Galvanic Corrosion (rust) in Automatic Fire Sprinkler & Hydrant Systems

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.Wikipedia 2020 “Galvanic series” 27, August 2019 - https://en.wikipedia.org/wiki/Galvanic_series

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:

1. Pitting in pipe walls and joints that leads to perforations and leaks; and
2. Excessive corrosion products and scale including tuberculation that adversely affect the the efficient flow of water through a fire protection system; and
3. Catastrophic failure of a component of a fire protection system and associated equipment. This can affect the structural integrity of the system under high water pressure or demand.

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.  

1. Minimise the use of dissimilar metals used in the installation of the fire protection systems. For example avoid unnecessarily mixing galvanised steel pipe with copper pipe.
2. Ensure that we are using the correct forms of pipe support and avoid dissimilar metals coming into contact with one another.
3. Where exposed pipe may come in contact with other metals, paint may act as a suitable barrier to help slow the galvanic reaction.
4. If two metals are required to be connected together, electrical isolation (typically in the form of a gasket) between the two metals must be employed.
5. Conduct a periodic review of the water supply, automatic fire sprinkler, fire hydrant and hose reel systems in the building.  Firewize can provide a systematic survey of your building to identify the early, intermediate or late onset affects of corrosion. 

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;

1. Periodic survey of the fire protection system including pipe supports for any condition that could adversely affect the function of the system.  This includes obvious signs of corrosion that may be present;
2. Checking and cleaning of in-line strainers for evidence of corrosion;
3. Operation of all water supply stop valves including backflow prevention stop valves and verify they are fully open, secure in the open position (relaxed 1⁄4 turn if appropriate) and are correctly labelled as indicated on the valve list. This helps loosen any corrosion that may have been built up in the preceding period and helps enable the valve to be fully closed;
4. Flow test or the water supply proving test to verify the performance of the fire protection system has not been adversely affected in the previous 12 months, typically due to a partially closed valve or the accumulation of debris within the water supply.
5. For water supplies such as rivers, lakes, dams, etc check the suction inlet strainer(s) or screen(s) for any condition likely to affect its function.  This could include debris, waste or corrosion;
6. Verify that pressure readings on the low pressure side of the valves are within the range stated at the pressure-reducing valve station and pressure gauge schedule. This could change if there is corrosion build up within the pressure gauge;
7. Verify the operation of pressure-relief valve or pressure reducing valve and note operating pressure is within the range stated on the nameplate provided at the pressure gauge schedule.  The accuracy of a pressure relief or pressure reducing valves may be adversely affected by corrosion.
8. Check and test each pressure switch or flow switch and ensure that the cover is in place, correctly labelled, securely mounted and free from any condition likely to adversely affect its function. Like pressure gauges both pressure switches and flow switches are vulnerable to not working correctly due to  the presence of corrosion.

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.

• Corrosion of Mixed Metal Fire Sprinkler Systems, Copper Development Association Inc.
• How To Prevent Corrosion in Fire Sprinkler Systems, Facilitiesnet
• Electrochemistry, Khan Academy
• My Fire Sprinkler Pipes are Corroded. Now What?, Quick Response Fire Supply
• YouTube: https://www.youtube.com/watch?v=aEwD8lPdtoA