Giving you personal and shared information on science, technology, technical & Investments in Nigeria

THE PHYSICAL PROPERTIES OF PENETRANTS SUITABLE FOR NDT

The physical properties of the penetrant 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 as 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 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.

Advertisement

Variations in viscosity

Effect of temperature is a factor, which changes the viscosity of a penetrant. Viscosity tends to decrease with increased temperature.

Effect of pressure is also a factor which causes considerable change in the 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.                           

Advertisement

CAPILLARITY: 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-section. 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 which offer narrow openings such as a tight crack or fatigue cracks of hairline type are best detected with 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: the ability of a liquid to flow is termed as fluidity. The penetrant should have the ability to drain away from the component without dragging its original marks that revealed the defects.

SURFACE TENSION: 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 but however, low surface tension liquids provide the penetrating power and spreading properties necessary for a good penetrant.

WETTING ABILITY: 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 contact angle and surface tension of the liquid. The lesser the contact angle, the higher is the wetting ability. The contact angle of the penetrant should preferably be less than 5 degrees.

Advertisement

VOLATILITY: penetrants should essentially be non-volatile liquids. A small amount of evaporation at the discontinuity could help to intensify dye brilliance and also prevent excessive spreading of indications. However, low volatility is desirable to minimize the losses due to evaporation of penetrants stored in open tanks.

FLAMMABILITY: 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. Flashpoint of a penetrant should not be less than 135 degrees F.

CHEMICAL ACTIVITY: the ability of the penetrant to cause corrosion on the metals which are tested. This is due to the presence of halogens, elements which 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 a presence of halogens are usually restricted on austenitic steels, titanium, and other high-nickel alloys.

DRYING CHARACTERISTICS: the penetrant must resist drying out, and complete 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: specific gravity has no significant bearing on a penetrants performance. Most penetrant 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

Originally posted 2018-12-19 09:12:09.

Advertisement
Advertisement
Spread the love
  • 2
  •  
  •  
  •  
  •  
  •  
  •  
  •  
  •  
  •  
  •  
  •  
  •  
  •  
  •  
  •  
  •  
    2
    Shares
Advertisement

Leave a Reply

Your email address will not be published.

Advertisement
WP2Social Auto Publish Powered By : XYZScripts.com
error: Content is protected !!