The Importance of Pre Cleaning In Penetrant Testing

Pre Cleaning in Penetrant Testing

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.

Water Washable Penetrants & Their Sensitivity Levels

Water washable penetrants are all classified into sensitivity levels based upon their performance.

The sensitivity levels as per ASTM 1417 and AMS264 are classified as:

  • Sensitivity Level ½ – Very Low
  • Sensitivity Level 1 – Low
  • Sensitivity Level 2 – Medium
  • Sensitivity Level 3 – High
  • Sensitivity Level 4 – Very High

The reason for the various sensitivity levels of penetrant is mainly attributed to the need to have various levels of sensitivity to allow for the testing of multiple types of components, to which different levels are suited to the task at hand.

For example when testing an as cast rough surface whilst looking to identify large open defects such as inherent casting defects, a relatively low sensitivity penetrant would be the most suited to the inspection, this Is because a higher sensitivity penetrant would first of all be extremely difficult to wash off the surface of the parts and secondly the number of non relevant indications that would be picked up coupled with the excessive amount of background could actually reduce the sensitivity of the test.

On the reverse of this argument, if you were to be carrying out testing on a highly machined turbine blade with a smooth surface finish looking for stress cracking and micro fractures, a penetrant with a high degree of sensitivity would be the best penetrant for the task. A level 3 or 4 sensitivity penetrant would allow for the detection of even the smallest of surface breaking defects.

In addition to the penetrant sensitivity classifications penetrants are also further classified into Types which include:

  • Type I – Fluorescent Dye
  • Type II – Visible Dye

Penetrants are also classified depending on the method of penetrant removal from the components

  • Method A – Water Washable
  • Method B – Post Emulsifiable Lipophilic
  • Method C – Solvent Removable
  • Method D – Post Emulsifiable Hydrophilic

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Electrostatic Penetrant Application

Penetrant testing is one of the most commonly utilised non destructive testing methods used in all industries. This can largely be attributed to its relatively low cost and high adaptability meaning it can be applied in all manor of ways without dramatically affecting its sensitivity.

The one application method however which is believed to marginally increase the sensitivity of a penetrant inspection is the electrostatic penetrant application method.

When penetrant is applied via electrostatic spray application there several benefits including:

  • Reduced consumption of penetrant
  • Ability to test large components
  • Ability to test complex geometry
  • Increase in sensitivity attributed to electrostatic attraction
  • Reduced in service tests on consumables

One of the easiest advantages to quantify is the reduced cost of consumables that occurs, to immersion test a large component with dimensions of 1000mm x 1000mm x 200mm would require hundreds of litres of penetrant in a dip tank to ensure adequate penetrant to cover the parts, this could cost thousands of pounds. Electrostatic penetrant application allows a small singe 25 litre barrel to be used and apply via spray which can vastly reduce initial outlay costs.

Another easy to see advantage is the reduction of in service tests that are required when using electrostatic penetrant application. When using immersion methods, the penetrant is classified as re used, as a result there are checks to be carried out including daily contamination checks, monthly water content checks and quarterly fluorescent intensity checks. As electrostatic application is classified as a spray to waste system, this negates the need to have these checks performed. Not only does this reduce the amount of work to be carried out by the operator but also removes the chance of any of these tests constituting in a failure which could subsequently lead to down time on the penetrant line.

The final and possibly most important advantage of electrostatic penetrant application is the slight increase in sensitivity when using electrostatic penetrant application. Penetrant naturally is “drawn” into surface breaking defects by capillary action, when using the electrostatic spray method this action is further aided by the electrostatic effect which further draws penetrant to the surface of the part. As such it is believed that this application method increases the overall sensitivity of the penetrant inspection.