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

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