Inroduction

Corrosion is the gradual destruction of material, usually metals, by chemical reaction with its environment. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen. Rusting, the formation of iron oxides is a well-known example of electrochemical corrosion. This type of damage typically produces oxide(s) or salt(s) of the original metal. Corrosion can also occur in materials other than metals, such as ceramics or polymers, although in this context, the term degradation is more common. Corrosion degrades the useful properties of materials and structures including strength, appearance and permeability to liquids and gases.

Many structural alloys corrode merely from exposure to moisture in air, but the process can be strongly affected by exposure to certain substances. Corrosion can be concentrated locally to form a pit or crack, or it can extend across a wide area more or less uniformly corroding the surface. Because corrosion is a diffusion-controlled process, it occurs on exposed surfaces. As a result, methods to reduce the activity of the exposed surface, such as passivation and chromate conversion, can increase a material's corrosion resistance. However, some corrosion mechanisms are less visible and less predictable.

We had planned to choose the corrosion on the ships for our experiment

Corrosion by sea water 

Corrosion by sea water, aqueous corrosion, is an electrochemical process, and all metals and alloys when in contact with sea water have a specific electrical potential (or corrosion potential) at a specific level of sea water acidity or alkalinity - the pH.   

This typical diagram shows the regions where the metal will freely corrode; the region of passivation where stable oxide or other films form and the corrosion process is stifled; the region of pitting corrosion where the corrosion potential of the metal exceeds that of its oxide; and the region of immunity where the metal is normally fully safe to use.  More resistant alloys mean less corrosion, metals like gold platinum and tantalum can resist virtually all corrosion, but for marine service the final choice will always be a compromise with cost. 

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Sea water, by virtue of its chloride content, is a most efficient electrolyte.  The omni-presence of oxygen in marine atmospheres, sea spray and splash zones at the water-line, and sometimes surprisingly at much greater depths, increases the aggressiveness of salt attack.  The differential concentration of oxygen dissolved at the waterline or in a droplet of salt spray creates a cell in which attack is concentrated where the oxygen concentration is lowest.  Crevices which allow ingress of water and chlorides but from which oxygen is excluded rapidly become anodic and acidic and are hidden start points of corrosion. 

Problem Statement: How does corrosion affected the ship and the solution for prevent corrosion in ships?

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i. SURAFACE ENGINEERING METHOD

We have chosen ship as subject for our experiment. We chose ship because this subject has many corrosion problems which give the ships so many problems due to corrosion on the body and certain important parts of the ship. There is few big problems faced by the ships. Now we will show few examples:

Corrosion of turbine fan.

(One of the most important parts of ship)

Corrosion of ship.

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ii. Importance of selected process

We have chosen one widely used method that is the painting method. We choose this method because this is one of oldest and most effective method to prevent ship corrosion. Painting system still effective in prevent corrosion because the material used for this system is widely available in every country. These are the importance of painting system:

Paints contain “binders” it causes the paint coats the physical surface of the ship. This resists the water and humidity which causes corrosion.

Paint forms an upper layer on the engineering surface. Basically the paint contains oil. In the general science oil and water cannot be mixed hence this method is used so the water and the paint not mixed and slow down the activity of corrosion.

There are two types of paint convertible that involve coating by chemical reaction and non-convertible coating that not involve chemical reaction.

-In convertible coating that involve chemical reaction, it chemically react with water and oxygen and produce a chemical shield to prevent corrosion

-In non-convertible coating there is no chemical reaction. It produces a protective layer which is solvent layer. Solvent is lost from the film by evaporation and the film dry to the touch

Paints contain pigment and extenders that are used in the form of fine powders which are disperse into the binders to various particle sizes. It makes the painting layer thicker.

Paints are anti-corrosive. Paints contain strength hard wearing impact.

iii. Details of the process

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Underwater anti-corrosion paints

• The outside of a ship’s hull is generally coated with an anticorrosive paint system and an antifouling paint.

• The better the anticorrosive system the longer the vessel will be protected against corrosion, and the less will be the reliance on cathode protection.

Underwater anti-corrosion coatings

There are essentially 4 mechanisms by which a coating can provide protection:

– Physical barrier properties

– Ionic resistance

– Adhesion

– Chemical inhibition

Physical Barrier Properties

• Corrosion cannot proceed more rapidly than the slowest mechanistic step. Therefore limiting the arrival of one of the reactants will slow the overall corrosion process.

• It was assumed for many years that organic coatings protected byacting as a barrier to water and oxygen.

Ionic Resistance

• In many cases, it is found that the corrosion rate of painted steel is much slower than that allowed by the supply of water and oxygen through the coating. Some other process must therefore be rate controlling.

• Movement of ions is critical in the charge balance required for the corrosion reactions: Fe → → → → Fe2+ + 2 e- H2O + ½ O2 + 2 e- → → → → 2 OH-

• Transport of Na+ and Cl-ions to the cathodic and anodic sites is also important.

• Research indicates that where the electrical resistance of a paint is high, good corrosion protection is more likely because the ionic transport is impeded.

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iv. Comparison between Painting method and Sacrificial Anode method.

Sacrificial Anode CHARACTERISTICS PAINTING METHOD

Expensive Cost Cheaper

Simple to install Installation Hard

Simple to maintain Maintenance Must look after for certain period

No power supply needed Power Supply Workers

Until the metals solvent Last long Until the paints layer dissolve

Advantage Disadvantage

Produce a barrier upper layer

A very simple method

Low cost than other preventative methods

Easy to maintenances

Easily dissolve

Very short durability

Need a effective maintenance

Take time to the wet paint dry

Corrode easily when the mixture of the paint is not in correct odour

v. Advantages and Disadvantages

vi. Characterisation technique

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Introduction to Paint

• Paints are mixtures of many raw materials. The major components are:

– Binder (other terms used include: vehicle, medium, resin, film former, and polymer)

– Pigment or Extender.

– Solvent.

• The first two form the final dry paint film.

• Solvent is only necessary to facilitate application and initial film formation, it leaves the film by evaporation and can therefore be considered an expensive waste product.

Binders

• Binders (or “Resins”) are the film forming components of paint.

• They determine the characteristics of the coating, both physical and chemical.

• Paints are generally named after their binder component, (e.g. epoxy paints, chlorinated rubber paints, alkyd paints etc.).

Liquid Solid Solid epoxy with Liquid curing agent

Polyester Coal tar resin

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Binder types

• Binders fall into two classes: - convertible - non-convertible

• In convertible coatings there is a chemical reaction involved.

• In non-convertible coatings there is no chemical reaction, only loss of solvent.

Binders - Convertible

• Stage One: Solvent is lost from the film by evaporation and the film becomes dry to the touch

. • Stage Two: The film progressively becomes more chemically complex by :

– Reaction with atmospheric oxygen, known as oxidation

– Reaction with an added chemical curing agent.

– Reaction with wate (generally moisture in the atmosphere)

– Artificial heating

– Radiation curing (e.g. ultra violet)

Binders - Convertible

Generic types of binders which are in this category include:-

• Oleo resinous varnishes }

• Oil modified alkyd resins} Dry by oxidation

• Urethane oil/alkyd resins} (air drying resins)

• Epoxy ester resins}

• Two component epoxy resins} Dry by chemical

• Two component polyurethane resins} curing

• Moisture cured polyurethane resins} Dry by water

• Silicate resins} absorption

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Binders - Non convertible

• Simple solutions of various resins or polymers are dissolved in suitable solvent(s).

• Drying is by solvent evaporation – there is no chemical change

• Generic types of binders which are in this category include:-

- Chlorinated Rubber

- Vinyl

- Bituminous

- Cellulose Note: Antifouling’s are a special chemistry,

Pigments and Extenders

• Pigments and extenders are used in the form of fine powders which are dispersed into the binder to various particle sizes.

• These materials can be divided into the following types:

- – Anticorrosive pigments

– Barrier pigments

– Colouring pigments

– Extenders

Pigments and Extenders - Anticorrosive pigments

• These prevent corrosion of metals by chemical and electrochemical means (chemical inhibition):

– Red lead

– Zinc

– Zinc phosphate

– Zinc chromate

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Solvents

• Solvents are used in paints principally to facilitate application.

• Their function is to dissolve the binder and reduce the viscosity of the paint to a level which is suitable for the various methods of application, i.e. Brush, roller, conventional spray and airless spray.

• After application the solvent evaporates.

• Solvent therefore is a high cost waste material.

Examples of solvent:

• White spirit

• Xylene

• Butane

• Iso Propyl Alcohol

• 1, 1, 1 trichloroethylene

• Acetone

• Methyl ethyl ketone (MEK)

vii. OTHER ALTERNATIVE

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Insulation

Many schemes are used to insulate metal boats.  Insulation is mentioned here in the context of corrosion prevention mainly to point out that regardless of the type used, insulation is NOT to be considered an effective protection against corrosion.  As with anywhere else on a metal boat, epoxy paint is the best barrier against corrosion.

 

Zincs

Zincs are essential on any metal hull for galvanic protection of the underwater metals (protection against galvanic attack of a less noble metal by a more noble metal), as well as for protection against stray current corrosion.

 

Aluminium Hulls

All manufacturers of out-drives and outboard motors provide for installation of zinc anodes of a specific type and size on their equipment, for protection of the aluminum and other metal components immersed in sea water.    

 

Bonding

Bonding is the practice of tying all of the underwater metals together with wires or bonding strips.  It is done in order to 'theoretically' bring all of the underwater metals to the same potential, and aim that collective potential at a single large zinc.  It is also done in order that no single metal object will have a different potential than surrounding metal objects for the sake of shock prevention.  

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CONCLUSSION

As a conclusion we know that painting is one of the best corrosion preventative methods. It is commonly used because it is an effective method also. This method has easier maintenance then other and this method also so simple to handle. If we do not have effective maintenance on the corrosion it will damage the object. The main cause of corrosion is combination of oxygen and water (H2, O). The corrosion preventative is having a great resistance to the water and oxygen so it can prevent the corrosion.

REFERENCE

Internet: http://en.wikipedia.org/wiki/Corrosion Book:  handbook "Corrosion Prevention and Control

Friends Star newspaper (25 march 2003) Library of POLITEKNIK IBRAHIM SULTAN

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