Introduction
On March 13, 1998, the US Air Force issued a new qualified products list (QPL). The new QPL-AMS-2644-1 superseded QPL-25135-17, the inspection QPL that had been in effect since February 17, 1994. Included in the new QPL were water based (WB) penetrants, fluorescent penetrants that are diluted or extended with water instead of with a petroleum product such as a solvent or light oil. The newly approved WB penetrants are fluorescent (type I), method A (water washable), in sensitivity levels one (low) and two (medium).

These are the first fluorescent WB penetrants to be Air Force approved for general use since MIL-I-25135 version C was superseded on June  21, 1984, by the D revision. Until this recently published QPL, approved method A fluorescent penetrants were either undiluted surfactant ester formulations (biodegradable) or such formulas diluted with about 40 percent to 60 percent of a petroleum distillate or a glycol type solvent. WB fluorescent penetrants were not listed as Air Force approved.

 

Benefits of Water Based Penetrants
Certain advantages of using water, an inexpensive, nonpolluting, and renewable resource, instead of a petroleum product as a diluent, are obvious. Environmental considerations forced paint manufacturers to substitute water for volatile petroleum thinners in many of their coatings. Cleaning compound manufacturers switched from petroleum solvent to aqueous cleaners. The benefits of using water as the diluent which represents approximately 50 percent of a penetrant are several and include:

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Approved water based penetrants should find a wide range of applications.

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• Lower costs: Light oils cost $0.48 per L ($2 per gal) and up. The savings in material costs on 55 gallons of penetrant diluted 50 percent with water would be 208 L (55 gal) or more.

• Sewer system acceptance: Waste treatment authorities accept water without question while petroleum oils resist biodegradation and, if accepted, are often subject to high disposal fees.

• Smell: Some grades of petroleum distillates have an unpleasant odor.

• Contamination: Petroleum distillates are a source of halogens and sulfur.

• Reduced fire risks: Although petroleum distillates used in inspection penetrant formulations are high flash point, they are combustible and fuel fire. Water does not.

• Lower shipping expense: A WB penetrant can be shipped as a concentrate, and the water can be added at the work site. This is not practical when a petroleum distillate is the diluent.

• Aqueous cleaner compatibility: WB penetrants are compatible with water based cleaners, e.g., hot alkaline, which for environmental reasons have replaced petroleum solvent and vapor degreasers. Being compatible with the cleaner, WB penetrants have tolerance for rinse water carry over. Also, flaw entrapped water is less likely to impair the inspection process, such as would happen when flaw entrapped water repelled a petroleum based penetrant.

When 50 percent of the diluent is water rather than the conventional petroleum distillate, cost savings are realized in the areas of  waste disposal, shipping, storage and materials. Further, using water means a more pleasant and healthier work environment free of petroleum vapors. So, the questions regarding the use of WB penetrants are "Why haven't penetrant manufacturers switched from petroleum diluents to water diluents, and why haven't WB penetrants appeared earlier on the QPL?"

 

Overcoming Technical Problems
There is more than one good reason why WB penetrants have not appeared on the QPL before now. First, petroleum distillates, which are used to dilute both water washable and nonwater washable penetrants, contribute to penetrant performance in several ways. Second, water has inherent negative properties in a penetrant formulation. Furthermore, under current specification restrictions, petroleum distillates function as diluents in both water washable and nonwater washable (method B and D) penetrants, whereas water's use is restricted to water washable (method A) penetrants.

As a diluent, a light petroleum distillate has desirable characteristics; it dissolves light organic soils on surfaces and in flaws, and it assists crack penetration with its low surface tension and affinity for metal surfaces. Also, petroleum distillates naturally fluoresce a light blue color and, in a minor way, contribute to penetrant brightness.

This contrasts with water, which, by itself as a penetrant diluent, does not dissolve organic soils. Water's high surface tension, if not corrected, adversely affects penetrant performance. Furthermore, water does not have intrinsic fluorescent properties, and, while there are water soluble fluorescent dyes, none have been found which give the needed brightness. Moreover, using water soluble dyes may not be a practical approach; when a water based penetrant is placed on a surface in a thin coat, the water evaporates relatively rapidly and, theoretically, will cause water soluble dye to precipitate, diminishing the fluorescence of the penetrant.

 

Water Based Penetrant and Evaporation
The typical light oil used to dilute conventional penetrants is relatively nonvolatile, and evaporation loss is a concern. On the other hand, water in WB penetrants will evaporate, and simple, but regular, testing with a refractometer is necessary to measure the loss. Periodic water additions are part of the maintenance criteria for WB penetrants.

Monitoring water loss with a refractometer is not new to the penetrant industry. Currently, hydrophilic emulsifiers, which typically are diluted with about 80 percent water, must be monitored, and water content must be restored to a minimum level. Water loss in hydrophilic emulsifiers results in more concentrated and more active solutions, a cause for overemulsification with resultant failure of the penetrant process. In contrast, water loss with a WB penetrant results in a more concentrated and more sensitive penetrant.

 

Passing Qualification Tests - A Balancing Act
Qualifying a penetrant at a particular sensitivity level, among other things, requires two tests performed by the Air Force. One test is to quantify the brightness of the flaw indication produced by the candidate penetrant with instrumentation. The other test is to measure the degree of fluorescent background left on the surface after the penetrant removal step. Insufficient flaw indication brightness is cause for nonapproval of the candidate penetrant. A too-high level of background fluorescence is also cause for rejection because intense background fluorescence obscures the fluorescing flaw indication.

These requirements are at cross purposes. Penetrant lodged in a surface flaw must resist overwashing, while penetrant remaining on the surface must wash readily. As a WB penetrant's diluent is water, balancing its formula to resist overwashing from flaws while washing readily from part surfaces is difficult.

To accept water as a diluent, the penetrant formula must be hydrophilic. It must readily accept water. If the formula too readily accepts water, the flaw entrapped WB penetrant will too readily accommodate rinse water. The flow entrapped penetrant will then be diluted and displaced, weakening or eliminating fluorescent flaw indications. A near perfect balance must be struck.

It is possible to vary the hydro-philic/lipophilic balance (HLB) to reduce water acceptance and retard flaw mark weakening. However, in so doing, the product becomes more resistant to washing from rough surfaces. The result is success in meeting sensitivity or flaw indication brightness but failure in meeting low background requirements. The HLB must be precise. The penetrant must resist overwashing while being sufficiently free rinsing to meet the low fluorescent background requirement.

 

Water Based Penetrants and Dry Powder Developer
Another major obstruction to Air Force approval of water based penetrants is the hydrophobic character of dry powder developers.

The processing procedure for QPL qualification of a method A penetrant includes a five minute penetrant dwell, a plain water spray wash, oven drying, and application of a dry powder developer by immersion of the test piece. The powder intensifies and adds brightness to fluorescent indications. Crack retained penetrant is attracted to the powder. It coats the white, reflective powder surfaces. Ultraviolet light shines through the penetrant layer, activates its fluorescence, hits the white powder reflective surfaces, and bounces back through the penetrant layer, reactivating it and generating a high cascading effect. Powder developer increases the brightness of the penetrant indication.

Dry powder developers must be hydrophobic; they must repel water. If they were otherwise, they also would be hydroscopic and attract moisture from the air, resulting in a damp, lumpy, and ineffective developer. This is unfortunate for WB penetrants.

WB penetrants are necessarily hydrophilic while dry powder developers are hydrophobic. Thus, they repel one another. Dry powder developers' contribution to increased WB penetrant brightness, therefore, is substantially less than its contribution to petroleum based penetrant brightness. Even though WB penetrants seemingly are incompatible with dry powder developer, the recently QPL listed water based penetrants did pass the qualification tests using dry powder developer as the specification mandates.

 

Water Based Penetrants and Dwell Times
As mentioned, the penetrant dwell time under test conditions is five minutes, whereas MIL-STD-6866 and ASTM E-1417-95a specify a minimum dwell of ten minutes. In practice, dwell times are usually longer than ten minutes.

When a WB penetrant is applied in a thin layer over the surface and allowed to drain dwell, the water evaporates and the penetrant becomes concentrated. Sensitivity increases with concentration. In fact, with a 30 minute drain dwell, an Air Force test found that an approved WB penetrant's sensitivity increased from level two to level three. A short dwell time works to the disadvantage of WB penetrants, whereas the longer dwell time increases their effectiveness.

 

Water Based Penetrants Compare Favorably with Conventional Penetrants
By virtue of their meeting ASTM E-1417 specifications and their having been tested by an independent third party, WB penetrants compare favorably with conventional penetrants.

In Figure 1, a level two WB penetrant was applied to the left section (section A) of a cracked aluminum block, while a conventional level two was applied to the right section (section B) of the block. The penetrants were Sherwin Inc.'s WB-200 and HM-3A, respectively. No developer was used. In Figure 1, after a 10 minute dwell, it is clear that the water based penetrant has superior self developing properties.

The remaining figures show similar comparisons between WB-200 and either HM-3A (conventional) or HM-602 (surfactant based) when tested using either cracked aluminum blocks, as in Figures 2-4, or using the Twin KDS Panel, as in Figures 5-7. As indicated by the figures' captions, the varied parameters include the specific penetrants used, application of developer, and dwell times. The results clearly indicate that water based penetrants are superior or equal to already approved conventional or surfactant based penetrants.

 

Conclusion
The principal drawback of water based penetrants is that they require monitoring to maintain the correct water proportion. Nevetheless, QPL approved water base penetrants should find a wide range of applications. They are more environmentally friendly than traditional penetrants; additionally, they generally cost less, conserve resources, are more compatible with water based cleaners, have self-developing properties, and, in concentrate form, are less expensive to ship and store than traditional penetrants.

Figure 1 - Ten minute dwell without developer; (a) WB level 2 penetrant; (b) biodegradable level 2 penetrant.

 

Figure 2 - Ten minute dwell with powdered developer; (a) WB level 2 penetrant; (b) biodegradable level 2 penetrant.

 

Figure 3 - Thirty minute dwell with powdered developer; (a) WB level 2 penetrant; (b) biodegradable level 2 penetrant.

 

Figure 4 - Thirty minute dwell without developer; (a) WB level 2 penetrant; (b) biodegradable level 2 penetrant.

*   Sherwin Incorporated, 5530 Borwick Ave., South Gate, CA 90280; (562) 861-6324; fax (562) 923-8370; e-mail sherwininc@aol.com.

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