Hello!
You receive this letter for the first time? Is the topic of some interest for you?
Then please, validate your subscription to receive further issues!

May 2008

Post-emulsifiable penetrant testing: Hydrophilic emulsifier

1. Emulsifier: agitate or do not agitate?

Fussy question! A subcontractor doing business with several primes may have both requirements for the same emulsifier tank!

In fact, the right answer is directly connected to the emulsifier formula.

An emulsifier mainly based upon subvent action shall be agitated.

An emulsifier mainly based on surface action should not be agitated.

As a general rule, in an hydrophilic emulsifier tank, parts or baskets are agitated:

- to get rid of the thin film of post-emulsifiable penetrant emulsified by the lipophilic part of the surfactants molecules incorporated in the emulsifier.

- to have a fresh film of emulsifier coming in contact with the parts’surface.

In fact, emulification of a post-emulsifiable penetrant is carried out through the interfacial action. On the other hand, when using a lipophilic amulsifier, this emulsifier is unnecessary, even counter productive.

Agitation shall always be moderate. If a thick layer of foam appears, agitation shall be ... weaker.

Nevertheless, this question may lead to a more ... ambiguous answer.

In fact, an emulsifier tank is prone to anaerobic bacteria growth (bacteria which proliferate in low oxygen concentration) : water, organic chemicals, an appropriate and fine temperature (the best temperature for bacteria is generally... +37°C, ie 100°F, with the noteworthy exception if listeria which grows very well when faced to +6°C/8°C – 43°F/46°F ; it is an aerobic bacteria, that means it needs oxygen for growing).

Anaerobic bacteria give rise to hydrogen sulfide (H2S), the rotten eggs smell. These sulfato-reducing bacteria are usually found in light alloys foundries and other metal transforming industries. Humans are very sensitive to it ! 1ppm in air.

That’s why emulsifiers currently contain some kind of biocides. For many years, hexahydrotriazines have been used, due to their efficiency even when highly diluted. But as they are metabolized to formol (formaldehyde) they are now taken out of formula.

To prevent bacteria from growing, two methods come to mind:

1. The WMD (Weapon of Massive Destruction) bactericide. By the way, is there any AMS 2644 approved bactericide ?

2. The soft method: to help aerobic bacteria growth, which produce only water and CO2 and prevent anaerobic bacteria growth.

But, how to do that?

Introducing oxygen in the tank is the simple way. Compressed air, bubbling is the tank 24h a day, including Saturdays, Sundays, Ban holidays, enough to supply oxygen, not too much to prevent excessive foam.

We have a good example with a large user of postemulsifiable penetrants. Inspectors complained about bad smelling of the large hydrophilic emulsifier. We recommended to put a fine tube drilled with many tiny holes, at the bottom of the tank, and to have compressed air bubbling, 24 hrs a day, 7 days a week. We suggested to put this device in service on a Friday evening : in fact, for the several first hours, odour would be far stronger, as oxygen would displace H2S from water, H2S is highly soluble in water.

This worked very well for about 18 months, when we received a phone call stating bad smelling was back. After some questions (“nothing had been changed in the installation” was the first answer) we understood that the “Environment Service” did not understand why compressed was “lost” on the installation when no personnel was working. Automatic time-controlled valves stopped bubbling during week-ends and Ban holidays. So, bad smelling was back !

We had to explain again thoroughly that money spared while shopping air was in fact a bad idea ! Less comfortable working area, complaints from users and unexplained corrosion on some parts (H2S being the sulphydric acid).

2. Emulsifier concentration? Which percentage?

First American primes, then almost all the specifications included in the PRI-Nadcap questionnaires, state that the maximum concentration in the tank shall be 7, 17 or 27% respectively for emulsifiers qualified at 10, 20 or 30% maximum.

An other point: at any moment, the concentration shall be within ± 3 percentage points of the nominal figure for the tank.

Let us have a thorough look to these requirements.

2.1- Let us be malicious

If we combine requirements, say : “5% in the tank, nominal, ± 3 percentage points allowed”, we go to a 2% to 8% allowed range. It seem almost a small difference: but the ratio is 1 to 4!

Do you think it's the same difference for 27 ± 3%, i.e. from 27 to 30%, hence a 1.25 ratio?

In the first example, efficiency and reliability are not comparable between the two limits (2% and 8%). But these figures are within the allowed range!

2.2- Let us be far more malicious!

When emulsifier is sprayed instead of used by immersion of parts, it is a good idea to use half the concentration which would have been used by immersion – or less.

Go back to the 5% figure. Half of 5 is 2.5. A range of ± 3 percentage points is allowed, a figure of 5% being a maximum (for some primes).

Therefore, combining these two requirements, concentration may be from 5% (max) down to 2.5-3<0.

This being less than zero, the bottom limit is ... zero.

While strictly meeting requirements, one may go to the conclusion that pure water is OK. And IT IS OK!

Now, is pure water efficient? One may be skeptical ... but in complete compliance to the requirements!

---

This paper purpose is only to make specs writers and auditors think twice.

Quite as when the ISO 9001/9002 1984 version was used: no wonder about the QUALITY of the parts. If, following the procedures, the company manufactured defective parts, defective parts should be always and forever manufactured.

The day when parts good for service were manufactured, that meant procedures were not adhered to and, if an audit was then performed, NCRs were issued because procedures were not followed !

We then with other concerned people, had a strong comment when these standards were published. About 16 years were needed to have this way of thinking modified.

Questions asked in this paper have been pointed out for years. Hope for adjustment is still "on".