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Any system is prone to corrosion when put into a state of non-operation. For boilers, shutdown can occur for various reasons, including equipment inspection, maintenance outages, fluctuating production requirements, or seasonal and holiday layups. When the boiler is started again, a spike in iron levels in the feedwater or condensate samples signals unchecked corrosion and potential short- and long-term service challenges. Corrosion protection is therefore critical to successful layups and startups.

The need for boiler layup is driven by an institution’s demand for steam, the key byproduct of boiler operation. Steam demand fluctuates on a seasonal basis. If out-of-use systems are not adequately protected, corrosion and iron can affect a water treatment chemical program, tie up the system and become a foulant. Iron chips can plug up sensor lines and interfere with heat transfer efficiency. Corrosion sites also could become eventual points of leakage.

One of the most common corrosion-protection methods during boiler layup is applying desiccant to a dry, empty boiler. Desiccant may seem like an inexpensive commodity chemical for layup. This layup philosophy, however, hinges on the ability of desiccants to remove moisture from the air and limit corrosion by keeping the space dry. Protection, therefore, is limited and depends on the adequate absorption of moisture, which means the desiccant may need to be removed and replaced periodically. Otherwise, the system may be unprotected (if the desiccant cannot absorb any more moisture). In the worse cases, a saturated desiccant can lead to corrosion at the point of metal contact if not removed quickly enough.

2 PHC 1022 Cortec Controlling Corrosion on Fireside

Extreme corrosion on the fireside affects the integrity of the boiler waterside from the outside in. Proper layup of both is important. | Photo credit: Cortec Corp.

Desiccant also needs to be removed before startup. This step is sometimes forgotten until it is too late, unfortunately. If the boiler has been refilled and started with the desiccant inside, the operator will have a serious mess to clean up.

Nitrogen blanketing is another method used for boiler layup. While it can be effective for displacing oxygen from the boiler internals with inert gas, success is dependent upon maintaining an airtight seal and constant pressure and removing all moisture. If a leak occurs, the nitrogen will need to be reapplied on an almost continual basis. Furthermore, nitrogen leaks can pose a threat to worker safety, and the nitrogen must be purged from the system should there be any reason to enter the boiler during the layup period.

An alternative to desiccants and nitrogen blanketing is vapor-corrosion inhibitor technology. Vapor-phase corrosion inhibitor chemistry provides active corrosion protection; by contrast, moisture-absorption or oxygen-displacement methods attempt to remove the corrosive element but cannot do anything further if moisture or leaks enter the system. Also, vapor-phase corrosion inhibitor layup materials typically do not need to be removed before startup.

3 PHC 1022 Cortec Controlling Corrosion Failed Firetube

A failed firetube was removed from a fireside boiler that had been shut down for several months without preservation. The external surface shows pitting corrosion, likely due to an oxygen attack on the waterside. This is an example of what can happen when measures are not taken to protect against an increase in dissolved oxygen during startup. | Photo credit: Cortec Corp.

Vapor-phase corrosion inhibitor technology comes in multiple forms, but the underlying concept is the same: The inhibitors vaporize and disperse throughout a void space. This allows them to travel to any exposed areas within the enclosure and form a molecular corrosion-inhibiting layer on the metal surfaces, to which they are naturally attracted. As long as the space remains closed, the vapor-phase corrosion inhibitor remains inside, and corrosion protection continues. Protection is active even in the presence of residual moisture and oxygen, eliminating concerns that the boiler must remain completely dry.

One form of dry layup is a water-soluble tube containing vapor-phase corrosion-inhibitor powder. This tube can be placed inside the boiler and slit open to allow the protective vapors to escape. When the boiler is refilled, the water-soluble packaging and the vapor-phase corrosion inhibitors dissolve in the makeup water. Product removal typically is unnecessary.

In some cases, depending on boiler size, metal type or boiler components, a different form of vapor-phase corrosion inhibitor may be preferred. Very large boilers often are protected by fogging a waterborne corrosion inhibitor into the boiler internals. Fogging of vapor-phase corrosion inhibitors also may be preferable for small boilers (100 gal [380 L] or less) or for protecting fire-side components where the tube-based product would have to be removed manually.

4 PHC 1022 Cortec Controlling Corrosion Closeup of Failed Tube with Oxygen Pitting

This tube failed due to oxygen pitting. | Photo credit: Cortec Corp.

Startup Situations

Boiler layup is an appropriate time to prepare for startup concerns that can arise in the period between filling and firing off. Boiler startup can be a sensitive time because the normal chemical water treatment program has not yet been started, and feedwater often is unheated. This results in a higher level of dissolved oxygen, which raises the risk of pitting corrosion.

Boiler operators can prepare for this by adding an oxygen scavenger or metal passivator during layup. Some passivators are packaged in a water-soluble bag to simplify adding it when the vapor-phase corrosion inhibitor is applied. This passivation product remains intact and undiluted until the boiler is refilled. Then, the packaging dissolves, and the corrosion protection product passivates the metal and scavenges oxygen.

The prepackaged passivator can be added to the condensate receivers, feedwater tank and deaerator tanks. Adding a passivator product in those locations helps protect the system. (Even one unprotected line could contribute to a large amount of iron transported to the boilers at startup.) Adding a passivator product also can help in the not-uncommon situation where boilers are brought back into service over a weekend when the chemical pumps may not be plugged in, exacerbating the corrosion problem. Planning ahead helps ensure good startup chemistry and vapor-phase corrosion-inhibitor application, and it does not require followup, maintenance or product removal if the boiler is brought into service unexpectedly.

5 PHC 1022 Cortec Controlling Corrosion Waterside of Firetube Boiler

The waterside of a firetube boiler that was shut down and left full of water is shown. Brown areas show where the pipes and walls were covered with water. The orange line shows where corrosion has occurred above the water level because the boiler was not laid up with a vapor-phase corrosion inhibitor. | Photo credit: Cortec Corp.

Wet Layup Tips

While dry layup generally is recommended for a seasonal or long-term layup, wet layup sometimes is required. One common example is to ensure boiler redundancy, especially for facilities that operate continuously and need a contingency plan if one boiler goes offline. Keeping the backup boilers full and on low fire eliminates the time needed to refill and reheat the water in the case of emergency failure or repair on another unit.

One method of wet layup is a cascading blowdown. Normal water treatment boiler controls mean that every operating boiler has some water bled out during operation. That treated water can be piped into an offline boiler to keep it protected with the same active chemical program as the operating boiler. While this method can work, it is complicated, and protection may be inadequate. Isolated areas of corrosion can occur if not properly monitored.

The most common wet layup treatment is maintaining a high pH and scavenging oxygen by adding a high level of sulfite. The downfall of this approach is that it is maintenance intensive. If sulfite and pH levels are not monitored frequently, the result can be inadequate treatment and corrosion. Unfortunately, this maintenance task is easy to overlook, leading to the failure of the treatment program.

Operators can eliminate frequent monitoring of standby boilers by using multi-phase inhibitors; that is, a combination of waterborne contact and vapor-phase corrosion inhibitors. These organic chemistries offer comprehensive treatment because they protect metals below and above the water level. Traditional corrosion inhibitors used during boiler operation generally only protect metal surfaces in direct contact with the water. When using multi-phase inhibitors, while most of the protection takes place on the water-contacting surfaces, the vapor component diffuses into the air to form a protective molecular layer on overhead surfaces that are vulnerable to corrosion from water vapor condensation.

While some of the same vapor corrosion inhibitor formulations used for dry boiler layup could theoretically be used in these applications, it is better to use those designed for wet layup because of dosing differences. For example, using a dry layup vapor corrosion inhibitor powder to perform a wet layup would require four times as much product to chemically saturate the water to a point where the remaining chemistry could diffuse and protect the void space above the water. Liquid corrosion inhibitor blends specially formulated for wet layup are the best option when used as designed.

For boilers on standby, the operator must choose a corrosion inhibitor package that can withstand the moderately high temperatures of a boiler kept on low fire. Whether or not the water is hot, wet layup vapor corrosion inhibitors and contact corrosion inhibitor blends are generally compatible with normal water treatment programs and do not need to be drained or flushed out before startup. Operators can simply follow their normal startup procedures and proceed with the normal treatment program minus the corrosion problems.

In conclusion, taking measures to prevent corrosion during layup is important to maximize asset service life and reduce problems. Low maintenance layup with vapor-phase corrosion inhibitor technology provides effective boiler protection in dry and wet layup.