Did you know that all penetrants are solvent removable?
Water washable penetrants can be removed using water or solvent wipe methods (Methods A & C). Water washable penetrants have had an emulsifier added to them before they are packaged and sold. An emulsifier is a chemical added to an oily substance that makes it removable by water, most oils cannot be rinsed away with straight water. Most penetrants are oil based, oils can be removed using solvents or by adding an emulsifier prior to rinsing with water.
Which brings us to post emulsified penetrants. These are also oil based penetrants, as such they can be removed with a solvent wipe. However, to make them water washable we add an emulsifier to them as part of the removal process.
The basic process of penetrant testing is 1) preclean the part 2) apply the penetrant 3) allow it to dwell for the appropriate amount of time 4) remove the excess penetrant 5) apply developer 6) evaluate the indications 7) post clean the part.
Step 4 is where we see big differences in the process. As discussed previously water washable penetrants have an emulsifier already added to them, solvent removable only are oil based with no emulsifier. Lipophilic Post emulsified (Method B) penetrants are oil based penetrants with no emulsifier added, so to make it water washable we dip the part in an oil based emulsifier, this emulsifier diffuses into the penetrant making it water washable. Timing is critical, the longer the penetrant is in contact with the emulsifier the more of the penetrant that will be removed when you rinse with water and you might remove small shallow indications. Hydrophilic emulsifier is water based and is actual more of a detergent with mechanical action, it can be flowed or dipped (Must be agitated). Timing is critical, too much exposure to the emulsifier will result in the removal of small shallow indications. When flowed the movement of the flow provides the mechanical action, when dipped the agitation provides the mechanical action. The other big difference is that with hydrophilic emulsifiers there is a pre-emulsification rinse with straight water which will remove 60% or more of the oil based penetrant.
The post emulsified process provides greater sensitivity when performed correctly, because it limits the amount of penetrant that can be removed. Water washable penetrants make it easy to remove excess penetrant from rough surfaces, but you also run the risk of over washing the part and washing penetrant out of the discontinuities. Solvent removable is easy to use on smooth polished surfaces and as welded parts that have smooth transitions into the base metal.
You can only add so much stuff to the oil of a penetrant, which means that if you add an emulsifier to the oil base you have less room for dye stuff (color). If you do not add the emulsifier you have more room for dye stuff. The additional dye stuff makes for more intense color, and greater contrast.
Dry powder developers and Non-Aqueous (solvent based, wet) developers are applied after the part has completely dried from the excess penetrant removal process. Dry developer is the least sensitive, should only be used on Fluorescent penetrants, it provides good coverage on rough surfaces and is normally used in a penetrant line. Non-Aqueous developer is considered the most sensitive of the developers, works well with both visible and fluorescent penetrants, works well on most surfaces. The non-aqueous developer solvents diffuse into the penetrant increasing the volume of penetrant, then as the solvent quickly dries it helps pull the penetrant to the surface and into the developer.
There are also water based developers, water soluble, and water suspendable. Water soluble developers are added to water in the correct quantities, once dissolved into the water it stays in solution. Water suspended developers will need to be agitated constantly to keep it suspended. These are more commonly used in penetrant lines than in the field. They both a leave a thicker layer of developer than dry or non-aqueous, and are both less sensitive than the non-aqueous. Both can be applied before the part is completely dried from the penetrant removal process.
Lighting is critical, most application of visible penetrant will need 100 foot-candles (1076 lx) of visible light. Most fluorescent penetrants will require 1000 micro-watts/cm squared of UV-A at the surface and no more than 2 foot-candles (21.52 lx) of visible light.
UV-A has a wave length of 320 – 400 nanometers, peak frequency needs to be 365 nm. UV-A lamps need to comply with ASTM E3022.
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