From Rasmussen Mechanical
If you’re in charge of a boiler, you likely know that boiler corrosion is the enemy. Exposure of iron in the system to water and oxygen creates corrosion. The metal reacts chemically and disintegrates, forming rust.
The Science of Corrosion
When Iron is in contact with water, they combine to form ferrous hydroxide.
Fe + 2H2O=Fe(OH)2 + 2H+
Iron + Water=Ferrous Hydroxide + Hydrogen
(The top layer of ferrous hydroxide protects the remaining iron.)
If dissolved oxygen is present, it combines with the ferrous hydroxide to form an insoluble compound, ferric hydroxide, which is rust.
4Fe (OH)2 + O2 + H2O=4Fe(OH)3
Ferrous Hydroxide + Oxygen=Ferric Hydroxide (Rust)
If the system continuously cycles dissolved oxygen, it will continuously remove ferrous hydroxide from the system until the metal has completely dissolved.
This can create holes in economizers, boiler tubes or feedwater piping resulting in boiler leaks and even break downs. But not every form of corrosion is equal. Let look at the different types of corrosion you can see in a boiler.
The Different Types of Corrosion
[Need help fighting boiler corrosion? Download Rasmussen’s Boiler Safety: Annual Inspection Checklist to help rid your boiler of any unwanted corrosion.]
Caustic Corrosion
When a concentrated caustic substance dissolves the protective magnetite layer of a boiler. This commonly occurs when the boiler water pH is too high, when there is steam blanketing (poor circulation), or during local ‘film boiling.’ If your boiler has a porous scale, then under deposit corrosion is also possible. Boiler water pH should be a part of your logbook.
Acidic Corrosion
This occurs due to mishandling of chemicals during acid cleaning or running the boiler pH too low. This will passivate the carbon steel surfaces of the boiler.
Pitting Corrosion
This is one of the most destructive types of boiler corrosion, as it can be hard to predict before a leak forms. Pitting is a localized form of corrosion. Either a local anodic point or more commonly a cathodic point, forms a small corrosion cell within the surrounding normal surface. Oxygen in feedwater is a common cause of boiler tube pitting. If your boiler is pitting, investigate the proper operation of your deaerator or feedwater tank and chemical treatment. If you have a hot water system, oxygen pitting can occur if the system has a leak and is bringing in freshwater.
Crevice Corrosion
This localized form of corrosion usually results from a crack in the boiler that does not get good circulation to rinse away caustic corrosion.
Galvanic Corrosion
Galvanic corrosion is the degradation of one metal near a joint or juncture. This occurs when two electrochemically dissimilar metals are in electrical contact in an electrolytic environment. So, dissimilar metals may need a special dielectric joint, sacrificial anode, or active cathodic protection system to prevent this phenomenon.
What Can You Do About Boiler Corrosion?
Even the most aggressive forms of prevention can’t stop minor corrosion from eventually happening. But, with the right approach, the effects of corrosion can be minimized to extend the life of your boiler.
Here’s what to do to minimize the effect of corrosion before they happen:
Use a boiler logbook. Regularly tracking the normal operation of your boiler room equipment makes it easy to spot when something critical changes. Deaerator pressure or feed-tank temperature changes will give advance warning of a more expensive corrosion problem. pH changes could indicate problems with water treatment or process contamination.
Treat feedwater. Additives can ensure that any oxygen that makes its way to the boiler in the feedwater is rapidly absorbed. These additives remove oxygen before it has the opportunity to form corrosive cells and blisters. Work with a good water chemistry company to stay on top of your boiler water.
Implement a regular service program to ensure the boiler stays clean and free of scale and corrosion problems. This will allow you to catch problems early before they turn into costly repairs.
Install a Deaerator to help remove gasses from feed water prior to entering the boiler.
Check for leaks and monitor the quantity of make-up water. Hot water heating systems shouldn’t need make-up water unless something is wrong. Call your service provider to fix the leak right away, or you may be replacing the boiler next year.
Here’s what to use after corrosion has already reared its ugly head:
Oxygen scavengers to prevent pitting
Scale inhibitors to prevent deposits
Alkalinity to control pH
Condensate line protection to control condensate pH
Train your crew on boiler preventative maintenance and water chemistry tests
Document and report any signs of corrosion to your boiler service provider and your water chemical company, so they can help prevent further damage.
Use our tips to ensure the longevity of your boiler. Need some expert advice or repair services? Contact Rasmussen Mechanical today to schedule a free consultation.
FAQs About Boiler Corrosion
1. Can a corroded boiler be repaired?
Yes, you can often repair a corroded boiler, but the extent of the repair depends on the severity of the corrosion. You can address minor corrosion by cleaning and treating affected areas. Extensive corrosion on the other hand may require the replacement of damaged components or even the entire boiler. Regular inspections and maintenance can help identify corrosion early, increasing the chances of successful repairs.
2. How do you treat corrosion in a boiler?
Boiler corrosion treatment involves using a combination of preventive measures and corrective actions. Prevention involves maintaining proper water chemistry, using corrosion inhibitors, and ensuring the pH levels of the water are within the recommended range. Corrective actions include cleaning and passivating the boiler’s interior surfaces, replacing corroded parts, and implementing water treatment programs to mitigate further corrosion.
3. How does corrosion affect the heat exchanger?
Corrosion can have a detrimental impact on the heat exchanger of a boiler. It can lead to the formation of rust and scale on the heat exchanger surfaces, reducing its efficiency. Over time, corrosion can cause leaks and cracks in the heat exchanger, leading to safety hazards and decreased heating performance. Regular maintenance and corrosion prevention measures are essential to prolong the life of the heat exchanger.
4. What are the signs of boiler failure?
Several signs may indicate impending boiler failure, including:
Reduced heating efficiency: A sudden drop in heating performance or increased energy consumption.
Unusual noises: Loud banging, hissing, or popping sounds coming from the boiler.
Rust-colored water: Discolored or rusty water in the system or on the boiler’s exterior.
Leaks: Visible water or steam leaks around the boiler or its piping.
Pressure fluctuations: Frequent pressure fluctuations or irregular pressure readings.
Decreased heat output: Inadequate heating or inconsistent temperature control.
Frequent breakdowns: Repeated malfunctions or boiler shutdowns.
If you observe any of these signs, conducting a thorough inspection becomes crucial. Address the underlying issues promptly to prevent further damage and ensure boiler safety and efficiency.
Corrosion of Materials Used in Steam Generating Boiler Systems
Below is a summary of a study completed by the US Department of Commerce for the US Department of Energy.
“Five alloys, SA178, SA192, SA213-T11, SA213-T22, and Type 304 Stainless Steel, were evaluated on their resistance to pitting. This was done in a coal burning boiler and in a residential refuse burning incinerator. The materials were introduced into the vicinity of the boiler tubes using a probe. The probes temperature was controlled and monitored to simulate conditions of the boiler tubes. After three to six months, the probes were withdrawn and the alloy specimens removed for evaluation.
The data indicate that the environment of the refuse burning incinerator was considerably more aggressive than that of the coal burning boiler. Chloride was found in practically all the pits examined in the alloys from the refuse burning system. No chloride was found in the pits examined on the materials exposed in the coal burning boiler. The data suggest that the moisture from lawn clippings increases the rate of attack which is further aggravated by large temperature fluctuations. Type 304 stainless steel was the most resistant to pitting in both environments. SA213-T11 and SA213-T22 were less resistant to pitting than the lower alloy SA178 and SA192 in the refuse burning incinerator.” Corrosion of Materials Used in Steam Generating Boiler Systems E. Escalante, D. Mathews, J. Fink National Bureau of Standards Gaithersburg, MD 20899
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