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Post-emulsifiable penetrants contain no built-in emulsifier, so water alone will not lift them off the surface. That is deliberate: because the penetrant cannot be washed out of a discontinuity by accident, these systems are the most sensitive available and are used where shallow, wide, service-induced defects must be found. Removability is switched on only when a separate emulsifier is applied, and the emulsification time is the interval during which that emulsifier is allowed to act before the final rinse stops it.
The two families work differently. A lipophilic emulsifier (Method B) is oil-based and removes penetrant by diffusion: it must physically mix into the surface penetrant film, and the process cannot be stopped once it has started diffusing, so times are short and control is coarse. A hydrophilic remover (Method D) is a detergent that acts by displacement at the surface rather than by diffusing in, which makes it far more controllable and is why it dominates modern practice. Hydrophilic processing also requires a pre-rinse before the emulsifier, to strip the bulk of the surface penetrant so the emulsifier has less work to do.
The failure mode is one-sided and unforgiving. Under-emulsify and background penetrant remains, giving fluorescent haze that hides indications - annoying but visible and recoverable by re-processing. Over-emulsify and the emulsifier reaches into shallow discontinuities and strips the penetrant out of them, and the part then looks perfectly clean while the defect has been erased. Nothing downstream will reveal that error.
For that reason the times below are nominal starting points only. The actual time must be established experimentally on the real part geometry and surface finish, usually with a known-cracked reference panel, and then fixed in the written procedure. Timing is from the moment the emulsifier contacts the part until the final rinse starts.
Hydrophilic (Method D): 30 s to 2 min, pre-rinse required before the emulsifier Lipophilic (Method B): 10 s to 3 min, no pre-rinse Total removal time = pre-rinse (hydrophilic only) + emulsification + final rinse Times are nominal - establish the actual time on a cracked reference panel
- These times are nominal. Establish the real time on the actual part with a cracked reference panel and fix it in the written procedure.
- Over-emulsification strips penetrant out of shallow discontinuities and leaves a part that looks clean but is not. It cannot be detected downstream.
- Under-emulsification leaves background fluorescence. It is recoverable - re-clean and re-process from the beginning.
- Hydrophilic removal needs a coarse pre-rinse before the remover; lipophilic emulsifier is applied directly to the drained part.
- Emulsifier is applied by immersion or foam, never by brushing or wiping, which would work it into discontinuities.
- Time from emulsifier contact to the start of the final rinse. Drain time counts.
- Hydrophilic remover concentration and contamination must be checked at the interval set by the procedure - a drifting bath changes the effective time.
Reference: ASME BPVC Section V (2023 Ed.), Article 6 - post-emulsifying techniques (Method B lipophilic, Method D hydrophilic), which require the emulsification time to be established and stated in the written procedure. The 30 s to 2 min hydrophilic and 10 s to 3 min lipophilic bands are the nominal ranges given in ASTM E165/E165M-18; they are starting points, not acceptance limits.
