technical-information/other-ways-of-protection-against-corrosion/



Technical Information

What is corrosion?

It is the destruction of a material caused by a chemical, or electrochemical reaction with its environment. The material and its environment form an element of the corrosion which is influenced by the electrical potential of the metallic materials, and also by the differences in temperature and concentration.

All metals have a tendency to return to the stable condition in which they are found in nature, that is to say, thermodynamically speaking, a stable state.

This tendency is much stronger in the less stable metals, which means that they can be classified in accordance to their stability in order of decreasing activity and increasing potential.


Corrosion, the worst enemy

Many people think that with the use of stainless steel and reinforced plastic hulls there’s no need to worry anymore about the protection of their vessel. But, the modern materials have still not managed to defeat our boat’s worst enemy, corrosion.


Causes of corrosion

In reality, corrosion is a superficial deterioration which metals suffer due to electrical, chemical or mechanical phenomena.

The potential difference between the metals and the existence of an electrolyte, in contact with them, makes that one of the metals has to act as an anode, suffering from ionization or deterioration. The deterioration that can be produced in a metal, is proportional to the flow of electrons it receives, and this depends on the potential and the resistance of the metal. The most important agent of corrosion between two metals in contact, submerged in an electrolyte, is the potential, which depends on the natural chemistry of the anode and the cathode, the former being attacked or dissolved while the latter remains unaffected.

The surroundings, in this case seawater, can obtain a number of chemical products more or less aggressive which, when combined with the change of temperature, attack various materials, corroding the surface or the structure. We find ourselves, therefore, faced with a chemical type corrosion which we must combat with the appropriate protection. The fatigue of the pieces, due to rubbing, abrasion or movement, reduces the protective layer and they deteriorate. This is also known as mechanical corrosion.


Types of corrosion

Not all corrosive phenomena are identical duet to the fact that there exist various types of corrosion. We can distinguish:

Global corrosion

This type attacks the whole surface of the piece uniformly. In general, this is not a serious form of corrosion as the rust produced serves as a protective layer to stop the corrosion advancing and weakening the attacked piece.

Porous corrosion

This type generally attacks determined areas of the piece, forming cracks or fissures. It has to be classified as serious when the crack is produced in certain parts of the piece and especially if it appears in joints or grooves as it produces a fatigue in the metal which can cause its breakage.

There is no doubt that this is the most serious kind of corrosion as it’s impossible to know the depth the corrosion has reached in the damaged area.

Selective corrosion

Defects or defects in the alloy cause the piece to become porous and finally yield. This type of corrosion is more commonly found in melted metals or alloys.

         


Protection against corrosion

All the problems related to corrosion must be considered from 3 fundamental aspects:

1. Undoubtedly the most important aspect is the one concerning the technical drawings. As the corrosion usually starts in the same moment as the design and construction of the vessel, it its different integral parts, it should already then been take into account. It’s very important to oversee the positioning of the different metals, making sure that the correct screwing method is used and avoiding the different metals from coming into direct contact.

Also the engines and all types of electrical installations demand the correct connection to the bulk. It’s necessary that the radio equipment has its own earth connection consisting of a special plaque because it’s a serious error to use a passing valve or other means whose deterioration could be vital for the security of the vessel. Another important factor is the adequacy of the paint and antifoulings with the metal of the full and the stern drives.

2. The cause of the corrosion is the second aspect to be bear in mind. In general terms, the causes are always due to the failure in compliance with the norms concerning the electrical installations of the boat, which permit the passage of current through vital parts of the vessel. The first thing to do is to check the bulk polarity of the diverse circuits, then it is necessary to verify if current is lost when various devices are used, measuring with a tester if the consumed current is higher than the current that should be used by each device.

You also have to make sure that there are no different metals in contact on board. Something that is rarely taken into consideration, but is very important, is the effect of the ventilation differential produced by humidity in hidden areas, which is a phenomenon that causes a high level of corrosion. This type of corrosion is frequent in the screws that go through wood or plastic and that allow humidity to penetrate due to a lack of adequate waterproofing.

3. Lastly, the third point, is the protection. This protection is particularly important in hulls where it has not been possible to avoid some of the causes of corrosion originated from the construction, for example the necessity to put 2 different metals very close to each other. The methods of protection against corrosion are based on the appropriate selection of an alloy (pure metals or with chrome and inhibitors) and an adequate structure, and also covering the surface with special materials.

For years, the covering has been applied to the pieces susceptible to be attacked, such as the galvanization, but the treatment has to be done very well, in order to offer the necessary protection and besides that, rubbing or knocks can deteriorate the layer leaving the corrosion to act underneath it, which is frequently more serious.

Another type of covering is paint that, when it’s applied correctly and regularly, is one of the best protectors. However, for certain defined pieces, the cathodic protection is the most efficient one. As we will explain, it consists in sacrificing metals which can be controlled and substituted in favour of those metals to be protected.


Electrical potential of the metals

As we have seen, the most important corrosive phenomenon is the electrical type. It is especially with the use of modern metals that practically all corrosions that have to be combated in the sea, are of this type.

When 2 metals are in contact through a liquid, a galvanic or electrical corrosion is produced. The degree of this corrosion depends fundamentally on the difference in electrical potential existing between the 2 metals in contact.

The lower (more negative) the potential of the metal, the easier it is corroded. Equally, the bigger the difference in the potential between the 2 contacting metals, the higher the galvanic corrosion between them. As a result the metal with the lower potential is being damaged.

The table indicates the electrical potential, of the most interesting metals, that is produced when they are submerged in salt water at a temperature of 25º C.

Electrical potential of some metals in salt water at 25 °C

Metals

Electrical potential in V

Sodium -2.71
Magnesium -2.38
Aluminium -1.67
Manganese -1.05
Zinc -0.76
Chrome  -0.71
Iron -0.44
Cadmium -0.40
Nickel -0.25
Tin -0.14
Lead -0.13
Hydrogen 0
Copper +0.35
Silver +0.80
Mercury  +0.85

The correct positioning of the anodes

To protect a certain piece, we use metals with a negative electrical potential, and lower than the electrical potential of the metal of the piece, placed in contact with the metal to be protected. These metals that are used for protection receive the name of anodes.

Anodes are manufactured in different shapes and sizes, specifically built for their use in vessels. As a general norm, Zinc or Aluminium are used for anodes to be used in salt water and Magnesium alloys are used for boats navigating in fresh water. All the metal parts of the vessel must be in contact with an anode for which we use bolts and flexes or connection cables directly with the piece to be protected. These connectors need to have a cross section of at least 4 or 5 mm.

Something to bear in mind is the position of the anodes. The anodes must always run parallel to the longitude of the boat, because only in this way the maximum performance is obtained. The anodes and the points of contact with the pieces should in no way be painted. Bolts or screws should be used to attach them to the object to facilitate their replacement.

Amongst the elements which require special protection are:

Propeller and transmission shaft: in the case of the steering shaft, a special anode must be used. This anode has to be placed about 3 or 4 mm from the support pad of the propeller. But, if the horn is metallic, the anode has to be placed near it. If the horn is made of a non-conducting material (nylon, rubber…) the fixing bolt of the anode should be connected with the engine block.

Metallic rudders: These require a circular anode to be fixed in the centre of the blade.

Metallic keels: To protect this part of the boat, an anode has to be placed on each side, held with bolts screwed onto the keel.

Flaps of motorboats: An anode is fixed onto the surface of each flap, longitudinally with respect to the boat. If the flaps are made of Aluminium, the screws used to fix the anodes should be galvanized.

Anodes should be replaces as soon as they show signs of wear and it’s essential that they are replaced when they reach only 20% of their original weight.


What can be protected by cathodic protection?

  • ENGINES AND THEIR COOLERS.
  • SHIPS.
  • SUBMARINE PIPES.
  • BUOYS.
  • CHAINS.
  • JETTIES.
  • TANKS.
  • CONDENSERS.
  • BURIED TUBES.
  • CISTERNS.
  • METAL TOWERS.

And, in general, any metal which is in contact with a hostile medium.


How can we fight corrosion?

To get corrosion in a metal structure, it has to be in contact with a corrosive medium. This medium can be the atmosphere as in the case of atmospheric corrosion, or an electrolyte (earth, water or another hostile medium) in the case of galvanic corrosion.

The first method we can use to avoid corrosion can be to insulate the metallic structure from the corrosive medium using an insulating or more stable coating. This type of protection is called PASSIVE PROTECTION.

In the modern industry we use many types of insulated coverings: resins, asphalt, vinyl paint, epoxy, chloro-rubbers… In all of these coverings, the value of resistance, flexibility, adherence, point of softening, power of absorption of water… play an important role in the selection of this type of protection.

PASSIVE PROTECTIONis the system in which the metal is covered by another metal of higher resistance to the corrosion, or is capable of neutralizing the medium which surrounds it.

There are many ways of achieving it: by using electrolytes, by immersion, by sprinkling… To choose the metal and the way of covering, a series of factors have to be taken into consideration. For example: the porosity of the carrier metal and its electrochemical behaviour with respect to the base metal.

Another method of anticorrosive protection – the most important one – is CATHODIC PROTECTION.

In the formation of galvanic batteries, by introducing a metal structure in an electrolyte, the destruction of the structure is caused by the dissolving of the anodes apposite the cathodes, as electrons are carried from one to the other and this in the end starts dissolving the metallic ion.

Therefore, this process gives us an idea of the method that we have to use to protect the cathode, converting the metal structure that needs protection, into the cathode of a galvanic battery or electric circuit. We can do this by turning to the electrochemical series of the metals and choosing a more electronegative metal to act as the anode or by connecting the structure to the negative pole of a continuous current generator, whose positive pole is introduced into the electrolyte in question, by using an anode which generally doesn’t dissolve or dissolves very slowly. With this method we can pass a controllable voltage to the structure to be protected at any moment in the installation’s life.

The first method described is known as PROTECTION BY SACRIFICIAL ANODES and the second method is known as IMPRESSED CURRENT.

There are various raw materials used for the manufacturing of sacrificial anodes, however alloys of Zinc, Aluminium and magnesium are the most common ones. Pure Magnesium can’t be used in systems of cathodic protection in seawater due to its rapid deterioration, although some of its alloys are used.

Certain alloys of Aluminium are used, but the most commonly used sacrificial anodes are of Zinc, which you don’t have to control constantly and which also supplies a continuous and efficient current. An imperative for this type of anode is the purity of the base metal. The composition must be made following the current specifications. Iron is one of the most damaging impurities to Zinc’s activity as an anode. A maximum of 50ppm of Fe can be tolerated if at the same time certain quantities of Cd and Al are present.


External protection of a vessel with sacrificial anodes

To obtain a good anticorrosive protection for a ship, we recommend you to use our Zinc or Aluminium anodes. In that way you can protect:

  • The stern, the rudder and the axle.
  • The hull.
  • The boxes and sea valves .
  • The propellers.
  • The nozzles.

To obtain a quotation for the protection of the hull, we’d need to receive the below indicated information :

1. General layout of the ship.
2. Wet surface.
3. Paint specification of the wet surface.
4. Date of entering the dock.


Is painting enough?

Everyone who has experience in the maintenance of ships or structures is convinced that, passed some time, there is a corrosive action, usually in very severe meteorological conditions, especially in the submerged parts of a ship or structure, due to the quantity of paint that peels off and causes a degradation in the protection. This peeling off can be caused by collisions against q quay, a tug, anchors…

In general, if we want to obtain a good protection, it’s absolutely necessary to combine the paint with another protection system. The best paint can never prevent completely a diffusion of water and oxygen in the areas of the submerged steel, which helps the rusting process.

The best protection against corrosion is a combination of high-quality paint and a good protection with sacrificial anodes, because the paint is a barrier which reduces the current that has to be delivered by the cathodic protector. It should be taken into consideration that all paints that are used must have a high alkaline resistance because the cathodic protection is accompanied by a light alkaline.


Cathodic protection in a ship´s tanks.

ZINETI’s Zinc or Aluminium anodes for the protection of ballast tanks and ballast and cargo tanks.

Since the Second World War, the classic anode used for the protection of ballast tanks was made of Magnesium. This metal was extraordinarily well suited, having an electronegative force of 700mV over the polarized steel.

The greatest advantage was its rapid polarization power. The problem was an overproduction caused by the emission of hydrogen and its electrochemical power, approx. 55%.

Nowadays, this metal is rarely used due to the restrictions imposed on it by the classification associations. However, it is still used in tanks with platforms.

Zinc and Aluminium show a relatively low potential, the condition voltage being in the order of 230 till 300mW over polarized steel. However they have an efficiency of 80%. Experience shows, regarding Zinc Anodes, that the importance of the steel contamination is very little.

The result of the Aluminium anodes depends largely on the additives (Indium and Zinc), which immunize the tendency of the steel to form a rust film with passive effect.

Both Aluminium and Zinc have a large and satisfactory history as a cathodic protection material. One of the advantages of Aluminium is that, in its installation, only a third of the weight is used compared with Zinc, which is important when considering the dead weight of the ship and also taking into consideration that the cost of the installation is calculated by the shipyard according to the installed weight. One disadvantage, in agreement with the classification associations, is the possibility of sparks, which gives rise to the situation whereby the aforementioned associations impose certain restrictions on the use of Aluminium anodes, but not for those made of Zinc. The distribution of theses Aluminium anodes, due to the possibility of sparks, must be studied so that they are placed in low areas of the tanks.

Precise data needed to make a calculation for the protection of the tanks:

1. General layout, master frame, longitudinal profile, principal screens and plan of capacity.
2. Type of tanks, if only ballast, or segregated ballast in petrol tanks.
3. Density of current required.
4. Life expectancy of the anodes.
5. Paint used on tanks.
6. Expected ballast.


Other ways of protection against corrosion

The most common ways to protect an embedded or underwater structure are:

-          By applying a correct painting scheme

-          By the use of Cathodic protection

o   With sacrificial anodes

o   With impressed currents

-           By  implementing a combination of painting and cathodic protection


Methods for calculating the hull's protection

1. Density of the current:

The electrochemical and mechanical conditions have a huge influence on the design process of the Cathodic Protection systems.

Other conditions to consider are the temperature, the salinity, resistance to the oxygen dissolution,…

The Cathodic Protection system´s specifications are generally expressed by the required current density to give a sufficient potential to the surface that needs to be protected.

The normal current density for a hull can vary from 10 m A/m2 to 30 m A/m2,  though, this can also increase or reduce in special cases, depending on the painting scheme of the underwater side.

2. Service life of an anode

Zinc Anodes are generally calculated for being used during one, two or three years, depending on the size, form and weight of the anode, and this also applies to the Aluminium Anodes. 

3. Quantity of anodes

Quantity:


 

The total weight of the anodes:

Quantity and type of Anodes to compensate the entire current and the required weight:


4. Location of the anodes:

 

 

The Anodes should be conveniently installed around the underwaterline of the hull, increasing the quantity of the Anodes in the stern side because that area has a higher current density generated by the propeller. It is also recommended to install anodes on the sea chest.

This practice may vary depending on the vessel´s geometry, the painting scheme, and even depending on the vessel´s duty. 


5. Practical recommendations for the installation:

·         Our Anodes have an insert for direct welding to the hull

·         The anode distribution should be done according to our design

·         In no case the anodes should be painted 


Contact
ZINETI, S.A.U.
Larrondo Beheko Etorbidea
Edificio 3 - Nave P-4
48180 - Loiu (Vizcaya) - España

43º17'33''N 2º54'46'' W
+34 944 53 59 16
+34 944 53 61 49
info@zineti.com
Web: www.zineti.com
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