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Textes sur la magnétoscopie

December 2013 - Magnetic Benches and Fires

Written by Dubosc
Tuesday, 12 November 2013 16:47

Safety rules should be rather stringent when working on magnetic benches: very high current intensities, strong magnetic fields. These specific risks are known, and generally well taken into account.

The topic of this paper is more down-to-earth: the magnetic ink.

Many magnetic benches in the world use oil-based magnetic inks, and not water-based ones.

Why is oil preferred as the carrier liquid (carrier fluid in the USA) of magnetic particles?

Oil-based carrier liquid has been used for...decades, while the first water-based magnetic inks were launched at the beginning of the ‘70s. The main reason was the very rapid increase of the oil price (by 300%, i.e. multiplied by four within less than three years.)

However, water is renowned for causing “flash rusting” and possibly inducing corrosion on carbon steel-based parts! Thus, water-based magnetic inks, in addition to the surface-active agents (surfactants) needed to make the ink wet the surfaces, had to be completed with a corrosion inhibitor package. Surface-active agents (surfactants) produce foam, when agitated. In magnetic benches, circulation pumps are prone to produce huge amounts of foam! Therefore, anti-foam additives had to be used. Unfortunately, during the first years, many corrosion-inhibited formulae had a high pH (well higher than 10, causing dermatitis)...and many of the anti-foam agents did not resist such a high pH or were expensive (like silicones with generally a short-term effectiveness!)

In addition, the fluorescent coating of the magnetic particles was partially released by the basic carrier liquid. Thus, it became fluorescent while the magnetic particles lost some of their fluorescence under (UV-A) ultraviolet radiation.

In short, at that time, the water-based fluorescent magnetic inks caused a lot of problems. Furthermore, taking into account all these additives...their cost per litre ready-for-use could be somewhat similar to their oil-based equivalents, with a far shorter shelf-life!!!

When straight replacing the oil-based magnetic ink by the water-based one, an unpleasant surprise appeared: the magnetic ink turned into something which looked like mayonnaise. The incompatibility, unsuspected by the users, of these two magnetic inks was highlighted, hence the need to thoroughly clean the magnetic bench before pouring the magnetic ink into the reservoir.

In addition, magnetic bench fouling occured with some water-based magnetic inks. Gummy deposits appeared, for instance, on the head- and tail-stocks, along with moving components seizure.

Apart from the concerns about corroded parts, there was another source of corrosion. Indeed, the then current magnetic benches were not designed for water-based magnetic inks, and corrosion of the equipment (such as reservoirs, pumps, pipes, etc.) occurred. Thus, corrosion residues were found on parts at the inspection stage.

Another concern is that, especially in hot environments, water evaporates quickly. This phenomenon is enhanced by the heat due to the high current intensity flowing through the parts and the electrical components.

So, at that time, some industries, such as the aerospace industry, very soon came back to oil-based magnetic inks only, after some disappointing tests.

Since then, water-based magnetic inks have been greatly improved. The only constraint is a thorough degreasing of parts to be processed to obtain a good wettability. Some aerospace spare parts suppliers, after satisfactory tests, use these magnetic inks without any further trouble.

However, some primes are still reluctant, but we may hope that the aerospace industry users of water-based magnetic inks will succeed in making some specifications change.

A major advantage of water-based magnetic inks is that they are non-flammable. Many are users of oil-based magnetic inks on magnetic benches who have seen a fire starting from a magnetic bench, due to electric sparks. Indeed, in the ‘70s, some oil-based magnetic inks had a (Pensky-Martens closed cup) flash point of 70 °C (circa 160 °F) or even less. Along the years, more and more of these magnetic inks had a flash point not lower than 93 °C ( circa 200 °F); this allowed for the reducing the fire hazards. Note that the higher the flash point, the higher the price of dearomatised hydrocarbons.

However, we want to draw the readers’ attention to another fire risk.

Many users have the habit of using cotton rags to wipe parts, or some areas of the equipment, either to prevent too much magnetic ink falling down on the ground, or to remove carefully an excess of magnetic ink that could disturb the inspection, for instance.

When oil-dampened, these rags are usually thrown into a dust-bin (trash can, for our American readers...), and piled up along the working day.

When the user prepares his own magnetic ink, he may use, due to a lack of knowledge or to a lack of availability, almost any “oil” as a carrier liquid: petrol (gasoline in the USA), non-dearomatised hydrocarbons (by catalytic hydrogenation), recycled or regenerated oil, which  can even contain other contaminants.

This is why it is preferable to get the magnetic inks, as well as the carrier liquids, from distributors authorized by renowned manufacturers of MT materials.

Furthermore, the organic pollutants drag-out by parts should be taken into account, these pollutants being mixed with the magnetic ink.

In such a case, there are risks to think about.

Especially in a hot and humid atmosphere, oxidation and hydrolysis of these organic contaminants could occur, increased by bacteria, which find a very interesting food supply! Any oxidation releases heat. Heat increases the volatility of the oil. Oxidation increases the temperature of the rags. And what could occur, finally, occurs: a “spontaneous fire” begins in the dust bin, at the surprise of the surrounding people who do not smoke!!! Alternatively, it This may even happen at night, when nobody is there to react immediately: the may imagine!!!

What did happen? What to do to avoid such an event?

At a moment, the dust bin’s content temperature rises up to the auto-ignition temperature of the rags: there is oil and there is oxygen enough. No need of a spark or of a cigarette!

The prevention measure to be taken is very easy: if rags are to be used, the only other “ingredient” needed for this ignition is...oxygen. Prevent oxygen entering the dust bin, and fire will never occur. Therefore, use closed, metal dust bins, and put the lid all the time on the top, except for throwing additional rags into the bin. So simple!!!

Sometimes, in fact, even very often, the simpler, the better.

Last Updated ( Monday, 13 January 2014 16:01 )

Wood and Magnetic particle inspection

Written by Administrator
Saturday, 16 February 2013 11:08

Everyone involved in NDT, and especially in magnetic particle inspection (MT), knows that a magnetic particle inspection (MPI) can be performed only on ferromagnetic materials.

What if?

Let us take the example that we saw… many times, before retiring.

Last Updated ( Saturday, 16 February 2013 11:25 )

MT and the European Directive 2004/40/EC (follow-up)

Written by Administrator
Saturday, 05 May 2012 11:09

Magnetic particle testing and the European Directive on the exposure of operators to low frequency magnetic fields (follow-up)

June 2012

We have to come back to this topic, as the implementation of the European Directive 2004/40/EC(1) has been postponed, and the threshold limits will be modified.

Further, is it useful to remind users, Occupational Medicine, Health and Safety responsible persons in companies, that since MT is widely used (i.e. back to the ‘30s), no correlation has ever been established between exposure to – for a long time and unknown – to magnetic fields and to any disease of the operators.

Last Updated ( Saturday, 05 May 2012 13:25 )

Guide for good practices in MT

Written by Administrator
Sunday, 12 February 2012 11:37

A help to choose the most suitable technique

March 2012

1- Introduction

Magnetic Particle Testing (MT) is a non-destructive testing (NDT) method used to detect surface-breaking or just-below-surface (generally down to some mm deep) discontinuities, only on ferromagnetic materials. This method requires following the rules to get the best results.

You will find in this paper some useful advice.

Last Updated ( Wednesday, 22 January 2014 08:43 )
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