid-etching process, the anodizing process, the fluorescent penetrant process, or the dye penetrant process. Where applicable, one process is supplemented with another.

Etching Aluminium.

Over the past several decades, etching has proved to be a versatile and cost effective way of fabricating thin-gauge metal parts in many alloys.

One of the lightest metals, aluminium etching is challenging. Aluminium is a very active and reactive metal. It oxidizes readily and actually becomes fuel for the reaction.

Etching aluminium finishes the surfaces of forged aluminium propeller hubs. The process provides a solution to the problem of edge consistency, sometimes the edge of a blade looks smooth and other times it appears very rough. It is difficult to control the quality of the etchant from one bath to the next and from one alloy to the next.

Etching is accomplished by immersing the blade in a warm 20 percent caustic soda solution and cleaning with a warm 20 percent nitric acid solution, with a warm water rinse between the caustic bath and the acid bath, and also a warm water rinse following the acid bath. Remove all effects of the etching by polishing. Maintain the caustic and acid solution at a temperature of from 160° F. to 180° F. Some blades incorporate parts made of steel and other materials, and the caustic soda and the nitric acid must not be allowed to come in contact with these parts. The blade surfaces are then examined with a magnifying glass of at least three-power. A crack will appear as a distinct black line. The fluorescent penetrant method is recommended as a supplement to the caustic etch for the inspection of the shanks (roots or hub ends) of adjustable pitch blades.

Suspected cracks or defects should be repeatedly local etched until their nature is determined. With a No. 00 sandpaper, or fine crocus cloth, clean and smooth the area containing the apparent crack. Apply a small quantity of caustic solution to the suspected area with a swab or brush. After the area is well darkened, thoroughly wipe it with a clean (dampened) cloth. Too much water may entirely remove the solution from a crack and spoil the test. If a crack extending into the metal exists, it will appear as a dark line or mark, and by using a magnifying glass, small bubbles may be seen forming in the line or mark. Immediately upon completion of the final checks, remove all traces of the caustic solution by use of the nitric acid solution. Wash the blade thoroughly with clean (fresh) water.

Chromic acid anodizing.

The chromic acid anodizing process is sometimes considered superior to caustic etching for the detection of cracks and flaws and should therefore be used, whenever it is available, for general inspection of blades, for material defects, and for final checking of repairs performed during overhaul. Immerse the blades in the anodizing bath as far as possible, but all parts not made of
aluminium alloy must be kept out of the chromic acid bath or be separated from the blade by nonconductive wedges or hooks. Follow the anodizing treatment by a rinse in clear, cold, running water from 3 to 5 minutes, and dry the blades as quickly as possible, preferably with an air blast. Allow the dried blades to stand for at least 15 minutes before examination. Flaws (cold shuts or inclusions) will appear as fine black lines. Cracks will appear as brown stains caused by chromic acid bleeding out onto the surface. The blades may be sealed for improved corrosion resistance by immersing them in hot water (180° F. to 212° F.) for one-half hour. Never, of course, immerse the blades in hot water before the examination for cracks, since heat expands cracks and allows the chromic acid to be washed away.