As the nature of penetrant testing is to locate surface breaking defects this means that the preparation of the surface is one of the most critical parts of the entire penetrant testing operation.

With this in mind many specifications and end users feel the need to specify pre penetrant etching of parts which have been subject to operations which could potentially smear or otherwise peen the surface of the parts.

These operations include but are not limited to Sand, grit, vapour, plastic or other media blasting, burnishing, lapping or honing surface finishing by tumbling, vibration or grinding.

During these operations and others with similar features softer metals such as aluminium and titanium can have small parts of their surface peened which could potentially lead to a section of material being pushed over the top of the open surface of a surface breaking defect, should this occur and be left in this condition it would essentially stop dye penetrant from entering the open surface of the defect which would render the test unable to locate the defects.

Due to this the parts must be etched prior to the dye penetrant testing process.

The process of etching is a chemical operation using a suitable chemical to remove a pre determined quantity of the material from the surface of the component.

An example type of this operation would be the pre penetrant etching of aluminium for Airbus UK. The common method to carry this out is via the use of a deoxidiser such as Henkel 6/16 deoxidiser in nitric acid, in this process an etch rate is first determined by submerging a 2024 aluminium test plate in the etching solution for a pre determined length of time and performing stock loss and weigh measurements in order to calculate a removal rate of the material.

Once the removal rate has been calculated by an approved member of staff they are then ready for the etching of production parts.

For Airbus UK parts the total amount of material that shall be removed from the surface prior to the penetrant testing operation is 5 μm (0.0002 inches).

Once the parts have be submerged in the etching solution for the correct amount of time to remove this amount of material the parts are then thoroughly dried usually at an elevated temperature for a minimum of 45 minutes before being moved on to the dye penetrant testing process.