chemical milling (underside of leading edge of aircrqu wtng section « a skin is attached to the faces of the stripes and Iorenges left by chemical milling and hot air is passed between for tie-icing the wing)
outline of the areas to be etched is marked out. and. by means of a sharp knife, the masking is stripped frotn the component. When mark~ ing out. allowance must be made for the undercutting penetration which occurs at the edge ofthc mask during etching. This is shown in Fig. I48.
2. Etching. For the etching process. the components are Suspended in a tank of the solvent with about 150mm clearance at the bottom, and about the same amount below the surface of the bath. Tanks must be constrlicted of materials not affected by the ctchant. The procedure and the etehants used depend on the metal being treated, and are briefly as follows.
Aluminium and its alloys. Etchant l0f3rt; caustic-soda solution seeded with l% aluminium by dissolving aluminium pieces in it. Rate of attack about lmm depth per hour. Working temperature 80 After etching, the component is thoroughly washed and transferred to a bath of 10% nitric acid to remove a smut which forms during etching. Finally the com- ponent is rinsed for two minutes in clean water. '
Magnesium and its alloys. Herethc etchant is "/1; nitric acid used at room