Because the air holding the valve closed contains oxygen and moisture, and residual water remains in the pipe.

That combination attacks steel from the inside where nobody can see it.

Testing and previous trips leave water at low points regardless of grading.

Wet and dry surfaces alternating inside the pipe accelerate the process.

Systems have failed prematurely on a large scale for this reason.

It is a problem of the technology rather than of any particular installation's quality.

Corrosion debris obstructing sprinkler heads - a failure nobody discovers until the system is asked to work.

Pinhole leaks are the obvious symptom and at least announce themselves.

Loose scale travels through the pipework and lodges in heads and small orifices.

An obstructed head is a head that will not discharge properly on a fire.

Obstruction investigations are required periodically in many regimes precisely because of this.

The debris also fouls valves, switches and trim components.

It supplies nitrogen instead of air, so the oxygen that drives the corrosion is largely removed.

Nitrogen is inert enough that the reaction slows dramatically.

Systems either purge and fill to a high nitrogen purity, or maintain a supply that displaces the remaining air over time.

Purity is monitored, since a system slowly readmitting air loses the benefit.

It does not reverse existing corrosion, only slows further damage.

It is most valuable on new systems and on existing ones caught early.

It regulates supply pressure and restricts how much fresh air enters, so oxygen is not continuously replenished.

A leaking system otherwise draws in new air and moisture constantly.

Restricting that slows the corrosion without removing the oxygen already present.

It is far cheaper and simpler than a nitrogen generator.

It also prevents over-pressurisation of the system from an unregulated compressor.

For smaller systems it is often the proportionate control.

Large dry systems, high-value contents, and any existing system already showing corrosion.

A large warehouse dry system represents a substantial pipework asset worth protecting.

Systems where replacement would be extremely disruptive justify it on that basis.

An existing system with leaks or scale found at inspection is the classic retrofit case.

A small dry system on a canopy usually does not justify the cost.

Insurers sometimes drive the decision where corrosion has already caused losses.

Both can, and retrofit is most of the market because the problem is usually discovered rather than anticipated.

A leak, an internal inspection finding scale, or an obstruction investigation is typically what prompts it.

Retrofitting nitrogen involves purging the existing air, which takes time.

Air maintenance devices are a straightforward addition to the air supply.

Neither addresses corrosion that has already occurred.

Where damage is advanced, pipework replacement may be needed regardless.

The regime is unchanged, but internal inspection findings are what usually justify these devices.

Periodic internal inspection of pipework is required in most regimes.

Obstruction investigations are triggered by specific findings such as debris or corrosion.

Nitrogen purity monitoring becomes an additional routine check where a generator is fitted.

Compressor run time remains a useful indicator of leakage in either case.

The governing document is the local standard, and it differs enough between markets to be worth checking.