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Fire Sprinkler Corrosion

Corrosion activity in fire systems is greatly dependent upon a variety of factors related to its function, design, construction, materials of construction, pipe quality, testing frequently, renovation performed, and overall maintenance.  In most examples, the fire system will provide decades of reliable and trouble free service.  But for others, problems can arise within just a few years.

In an unfortunately large number of examples, and primarily for dry and pre-action systems, corrosion can quickly deteriorate the piping to the point requiring replacement.  In the worst of those cases, as we have documented in many of our own investigations, the fire system was likely not functional when the need for replacement was finally discovered – a frightening reality most prefer not to consider…

While no introduction to corrosion in fire protection systems is likely necessary to other professionals in the field, we offer a large Photo Gallery on this site showing specific problem areas for wet and dry fire systems, as well as for fire water storage tanks.

Various responses to the problem of corrosion at fire systems have been considered and some put into practice.  Inherent to the difficulty of employing any proposed solution is the design layout of fire systems, and the fact that most fire systems are one way networks of mostly dead end pipe sections – thereby limiting the movement and effectiveness of any mitigation measure throughout the entire system.

We offer below various corrosion control strategies and options along with what we view as the pros and cons of each.  Most have been tried with varying levels of success.  For some the theory is excellent but the practical aspect of making it work is questionable.

Possible Preventative Actions

• Galvanized Steel Pipe

The reasonable response to the corrosion threat at any fire system is to use galvanized steel pipe.  Galvanized steel can still be found at very old threaded fire systems from the 1920’s in still near new condition, and with 100 or more years of service life remaining.

A significant volume of dry and pre-action fire systems are now constructed of galvanized steel due to this presumption that it will not rust.  In fact, today’s galvanized piping products are not what they were decades ago and suffer accelerated failure often negating any possible benefit.

We have documented failed galvanized steel dry fire systems in under 2 years.  Although foreign produced galvanized steel pipe and thin wall schedule 10 are important contributing factors, there is no question that today’s galvanized steel pipe products will not provide the level of protection expected.

This is due to the widely divergent mechanism of corrosion for galvanized pipe.  Where the zinc protective finish holds, the pipe will last.  Wall thickness measurements decades later will show the pipe still at new specifications.

Where failed, however, all potential corrosive forces that would otherwise attack the entire surface of carbon steel pipe to a more generalized degree now attack the galvanized finish in selective areas.  Any area of weakened finish becomes a focus site for pitting activity that can easily exceed 50 MPY.

Generalized corrosion against steel pipe may take decades where the same wall loss focused in one or a few areas may produce drill bit like pipe wall penetrations through galvanized pipe in just a few years.

One clear benefit of galvanized pipe is the far lower internal deposits produced as a result of any corrosion mechanism, and therefore the lower threat of constriction and loss of water flow.

The higher rate of failure of newer galvanized steel fire sprinkler systems has resulted in a shift back toward the use of carbon steel.

• Heavier Schedule 40 Pipe

Although not in any way reducing the level of corrosion activity which may be present, heavier pipe will provide greater service in comparison to thinner pipe under the same corrosion conditions.

The trend toward using thin wall schedule 10 and even schedule 7 pipe in all fire systems has been probably the largest factor in the more advanced failures we investigate.  Proponents arguing greater water flow, lower cost, and less shipping weight for the same pipe size fail to recognize the substantially lower wall thickness available – 50% less for schedule 10 pipe in comparison to schedule 40.

Returning to the use of standard schedule 40 pipe is a trend we have been seeing lately, although limited.  Substantially higher material costs exist, although with approximately double the pipe wall thickness, greater service life is assured.

One concern is that while lasting longer under high corrosion conditions, a longer period of time is also available for internal rust deposits to accumulate.  Standard flushing techniques are useless at moving heavy iron oxide rust deposits along long horizontal dead end runs – thereby allowing most corrosion problems to remain and accumulate.

We favor the use of heavier schedule 40 pipe for all fire protection applications and especially where threaded.  We also strongly recommend close monitoring of internal corrosion activity in order that the heavier pipe provided is not also creating a larger hidden threat.

• Nitrogen

Dry fire systems are under significantly greater threat due to the abundant air and oxygen above the remaining water to drive the corrosion mechanism to maximum rates.  And with every test cycle, new air and oxygen is introduced to increase corrosion activity further.

Recognizing that the air covering zones of trapped water is the mechanism of pipe destruction, an obvious focus has been to replace the air with an inert gas – nitrogen.

Adding a nitrogen blanket to stop the effects of air supported corrosion is a well proven method used in various other industries such as nuclear and petrochemical.  It promises some benefit to the fire protection industry but requires a greater than 95% replacement of the air with nitrogen.

As the nitrogen is introduced, it mixes and dilutes the existing air volume rather than simply replacing it in a laminar flow as might occur if sent through a very small diameter 1 mm hose, for example.  Creating a nitrogen environment may require extensive gas product and venting of the air/ nitrogen mixture depending upon the size of the system.  Bringing compressed nitrogen bottles into a site is one added difficulty.

Larger systems not only require greater nitrogen in volume, but require more time due to slower dilution by the original nitrogen supply.  If accomplished, the oxygen available above to the water may be removed and corrosion rates slowed.  Corrosion activity below the large volumes of water in most “dry” fire systems, however, remains to continue deteriorating as a wet fire system.

• Antifreeze

The threat of freezing is the primary concern resulting in the design of “dry” fire systems, although other reasons exist.

Commonly installed in parking garages, loading docks, and other exposed areas in colder climates, a very reasonable alternative to removing the water would be to instead fill the system with antifreeze.  Antifreeze is very well inhibited to stop corrosion and if maintained will control corrosion to under 0.5 MPY.  It is commonly used in circulating cold water systems with outstanding results.

Antifreeze use in any piping system is highly regulated by state and federal authorities due to toxicity should it be drained, and therefore requires higher grades known as hydronic fluid at higher cost.  Only food grade propylene glycol products are typically allowed.

Antifreeze based fire protection systems are not widely used, but are an approved fire system design – with one very critical flaw.  The propylene glycol antifreeze commonly used is a petroleum based material which is highly flammable.

Furthermore, concentrated antifreeze is potentially explosive when atomized or turned into a fine mist, as would occur if discharged under pressure through a fire sprinkler head.  Warning to the potential explosive nature of antifreeze if atomized and ignited are provided with every product MSDS sheet.

In a prior example at a loading dock for a downtown New York City office building, a small truck engine fire became an inferno as soon as the overhead fire sprinkler system was activated.  Discharging antifreeze accelerated and spread the fire throughout the loading dock area until a combination of fresh water replacing the antifreeze flow and the NYC fire department extinguished the blaze.

In a more recent 2009 example from Califormia, a small commercial kitchen frying pan fire activated an overhead sprinkler head.  A violent explosion resulted – killing one individual and hospitalizing 4 others with severe burns.  Metal doors were blown from their concrete block frames and window frames blown 86 ft. across an adjacent parking area.  Initial impression of the scene was such that local authorities called in bomb squad and ATF explosives units thinking a deliberate murder had taken place.

In fact, it was the antifreeze in the fire system which atomized and then ignited to cause the blast.  After a thorough investigation, a higher than recommended concentration of antifreeze was suspected as the cause.

This leaves the use of such antifreeze systems a risky effort at best.  Safe application requires maintaining the glycol concentration precisely.  Too low and freezing may occur, along with little protection against the corrosion activity it was intended to provide.

Too high, and far more damage may result than if no fire system was even installed.

• Air Drying

As an electro-chemical reaction, corrosion can only occur in the present of an electrolyte such as water or moisture.  Corrosion control chemicals typically add a monomolecular film at the metal surface to resist the movement of electrons.  Where no water or moisture exists, corrosion is prevented.

Turning a “dry” fire system into a true dry fire system is one method having a theoretical 100% effectiveness.  Dry fire systems are never dry, of course, and some dry systems allow as much as 50% of water to remain in some examples – producing a large variable to the extent and hidden unknown for any fire system.  For a large and poorly draining dry fire system, 1,000 or more gallons of water may remain trapped within its network of lines.

Drying away this water is possible through the introduction of large volumes of ultra dry air vented through the furthermost extent of each fire zone.  Rough calculations suggest a time frame in weeks to months based upon reasonable assumptions of remaining water and reasonable air flow rates.

The unfortunate flaw to this option is the need to water flow test dry fire systems on a regular basis as per current fire codes.  This same introduction of fresh oxygenated water, which is a source of deterioration to all fire systems, then replaces water just dried from the system.

Depending upon the finer characteristics of a dry fire system and the volume of water which is regularly replenished, no drying method may be sufficient to produce any worthwhile benefit.

• Chemical Protection

Anti-corrosion chemicals have more than proven their effectiveness in circulating water systems.  For closed systems especially, they can extend the useful life of most piping to virtually unlimited.

It is the impossibility of providing a chemical inhibitor to all extremities of the one way network of branch lines and run-outs to the individual sprinkler heads which leads the difficulty in applying this option.  The chemicals are generally excellent and very effective; the problem being getting the chemical throughout the system.

Most chemical inhibitors have a shelf life or effectiveness of only a few years – meaning that it may not have any protective capacity once finally reaching the furthermost dead end zones.

Batch fed chemicals to an inlet line may prevent or slow corrosion in that immediate environment, but will not assist where it cannot reach.

As for most issues with fire protection systems, it is one thing to state or specify the initial cleaning, disinfection, or flushing of rust deposits from a fire system, and an entirely different issue to fully succeed in that task.

• Internal Coating

The ultimate solution to corrosion is to protect the metal from any contact or interaction with the water.  This is difficult with larger piping systems which are welded since any heat will destroy the internal coating near the weld zone.

For fire protection systems typically assembled from pre-fabricated pipe sections, internal coating is a far more reasonable alternative to ensure extended service life.  Once the pipe is cut, grooved, and with its side ports welded on, it can be sent out for cleaning and the internal coating then factory applied.

When assembled by threads or clamped grooved construction methods, no damage is done to the internal coating and the pipe can then provide long corrosion free service.  While there is an added cost for the coating and shipping, it is unquestionably lower than fully replacing the entire fire system should a severe corrosion problem develop a few years later.

• Engineering And Design

Many corrosion problems we document at fire protection systems are self-inflicted through design and maintenance.

Drains for dry fire systems are inadequate, improperly installed and rarely maintained.  For wet systems, air vents are lacking.

Flow testing, of course, is the fundamental source of most problems.  Substantially inadequate pitch for dry systems allows more water to remain, in the same way that ascending pitch at the roof of many wet systems trap air to create a partially dry system.

Cleaning and flushing fire systems may be defined, but is practically impossible to fulfill to any degree of effectiveness.

Welding should not be performed at galvanized pipe without then re-galvanizing it to protect the now burned off interior zinc coating.  Spray painting the outside of the pipe with silver paint or cold galvanizing compound simply does not provide any benefit.

Thin wall pipe, and even threaded schedule 7 produces an almost guaranteed failure months to a few years in the future except under the lowest corrosion conditions.  Add in low cost foreign pipe products, seamed pipe, higher corrosion vulnerability compared to products used decades ago, and the current higher level of failure is not at all difficult to explain.

© Copyright 2023 – William P. Duncan, CorrView International, LLC