water-washable and post-emulsified penetrants _ 2012-02-08 _ quality magazine
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Home » WaterWashable and PostEmulsified PenetrantsNDTNDT Exclusives
WaterWashable and PostEmulsified Penetrants
February 8, 2012Cheri StockhausenKEYWORDS liquid penetrants / ndtReprints
No CommentsAlthough Liquid Penetrant Inspection is recommended for the detection of all types of surface fl aws, theselection and use of the penetrant is going to depend on a number of factors.
A liquid penetrant inspection under black light shows the detection of surface flaws. Source: Magnaflux
If working to any specification, whether military or private, the penetrant selection for a process will be
spelled out. Other selection criteria include the surface and configuration of the part to be tested. Also,the size and location of the defect determines the sensitivity level required. Also, how critical is thefunction of the part to be inspected, such as used in rotating or moving components, and is it subject toexcessive heat.
All penetrant materials do not perform the same and are not designed to do so. Different formulationshave been developed to address a variety of inspection applications. For the purpose of this discussion,we will look at both waterwashable and postemulsified penetrants.
As pictured, the liquid penetrant will fluoresce under black light. Source: Magnaflux
WaterWashable Penetrants
Waterwashable penetrants (Method A) are available as Fluorescent (Type 1) or Visible (Type 2) penetrants and in all sensitivity levels. The waterwash method is popular for several reasons. It is less expensive, the process time is quicker, there are fewer processing steps and timing variables are reduced.
Waterwashable penetrants work best on rough surface parts, threaded or grooved parts and those withholes and orifices, which may be difficult to remove with the post emulsification method. It is especiallysuitable for automation, larger parts, leak testing and use on parts that are incompatible with oilbasedsystems.
Waterwashable penetrants are removed by manual or automated water spray, manual wipe or airagitated immersion wash. If a manual spray is used, the water pressure should not exceed 40 psi; thetemperature should be between 50 –100 F using a coarse spray at a minimum distance of 12 inches. Although water washable processing has its advantages, certain specifications will restrict its use becauseoverwashing can occur in shallow discontinuities, which makes rinsing time critical to the process.
Recommended Developers for WaterWashable Penetrants
Developers increase the visibility of both the visible and fluorescent penetrants. They draw the penetrantout of the flaws and spread the indication for brighter results. The developer you choose should be able toprovide a good, uniform contrast; be easy to apply and remove; and work well with the penetrant chosen.
Dry developers are the most effective for fluorescent waterwashable penetrants. It is the easiestdeveloper to apply. However, before using a dry developer, it is important that the part be completelydried after the rinse before applying the dry powder. It can be applied by immersion or by dusting withhandheld powder bulbs. In automated systems, developers are applied in a swirl cloud developer sta
tion. The use of a dust collector to evacuate the powder from the chamber is also recommended. A nonaqueous developer is a convenient solvent developer for portable fluorescent aerosol penetrant inspections.
Watersoluble developers are not recommended for use with waterwashable penetrants because it ispossible that when used with these penetrants, it will act as an additional emulsifier and wash thepenetrant away. Many specs will prohibit the use of these developers with water washable penetrants.
With visible waterwashable penetrants, a nonaqueous or solvent developer is recommended because itgives the best contrast to the penetrant.
Advantages to WaterWashable Penetrant
Lower cost (no emulsifier needed)
Fewer processing steps
Process time reduced
Variables associated with PE dwell time eliminated
Disadvantages to WaterWashable Penetrant
Over washing can occur in shallow defects
Water rinsing time is critical
Water contamination is susceptible
PostEmulsifiable PenetrantsWhen removal of the penetrant from the defect due to overwashing of the part is a concern, a postemulsifiable penetrant system can be used. These parts are usually smoothed surfaced and performcritical functions that require higher sensitivity to smaller defects. Postemulsifiable penetrants require aseparate emulsifier to break the penetrant down and make it water washable. Method C relies on a sol
vent cleaner to remove the penetrant from the part being inspected. Method B and D penetrants requirean additional processing step where a separate emulsification agent is applied to make the excesspenetrant more removable with a water wash. Lipophilic emulsification systems are oilbased materialsthat are supplied in readytouse form. Hydrophilic systems are waterbased and supplied as aconcentrate that must be diluted with water prior to use.
Emulsifiers
Lipophilic emulsifiers (Method B) were introduced in the late 1950s and work with both a chemical andmechanical action. After the emulsifier has coated the surface of the object, mechanical action starts toremove some of the excess penetrant as the mixture drains from the part. During the emulsification time,the emulsifier diffuses into the remaining penetrant and the resulting mixture is easily removed with awater spray.
Hydrophilic emulsifiers (Method D) also remove the excess penetrant with mechanical and chemicalaction but the action is different because no diffusion takes place. Hydrophilic emulsifiers are basicallydetergents that contain solvents and surfactants. The hydrophilic emulsifier breaksup the penetrant intosmall quantities and prevents these pieces from recombining or reattaching to the surface of the part. Themechanical action of the rinse water removes the displaced penetrant from the part and causes freshremover to contact and lift newly exposed penetrant from the surface.
The hydrophilic postemulsifiable method is more sensitive than the lipophilic post emulsifiable methodand has made the latter method virtually obsolete. The major advantage of hydrophilic emulsifiers is thatthey are less sensitive to variation in the contact and removal time. While emulsification time should becontrolled as closely as possible, a variation of one minute or more in the contact time will have littleeffect on flaw detectability when a hydrophilic emulsifier is used. However, a variation of as little as 15to 30 seconds can have a significant effect when a lipophilic system is used.
Advantages of the PostEmulsifiable Penetrant Process
Higher sensitivity to smaller defects
Shows wide, shallow defects
More controlled removal of surface penetrants
Disadvantages of the PostEmulsifiable Penetrant Process
Extra processing step
Emulsification time control is critical
Penetrant removal is difficult in threaded parts, holes and slots
Not good on rough surfaces NDT
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Cheri Stockhausen is a product manager for Magn flux, a division of Illinois Tool Works, Inc.www.magnaflux.com
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