The Penetrants Use in NDT Inspection
The Physical Properties Of Penetrants Suitable For NDT are what makes the difference from other solvent solutions. Penetrant suitable for NDT has specific qualities and properties. Corrosion and rust of the metal surface after an inspection has been done on it could be a result of the wrong penetrant, and that could include other effects. Below is a list of known physical properties of the NDT penetrant:
• Viscosity
• Capillarity
• Fluidity
• Wetting ability
• Surface tension
• Volatility
• Flammability
• Chemical activity
• Drying characteristics
• Specific gravity
VISCOSITY
viscosity is one of the Properties Of Penetrants Suitable For NDT inspection. It is the property of penetrant by virtue of which it offers resistance to relative motion between its different layers. The layer in contact with a fixed horizontal plane will be at rest due to adhesive forces between molecules of liquid and plane.
Variations in viscosity
The effect of temperature is a factor, which changes the viscosity of a penetrant. Viscosity tends to decrease with increased temperature.
The effect of pressure is also a factor that causes a considerable change in viscosity. Viscosity increases with an increase in pressure. Viscosity has no actual bearing on the ability of a penetrant to ‘penetrate’, although it does affect the rate of penetration.
Kinematic Viscosity is generally expressed as a ratio of dynamic per second.
One square centimeter per second is equivalent to one ‘stroke’. Viscosity is measured in terms of centistokes (csts). The viscosity of the penetrant should not exceed 5cst. High viscous penetrants have a slower rate of penetration; low viscous penetrants drain away easily.
CAPILLARITY
This is one of the Properties Of Penetrants Suitable For NDT inspection. The ability of a liquid to rise or fall in narrow openings is due to capillarity action. It is demonstrated by a simple experiment, which uses two tubes of varying cross-sections. The tubes are then immersed in a container filled with water. Observation showed that the level of water in the thinner tube rises faster. His experiment proves that a liquid with a low viscosity penetrates much faster into narrow openings.
Therefore, cavities that offer narrow openings such as a tight crack or fatigue cracks of hairline type are best detected with a penetrant test system as it penetrates readily into narrow openings. When repeating the experiment with a liquid such as mercury, the level of mercury falls down in the thinner tube.
The height to which the liquid is raised is determined largely by surface tension and the wetting ability of the liquid. Also, lifting ability due to capillary action increases, as the diameter of the bore decreases.
Capillary forces will be less in a closed tube than in an open tube, because of the air that is trapped in the former. This can be compared with a discontinuity, which has a closed-end on one side. The air that is trapped inside will be dissolved by the penetrant and is diffused at the surface.
FLUIDITY
This is one of the Properties Of Penetrants Suitable For NDT inspection. The ability of a liquid to flow is termed fluidity. The penetrant should have the ability to drain away from the component without dragging its original marks that revealed the defects.
SURFACE TENSION
This is one of the Properties Of Penetrants Suitable For NDT inspection. The force acting per unit length of an imaginary line drawn on the surface of the liquid being normal to the surface is known as the surface tension. It is the property of a liquid that acts like a stretched membrane. Surface tension plays an important role in the effectiveness of a penetrant.
High surface tension liquids are usually excellent solvents and will easily dissolve the dyes, however, low surface tension liquids provide the penetrating power and spreading properties necessary for a good penetrant.
WETTING ABILITY
This is one of the Properties Of Penetrants Suitable For NDT inspection. The ability of a penetrant to wet the surface of a material is known as its wetting ability. It is a measure of the area of contact with the surface.
This property is dependent on the contact angle and surface tension of the liquid. The lesser the contact angle, the higher the wetting ability. The contact angle of the penetrant should preferably be less than 5 degrees.
VOLATILITY
This is one of the Properties Of Penetrants Suitable For NDT inspection. Penetrants should essentially be non-volatile liquids. A small amount of evaporation at the discontinuity could help to intensify dye brilliance and also prevent the excessive spreading of indications. However, low volatility is desirable to minimize the losses due to the evaporation of penetrants stored in open tanks.
FLAMMABILITY
This is one of the Properties Of Penetrants Suitable For NDT inspection. The penetrants should have a high flash point as a matter of safety while in use. Flashpoint is defined as the lowest temperature at which the liquid gives a flash. When a small flame is passed across its surface, the liquid gets heated up and subsequently burnt. The Flashpoint of a penetrant should not be less than 135 degrees F.
CHEMICAL ACTIVITY
This is one of the Properties Of Penetrants Suitable For NDT inspection. The ability of the penetrant to cause corrosion on the metals which are tested. This is due to the presence of halogens, elements that belong to a highly reactive group. Some examples are; chlorine, fluorine, bromine, and iodine.
No penetrant is without the presence of halogens and should be noted that these can cause corrosion on the metal surface, if not removed. This is the primary reason why the post-cleaning operation is essential in penetrant testing. Therefore, penetrants with the presence of halogens are usually restricted on austenitic steels, titanium, and other high-nickel alloys.
DRYING CHARACTERISTICS
This is one of the Properties Of Penetrants Suitable For NDT inspection. The penetrant must resist drying out, and completely bleed out, during hot air drying of the component after the wash operation has been completed. Ideally, heat should aid the penetrant in promoting a return of penetrant to the component surface in order to produce a sharply defined indication.
SPECIFIC GRAVITY
This is one of the Properties Of Penetrants Suitable For NDT inspection. Specific gravity has no significant bearing on a penetrant’s performance. Most penetrants should have a specific gravity of less than 1. In general, the penetrability depends on three important factors which are;
1) The surface condition of penetrant
2) Type of penetrant
3) The temperature of part/penetrant
