Pure water, that is water that doesn’t contain any metal ions, is usually only found as distilled water. Even this may contain traces of metal ions from the distillation equipment, which is often glass but can be fabricated from metal. We are unlikely to use distilled water due to cost and availability.

Rain water? Well, that’s good for us to use. It doesn’t have much in the way of metal ions because it has come from the skies, but it does have dissolved gases such as oxygen and carbon dioxide, sulphur dioxide, and nitrogen oxides, the latter three from burning fossil fuels.

It will contain particulates from the dust in the air. Oh, it also contains among other things living organisms! Bacteria and spores can cause corrosion in our models.

Softened water is probably the best water for us to use, providing the equipment is used according to the manufacturer’s instructions, and it is recharged at the appropriate time.

Corrosion accelerators

1. High velocity and/or turbulence

2. High temperature and pressure

3. Low Total Dissolved Solids (TDS)

4. Dissimilar metal contact

5. Low pH (pH measures acidity or alkalinity. Neutral pH is 7. Low pH is less than 7)

6. Carbon dioxide

7. Biofilm accumulation: microbially influenced corrosion

8. Chemical agents such as chlorine and dissolved oxygen

9. An elevated chloride-to-sulphate mass ratio (CSMR)

High velocity and/or turbulence

Don’t try this at home! Water jet cutting of steel is a well-established process. Karcher (normal disclaimer applies) and other high velocity water cleaning systems strip metals. See what it does to concrete. Cavitation often occurs in pumped water systems and can accelerate corrosion of valves and pumps. The smaller the tube diameter the greater the flow resistance. In pressure systems this creates turbulence and high velocity flows.

High temperature

Chemical reactions (corrosion) are accelerated at higher temperatures. Typically, a reaction at 20 degrees Celsius will be eight times faster at the boiling point of water at sea level. This is because higher temperatures create more collisions of molecules. Pressure increase also will occur in a sealed vessel such as a boiler. More pressure again causes more collisions of molecules.

Low TDS

OK, this is what I referred to when talking about distilled water earlier in this article. Ultra-pure water is very corrosive as it will dissolve limestone, metals (and gasses).

Dissimilar metals

From school physics or biology lessons you might remember a person called Luigi Galvani. If you don’t, he discovered the effect of an electric spark on dead frog muscles (no reptiles were hurt in this article). The name Galvani is linked to everyday corrosion in many ways. Galvanisation, galvanic potential, galvanic action etc. Metals are more or less reactive. Gold is unsurprisingly,  the gold standard and can be attributed the lowest reaction rate and copper and bronzes less reactive than brass or lead.

Soft-soldered joints may contain lead, tin, antimony, cadmium etc. All these metals are more reactive than copper. Hard-soldered joints may contain similar metals but will have higher silver content. The manufacturers data sheets give good information outside of the scope of this article. The point is that dissimilar metals create a galvanic cell which is improved by the addition of - you’ve guessed it - water.

Scale is made up of metal salts and these form mini galvanic cells and this is why we have washout plugs and ‘mud’ holes in boilers to get rid of scale.

Low pH

We all know the corrosive nature of acids. We use acidic ‘pickles’ to clean metals after silver soldering etc and fluxes can be acidic, as well. Long term use of low pH water <pH 7 at high temperatures and pressures increases corrosion.

Softened water is usually around pH 7.2 to 7.8 and is very slightly alkaline. Unless you have an aluminium element in your model water with this alkalinity is unlikely to cause a problem. Aluminium also reacts very rapidly with oxygen creating a very fine monolayer of aluminium oxide (the grey tarnish seen on aluminium).

Carbon dioxide

There is no getting away from it. It is everywhere. In the fizzy drinks like ‘Coke’ and in the atmosphere. It is readily soluble in water to form carbonic acid. When heated carbonic acid disassociates into water and carbon dioxide. The acid steam produced will corrode.

Chemical agents such as chlorine and oxygen

These gases are very reactive. Chlorine is added to water to sterilize and kill off harmful to us bacteria. Oxygen dissolved in water allows fish to breathe. It again is very soluble in water. Both gases cause corrosion either by forming chlorides or oxides with metals. The greenish blue coloration seen on copper and brass is due to these gases.

Biofilm

Bacterial slime buildup in water systems is difficult to control. This biofilm causes corrosion by mechanisms outside of this article. It also causes blockages in pipes and is particularly important when involved in the injector stream. Recommended methods of removing biofilm from the like of salt recharged water softeners involves the use of strong oxidizing acids such as peracetic acid. Regular use and timely maintenance of the water softener is therefore of utmost importance.

Finally, proper flushing of the water softener and the correct recharge interval will minimize the chloride sulphate corrosion potential. If salt remains in the softener bed this will increase the likelihood of corrosion. This is particularly important where lead is present (such as in soft soldered joints).

Well, a bit long winded, but it is important for everyone to know that water and the treatment we give it is important to the running of models especially those using steam.

This article first appeared in the newsletter of the Bournemouth and District SME.