What are the required properties of a penetrant?
TWI Frequently asked questions
by Ivan Pinson
- Wetting ability
Affects penetrability and bleed-back characteristics.
- Specific gravity
Penetrant chemicals used in a tank system must have a specific gravity less than one, to assure that water will not float on top of the penetrant and prevent the penetrant from covering the test object.
- Flash point
The flashpoint of a material is the temperature at which enough vapour is given off to form a combustible mixture and a minimum value of around 93°C is typical.
- Volatility
Highly volatile penetrant chemicals would evaporate too quickly to be practical.
- Chemically inert
Penetrant materials must be as inert and non-corrosive as possible. Maximum sulphur, sodium and halogen levels are often specified by the nuclear and aerospace industries, to avoid the possibility of embrittlement or cracking.
- Viscosity
Viscosity relates to the thickness or body of a fluid and is a result of molecular or internal friction. Excessive viscosity results in long dwell times, low viscosity leads to reduced dwell times but makes the penetrant prone toover washing.
- Solubility
A penetrant must hold sufficient dye at ambient or high temperature and the dye must not come out of solution if the temperature drops.
- Solvent ability
Having applied the penetrant, it becomes necessary to remove the surplus from the test specimen to ensure a clean, clear background. Volatile solvents, some flammable, some not, are often used. These must not dissolve thepenetrant in defects.
- Health hazard
Chemists developing new penetrant materials must comply with or exceed the most stringent health and safety requirements.
- Electrical conductivity
Electrostatic spraying of penetrant is becoming increasingly popular in large automatic processors and even where electrostatic hand-spray guns are used. To be adaptable to electrostatic spraying, penetrant must readily acceptand hold the electrical charge placed on the liquid particles.
