Boiler Corrosion & Its Effects
Boiler corrosion causes half of all forced outages and almost all tube failures of boilers in power plants, making it crucial to prevent. However, power generation plants are far from the only ones affected by corrosion and failure of boilers. Any facility that uses steam boilers can experience disruptions from corrosion damage.
Since repairing the effects of corrosion is both costly and challenging, you’ll want to take steps to prevent corrosion. By reducing corrosion, you can save on repair and replacement costs. Besides helping your facility save on money in repair costs, you can reduce downtime by ensuring your boiler stays free from corrosion.
As you look to reduce boiler corrosion, review some of the primary corrosion causes and learn more about the effects of corrosion. Finally, you can create a plan to prevent boiler corrosion in your facility.
Corrosion has several causes, but many relate to the chemistry of the water. Acidity and dissolved oxygen and solids can contribute to corrosion in a boiler. Maintaining a balance of these substances can prevent damage to the system if you understand why out-of-range levels can create metal pitting.
Some of the top corrosion causes include the following:
1. Dissolved Oxygen
In the presence of oxygen, steel breaks down into either insoluble or soluble iron compounds. Oxygen will cause pitting in the preboiler section and in the tubes. Scavenging the oxygen through the use of catalyzed sodium sulfite or hydrazine chemically removes oxygen. This process helps either instead of mechanical deaeration or after the process.
Acidity also impacts the corrosion of boiler material. Acid corrosion often occurs in the condensate return portion of the system. A low pH level, indicating more acidic water, is among the top three causes of corrosion, along with dissolved oxygen and weakened metal areas. Though water seems benign, dissolved gases can affect its pH level, which leads to wear of the surfaces inside your boiler. For example, carbon dioxide can make water acidic, which contributes to the pitting of the metal.
Acid attacks can happen from chemicals other than carbon dioxide. In high-pressure boilers, soluble nickel or magnesium salts can hydrolyze into acids. These acids attack the surfaces inside the boiler, leading to further pitting and corrosion.
3. Dissolved Solids
Solids in the water may also contribute to corrosion formation. For example, calcium and magnesium salts, silica, manganese and iron can all form scale in a boiler. When dissolved solids deposit onto the metal, these scale-forming minerals can trap sodium salts under them. While sodium salts do not cause buildup, they can cause pitting and corrosion under the scale, which will remain unseen until you remove the scale.
While sodium causes problems under scale, it can also contribute to other corrosion problems in the system. Sodium carbonate can convert into sodium hydroxide through hydrolysis. The latter compound reacts with iron in the boiler, dissolving it and producing sodium ferrate. This product then undergoes hydrolysis again, converting back to sodium hydroxide, continuing the process. Joints and bends are especially susceptible to this type of damage from sodium in the water.
While reusing as much condensate as possible to save on fuel costs is an increasingly popular option, it can lead to more problems than it solves. Another cause of corrosion can happen when untreated condensate returns to the system, bringing dissolved iron and copper oxides with it back into the boiler. To prevent this type of corrosion, plants often use volatile amines in the form of either neutralizing or filming treatments, which prevent contaminants in condensate from producing damage.
Neutralizers turn into steam where they can react with carbon dioxide and neutralize it, reducing acidity by raising the pH of the condensation. Filming agents also turn into steam but condense into a protective film that coats the metal and keeps corrosion from damaging it.
Effects of Boiler Corrosion
Corrosion can damage the internal workings of your boiler in two possible ways — general and pitting. General corrosion creates damage all over the system, while pitting creates localized erosion of small parts of the boiler, such as in tubes.
You must watch closely for corrosion in times of minimal use of the boiler, but damage can occur anytime. Keeping up with the water chemistry and regularly treating the system will prevent corrosion that can shut down your system or reduce its efficiency.
Here are the most common effects of boiler corrosion:
1. Lost Efficiency
Corrosion and scale deposits ruin the efficiency of the system. Corrosion products also contribute to scale deposits. So, even if you don’t have any scale yet, the more corrosion occurs, the more likely the metal pulled out will cause efficiency-robbing deposits. Even scale buildup as small as an eighth of an inch can significantly reduce efficiency.
Scale buildup cyclically contributes to more corrosion. It can trap sodium under the scale, pitting the inside surface and leading to further damage inside the boiler and its tubing.
Corrosion that eats through metal also reduces system efficiency. Holes in the metal create leaks that can cause severe operational problems and shut down the boiler. If you don’t repair the damage, these leaks can unexpectedly cause the failure of the system.
2. Shorter System Life Span
Failing to control corrosion shortens the life span of the entire system. Corrosion will worsen with time, especially if water chemistry does not receive attention. The loss of efficiency you experience will only continue to degrade until the system shuts down.
For example, corrosion caused by iron or copper oxides from condensate can reduce water circulation, possibly leading to tube starvation. This event can severely damage tubes and other parts of the boiler system. The more parts of a boiler need replacing, the more a replacement system makes financial sense. Such a purchase, though, will cut into your facility’s bottom line more than preventing corrosion.
3. Higher Costs
Corrosion can lead to high costs to repair the system or replace damaged parts. In many cases, pitted tubes or parts require replacing instead of repairs. To fix the damage, you will need to shut down your system, which will reduce your facility’s productivity. The more often you need to make repairs, the less efficient your facility becomes, resulting in lost profits from an inability to operate your boilers at their peak.
Additionally, the downtime cuts into your operations and profits. For instance, over five years, indirect corrosion costs from Pacific Gas and Electric Co. totaled $80 million. Another example that shows the severity of the costs incurred from failing to maintain proper boiler water chemistry is in power outages that could add up to more than $1 million daily.
Lost productivity and repair costs both add up. If you must replace the entire system due to corrosion damage, you will pay an even higher price for your negligence.
Holes happen when you keep operating a system that already has severe pitting. The pits will not repair themselves or reverse in their severity. Instead, they worsen as the chemical reaction that caused them to erode continues until the metal has a breech. Since a breach can be incredibly damaging to your boiler system, holes can cause an entire system failure.
Pitting is more than a minor problem. In areas of the boiler with high oxygen levels, oxygen reacts with the metal to cause pits in the surface. If your chemical treatment is off or your feedwater tank or deaerator isn’t functioning properly, these pits can form and continue to deepen until they create holes in the metal. As noted, holes can lead to the failure of the system.
Pitting that occurs under scale is called “under-deposit corrosion.” This type of corrosion, coupled with the scale, produces severe damage to the system.
At a plant with deposit rates between 7.8 g/ft2/yr and 8.9 g/ft2/yr, the deposits caused severe scaling on the surfaces of the tubes. The extra layer increased the temperatures in the area and trapped minerals under the scale. The combined effect of the additional stress and under-deposit corrosion resulted in the complete failure of the boiler tube. Had the facility kept its deposit rates below 1 to 2 g/ft2/yr, it would not have experienced such severe damage.
How to Prevent Boiler Corrosion
As severe as the effects of corrosion are, you can prevent them through proper monitoring and treating the water in the system. For example, both pH monitoring and adjusting the water feed are vital components of a corrosion prevention program. These both keep acid and dissolved gases from causing damage to the metal components of the boiler and its tubing. Additionally, keeping a boiler logbook and scheduling regular boiler system service can help prevent boiler corrosion.
Some of the best ways to prevent boiler corrosion include the following:
1. Monitor the System Water’s Acid-Base Balance
Monitoring the acid-base balance of the water is an essential step to keeping acidity from damaging the boiler. Even with precise adjustments of the feed water, you may still have contaminants in the boiler itself. As you boil the water off, the contaminants remain behind, concentrating over time and potentially causing scale deposits and corrosion.
As you monitor your system for acidity and contaminants, pay attention to the following levels:
- Iron level: Measuring the amount of iron in the water will identify if excessive amounts are present. A high level of iron can deposit in the system and reduce its efficiency.
- pH level: Monitoring your boiler water pH will help you determine when to blowdown the system to remove some of the contaminated water and reduce the effects of those products. The ideal pH for feedwater is between 7 and 9, slightly alkaline. To keep the water within this range, add sodium phosphate salts or sodium hydroxide. Unfortunately, you cannot directly monitor the pH inside the heat of the boiler. Rather, you must measure pH from a cooler, lower-pressure side stream.
- Sodium level: Monitoring the water and steam for sodium will also help you prevent corrosion. If you find out you’re approaching dangerous levels, you can better control this mineral’s presence in the system.
2. Utilize a Regular Service Program
A simple way to prevent boiler corrosion is to schedule regular inspections and services for your boiler system. With a service program, you can devote staff to keeping your boiler clean. These programs can also help your company ensure your boilers stay free of corrosion and scale issues. When you have regular checkups and services, you’re more likely to catch issues early and take action before significant damages occur.
3. Adjust Feedwater
Feedwater adjustment prevents dissolved oxygen from affecting the metal surfaces of the system. Depending on your monitoring results, you may need to add scavenging agents or use a deaerator to eliminate oxygen.
If you want to remove oxygen with scavenging agents, you can use three common chemicals, depending on the system.
- Sodium sulfite: Companies usually use sodium sulfite in medium or low-pressure systems. When used in systems with pressures greater than 1,000 psi, sodium sulfite will convert to hydrogen sulfide or sulfur dioxide, both of which cause corrosion. Extra pressure makes sodium sulfite increase the solids in water, too, which is why this chemical works best in systems that operate under 1,000 psi.
- Hydrazine: Typically, companies prefer to use hydrazine for high-pressure boilers. For higher-pressure systems, hydrazine works well to react with oxygen to create water and nitrogen. However, the Food and Drug Administration (FDA) bans this product from use in food processing plants.
- Sodium erythorbate: Sodium erythorbate is non-toxic and can replace the other two chemicals for use in food processing plants. Since this chemical is a non-toxic alternative to the two scavengers above, it’s safe for food processing facilities.
Alongside scavenging agents, mechanical deaerators can also remove oxygen, though they cannot eliminate it. These mechanisms use a reversal of the same process that leads to the dissolved gases getting into the water. Many mechanical oxygen removers will reduce the amount down to 7 ppb, but they can eliminate free carbon dioxide in the water. Keep in mind that reducing the oxygen levels below 7 ppb usually does not lead to any improvements in the system’s operation.
This method of getting oxygen out starts by warming the water and using low-oxygen air above the deaerator. Heating the water reduces how much oxygen it can hold, while the low-oxygen air above the water gives the oxygen a place to go. By adjusting the oxygen in the feedwater via this method, you can reduce the amount of dissolved oxygen that can harm your system if left unchecked.
4. Utilize a Boiler Logbook
Another way you can minimize boiler corrosion is to keep a boiler logbook. With a logbook, you can track your boiler room equipment’s proper operation and more easily identify when something changes. For example, a logbook can help you see if a feed tank’s temperature or a deaerator’s pressure changes, helping you take action before more expensive corrosion damage occurs. Additionally, you can use a logbook to track pH changes, which could indicate you have process contamination or need to adjust your water treatment program.
How Chardon Laboratories Can Help
You are not alone in corrosion prevention. At Chardon Laboratories, we have the chemicals you need to maintain water pH and parts to replace any damaged components. Additionally, we can provide the parts you need for keeping up your water’s proper chemistry to reduce corrosion. Some of the boiler equipment we have includes the following:
- Blowdown solenoid
- Chemical mix tanks with agitators
- Chemical pumps
- Contact headwater meters
- Corrosion coupon pack
In addition to equipment to help you maintain water chemistry, we’ll also treat the water and schedule regular return visits to help you keep the appropriate balance of chemicals. To absorb dissolved oxygen, we use sulfite. Additionally, our mixing process prevents deposits from settling, which can lead to future pitting.
We recognize the importance of preventing deposits that contribute to scale formation. Increased scale on the interior surfaces costs you money through wasted fuel. A scale thickness of just 0.03 inches requires 7% more fuel to achieve the same heat as a unit without scale. While the wasted fuel costs you money, the scale can also contribute to corrosion. When you choose us to help you prevent buildup and keep your unit free of corrosion, you’ll save money.
Finding the right boiler water treatment and chemical balance for your boiler takes experience and knowledge. Trust our ISO-certified technicians and procedures to get the corrosion-resistant water balance you need for your system to last longer.
Find out More About Optimizing Your Boiler
Keep your boiler from the damaging effects of corrosion through preventive measures that will save you money over time. Let our team at Chardon Laboratories help. You can trust all our processes and technicians, both of which carry ISO certification. Additionally, we guarantee our results and install any necessary equipment. We don’t just sell chemicals. We sell clean systems.
If you want to prevent the damage done by gases or minerals in your boiler water, contact us at Chardon Laboratories.