Basic causes of corrosion on ships

Basic causes of corrosion on ships

The ship has never been spared from corrosion. Generally, those affected by corrosion on ships are ship hull plates, ship propellers and ship steering because they come into direct contact with sea water. Understanding corrosion is a redox reaction between metals and various substances in their environment so as to produce unwanted compounds. In everyday life, we know corrosion as rust or corrosion.

One of the biggest sources of damage to ships is caused by seawater corrosion. Until now the use of iron and steel as the main material for shipbuilding is still dominant. In terms of cost and strength, the use of iron and steel for shipbuilding is indeed quite adequate. But iron and steel are very reactive and have a great tendency to be attacked by seawater corrosion. Corrosion is a process of degradation of a metal due to chemical reactions between the metal and its environment. Basically corrosion is the event of the release of electrons from metals (iron or steel) that are in an electrolyte solution such as sea water. While the atoms which are positively charged from metal (Fe + 3) will react with hydroxyl (OH-) ions to form ferric hydroxide [Fe (OH)3] known as rust.

Based on the construction aspect of a ship, the hull plate is the area that was first exposed to sea water. In this gastric area underwater or upper water ranges are subject to corrosion. Corrosion to the ship’s body plate can result in a decrease in strength and service life of the ship, reduce the speed of the ship and reduce the safety and security of cargo and passenger cargo. To avoid greater losses due to seawater corrosion, vessel maintenance and maintenance must be carried out periodically.

Form of corrosion that occurs in the hull is evenly distributed corrosion. Corrosion evenly is a type of corrosion in which this type of corrosion rate of corrosion that occurs on the entire surface of the metal or alloy exposed or exposed to the environment takes place with almost the same rate. Almost all metal surfaces show a corrosion process.

Until now, to protect the ship’s body plate against seawater corrosion attacks still use 3 (three) ways, namely to avoid the cause of corrosion, active protection (with Cathodic Protection method) and passive protection (by painting process). The cathodic protection method is a method that is very commonly used for corrosion protection on the hull of the ship, but sometimes this is not seriously considered so the desired results are usually off the mark and inefficient. One method of cathodic protection is the victim anode method.

Occasionally on the ground, hull plates are exposed to severe corrosion due to the lack of anodes installed both in the hull, in the rudder of the ship and in the propeller of the ship.

Marine corrosion

Corrosion is a subject that concerns various disciplines, or in other words, it connects the elements of physics, chemistry, metallurgy, electronics and recording. Most of those involved are those who have a background in one of these sciences but not all of them. So an electrochemist does not have to explore the corrosion aspects in terms of metallurgy or engineering, and vice versa (Tretheway and chamberlain, 1991).

Gustavo, 2003 defines corrosion as a chemical / electrochemical reaction between a material, which is often metal or mixture and its environment which results in detonation of the material and its properties.

Another definition of corrosion is a collection of whole processes by means of which the metal or alloy used for structural material changes its shape from being metallic to some combination of conditions caused by interactions with its environment. Thus corrosion is also interpreted as damage or wear and tear of the material due to reactions with the environment supported by certain factors (Supomo, 2003).

From the various opinions of the definition of corrosion above, explicitly different based on the development of the science of corrosion. But it can be said to have the same essence, namely the phenomenon of material degradation (generally in metals) because it reacts with the environment.

The principle of corrosion

Some experts argue that corrosion does not only occur in metals, non-metals also experience corrosion which is then classified as non-metal corrosion. For example, paint color fading due to sunburn, rubberiness due to heat and weather, weathered wood construction due to mold and many other examples of non-metal corrosion.

While corrosion in metals can be divided into two classes, namely “wet” and “dry”. In dry corrosion, corrosion occurs in gases / metal surfaces (metal surface) and water does not much affect the reactions that occur. In wet, the interface is metal / solution. While based on its shape, corrosion in metals can be divided into two groups, namely corrosion “general / general” and “Localized / centralized” (Fontana, 1986).

Broadly speaking, the factors that influence the fast or slow corrosion process are :

A. Construction material

The material used in construction has a fundamental value in determining the age and strength of construction. Selection of suitable material will reduce the negative impact of corrosion.

B. Environmental / media conditions

The environmental conditions in which construction material will be made must also be considered. Because it is closely related to the process and speed of the corrosion media. Material for sea water conditions is different from fresh water conditions. So it can be said that the corrosion that arises is influenced by the corrosive media that are around.

C. Construction / arrangement

The form of construction that is usually ignored by some people has an effect on the corrosion process which has not a few negative impacts. Because more or less influence on the speed of corrosion.

D. Construction function

Steel construction for high temperature operations compared to low temperature operations will have different treatments in consideration of determining the material at the beginning.

Corrosion in the water environment is formed due to the incorporation of iron and oxygen in the air can occur due to the presence of water in the form of water vapor.

Most metals produced on a large scale for engineering purposes have volume defects, even pure metals that are free of all defects from the production process can still undergo selective corrosion at grain boundaries. All corrosion reactions in the water environment can be considered no different from the example of a simple wet corrosion cell, although the cell is part of the same metal surface of the anode and cathode which can usually be distinguished. Where we can expect that iron will be an anode when compared with a solution of hydrogen ions (iron dissolves in acid). The equations for the reaction are :

• When iron is dissolved

Fe (S) → Fe2 + + 2e

• When hydrogen gas is formed

2H + 2e → H2 (g)

• Overall reaction

Fe (S) + 2H + → Fe2 + + H2 (g)

Types of corrosion

Type depends greatly on differences in chemical composition in electrolytes and differences in composition of metals. In general, the type of corrosion can be divided into 4 types, including: General Corrosion Attack (evenly distributed corrosion), Localized Type Attack (centralized corrosion), Stress Associated Attack (stress corrosion), Movement Associated Attack (flow corrosion). The types that have been mentioned have their own specifications, for example factors causing corrosion and handling of corrosion (Jones, 1996).

Uniform corrosion

Uniform or commonly called uniform attack is a form of electrochemical corrosion that occurs with a high equivalent level of all parts of the surface being tested and often leaves a scale behind the surface or sediment. With a microscope it can be seen that the reduction and oxidation reactions that occur on the surface look more random. In general, uniforms occur in iron, steel and items made of silver. uniforms are generally more acceptable than other corrosion because uniform corrosion can be predicted and designed for relative ease.

Crevice corrosion

In the past, the use of the term crevice corrosion was restricted to attacks on alloys where the oxide was passive by aggressive ions such as chloride in cracks or hidden metal surface areas. Attacks under similar conditions on unpassified metals used to be called differential aeration. A gap is an attack that occurs because a portion of the metal surface is blocked from the environment compared to other parts of the metal that face electrolytes in large volumes.

Pitting corrosion

According to Tretheway and Chamberlain, 1991, pitting corrosion is local corrosion that selectively attacks metal surfaces that :

(a) The protective membrane is scratched or cracked due to mechanical treatment;

(b) Having a protrusion due to dislocation or slip caused by tensile stress experienced or remaining;

(c) Has a heterogeneous composition in the presence of inclusions, segregas or precipitation.

Observation of holes due to crevice corrosion can sometimes cause us confusion about the difference between crevice corrosion and sink corrosion. Corrosion wells can be distinguished from crevice corrosion in the triggering phase. Crevice corrosion is triggered by different concentrations of oxygen or ions in the electrolyte, while pitting corrosion (on a flat surface) is only triggered by metallurgical factors.

Stress (stress corrosion)

Stress-corrosion cracking (SCC) is a term given to intergranular or transgranular cracking of metals due to the combined activity of static and special environmental stresses.

This form of corrosion is very common in industrial environments and, despite intensive research that has been carried out for decades, we have only come to an understanding of the processes involved, while efforts to control themselves have often failed. In boiling water reactor technology, intergranular SCC in stainless steel pipe systems (type 304) is a major corrosion problem, while in pressurized water reactors the same material turns cracked when used as boric acid filler pipes and fuel filler pipes. The failure of stress corrosion on turbine blades made of stainless steel (type 304) is said to reach a rate of 4% per year (Tretheway and Chamberlain, 1991).

In the chemical industry, SCC in stainless steel due to chloride decay from heat insulators continues to be a problem, even though the cause is well known. In 1973, a single failure in stainless steel components caused a loss of one million dollars (Tretheway and Chamberlain, 1991). A similar problem continues to haunt the oil industry because deep pipes and high pressure require the use of high-strength steel that is known to be susceptible to SCC, especially when accompanied by hydrogen sulfide.

A barrier material has been used consistently in an effort to reduce corrosion in such situations, but failures, even though there are inhibitors still continue to be reported for up to 10 years since the material has proven ineffective (Tretheway and chamberlain, 1991).

Characteristics of water bacteria

Bacteria have a way of reproducing asexually by dividing themselves binary. Bacteria can divide within 20 minutes if the environmental conditions are very supportive. The process of bacterial reproduction undergoes several phases, such as the phase of transformation, conjugation and transduction. The size of the bacteria can be determined by using a microscope equipped with an ocular micrometer lens. Bacterial size is expressed in microns (1 micron = 0.001 mm). The length of bacteria generally ranges from 0.5 to 3.0 microns with diameters ranging from 0.1 to 0.2 microns.

According to Holt et al., 1997 based on their way of life, bacteria are distinguished from being heterotrophic and autotrophic.

Heterotrophic bacteria

Heterotrophic bacteria do not have chlorophyll, their lives are highly dependent on the organic material that is around them, so that these bacteria cannot convert inorganic material into organic matter. These bacteria are classified into several types, such as :

1) parasitic bacteria

These types of bacteria obtain food from the organisms they are riding in (host). Example : family Spirochaetaceae

2) Saprophytic bacteria (saprobacter)

Sprofit bacteria are bacteria that need the rest of the organism that has been used as food. These bacteria remodel organic matter into inorganic through imperfect respiration and fermentation using gases such as: CO2, H2, CH4 (methane), N2, H2S and NH3. Examples of these bacteria are as follows :

Escherichia coli , in certain circumstances can decompose ant acid (HCOOH) to CO2 and H2O.

 Metahanobacterium omelianskii and methanobacterium ruminatum decompose vinegar (CH3COOH) into methane (CH4) and CO2.

Thiobacillus denitrificans, decomposes nitrates or nitrites and produces N2, this bacterium converts ferro to ferric so that it influences corrosion resistance.

 Desulfovibrio desulvoricans, can decompose carcasses and decompose sulfate in a wet state and produce H2S

3) Pathogenic bacteria

Is a parasitic bacterium that can cause various diseases in the host that is seized, as in the following example :

In humans: Salmonella typhi, Vibrio comma, Neisseria gonorrhoeae, etc.

In plants: Bacterium papaya, Pseudomonas cattleyae, etc.

 In Animals: Bacillus anthracis, M. Avium etc.

4) Apatogen bacteria

Is a bacterium that does not cause disease in the host, for example: Streptomyces griseus.

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