Steam Condenser on Board Vessels: Operation, Maintenance, and Troubleshooting

The steam condenser is a critical component on board vessels that plays a crucial role in the efficiency and operation of boiler and steam-powered systems. It enables the conversion of exhaust steam from the consumers or main propulsion or power generation systems back into water, allowing for reuse and maximizing energy efficiency. In this comprehensive article, we will delve into the correct operation of a steam condenser, highlight the importance of regular maintenance, and provide a troubleshooting guide for common operational malfunctions.

    1. Correct Operation of a Steam Condenser

The correct operation of a steam condenser involves several key steps and considerations:

1.1. Cooling Water Circulation

Adequate cooling water circulation is vital for the efficient operation of the steam condenser. Cooling water is typically drawn from the sea or a freshwater source and circulated through the condenser tubes, absorbing the heat from the exhaust steam. It is crucial to ensure a sufficient flow rate and temperature difference for effective heat transfer.

1.2. Air Extraction

To maintain optimum performance, it is essential to remove non-condensable gases, such as air, from the condenser. This is achieved through air extraction systems that continuously remove air from the condenser, preventing its accumulation and reducing the risk of performance degradation.

1.3. Vacuum Creation

Maintaining a vacuum within the condenser is vital to enhance the efficiency of steam condensation. A vacuum is created by condensing steam, reducing its pressure, and evacuating the condensed water. Efficient vacuum creation is crucial for maximizing power generation and optimizing overall system performance.

    1. Maintenance of a Steam Condenser

Regular maintenance is essential to ensure the reliable and efficient operation of a steam condenser. Neglecting maintenance can lead to reduced performance, increased energy consumption, and even catastrophic failures. Key maintenance activities include:

2.1. Tube Cleaning

Over time, fouling and scaling can accumulate on the condenser tubes, reducing heat transfer efficiency. Regular tube cleaning, using methods such as mechanical brushing, chemical cleaning, or high-pressure water jetting, is necessary to remove deposits and maintain optimal heat exchange.

2.2. Inspection and Repair

Routine inspections of the condenser, including visual checks and non-destructive testing, help identify any corrosion, tube leaks, or structural damage. Prompt repair or replacement of damaged components is crucial to prevent further degradation and ensure the condenser operates at its designed capacity.

2.3. Cooling Water Treatment

Effective treatment of cooling water is essential to prevent scaling, corrosion, and biological growth within the condenser. Regular monitoring and adjustment of water chemistry parameters, such as pH, alkalinity, and dissolved solids, help maintain water quality and prevent detrimental effects on condenser performance.

2.4. Air Extraction System Maintenance

The air extraction system must be inspected regularly to ensure proper functioning. This includes checking air extraction pumps, vacuum breakers, and associated valves for leaks or malfunctions. Any issues should be addressed promptly to maintain efficient air removal from the condenser.

Maintenance is of paramount importance for steam condensers due to the following reasons:

    • Efficiency and Performance: Regular maintenance ensures that the condenser operates at optimal efficiency, maximizing heat transfer and energy conversion. This results in improved overall system performance, reduced fuel consumption, and increased power generation or propulsion efficiency.
    • Equipment Longevity: Proper maintenance helps extend the lifespan of the steam condenser. Regular inspections, cleaning, and repair of components prevent premature wear, corrosion, and deterioration. This not only reduces the risk of costly breakdowns but also contributes to the longevity of the condenser and the entire steam system.
    • Energy Savings: A well-maintained steam condenser can significantly impact energy savings. When the condenser operates efficiently, more steam is converted back into water, reducing the need for additional steam generation. This leads to lower fuel consumption and operational costs, resulting in substantial energy savings over time.

3. Troubleshooting

Despite proper maintenance, steam condensers may experience operational malfunctions. Here is a troubleshooting guide for common issues:

3.1. Insufficient Cooling Water Flow:

      • Check the cooling water intake for any blockages or restrictions.
      • Verify that the cooling water pumps are operating correctly.
      • Inspect and clean the cooling water filters to ensure unrestricted flow.

3.2. Inadequate Vacuum:

      • Check for air leaks in the condenser or associated piping and repair any leaks.
      • Verify the proper functioning of vacuum pumps and vacuum breakers.
      • Ensure proper condensate removal to maintain the desired vacuum level.

3.3. Tube Fouling or Scaling:

      • Conduct a thorough cleaning of the condenser tubes using appropriate methods.
      • Review and adjust cooling water treatment to minimize scaling and fouling.
      • Consider implementing online cleaning systems for continuous tube maintenance.

3.4 Corrosion or Tube Leaks:

      • Perform regular inspections to detect any signs of corrosion or tube leaks.
      • Promptly repair or replace damaged tubes or components.
      • Consider using corrosion-resistant materials or protective coatings where applicable.

In conclusion, steam condensers are integral to the efficient operation of boilers and steam-powered systems on board vessels. Correct operation, regular maintenance, and timely troubleshooting are essential for optimal performance, energy efficiency, and prolonged equipment life. By following proper procedures, conducting routine maintenance activities, and promptly addressing operational malfunctions, ship engineers can ensure the reliable and efficient operation of their steam condensers, contributing to the overall success and safety of their vessels.

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What you need to know about cooling water expansion tanks

When it comes to maintaining efficient and reliable cooling systems, there are several crucial components that must be considered. One such component is the cooling water expansion tank.

Recently, I have been asked, by one of my followers, to write an article about cooling water expansion tank. I found the subject to be interesting and challenging as not many of us is really thinking about the importance of the expansion tank into the cooling water system. This is regarded, most of the time, as a simple tank with not so much importance.

If you want to learn more about engine cooling water system just follow the link in here and in here. Moreover, if you want to learn and understand more about vessel auxiliary equipment just click and have a look to Introduction to Marine Auxiliary Machinery.

Often overlooked, this vital piece plays a significant role in the overall performance and longevity of cooling systems. So, in this blog post, we will delve into the importance of cooling water expansion tanks, their function, and the benefits they offer in ensuring optimal cooling system operation.

Example of a cooling water expansion tank

Understanding the Function of Cooling Water Expansion Tanks

Cooling water expansion tanks are designed to accommodate the thermal expansion of water within a closed-loop cooling system. As water heats up, it expands, and without a proper means of accommodating this expansion, the system can experience excessive pressure buildup. Expansion tanks act as a reservoir for the expanding water, allowing it to safely expand and contract without compromising the integrity of the cooling system.

By providing a space for expanded water, cooling water expansion tanks help regulate system pressure. When the water expands, it enters the tank, reducing the pressure exerted on other components of the cooling system. This pressure regulation prevents potential damage to pipes, valves, pumps, and other system elements caused by excessive pressure. By maintaining a balanced pressure, expansion tanks contribute to the longevity and reliability of the entire cooling system.

Cooling water expansion tanks also play a crucial role in minimizing air and contaminant accumulation within the cooling system. The tanks are typically equipped with an air vent or an air separator, allowing trapped air to be released from the system. Removing air not only prevents airlocks but also enhances the efficiency of heat transfer, as air has a lower heat capacity compared to water. Additionally, expansion tanks can feature filtration systems that help capture contaminants and prevent their circulation within the cooling system.

Corrosion and oxidation pose significant threats to cooling systems, leading to reduced efficiency and potential system failures. Cooling water expansion tanks can include various internal coatings or linings that inhibit corrosion and oxidation processes. These protective measures help extend the lifespan of the expansion tank itself, as well as the overall cooling system.

Properly sized and installed cooling water expansion tanks contribute to enhanced system efficiency and performance. By maintaining optimal pressure levels, preventing airlocks, and reducing the risk of corrosion, expansion tanks ensure that the cooling system operates at its peak efficiency. This, in turn, translates to lower energy consumption, reduced maintenance needs, and increased overall system performance.

Example of expansion tank arrangement and fittings

Operation and Maintenance Guide for Cooling Water Expansion Tanks

Cooling water expansion tanks are crucial components of closed-loop cooling systems. To ensure the longevity and efficient operation of these tanks, proper operation and maintenance practices are essential. In the next paragraphs, we will explore the key aspects of operating and maintaining cooling water expansion tanks, providing valuable insights for engineers and operators.

      1. Regular Inspections: Regular inspections of cooling water expansion tanks are necessary to identify any signs of wear, damage, or leaks. Inspect the tank’s exterior for physical damage such as dents or corrosion. Additionally, check the tank’s connections, including inlet and outlet pipes, for any signs of leakage. Early detection of issues can help prevent costly repairs or potential system failures.
      2. Pressure Monitoring: Monitoring the pressure within the cooling water expansion tank is crucial for its proper operation. Utilize pressure gauges installed on the system to ensure the pressure remains within the recommended range. If the pressure consistently exceeds or falls below the recommended levels, it may indicate an underlying issue in the cooling system that requires investigation and correction.
      3. Ventilation and Air Release: Cooling water expansion tanks often incorporate ventilation systems or air vents to remove trapped air from the system. Ensure that the vents are clear and functioning properly to prevent airlocks, which can hinder the cooling system’s performance. Regularly inspect and clean the vents to maintain their effectiveness and promote efficient heat transfer within the system.
      4. Water Quality Maintenance: Water quality plays a vital role in the longevity and performance of cooling water expansion tanks. Implement appropriate water treatment methods, such as filtration and chemical treatment, to prevent the accumulation of contaminants that could lead to corrosion or blockages within the tank and the cooling system as a whole. Regularly monitor water quality parameters, such as pH levels and dissolved solids, and perform necessary maintenance and treatment actions based on the results.
      5. Periodic Flushing and Cleaning: Over time, sediments, debris, and scale may accumulate within the cooling water expansion tank. Regular flushing and cleaning of the tank will help remove these deposits, ensuring optimal performance. Follow manufacturer guidelines and industry best practices when conducting flushing and cleaning procedures, and use appropriate cleaning agents that are compatible with the tank’s material.
      6. Maintenance of Tank Supports and Mounting: Cooling water expansion tanks are typically supported by brackets or mounting systems. Periodically inspect these supports to ensure they are secure and in good condition. Any signs of wear, rust, or damage should be addressed promptly to prevent potential tank displacement or failure.

Proper operation and maintenance of cooling water expansion tanks are essential for the reliable and efficient performance of cooling systems. By conducting regular inspections, monitoring pressures, ensuring proper ventilation, maintaining water quality, and performing necessary cleaning and maintenance procedures, engineers can prolong the lifespan of the tanks and optimize the overall cooling system performance. Collaborating with maintenance professionals when needed will further enhance the effectiveness of the maintenance efforts, contributing to the long-term success of the cooling water expansion tank and the entire cooling system.

In conclusion, cooling water expansion tanks may be small in size, but their significance in cooling system operation cannot be overstated. From pressure regulation and system protection to minimizing air and contaminant accumulation, these tanks offer numerous benefits that help maintain the efficiency, reliability, and longevity of cooling systems. By understanding the importance of cooling water expansion tanks and incorporating them into cooling system designs and maintenance routines, engineers and operators can ensure optimal performance and maximize the lifespan of their systems.

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What you need to know about engine cooling system – part II

On my previous post we have discussed about how the engine cooling systems can be affected by scaling and corrosion and how different materials behave when they are exposed to corrosive environment. This post is a continuation and would like to discuss about how we can prevent and inhibit the scaling, fouling and corrosion in the engine cooling systems.

The majority of systems are designed and manufactured in such a way that a chemical inhibitor is required in order to control corrosion. This is the case despite the fact that there are many different options available for minimizing corrosion through improved design, the selection of materials, and improved construction techniques.

Example of anode  for corrosion protection purpose

As was said previously, each of the four components of a corrosion circuit needs to be finished in order for corrosion to take place. Therefore, any chemical treatment that is put to the water and stops the anodic reaction will halt corrosion, and any inhibitor that prevents the cathodic reaction will minimize corrosion. This applies to both types of treatments.

On the basis of the manner in which they influence the corrosion cell, corrosion inhibitors are divided into the following three categories:

    • Anodic inhibitors
    • Cathodic inhibitors
    • Combination inhibitors/organic inhibitors

Long-chained carboxylic acids form the foundation for the development of more recently discovered types of inhibitors. These have the benefit of depleting very slowly, which means that after the initial dosage, very little replenishment of the inhibitor is required and this is an advantage. Aluminum, along with the other metals that are frequently employed in a cooling system, is protected by the Organic Acid Technology inhibitors.

Example of Organic Acid Technology Inhibitor

Nitrites, which are good anodic corrosion inhibitors for mild steels, are by far the most frequent type of corrosion inhibitor utilized in engine cooling systems.
Nitrites cause mild steel surfaces to oxidize, which results in the formation of a layer of corrosion product that is exceedingly thin yet highly tenacious. Because of the very large dose rates that are necessary, nitrites are almost exclusively utilized inside of closed systems due to the economic benefits associated with this application method. There is no discernible effect that hydrocarbons or glycols have on the performance of nitrite.

Example of nitrate corrosion inhibitor

At the anode, silicates, which contain the anion SIO32-, react with dissolved metal ions. The gel that is formed as a result of the formation of the metal ion/silicate combination deposits itself on anodic sites. When compared to other regularly used inhibitors, the gel creates a layer that is adherent and essentially unaffected by pH. This layer is formed into a thin layer. The temperature and pH of an environment both contribute to an increase in the inhibitory qualities of silicates. In the normal case, silicates are employed to prevent corrosion of aluminum when the pH level is high (9.5 – 10.5). It is essential to keep the silicate in solution within this pH range in order to forestall any significant precipitation.
It is important to note that in this environment, if localized boiling causes cavitation and concentration mechanisms, then extremely rapid corrosion of the aluminum will occur because the protective oxide layer will be destroyed and the metal will be dissolved by the high pH conditions. This is something that must be taken into consideration.

In the presence of dissolved ferric ions, such as Fe2+, orthophosphate (PO4) can form an insoluble complex with these ions, which then deposits at anodic sites. When compared to other anodic inhibitors, ferric orthophosphate has a greater adhesion and is less sensitive to pH. When the pH is between 6.5 and 7.0, the film develops with more efficiency. Around 10 ppm is considered to be the standard minimum dosage rate in neutral seas.

In order to produce positively charged colloidal particles, polyphosphates must first form complexes with calcium, zinc, and any other divalent ions present. These move to the cathodic sites where they precipitate and form a coating that acts as a corrosion inhibitor. In order for polyphosphates to carry out their intended functions, calcium must be present in concentrations of at least 50 ppm. As is the case with zinc hydroxide, pH extremes have the potential to disrupt the stability of the inhibitor coating.

Zinc ions have a positive charge, which is why they are drawn to the cathodic region of the corrosion cell. Zinc hydroxide (Zn(OH)2) is formed when soluble zinc ions combine with the free OH ions found in the area surrounding the cathode to create a film. This protective coating is sensitive to shifts in the pH of the environment. If the pH is too low, then the film will dissolve and not reform, and if the water conditions are too alkaline, then the zinc hydroxide will precipitate in the bulk of solution rather than at the cathodic site. If the pH is too high, then the film will reform. The pH range that is typically considered acceptable for the use of zinc-based corrosion inhibitors is 7.4 to 8.2.

Phosphonates were at first implemented into the water treatment industry as scale inhibitors in order to take the place of polyphosphates. It has been demonstrated that they are capable of adsorption onto the surface of the metal by the P-O group, particularly in alkaline and hard fluids. In order to achieve the highest efficacy against corrosion, phosphonates are typically utilized in concert with other types of inhibitors.

Benzotriazole (BZT) and Tolytriazole (TTZ) are put to use as corrosion inhibitors that are tailored specifically for copper and brass. They attach themselves to the surfaces of the metals, which breaks the electrochemical circuit. These components are resistant to the temperatures and pH levels that can be encountered in engine cooling systems. They are typically mixed into unique corrosion inhibitor formulations because of their stability in these environments.

Once an appropriate corrosion inhibitor has been chosen and applied to an engine cooling circuit, general testing of the water quality would be carried out to measure the levels of inhibitor, pH and chlorides (as a check for contamination), and hardness salts. This would be done in order to determine whether or not contamination has occurred.

Example of water test kit used onboard vessels

Bacteria are uncomplicated forms of life that can be found in practically all of the water that can be found on Earth, which if they are already present in engine cooling circuits have the ability, under certain conditions, to adapt so that they can feed on the nitrite-based compounds that are employed in corrosion inhibitors and also on oils that are introduced into the cooling circuit as impurities. As a result of the bacteria’s actions, nitrite is converted to nitrate, which results in the loss of corrosion inhibition. This circumstance can result in rapid population increases of bacteria, which can then lead to the creation of insulating biofilms on the internal surfaces of the cooling system as well as the obstruction of filters and control devices. Because of the existence of the deposit, the effectiveness of the cooling system will decrease, and there will be an increased danger of corrosion as a result of the depletion of the corrosion inhibitor. Scale deposits produce the same effect.
The typical manifestation of this condition is the depletion of the nitrite reserve along with a steady or rising trend in the conductivity of the system. This is because the nitrate that has been generated continues to contribute to conductivity.
The high temperatures and pH levels that are generally experienced in engine cooling circuits contribute to the low incidence of microbial fouling that can occur. In the event that microbiological fouling is discovered, a specialized biocide should be used to eliminate the bacteria that are causing the problem and prevent further growth.

Example of biocide

Seawater is used to provide feed-water to the Evaporators, secondary cooling for the Engine Cooling Circuits and in some cases cooling for the Air Charge Coolers. Seawater is introduced into the ship through open inlets and stored in containing tanks called ‘sea-chests’ within the body of the hull. These cooling duties utilize a variety of Heat-exchanger designs which can become severely fouled due to impurities and contamination in the sea-water supply.

Example of seawater filter on the central coolers

As was said previously, corrosion and scaling are two issues that can be caused by the dissolved gases (oxygen and carbon dioxide) and salts of magnesium and calcium that are found in sea water. However, because these cooling systems have a high flow that only goes through them once, it is not economically feasible to continuously apply treatment to inhibit scale and corrosion like it is in boiler cooling circuits and engine cooling circuits. The easiest way to prevent scaling and corrosion is to make sure that the metallurgy of the heat exchanger is chosen correctly and to keep the temperature of the sea water below 50 ºC at all times.

The presence of microorganisms that are found naturally in sea water is the primary cause of the issue known as fouling, which is related with sea-water cooling circuits. The Mussel (Mytilus Edulis) and the Barnacle are the two troublesome species that cause the most issues (Balanus Balanoides).

Example of mussels and barnacles attached to the vessel sea water intake chest

These species are extremely numerous all throughout the world, and their spawning seasons change according to the environmental conditions that are present at the time. When the water temperature rises above 10-15 degrees Celsius, spawning activity begins to take place. Because of this, the majority of boats operating in deep sea environments are prone to pollution and fouling.
Filters and strainers are able to remove mussels and barnacles after they have reached full maturity; nevertheless, it is the newly produced species, known as veliger, that are the source of the issue. These veligers begin their lives in a very small size, making it simple for them to enter the cooling circuit and cause problems. Once they are inside the system’s pipes, they attach themselves to the surfaces utilizing protein fibers that are strong and elastic (Byssus). Once they have attached themselves, they are able to quickly feed and expand. Because of their growing size and population, streams are becoming fouled and blocked more frequently.

The presence of this type of fouling will lead to:

    • Reduced cooling efficiency
    • Risk of under deposit corrosion and failure
    • Risk of cavitation and impingement corrosion
    • Increased pumping and maintenance costs

Due to the fact that the issue organisms are alive, it is possible, in theory, to eliminate them through the use of a number of different biocide additives or the installation of Marine Growth Protection Systems (MGPS).
It has been tried to use chlorine, however it has been shown that fully grown species can require 0.2 to 1.0 ppm free chlorine over a period of up to 10 days, which can be difficult to accomplish on a ship at sea. Additionally, fully matured mussels and barnacles have the ability to recognize the presence of chlorine as an irritant. In response, they will ‘shut up’ and exude a mucous seal that is resistant to the biocide. Eliminating mussels and barnacles while they are in their most defenseless stage (as veligers) and preventing them from attaching and growing is the most effective strategy for dealing with these organisms. This can be achieved through the consistent use of a biocide and anti-foulant that is specific to the company.

If you have any questions regarding above, please feel free to use our existing forum Seafarer’s World and will try to answer to all your queries.

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Source and Bibliography:

  • WSS – Water Treatment Technical Manual


What you need to know about engine cooling system – part I

Diesel engines are the most common type of propulsion and power source for ships that are employed in the marine industry and they typically do not burn diesel fuel, despite their name, but rather heavy fuel oil that has been heated and purified by centrifugal filtering before being used. As a byproduct of the combustion processes, significant volumes of waste heat are produced and exhaust gas boilers are used to collect some of the energy that would have been lost as waste heat and put it to use in the production of steam. It is necessary to dissipate the bulk of the heat that is created by combustion in order to avoid the moving parts of the engine, such as the pistons, valves, and associated seals, from overheating and failing. The cooling of the combustion chamber and the cylinder head is made possible thanks to the incorporation of a water cooling circuit into the design of the engine. This allows for the heat to be dissipated more effectively.

Example of engine cooling circuit

The cooling system for the engine is intended to function as a “closed cooling system,” which means that the make-up is typically less than 5% of the total capacity of the system each week. On the other hand, make-up rates can be far greater than anticipated if the system is older, which increases the likelihood of leakage and the requirement for frequent and/or emergency maintenance. If the make-up water comes from the system that handles evaporated water, then scaling is not typically an issue because this water does not include any salts that can cause scale formation. Scale deposition is likely to occur in situations where shore water is used as the make-up water and the makeup rate is high. This might cause operational problems and put the facility at danger of having to shut down for repairs.

Deposits in engine cooling systems are caused when the solubility limits of contaminants dissolved in the make-up water are exceeded, which causes the water to become contaminated and contaminants become insoluble and deposit as the cooling circuit is heated and/or concentrated. This causes the scale to form. Calcium carbonate for example will deposit at 65°C – 70°C. It should be understood that, in general, the creation of scale will tend to be greatest in areas where the heat flux is greatest, such as around the cylinder lining and valve seats. This is something that may be expected. The amount of scale that is generated is proportional to both the rate of water loss and the amount of scale-forming salts that are present in the make-up water. The higher both of these factors are, the more scale will be formed.

Typical scaling contaminants are:

    • Calcium carbonate – scale appears as a pale cream. yellow deposit and is formed by the thermal decomposition of calcium bicarbonate at heat transfer surfaces.
  • Example of calcium carbonate scaling

    • Magnesium silicate – scale is a rough textured, tan to off-white deposit found in cooling circuits where sufficient amounts of Magnesium are present in conjunction with adequate amounts of silicate ions with a deficiency of OH Alkalinity. Silicate deposits are also a risk where silicate additives are used to as corrosion protection for aluminium metal in the cooling circuit. The corrosion mechanism relies on the silicate forming a protective film in the metal surface and the silicate is maintained in solution by maintaining a relatively high pH in the cooling circuit (9.5 – 10.5). If this pH is reduced due to ingress of shore-water or sea-water then the silicate will precipitate in the bulk of the cooling water (as magnesium or calcium silicate) and cause gross fouling of the cooling circuit. It is vital when operating a silicate treatment regime for cooling circuits containing aluminium that the system under consideration should be made up with evaporated or softened water to prevent this problem.
    • Iron oxide deposit like hematite (Fe2O3) – This type of red/brown iron oxide deposit is found because of active corrosion occurring somewhere within the cooling system.
  • Example of iron oxide scaling

    • Copper – The presence of copper deposits in a cooling system presents a very serious corrosion risk because of the initiation of galvanic corrosion mechanisms with steel. The steel in the cooling system is anodic to the copper and rapid loss of metal will occur. It is usual to incorporate specific copper corrosion inhibitors in the formulation of cooling system corrosion inhibitors.

Scale deposits have the function of providing thermal insulation on the water-facing side of a cooling surface and slowing the transfer of heat from the metal to the surrounding water. The only way that the heat can escape is if the temperature of the metal is raised, as well as the temperature of the gas that is being expelled from the engine. The first scenario could put the engine’s structure at risk, while the second scenario will lower the engine fuel economy. The presence of scale on the heat transfer surfaces of the cooling circuit might lead to a situation in which the alkalinity in the system begins to concentrate by evaporation within the scale deposit. This can happen if the scale is allowed to remain for an extended period of time. When present in high quantities, OH alkalinity is capable of corroding boiler steel and, in particular, contributing to the premature failure of aluminum pipe-work and components.

Components made of multiple metals are generally found in the engine cooling system. These metals typically include:

    • mild steel – in and aqueous environment mild steel will rapidly corrode by reaction with oxygen in the water. The variables pH, temperature and the concentration of oxygen affect the rate of corrosion.
      As temperature and/or oxygen levels rise the corrosion rate accelerates. Oxygen corrosion is usually observed as localized pitting on a metal surface often with large tubercles of iron oxide covering the pit. pH is inversely related to corrosion rate. This is why alkaline conditions are maintained in steam boiler systems and closed cooling circuits.
    • stainless steel – these are alloys of steel with variable levels of chromium (over 11%). This alloying process gives the material excellent corrosion and wear resistance compared to mild steel. Oxygen combines with the chromium and iron to form a highly adherent and protective oxide film. If this film is ruptured in an oxidizing environment then the oxide film is rapidly re-formed. Therefore the corrosion resistance of stainless steels is reduced in deaerated waters, which is the opposite to normal steels. The presence of chlorides and other strong ‘de-passivating’ ions can seriously reduce the corrosion resistance of stainless steels. The main drawback of more widespread use of stainless steels is extra cost and susceptibility to stress corrosion cracking in the presence of high chloride levels especially at elevated temperatures.
    • copper and its alloys – copper and its alloys are often used in heat exchangers because of their excellent heat transfer properties and relative low corrosion rate in water systems when compared to mild steel. In oxygenated water the corrosion of copper is slow as the metal forms a protective copper oxide film through which oxygen must diffuse for further corrosion to occur. Because copper is a softer metal than steel, water velocities and the scouring effect of suspended solids can act to disrupt the oxide film and increase corrosion rates. Corrosion of copper in the presence of steels is very serious as the copper can re-deposit on the steel and instigate very rapid galvanic corrosion of the steel and risk of failure. Copper and its alloys can be corroded by weak solutions of ammonia in the presence of oxygen.
    • aluminum – in water containing oxygen, aluminium, like copper and stainless steel is protected by an oxide layer. Aluminium is a amphoteric metal which means that it can be aggressively attacked in an aqueous environment with low or high pH. In water of relatively neutral pH at temperatures up to 180 °C, aluminium is essentially inert. In the engine cooling circuit, like in steam boiler systems, this corrosion mechanism can be exacerbated by localized boiling and the formation of high concentrations of OH ions at the metal surface. This will lead to aggressive corrosion and rapid failure.
    • zinc – is the principal metal used to protect steel as a galvanized coating. Here the zinc is anodic to the iron and preferentially corrodes at a slow rate thus protecting the steel. Zinc corrodes at a rate somewhere between that of iron and copper in most natural waters. Attack will occur in the absence of oxygen as with aluminium. Zinc is also an amphoteric metal and is corroded at high and low pH.

Caution is advised because the catholic-to-anodic connection with iron undergoes a reversion at temperatures above 60 degrees Celsius. When this happens, the steel piping becomes extremely anodic, which leads to rapid galvanic corrosion. Before the system can be put into use, the zinc coating on any galvanized pipework or tanks must be removed using a controlled acid wash because this is an essential aspect of engine cooling circuits. If galvanized pipework or tanks are present, this step is necessary.

When choosing a proprietary corrosion inhibitor, it is important to keep in mind that the mechanisms of corrosion that are specific to each metal are distinct from one another and that these mechanisms must be taken into account.

The factors that are affecting the corrosion rate are:

    • temperature – the pace of most chemical reactions increases by a factor of two for every 10 degree Celsius when the temperature is raised. Since of this, a rise in temperature will hasten the rate at which corrosion occurs because the processes that take place at the cathode will happen more quickly. Up to about 80 degrees Celsius, there is also an increase in the rate of oxygen diffusion. Because oxygen is less soluble in an open system, the rate of corrosion is beginning to decrease, which is why the solubility of oxygen is decreasing.
    • pH/alkalinity – as was said earlier, the rates of corrosion that metals experience with regard to pH might differ based on the electrochemical nature of the metal in question. For instance, the rate at which steel corrodes is significantly increased in acidic environments because these environments hinder the production of a protective oxide layer. As the pH increases up to roughly 13, the decreased solubility of iron oxide causes the corrosion rate of mild steel to decrease. This is because iron oxide is more soluble in lower pH levels. Copper behaves in a similar manner. Because of their amphoteric nature, aluminum and zinc both have elevated rates of corrosion when exposed to extremely low or high pH.
  • Example of aluminium pipe from engine circuit with corrosion damage due to high pH

    • chloride ions – at the anodic sites, small negatively charged ions like chloride ions have a tendency to congregate in order to electrochemically balance the positive ions (Fe2+ etc.) that are created as a result of the oxidation of the metal. These ions boost the locally enhanced conductivity, and as a result, the voltage gap between the anode and the cathode. Because of this action, the atmosphere becomes one in which corrosion can advance more quickly. When pollution of sea water has taken place, it is common practice in the maritime environment to make recommendations for increased amounts of corrosion inhibitor.
      Stainless steels are the most common materials to experience the phenomenon known as stress corrosion cracking owing to chlorides. The chloride ions in this situation are of a size that allows them to enter the atomic matrix of the metal, and the concentration of these ions speeds up the corrosion process and causes fissures to propagate further in the metal. Failure on a catastrophic scale is frequently the result of corrosion mechanisms of this kind.
    • galvanic corrosion – when two different types of metals are joined together and then subjected to an environment containing water, a kind of corrosion known as galvanic corrosion can develop. Because of this, what is known as a “galvanic cell” is created when one metal becomes anodic and the other becomes cathodic. When compared to the cathodic metal, the anodic metal is more likely to show signs of corrosion. The most typical manifestation of this kind of corrosion happens when copper and mild steel are joined together in water at the same time. The corrosion of the mild steel occurs as a result of the mild steel’s transformation into an anodic state, which occurs due to its greater propensity than copper to give up electrons.
    • cavitation – the immediate creation and collapse of vapour bubbles in a liquid that is subject to quick and strong localized pressure changes is an example of cavitation. In essence, this is localized boiling that does not involve an increase in temperature. Cavitation damage occurs when this phenomenon act at the metal surface and the hydrodynamic forces caused by the collapsing vapour bubbles form microscopic “torpedoes” of water, which are capable of reaching speeds of up to 500 meters per second, and when they make contact with a metal surface, they remove the protective oxide layer and deform the metal itself. Cavitation is common in the feed-pumps that are used on steam boiler systems. Additionally, it is becoming more common in engine cooling systems as engine manufacturers move toward producing more compact high efficiency diesel engines with smaller, more compact cooling circuits where the risk of localized boiling is increased. This is because cavitation can cause the water in the cooling circuit to boil more quickly.
  • Example of pump impeller cavitation

    • corrosion fatigue cracking – this particular kind of corrosion develops as a consequence of a combination of being exposed to an environment that is corrosive and being subjected to recurrent tensile stress. The application of stress in a repetitive and cyclical manner leads to metal fatigue, which, when combined with exposure to a corrosive environment, ultimately results in the metal cracking. These fractures typically manifest themselves as groups of fine to large cracks that run perpendicular to the direction in which the tension is being applied and are filled with thick corrosion product. The rate of damage that occurs as a consequence is higher than it would be if only the stress or corrosion mechanisms were taking place.
      Any type of metal can experience corrosion fatigue, and it can happen in a wide variety of corrosive situations.

Example of corrosion fatigue

This is the end of Part I with regard to engine cooling system and in next part we will talk about corrosion inhibition and its mechanisms.

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Source and Bibliography:

  • WSS – Water Treatment Technical Manual