Demineralised water is a form of purified water that has had all of its minerals removed. Despite this, demineralised water still corrodes steel – but this doesn’t mean it is a corrosive substance.
What is Demineralised Water?
When raw water undergoes ion-exchange demineralisation, nearly all of its mineral salts get removed. These include calcium, magnesium, sodium and potassium. This process is almost identical to deionisation, a process which produces deionised water.
Demineralised water and deionised water are virtually the same things. The main difference is that deionised water normally has more ionic contaminants removed, giving it a lower conductivity value. Demineralised is also slightly more corrosive to metals like steel.
Types of Steel
Steel is an alloy comprised of iron and carbon. While there are many different types of steel, all of these can be placed under two main categories:
- Carbon Steel: This has a higher carbon content
- Stainless Steel: This has a higher chromium content
Stainless steel is used more for decorative purposes in construction. Therefore, it contains elements that react with oxygen in the environment to form a protective film known as a chromium oxide layer. This passivation layer makes stainless steel very resistant to corrosion.
Carbon steel, on the other hand, doesn’t have a built-in protective layer to prevent it from corroding. This makes it more suited towards construction projects where the steel is for practical purposes, not decorative.
How Does Demineralised Water React with Steel?
Demineralised water has a huge range of applications, some of which may involve it coming into contact with steel. When this happens, depending on the type of steel and purity of the water, corrosion may occur.
This does not mean that demineralised water is inherently corrosive. Demineralised water will only corrode steel if it has been contaminated with dissolved gases like oxygen or carbon dioxide. It is very important, then, to always ensure that purified water is protected from atmospheric contact.
Mechanism of Corrosion
Corrosion is a process of degradation in a material that has a reaction with its environment. The extent of corrosion in carbon steel by water is dependent on several factors: the purity of the water, the acidity of the water and the conductivity of the water. Each of these factors can determine how aggressively demineralised water attacks steel.
Is Demineralised Water Entirely Pure?
Just because demineralised water has had all of its mineral salts removed, it doesn’t mean it’s 100% pure. While the ion-exchange is successful at removing solids, like charged mineral ions, from the water, this process doesn’t remove gases.
Not only does this make demineralised water less pure than distilled water, it also means that gases in the ambient air readily dissolve in it. This is because the water becomes hungry for gases, especially oxygen and carbon dioxide, in an attempt to compensate for its ionic imbalance.
When oxygen dissolves in demineralised water, an oxidation reaction corrodes the steel. But when carbon dioxide dissolves, carbonic acid is produced. This is dangerous because the corrosion rate of steel in carbonic acid is actually greater than in hydrochloric acid.
The presence of carbonic acid dramatically lowers the pH of the water from an alkaline reading to an acidic pH of 5. Any pH lower than 7 can cause acid corrosion to steel, which tends to prefer alkaline environments.
The pH of the demineralised water will then stay in the region of 5 for several reasons:
- Demineralised water can do nothing to remove the carbonic acid
- Demineralised water is greedy for ambient gases so will continue to absorb O2 and CO2
- The constant availability of O2 and CO2 will keep the pH at an acidic reading
Perhaps the most common form of corrosion to carbon steel is caused by an oxidation reaction between water, oxygen and iron. When the iron in carbon steel reacts with oxygen, an iron oxide is formed. This appears as a brittle powder and is more commonly known as rust.
Demineralised water is a hotbed for oxidation. This is not only because it readily absorbs ambient gases like oxygen, but also due to its lack of minerals. This means that the dissolved oxygen has nothing to interact with except for metal ions.
When oxygen combines with metal, electrons are released. Because demineralised water still has a slight conductive value, unlike deionised water, it behaves as an electrolyte and allows the newly liberated electrons to travel through it. Eventually, the metal from the steel will corrode and disappear into this electrical flow where it is converted into metal cations. These are what form rust.
How Can Corrosion be Prevented?
Perfectly demineralised water that has no dissolved contaminants will not corrode steel, but how can this be achieved if ion-exchange can’t remove gases? There are several ways to degas purified water and prevent steel corrosion.
A common method used to remove dissolved gases from purified water is degasification. This can be achieved via several different processes, including but not limited to:
- Pressure Reduction: According to Henry’s Law, the amount of dissolved gases in a liquid corresponds to its pressure. By placing demineralised water in a vacuum degasser, its pressure is reduced and the dissolved gases then become less soluble.
- Thermal Regulation: Water generally dissolves less gas at higher temperatures. Heating demineralised water can, therefore, expel any dissolved gases in the right conditions.
- Membrane Degasification: Gas-liquid separation membranes allow gas to pass through them, but not liquid. By passing demineralised water through these membranes, the dissolved gases will pass through and become separated.
New Gas Transfer Membranes (GTM) have been developed for use in water treatment and purification industries. These are much more efficient and cost-effective than existing degassing methods. GTM utilises hollow, polypropylene fibres that are hydrophobic and gas permeable.
What Effect Does Steel Have on Demineralised Water?
Gases in the ambient air not only compromise the purity of water by dissolving in it, but also by building up on the steel itself. Pollutants in the air accumulate on the surface of steel over time, meaning that no matter how pure the water is, it will usually always be contaminated if it comes into contact with metal.
This is why products like demineralised, deionised and distilled water are commonly stored in glass containers and distributed in glass tubes during transport. Unlike metal and even plastic containers, glassware is:
- Stable across a range of pH
- Difficult to oxidise
- Cheap and easy to manufacture
- Chemically inert
Some glassware products, like ampoules, can even be hermetically sealed to maintain the integrity of the product by protecting it from environmental contaminants. Storing demineralised water in glass containers is another effective and easy way to ensure that it stays as pure as possible.
For more information on demineralised water including its uses, benefits, how it’s made, specifications, hazards and MSDS, read our Complete Guide to Demineralised Water.
ReAgent stocks a range of purified water products to ensure that your business needs are being met. Whether you’re looking for demineralised, deionised, distilled or ultrapure water, we have the grade that’s right for you. All of our products are backed by a 100% quality guarantee, and we are even internationally recognised for our industry-leading services. Shop online today or contact us to see what we can do for you.
The blog on chemicals.co.uk and everything published on it is provided as an information resource only. The blog, its authors and affiliates accept no responsibility for any accident, injury or damage caused in part or directly from following the information provided on this website. We do not recommend using any chemical without first consulting the Material Safety Data Sheet which can be obtained from the manufacturer and following the safety advice and precautions on the product label. If you are in any doubt about health and safety issues please consult the Health & Safety Executive (HSE).