A boat or ship at sea is prone to corrosion by aggressive and environmental factors.
At Antala we have several anti-corrosives that can help you maintain optimal working conditions and safety of your vessels for long periods of time.
What is corrosion?
Table of Contents
- What is corrosion?
- What factors cause corrosion in the marine sector?
- What are the main causes that erodes the helm on a boat?
- What is marine corrosion?
- Anti-corrosives for the marine sector
Corrosion is an electrochemical process in which a metal or an alloy reacts with its environment, resulting in deterioration or destruction.
Corrosion phenomena can be classified according to many different criteria’s:
- Attack morphology: uniform, localized, intergranular, …
- Attacking medium: corrosion by acids, aerobic, atmospheric, …
- Physical actions that together with the chemical ones cause the deterioration: under tension, by fatigue and by cavitation.
- Reaction mechanisms: direct or electrochemical oxidation.
What factors cause corrosion in the marine sector?
Different metals must come into contact with each other. For corrosion to occur depending on the electrical potential of the metals, one of the metals must behave as an anode (oxidized) and another as a cathode (it is reduced).
The metals according to their internal constitution of phases can be:
With a uniform concentration, thus avoiding corrosion (the uniform concept is ideal because the metals are imperfect), or non-uniform, generating corrosion over time.
The phases of the metal itself contain potential differences that inevitably ends up showing signs of corrosion.
The metals according to their surface finish may show a greater or lesser degree of corrosion, if it contains chips, scratches or any defect that modifies the surface layer of the metal will cause corrosion, although polish or limemos corrosion will always manifest because the metals are not perfect, we can only control the corrosion rate.
All the tools or treatments for the surface finish cause a wear on the metal favoring the corrosion of the metal.
The superficial atoms of the metal to be cut, filed or treated, ultimately change their behaviour as far as potential is concerned.
The atoms that make up the metal in the most superficial layer behave like an anode (they are oxidized) and the internal grains behave like a cathode, producing corrosion at a microscopic level.
The tensions to which the cold metal is subjected generate a deformation in the metal, this deformed part will behave as an anode and the part not deformed as a cathode.
What are the main causes that erodes the helm on a boat?
The following factors are the main culprits of ship corrosion, although these vary according to the properties of the marine environment.
Corrosion of ships by friction
Friction corrosion (Fretting-Corrosion) is the deterioration produced at the interface between two surfaces in contact. One of them a metal is in contact with air and a certain degree of humidity, causing a relative displacement to take place, even if it is a minimum of one in respect to the other, this type of corrosion can still occur.
If we expand this on a microscopic scale, we can verify that friction corrosion is not performed uniformly, but only occurs at certain points (a certain % of the surface).
The corrosion process occurs as follows.
First imagine the union of two hull plates where vibration is a relative movement between them, at the time there is a lifting of particles of solid products from oxidized products.
Secondly, the protective layer area to be cleaned, favors the electrochemical corrosion by anodic and cathodic depolarization, this again generates solid particles of oxide in the area that finally comes off again by friction.
Factors that influence friction corrosion:
- Aggressiveness of the medium
- Abrasive nature of the detached particles
- Frequency of vibration
- Number of cycles
- Displacement length
- Applied load
Corrosion of boats by erosion
Erosion may be the main corrosion issue that affects the hull since it is defined as the deterioration caused by fluids with or without solids in suspension that move above a certain threshold value of speed on a metallic surface.
The higher the velocity of the fluid, the greater the attack of the corrosion on the material, since it produces the lifting of protective layers by erosion and even of metallic material.
Corrosion of vessels by cavitation
Cavitation corrosion occurs when a metal element in contact with liquids is subject to vibration, or the liquid circulates at a very high speed causing an extreme case of corrosion.
This type of corrosion in ships occurs in turbines, cooling jackets or in the area of the propeller and mainly adjacent surfaces, since the vibrations of the propeller or the high speed of the liquid in turbulent regime generates a small portion of liquid to steam due to the pressure drop being lower than its vapor tension.
Cavitation damage looks similar to pitting by corrosion, but the damaged areas are more compact, and the surface is more irregular in the case of cavitation. Damage by cavitation is partially attributed to the effects of mechanical wear.
Corrosion intervenes when the collapse of the bubble destroys the protective film as shown by the steps below:
- A cavitation bubble forms on the protective film
- The collapse of the bubble causes the local destruction of the film
- The unprotected surface of the metal is exposed to the corrosive medium and a new film is formed by a corrosion reaction
- A new bubble is formed in the same place, due to the increase in the nucleating power of the irregular surface
- The collapse of the new bubble destroys the film again
- The film is formed again and the process repeats indefinitely to form quite deep holes.
What is marine corrosion?
Marine corrosion happens to metallic materials that are in direct contact with seawater. To analyse the effects, we can identify certain influencing factors:
Factors that influence the corrosion of marine vessels
The salinity of the medium
The dissolved salts (mainly NaCl) in the salt water provide a high conductivity and an increase in the solubility of O2. Cl- anions can locally break passive films generating pitting corrosion phenomena. Corrosion due to salinity can account for 20% of total corrosion losses.
The temperature increase can facilitate the diffusion of the O2, this accelerates the speed of corrosion. Another point to take into account, is the greater proliferation of microorganisms, in areas of warm temperatures.
The Speed of Sea Water
As the speed increases, the likelihood of corrosion-erosion phenomena due to turbulence is greatly increased.
Corrosion in the marine environment also varies depending on the depth, in which the metal is submerged.
The point of greatest corrosion is the area called “Region of splash”, located just above the water line. From point two, as we increase the depth, the amount of O2 decreases until the depth reaches a minimum of 200 to 1000m. At greater depths a maximum occurs in the relative loss of metal due to galvanic currents and the presence of sulfate-reducing bacteria and anaerobic ferrobacteria that oxidize sulfur compounds causing sulfuric acid. Once point 4 has been passed, the phenomenon of cathodic polarization increases and corrosion decreases to 1500m. From this depth, the corrosion loss is constant since the presence of O2 is practically zero.
Anti-corrosives for the marine sector
Here are some of our recommended solutions to prevent corrosion problems on ships.
We recommend that you get in contact with Antala for more information and our experts will be able to help you choose the best solution for your corrosion problem.
in highly saline
|Protection of spare parts – boats|
Protection of windlass anchor
|TECTYL MULTIPURPOSE 506|
|Inboard and outboard|
motor protection, chrome plated
and mineral oils,
fats and coatings
that removes contaminants
such as grease, oils and coatings