Ultrasonic inspection uses short wavelength sound waves with high frequency to detect flaws or measure material thickness. Usually pulsed beams of high frequency ultrasound are used via a hand-held transducer which is placed on the specimen. Any sound from that pulse that returns to the transducer like an echo is shown on a screen which gives the amplitude of the pulse and the time taken to return to the transducer. Defects anywhere through the specimen thickness reflect the sound, back to the transducer. Flaw size, distance and reflectivity can be interpreted. CORROCONT performs ultrasonic testing of storage tanks, pipelines and other metal structures by its engineers holding at least UT level II certificate.
Liquid penetration inspection is a method that is used to reveal surface breaking flaws by bleedout of a coloured or fluorescent dye from the flaw. The technique is based on the ability of a liquid to be drawn into a “clean” surface breaking flaw by capillary action. After a period of time called the “dwell”, excess surface penetrant is removed and a developer applied. This acts as a “blotter”. It draws the penetrant from the flaw to reveal it’s presence. Colored (contrast) penetrants require good white light while fluorescent penetrants need to be used in darkened conditions with an ultraviolet “black light”. Penetrant inspection can be used on any material. It is essential that the material is carefully cleaned first, otherwise the penetrant will not be able to get into the defect. If surface penetrant is not fully removed, misleading indications will result.
This method is suitable for detection of surface and near surface discontinuities in ferromagnetic material, mainly ferritic steels and iron.
The principle is to generate magnetic flux in the article to be examined, with the flux lines running along the surface at right angles to the suspected defect. Where the flux lines approach a discontinuity they will stray out into the air at the mouth of the crack. The crack edge becomes magnetic attractive poles North and South. These have the power to attract finely divided particles of magnetic material such as iron fillings. Usually these particles are an oxide of iron in the size range 20 to 30 microns, and are suspended in a liquid which provides mobility for the particles on the surface of the test piece, assisting their migration to the crack edges. However, in some instances they can be applied in a dry powder form. The particles can be red or black oxide, or they can be coated with a substance which fluoresces brilliantly under ultra-violet illumination (black light). The object is to present as great a contrast as possible between the crack indication and the material background.
The technique not only detects those defects which are not normally visible to the unaided eye, but also renders easily visible those defects which would otherwise require close scrutiny of the surface.