Corrosion is the most insidious failure in a sprinkler system: invisible for years, the system looks like it is "working", and in a fire a blocked pipe or a perforated line appears.
Why it happens
In a wet system, water and the air trapped during filling sit inside the pipe together. Oxygen oxidises the steel; FM DS 2-1 (2.1) notes that in a wet system the oxygen is consumed over a relatively short period and corrosion ceases until fresh air or water is reintroduced. The problem is oxygen being continuously renewed:
- The system is drained and refilled often (testing, refurbishment, faults).
- Fresh water enters continuously because of a leak.
- In a dry system, moist compressed air carries oxygen continuously.
Dry and preaction systems are therefore more exposed to corrosion than wet ones (FM DS 2-1 2.1: in a dry system supervised with compressed air, water left in the piping causes rapid corrosion): the pipe holds both oxygen and condensing moisture, and water stands at low points that never drain fully.
Microbiologically influenced corrosion (MIC)
MIC is corrosion initiated or accelerated by a biofilm of microorganisms on the pipe surface. The typical damage is pitting under tubercles inside the pipe, ending in a pinhole leak (FM DS 2-1 3.8.2). Because you see local perforations rather than general thinning, wall thickness measurement can miss MIC. In the cases FM examined, MIC accounted for 10–20% of corrosion damage in fire sprinkler systems. Possible signs:
- Tubercle-shaped deposits on the inside of the pipe with pitting beneath them.
- Black, foul-smelling sediment in drained water.
- Repeat leaks at short intervals on the same run.
These signs alone do not prove MIC: pitting under tubercles also has non-microbial causes, and confirming MIC requires chemical, microbiological and metallurgical evaluation (FM DS 2-1 3.8.4). Source water quality is decisive: untreated water (which FM DS 2-1 2.2.1.11 says to avoid), stagnant water and dead ends (3.8.2), and pipework commissioned without flushing all raise the risk.
Blockage is the critical point. Corrosion products accumulate inside the pipe and narrow its bore. Small-diameter branch lines and sprinkler inlets block first. The hydraulic calculation assumes clean pipe; in a system with sediment build-up the real flow is below the calculated one.
Symptoms and what they mean
| Symptom | Likely meaning |
|---|---|
| Residual pressure on a main drain test clearly lower than in previous tests | An obstruction or partly closed valve in the supply upstream of the riser (the main drain test only checks upstream of the riser — FM DS 2-81) |
| Black, smelly sediment in drained water | MIC or advanced sediment build-up |
| Orange-brown sediment in drained water | General oxygen corrosion |
| Frequent compressor running on a dry system | Air leakage, circulating moisture and oxygen |
| Repeat leaks at low points | Local corrosion caused by standing water |
| Jockey pump running continuously | A system leak, bringing in fresh water and accelerating corrosion |
How an obstruction investigation is run
- Identify the trigger. An anomaly on the main drain test, pinhole leaks or observed sediment. FM DS 2-1 (2.2.1.5) calls for an obstruction investigation to be carried out promptly when these appear.
- Choose sampling points. The most remote point, the lowest point, and a point near the supply entry.
- Open the pipe and look inside. Inspect a sufficient sample of piping with a video borescope (FM DS 2-1 2.2.1.5 A); record sediment thickness, type and distribution, and photograph it.
- Remove sprinklers and check them. Is the inlet orifice blocked?
- Classify the finding. Surface deposit, sediment narrowing the bore, or active corrosion? Have a metallurgical examination done on a sample of the affected pipe to identify the mechanism (2.2.1.6).
- Decide. If obstructions are found, flush the whole system and replace any pipe section whose obstructions are not dislodged by flushing (2.2.1.5 B–C). Replace pipe whose remaining wall in any single pit is below Table 1 (Schedule 40: 25%, Schedule 10: 50%, Schedule 5: 75%) (2.2.1.8). Do not use chemical cleaners or corrosion inhibitors (2.2.1.11 B; 3.8.5).
Prevention
- Flush at commissioning. Installation debris, weld slag and soil are the seeds of corrosion.
- Do not drain unnecessarily. Every drain-and-fill cycle brings in new oxygen.
- Close out leaks. A continuously running jockey pump is a corrosion warning.
- Nitrogen or dried air on dry systems. Pressurising with an approved nitrogen generator reduces both moisture and oxygen (FM DS 2-1 2.2.1.12.1); an air-drying system reduces moisture only — the supply air dew point should be 20 °F (6 °C) below the lowest expected room temperature (2.2.1.12.2 F).
- Drains at low points. On a dry system, every point that does not drain fully is a corrosion focus.
- Air venting. Removing trapped air from a wet system reduces the oxygen source; FM DS 2-1 (2.2.1.9) calls for automatic air-release valves of at least 1/2 in. (13 mm) at system high points and removing the air after every drain-and-refill.
- Material selection. FM DS 2-1 says not to install internally galvanised steel pipe in wet-pipe systems (2.2.1.4), while for dry and preaction systems not using nitrogen it recommends galvanised or polymer-enhanced steel pipe (2.2.1.12.2 A). Water left in galvanised pipe can cause through-wall leaks within a few years through zinc and then steel corrosion (3.6).
The renewal decision
A single leak does not call for renewal, but repeat leaks in the same area over a short period show the general condition of the system. The decision rests not on the number of leaks but on the obstruction investigation findings, the remaining wall thickness and whether hydraulic performance is still intact. Repeat the obstruction inspection annually until the corrosion is under control (FM DS 2-1 2.2.1.7).
Frequently Asked Questions
Why do dry systems corrode more than wet ones?
The pipe holds both oxygen and condensing moisture, and water stands at low points that never drain fully. Moist compressed air carries fresh oxygen continuously.
How is MIC recognised?
Tubercle-shaped deposits inside the pipe with a pinhole beneath them, black smelly sediment in drained water, and repeat leaks at short intervals on the same run are possible signs; confirmation requires chemical, microbiological and metallurgical evaluation. General thickness measurement can miss it.
How does corrosion affect the hydraulic calculation?
The calculation assumes clean pipe. Sediment narrows the bore and real flow falls below the calculated value; small-diameter branches and sprinkler inlets block first.
What is the most effective prevention?
Flushing at commissioning, avoiding unnecessary drain-and-fill cycles, venting trapped air from wet systems, closing out leaks, and using nitrogen or dried air on dry systems.
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Download MEP Calc on the App StoreNFPA 25 · NFPA 20 (2025) · NFPA 13 (2025) · EN 12845:2015+A2:2026 · FM Global DS 2-0, DS 2-1, DS 2-81, DS 3-7. The findings here are typical defect patterns defined in the standards and survey guidance, not an account of events at any particular site.