There are many factors to consider when choosing the right penetrant for an application. The governing specifications are the top consideration, but along with those you must also consider the surface finish and configuration of the part, the size and location of defects to be found, and the in-use function and environment of the parts, such as parts used in critical rotating or moving components or parts subjected to excessive heat.
To meet the wide variety of application needs, penetrant materials are designed to perform differently, and they have different advantages and disadvantages. In this article, we will compare water washable and post emulsifiable penetrant and highlight the differences between these two types of penetrant
Water Washable Penetrants
The water wash method is popular because it is typically less expensive than post-emulsifiable and the process time is faster since there are fewer processing steps.
Water washable penetrants (Method A) are available as Fluorescent (Type 1) or Visible (Type 2) penetrants and in multiple sensitivity levels for fluorescent penetrants.
Water washable penetrants work best on rough surface parts, threaded or grooved parts and parts with holes and orifices. Water washable penetrants are suitable for automated penetrant systems, large parts, and leak testing.
Water washable penetrants are removed by manual or automated water spray, manual wipe or air agitated immersion wash. If a manual spray is used the water pressure should not exceed 40 psi; the temperature should be between 50–100°F / 10–38°C using a coarse spray at a minimum distance of 12 in / 30 cm from the part.
Although water washable processing has its advantages, certain specifications will restrict its use because over-washing can occur in shallow discontinuities, which makes rinsing time critical to the process.
Advantages of water washable penetrants
- Lower cost (no emulsifier needed)
- Fewer processing steps
- Process time reduced
- Variables associated with emulsifier dwell time eliminated
Disadvantages of water washable penetrants
- Over washing can occur in shallow defects
- Water rinsing time is critical
- Water contamination of the penetrant is possible
Learn more about Magnaflux WW Liquid Penetrants
Developers for Water Washable Penetrants
Developers increase the visibility of both visible and fluorescent penetrants. Developers draw the penetrant out of the flaws and spread the penetrant on the part surface for visually enhanced results. The developer should be able to provide a uniform coating, be easy to apply and remove, and work well with the penetrant chosen.
Dry developers, such as ZP-4B Form a powder are commonly used with fluorescent water washable penetrants. Before using a dry developer, it is important that the part be completely dried after the rinse step before developer application.
Dry powder developer can be applied by immersion or by dusting with hand held powder bulbs.
In automated systems, dry powder developers are applied in a swirl cloud or dynamic cloud developer station. Magnaflux recommends the use of a dust collector to evacuate the powder from the developer chamber.
A non-aqueous developer such as ZP-9F solvent-based developer is a convenient developer for portable fluorescent aerosol penetrant testing. With visible water washable penetrants like SKL-WP2, a non-aqueous solvent developer is recommended.
Water soluble developers are not recommended for use with water washable penetrants, it is possible that when used with these penetrants the water soluble developer will act as an additional emulsifier and wash the penetrant away. Many specifications prohibit the use of water soluble developers with water washable penetrants.
See the steps in water washable and post emulsifiable penetrant processing, plus 5 tips for penetrant testing, in our Penetrant Testing Process Guide Poster
Post Emulsifiable Penetrants
A post emulsifiable penetrant should be used when removal of the penetrant from the defect due to over-washing of the part is a concern. These parts usually have smooth surfaces and perform critical functions which require higher sensitivity to smaller defects.
Post emulsifiable penetrants require a separate emulsifier or remover to break the penetrant down so the excess penetrant can be rinsed off with water, while the penetrant in the indication stays put.
Method C, solvent-removable, penetrant removal technique relies on a solvent-based cleaner, like SKC-S NDT cleaner/remover, to remove the penetrant from the part being inspected.
Method B lipophilic and Method D hydrophilic penetrant systems require an additional processing step where a separate emulsification agent is applied to make the excess penetrant more removable with a water rinse.
Lipophilic emulsifiers are oil-based materials that are supplied in ready-to-use form.
Hydrophilic emulsifiers are water-based and supplied as a concentrate that must be diluted with water prior to use.
Advantages of post emulsifiable penetrants
- Higher sensitivity to smaller defects
- Shows wide, shallow defects
- More controlled removal of penetrant from the part surface
Disadvantages of post emulsifiable penetrants
- Extra processing steps depending on Method B or Method D
- Emulsification time control is critical
- Penetrant removal is difficult in threaded parts, holes and slots
- Not good on rough surfaces
Discover more about Magnaflux PE Liquid Penetrants
Emulsifiers for Post Emulsifiable Penetrants
Lipophilic emulsifiers like ZE-4E Method B emulsifier work through both chemical and mechanical action.
After the lipophilic emulsifier has coated the surface of the part, mechanical action starts to remove some of the excess penetrant as the mixture drains from the part.
During the emulsification time, the emulsifier diffuses into the remaining surface penetrant and the resulting mixture is easily removed from the part surface with a water spray.
Hydrophilic emulsifiers like ZR-10E Method D emulsifier also remove the excess penetrant with mechanical and chemical action but the action is different than with a lipophilic emulsifier because no diffusion takes place.
Hydrophilic emulsifiers are basically detergents that contain surfactants in their formulations. The hydrophilic emulsifier breaks up the penetrant into small pieces and prevents these pieces from recombining or reattaching to the surface of the part.
The mechanical action of the rinse water removes the displaced penetrant from the part surface and causes fresh emulsifier to contact and lift newly exposed penetrant from the surface.
Developers for Post Emulsifiable Penetrants
Most types of penetrant developers are suitable for use with PE penetrants including, Form a dry powder, Form b water-soluble, Form c water-suspendible, Form d nonaqueous for Type 1 fluorescent penetrant and Form e nonaqueous for Type 2 visible penetrant.
Dry powders, such as ZP-4B developer are also commonly used with fluorescent post emulsifiable penetrants. Before using a dry developer, it is important that the part be completely dried after the rinse step before developer application.
Dry powder developer can be applied by immersion, by dusting with hand held powder bulbs or by an automated system.
A non-aqueous developer such as ZP-9F solvent-based developer is a convenient developer for portable fluorescent aerosol penetrant testing.
Although less common than dry powder developers, water soluble developers may be used with post emulsifiable penetrant. ZP-14A water soluble developer is supplied as a white free flowing powder and is dissolved in water to form a developer solution. ZP-14A developer solution does not require agitation once the developer bath is prepared.
What other differences have you noticed between water washable and PE penetrants?
Do you have any guidelines you use when deciding which type to use for an application?
Share your knowledge in the comments section below.