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Mail inbox - December 2015 : Thixotropic magnetic inks ?

Written by Dubosc
Tuesday, 24 November 2015 22:52

Having to perform an MT inspection above his head, on an overhead structure, one of our readers wondered whether thixotropic detecting media exist for this method.

It seems that no thixotropic detecting medium exists. May we find why?

This makes us come back to remind you of the thixotropic penetrant testing materials that we presented in one of our papers(1).

In the British patent description N° 1,315,121(2) (3), we may find some formulae of thixotropic materials. Thixotropic penetrants may be easily formulated from virtually any type of conventional penetrant by adding a suitable thixotropic agent, for example, a clay-based one. Although this patent states that these materials may be applied by brushing or spraying, they are usually applied by brushing or by using a cloth, making them fluid from their gel normal, stable condition.

One way would be, for example, to replace the dyes by magnetic particles in a thixotropic penetrant.

Another way would be to add a thixotropic agent to a magnetic ink, by incorporating, if necessary, other additives to get a thixotropic magnetic ink.

Thus, formulating such materials does not seem difficult.

However, the following points should be considered.

In the penetrants, the dyes are, and remain, in solution. In a magnetic ink, the magnetic particles are in suspension, and eventually settle down.

A thixotropic carrier could prevent the sedimentation of the magnetic particles. They would remain dispersed in the gel ... but for which purpose?

However, might the homogeneous redispersion of the magnetic particles be guaranteed when applying such a magnetic ink?

The magnetic particles are generally spherical in shape and / or elongated. In magnetic inks, they shall be mobile, and this is why the ISO 9934-2 standard states that the dynamic viscosity of the carrier liquid, determined according to the ISO 3104 standard, shall not be higher than 5 mPas at 20 °C ± 2 °C.

In such low viscosity liquids, when the test piece is magnetized, the elongated particles are subjected to a torque due to the magnetic field and align with the magnetic-field lines. This torque is very weak. If the carrier liquid viscosity is increased, the torque applied to the magnetic particles is insufficient to "counter" the opposing forces due to the very high viscosity of the carrier liquid.

Even if the particles are left subjected to the magnetic field for fifteen seconds (which significantly increases the inspection time if using a portable electromagnet, for example: 15 seconds at each position of the equipment), it is highly unlikely that the performance of the magnetic ink may be guaranteed. If using a current generator or a magnetic bench to magnetize by current flow or magnetic flow, it is likely that processors and electronics would quickly overheat. The safety devices will stop the machine…for a while! Cooling will require long minutes. Again, for an unproven result!

Many questions that make us wonder what would be the performance of such thixotropic magnetic inks.

We found no patent dealing with thixotropic MT detection media.

We think that no MT materials’ manufacturer has so far considered the issue. What would the market size for such materials?

As for thixotropic penetrant materials, few manufacturers have them in their product range, which would tend to prove that this is a really marginal market.


Normative references

ISO 9934-2:2015, Non-destructive testing -- Magnetic particle testing -- Part 2: Detection media, International Organization for Standardization, Geneva, Switzerland, 2015.

ISO 3104:1994, Petroleum products -- Transparent and opaque liquids -- Determination of kinematic viscosity and calculation of dynamic viscosity, International Organization for Standardization, Geneva, Switzerland, 1994.



(1) Pierre CHEMIN and Patrick DUBOSC, DPCNews N° 017 – Special products for Penetrant Testing, (document updated on September 2014).

(2) Pierre CHEMIN and Patrick DUBOSC, Penetrant Testing history (updated in December 2014).

(3) British patent description N° 1,315,121, published on 26 April 1973. Improvements in or relating to process and materials for the detection of flaws and discontinuities in an article. Inventor : Norman Henry HYAM of Ardrox Ltd, Commerce Road, Brentford, Middelex, England.

Last Updated ( Tuesday, 05 April 2016 22:52 )