Penetrant testing is the testing of a part to look for the presence of surface breaking discontinuities, this can be achieved using both colour contrast and fluorescent types of penetrants with a variety of application and removal options.
Due to the nature of the process one of the most critical steps within the process is the pre cleaning / surface preparation step. The pre cleaning operation can be carried out using a variety of methods including solvent cleaning, aqueous alkaline cleaning and vapour degreasing.
One of the most important factors to consider when it comes to the selection of a pre cleaning method is the type of contamination you are looking to remove along with the surface condition of the part you are cleaning.
Vapour degreasing is one of the most utilised pre cleaning methods due to its ability to remove a large variety of contaminants specifically those such as cutting fluids and oil often used in CNC manufacturing of metallic components, the process does however have its limitations. Vapour degreasing using a product such as perchloroethylene is not capable of removing debris such as rust or scale and it cannot be used on all material types such as titanium which cannot be vapour degreased.
Another common method used for pre cleaning is the alkaline aqueous cleaning method, this method utilises an alkaline based cleaning solution such as Turco 4215NCLT or Adrox 6333A diluted with water. The cleaning solution is the operated in either an immersion tank where components are immersed and left for a time period of via a spray application in a purpose-built cleaning machine.
A final important pre cleaning step often used is pre penetrant etching. Many manufacturing methods used on softer materials especially aluminium can cause peening or smearing of the surface of the material. This can lead to a surface breaking discontinuity being partially of fully covered by the peened metal meaning that during the penetrant inspection the dye would not be able to penetrate the opening therefore the defect would likely be missed during the inspection.
To rectify this issue many specifications call for the etching of the surface prior to penetrant inspection, this involves removing a minimum of 0.0002 inches from the surface of the part using a chemical etchant.
The etching solutions are often controlled via a customer specification and can include things such as 6/16 deoxidiser or HN03 with the time of the operation determined by prior calculations using a representative test piece.