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.
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.
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.
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.
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.
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.
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).
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.
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Source and Bibliography:
- WSS – Water Treatment Technical Manual