A MATTER OF SCALE

By Crankpin

Whether its hard water scale in the boiler, or black oxide scale left after silver soldering, it can be dealt with - or perhaps left.


Boiler water scale

I am in one of the many areas in the British Isles with hard water. Opinions vary about the health benefits of drinking hard water but the negative effect of using it in boilers is obvious by the hard water deposits or scale which it leaves behind. It’s too easy to be dismissive about hard water scale in small boilers and in most cases its presence seldom causes obvious problems. Even so, if left for prolonged periods it will cause localised stress in the boiler in addition to blocking or causing malfunction of injectors and drain cocks etc. The explanation of hard water, its effects and treatment in model boilers was excellently covered by Robin Eycott in Engineering in Miniature (p 34, Aug 1999). It’s well worth reading..

We may conscientiously attempt to de-scale our boilers at the end of the running season but unless some basic principles are noted, more harm than good can result. Take for example the scale itself – it represents insoluble salts of mainly calcium though other metal salts are often there. In simple terms, we can  consider the  white scale to be calcium carbonate or ‘limestone’.  If you have some available, you might make up a solution of 10 or 15% sulphuric acid and pour this into the cold boiler through the safety valve bush. A good effervescence occurs and maybe 15 minutes later the gassing has stopped. A good wash through and youre left with a feeling of satisfaction that the boiler has been de-scaled for another year. However, the big problem is that the calcium carbonate has been converted to carbon dioxide and calcium sulphate (gypsum). The latter is insoluble and remains as an impervious white mass where the calcium carbonate was previously. Now we are decidedly worse off than before because the only way to remove the calcium sulphate is to scrape and chip it off which is clearly impossible inside the boiler.

Instead, we should use an acid which will leave behind a (water) soluble salt of calcium. Dilute hydrochloric acid would be excellent since calcium chloride is soluble and can be washed out. However, obtaining hydrochloric acid is increasingly difficult so unless you’re lucky enough to have a friend whose work involves handling hydrochloric acid – we’d better look for an alternative. Incidentally, the lavatory cleaner, Harpic liquid, contains about 8% of hydrochloric acid and this might be thought suitable. There are two problems though. Firstly the Harpic contains a lot of surfactants (strong detergents) and trying to get rid of them from the boiler will involve an awful lot of washing out. Secondly, it would be very expensive to even half-fill a 5” boiler with Harpic liquid at 750ml a plastic bottle full!

So, we face a dilemma.

There are weak acids and strong acids (those terms refer to the hydrogen ion concentration not their ability to attack say metals). Citric acid has been widely suggested as a safe de-scaler and whilst it’s true that it is relatively free of health hazards, it is a weak acid and will not do a thorough job of converting all the scale (calcium carbonate) to soluble calcium citrate and carbon dioxide. If it was effective then we would find it used in commercial de-scaling solutions. A compromise is necessary therefore – we need a strong acid but one which is readily obtainable, cheap and fairly safe to handle. The domestic de-scaling solutions sold by the DIY chains usually contain sulphamic acid and this acid is as good as most of us can get or need. It forms a soluble calcium salt, is rapid in action especially if warmed up and can be disposed of into the drains. Follow the manufacturer’s directions for dilution Unfortunately these de- scalers cannot be reused once the free acid has been depleted.


Oxide scale

In this case, it’s the situation where we’re heating up a metal to the melting point of a silver solder when making a boiler or fabricating some component from steel or a brass. Raising the temperature of the metal sufficiently in air will cause the oxygen to react with the metal surface to form the corresponding oxide. It is the oxide which creates the protective surface to prevent further oxidation and, unfortunately, prevents the silver solder or brass brazing wire from adhering to the metal. Silver solder fluxes such as Tenacity do an excellent job of both preventing oxide formation and removing any oxide which does form.

Having hopefully obtained a good bond between the two pieces of metal, we are confronted by an unsightly mess of black oxide elsewhere on the metal. It is rather unfortunate that we want to remove this oxide layer and see the pristine surface underneath. The oxide is protective and provides a good key for the subsequent primer. Nevertheless, we feel compelled to remove it and normally drop the component or immerse the boiler into our chosen ‘pickle’.

By contrast, here sulphuric acid is very effective solution, for it quickly removes the black copper or ferrous oxides. In the case of copper and the brasses, sulphuric acid reacts with the oxides and forms copper sulphate which is why the pickle turns blue/green. The blue colouration is not because the sulphuric is dissolving the copper which is not attacked by strong mineral acids. Some brasses are affected though, or more accurately, those containing zinc are (such as gunmetal but not phosphor bronze). This is why common brass turns a copper colour when in contact with a strong acid. The acid has dissolved the zinc on the surface leaving ‘pure’ copper. For this reason don’t thread bushes made of zinc-containing brass in  a new boiler before you’ve finished all the pickling or the thread crests will be weakened.

Concentrated sulphuric acid can be purchased as drain cleaner from many plumbers’ merchants. It is about 96% volume to volume,  and  must  be  diluted to  around  15%  to  be effective. Repeat of the warning: slowly ALWAYS ADD ACID TO WATER, stirring all the time and wear suitable protection. Drops of dilute mineral acids cause holes in clothing because the water evaporates leaving concentrated acid behind to eat through natural fibres like wool and cotton some days later.

Use citric acid solution if you wish (and can get it) but again, it is a weak acid and often leaves behind a tenacious green deposit of copper citrate which when dry and brushed off becomes an irritant dust.

An alternative to strong mineral acid is to use ammonia solution to remove the oxide. It works by forming complex chemicals with the copper oxide and then turns an intense blue. Though the pungent smell may put you off, it is effective and safe but you will need a source of strong ammonia (on Ebay at £12/litre).

Finally, why not just leave that protective layer of oxide in place? You’re probably going to paint or cover it anyway. If it’s necessary to clean part of the metal later then just paint on some rust remover it will work on copper, the brasses and of course mild steel.