The purpose of this advisory is to give an Insight to new technicians, trainees or managers or process owners of non-destructive testing who may only have administrative control. One thing to remember here, even though a method or technique maybe perceived as easy in application and “just a bit of cleaning”, without due care and attention it’s also very easy to get wrong, Also If it’s worth doing it’s worth doing right?
For the technician or manager of a process, there’s a lot of aspects to consider when selecting an appropriate cleaning technique, both for in-process examinations and for pre and post cleaning operations for a given NDT method, but the one method where this section is most critical has to be Liquid penetrant testing.
Essentially a visual surface examination based method, the removal of contaminants is one of the keys to the process and essential to help deliver liquid penetrants to the test area with enough dwell time to enter surface breaking flaws or potential crack openings, but without an operator’s experience and awareness the examination could lead to unreliable outcomes and even failure to locate critical defects.
Below are key points for consideration, and discussed in further detail:
- what stage of manufacture or service are we examining and what are we searching for?
- What material alloy or material group are we cleaning
- What manufacturing stage(s) precedes the NDT examination, and could these be detrimental to the outcome of the examination itself? What is the condition of supply?
- Do we select a detergent cleaner or chemical etchant solution which will remove up to 0.00025” from the surface of the material?
- And is the cleaner correctly utilised with the required steps during applications?
- What stage of manufacture or service are we examining and what are we searching for?
In service or manufacture, we are searching for widely different defects. In-service products may require removal of barrier coatings prior to Liquid testing, but not all. Manufacturers can also create more efforts where etchants are required for reworked surfaces, polished, burnished, or smeared surfaces. Most common material groups for this application are soft wrought aluminium alloys, and fully machined examples.
- What material alloy or material group are we cleaning? The chose chemical or process selected should be trialled before use in production for suitability, effectiveness, and the outcome. For harsh processes this may involve further examination magnification and measurement of chemical attack and end grain pitting caused. Reactive materials of aluminium and magnesium are typical examples.
- What manufacturing stage(s) precedes the NDT examination, and could these be detrimental to the outcome of the examination itself? What is the condition of supply? That is a question that should be asked by contract review…. even if you’re a sub-tear manufacturer. The choices made at this point can significantly affect the outcomes of the NDT examination and the cleaning processes used pre and post testing.
- Do we select a detergent cleaner or chemical etchant solution which will remove upto 0.00025” from the surface of the material? Etchants are not recommended for In-service components, where fatigue cracks are sought, most cleaning can be achieved by solvent swabbing or aqueously with the use of detergent cleaners.
- And is the cleaner correctly utilised with the required steps during applications? Ensure your aqueous cleaning process has a rinse stage or swill to remove residues, and a water break check to ensure the cleaning has been effective. Or if utilising solvent cleaners allow a clean tissue wipe after completion and allow a sufficient time for residues to evaporate before the application of Liquid penetrant testing materials.
Typical Cleaning considerations for:
Solvent swabbing: Ideal for local area examinations and the removal of light soils or contaminants. Carried out with pre-dampened towelettes, or aerosol applications with tissue or cotton rags to remove.
Remember to wipe across the surface after cleaning with a clean cloth to ensure a contaminant free surface, if we’re still removing soils and contamination, we’ll have to re-clean the area before testing. Also allow sufficient time for solvents to evaporate from the surface before applying Liquid penetrant oils and testing materials.
Aqueous immersion: Rapidly becoming the choice option following the on-going demise of Hot solvent techniques in recent years. And ideal for medium to high volume applications but will require some maintenance to monitor the solution strength and contaminant build ups before total replacement is required.
Achieved with the use of a low foaming detergent or mild alkaline chemical. Immersing component parts by basseting or jigged and suspended into solution for a sufficient time necessary to remove contaminants.
Surface rinsing is required following cleaning and the visual verification of a water break free surface. If the water film breaks during viewing, the surface may still be contaminated and require more time to clean before progressing.
Chemical etchants: A controlled chemical surface treatment used to evenly remove a surface layer of material from the test component. Utilised wherever burnishing, smearing of soft alloy surfaces (aluminium’s etc) or reworking or a component surface by polishing or dressing has caused an un-desirable surface for NDT examination.
well suited to complex geometries but be mindful of possible entrapment in bores, passageways, and assembled items. Ensure you can rinse away the etchant by rinsing and clean water immersion following the process
In-service: In many cases dependent upon the industry sector, components or assemblies for NDT are protective treated and do not require removal….and there are a myriad of treatments to contend with but most common is the use of primer and enamel paint finishes as a barrier coating against corrosion.
Most suitably stripped by dry media blasting with the use of plastic or crushed walnut shells, sufficiently low impact enough to leave any Anodise finishes intact when working with aluminium alloys. Chemical removals are available but will likely remove the anodise treatment and will require trials prior to widespread application to prevent corrosive attack to the substrate material.