Corrosion also known as rust or oxidation is a common cause of failure in automatic fire sprinkler systems and fire hydrant systems in buildings and structures. According to Wikipedia, “Corrosion (rust) is a natural process that converts a refined metal into a more chemically stable form such as oxide, hydroxide, or sulfide.1” In simple terms Corrosion is “It is the gradual destruction of materials (usually a metal) by chemical and/or electrochemical reaction with their environment”.
Another form of corrosion is called microbial corrosion, also called microbiologically influenced corrosion (MIC), microbially induced corrosion (MIC). This form of corrosion is affected by the presence or activity (or both) of microorganisms in biofilms on the surface of the corroding material 2. This form of corrosion and how it relates to fire protection systems is subject to another article on this website.
To understand galvanic corrosion we need to understand some basic atomic principles, and chemistry. In this article we will seek to explain these principles in simple terms (as they relate to Fire Safety) and how these principles affect the way we design, install and maintain fire protection systems in buildings.
In chemistry, “noble metals” are metals that are typically more resistant to corrosion (oxidation) in moist air relative to another metal or alloy.
An example of a noble metal is Gold (AU) that is very resistant to corrosion. The other end of the spectrum is a metal like Magnesium (Mg), which comparatively is very susceptible to oxidation. Generally speaking all metals and alloys fall somewhere between these two common metals on the spectrum. This spectrum is called the Galvanic Series.
When two dissimilar metals are in contact with each other in the presence of an electrolyte, an electrical circuit is created and current flow will occur. The electrolyte can be water or some other solution or material that is conductive. This is the basis for how a battery operates.
The galvanic series (or electropotential series) determines the nobility of metals and semi-metals. When two metals are submerged in an electrolyte, while also electrically connected by some external conductor, the less noble (base) will experience galvanic corrosion. The rate of corrosion is determined by the electrolyte, the difference in nobility, and the relative areas of the anode and cathode exposed to the electrolyte. The difference can be measured as a difference in voltage potential: the less noble metal is the one with a lower (that is, more negative) electrode potential than the nobler one, and will function as the anode (electron or anion attractor) within the electrolyte device functioning as described above (a galvanic cell). Galvanic reaction is the principle upon which batteries are based.1
Over time, the pipework for automatic fire sprinkler systems and fire hydrant systems corrode and while they may look ok on the outside, on the inside there could be a blockage, leak or flood waiting to happen.
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It is inevitable for corrosion (oxidation) to occur within an automatic fire sprinkler, fire hydrant hydrant or fire hose reel system over time. For this to occur, there are always four factors that exist; An anode (less noble metal), in the presence of a cathode (more noble metal), and a conductive electrolyte (typically oxygenated water) that facilitates the flow of electrons between the anode and cathode.
The speed of the corrosion can also be accentuated by the following factors; (1) the difference between the less noble and more noble metals; (2) the electrical conductivity of the electrolyte; (3) the surface area of the anode and cathode (4) the installation environment; and an oxidiser.
Another way to put this is that corrosion may be accelerated by intermixing dissimilar metals such as black steel or galvanised steel (anode) in the presence of copper (cathode). This can also be affected by the electrolyte (water) present, such as “sea water” (higher level of dissolved sodium chloride) or “hard water” (water with a high content of dissolved calcium and magnesium in the water).
“If an installation requires contact between galvanized materials and copper or brass in a moist or humid environment, rapid corrosion of the zinc may occur.”
There are an array of corrosion prevention technologies that may be employed to help reduce corrosion however these technologies are not covered in this article.
There are three common types of failures caused by corrosion found in fire protection systems:
In fire protection we have a few simple strategies available to us that may be employed to reduce the cause and impact that corrosion plays.
Australian Standard AS1851 - Routine Servicing of Fire Protection Systems and Equipment does not detail any specific requirements for the adverse effects of corrosion on a fire protection system however there are some tests that are conducted that if done correctly can help identify the potential for damage caused by corrosion and further mitigation strategies that may be required. These include;
This has been a very high level overview of the cause of corrosion within a fire protection system and is intended to provide a guide for consideration for people designing, installing and maintaining fire protection systems in buildings.
If you are looking for more information about this topic, we have compiled a list of useful resources below that may be of benefit. If your building is in an area where we service and you're looking for competent and reliable advice, feel free to contact Firewize via our contact page and we will do our best to help.
In preparing this article, we came across a number of useful resources that we think you might find useful. Some of these have been include referenced in our footnotes in this article.