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If you’re in charge of a boiler, you likely know that boiler corrosion is the enemy. Corrosion is created when the iron in the system is exposed to water and oxygen. 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, the ferrous hydroxide will be continuously removed from the system until the metal has been completely dissolved!
This can create holes in economizers, boiler tubes or feedwater piping resulting in boiler leaks and even break downs. But not all corrosion is created 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 our 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 is commonly caused by boiler water pH is too high, steam blanketing (poor circulation) or 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 results from the mishandling of chemicals during acid cleaning or the boiler pH being run too low. This will passivate the carbon steel surfaces of the boiler. Boiler water pH should be a part of your logbook.
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 and 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 your free consultation.
