Why do water-washable penetrants resist removal at surface openings on parts?

• A. Due to formation of plugs in the openings of flaws from gelling when in contact with water
• B. Because they are strongly hydrophobic
• C. Because they bond to metal surface
• D. Because they evaporate rapidly

Answer: (A)

When using water-washable penetrants, the washing step is designed to remove surface penetrant while leaving penetrant inside flaws to reveal them later with developer. At the mouths of surface openings, contact with water can cause the penetrant to gel and form a thin plug right at the flaw entrance. This gel acts as a physical barrier, preventing the penetrant near the opening from being rinsed away as easily as the rest of the surface material. The result is residual penetrant trapped at or just inside the surface openings, which helps highlight flaws when the developer is applied.

Other ideas don’t fit as well: the system isn’t defined by strong hydrophobicity or by bonding to the metal surface in a way that would prevent removal; nor is rapid evaporation the mechanism. The key behavior is gel formation that plugs openings and resists removal during washing.