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 installation sit inside the pipe together. Oxygen oxidises the steel; as it is consumed the reaction slows. 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 pre-action systems are therefore more exposed to corrosion than wet ones: the pipe holds both oxygen and condensing moisture, and water stands at low points that never drain fully.
Microbiologically influenced corrosion (MIC)
MIC is pitting corrosion of the pipe wall caused by bacterial activity. Instead of general thinning you see local pinhole perforations — which is why wall thickness measurement can miss MIC. Typical signs:
- A local tubercle-shaped deposit on the outside of the pipe with a hole beneath it.
- Black, foul-smelling sediment in drained water.
- Repeat leaks at short intervals on the same run.
Source water quality is decisive: well water, stagnant tank water 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 |
|---|---|
| Pressure slow to recover on a main drain test | Partial blockage in the supply line |
| 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, repeat leaks or observed sediment.
- Choose sampling points. The most remote point, the lowest point, and a point near the supply entry.
- Open the pipe and look inside. 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?
- Decide. Flushing, chemical cleaning, partial replacement or full renewal.
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.
- Air dryers or nitrogen on dry systems. These reduce both moisture and oxygen.
- 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.
- Material selection. With risky water quality, consider a suitable alternative to galvanised pipe; where the galvanised coating is damaged, local corrosion can accelerate.
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 and whether hydraulic performance is still intact.
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?
A tubercle-shaped deposit on the outside of the pipe with a pinhole beneath it, black smelly sediment in drained water, and repeat leaks at short intervals on the same run. 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, closing out leaks, and using air dryers or nitrogen on dry systems.
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Download MEP Calc on the App StoreNFPA 25 · NFPA 20 (2025) · NFPA 13 (2025) · BS EN 12845:2015+A1:2019 · FM Global DS 2-0, 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.