Ship corroison

Protection against Corrosion the

ships

Present: Erfan Zaker EsfahaniEmail: [email protected]: ---------------Lecturer: Dr. shams

University: najaf abad university of

iran-esfahanNumber of student: 890914688

Introduction1

Corrosion Processes2

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Introduction to Paint4

Anticorrosion Coatings5

outline

Control of Corrosion

Introduction

Corrosion can have disastrous consequences ….

Protection of Ships against Corrosion

---Effective corrosion control in hull structure is one of the most important features for the shipping industry to ensure reliability and minimize risk and major removal of hull structure.

---The problem of hull corrosion has become a great concern for ship owners as they have increasingly recognized the need to protect their investment and demonstrate a commitment to safeguard the structural integrity of ships.

Protection of Ships against Corrosion: Ballast Tanks

Corrosion management is concerned with the development, implementation, review and maintenance of the corrosion policy. The corrosion policy provides a structural framework for identification of risks associated with corrosion and the development and operation of suitable risk control measures.

“Erika” was a 25 years old, single hull tanker, which sank 40 miles off the Brittany coast, causing widespread

pollution, in 2000.

The design and maintenance of ships requires a knowledge of corrosion:

– Specification and life-time (Fatigue life, use of HTS etc.)

– Age-related defects of single and double hull tankers

– “Survey-friendly” design (ease of access to inspect and repair)

– Quality of build and outfit (fit up of blocks, quality of finishes)

– Tank cargoes (and tank ullage space)

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• The driving force behind corrosion of metal objects is the desire of metallic elements to return to the state they are predominantly found in nature e.g oxides.

• Extraction of a pure metal from its ore state requires energy, and it is this energy which makes metals inherently unstable and seek to react with their environment.

• Metals which have a higher energy input in their production processes are more susceptible to corrosion, and have a lower electrical potential.

Corrosion Processes

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– Atmospheric

– Immersed

– Water

– Contaminants in the water (eg salts)

– Oxygen

• There are 2 main types of corrosion on ships:

• The three essential elements necessary for corrosion to occur are:

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–Diffusion

– Temperature

– Conductivity

– Type of ions

– Acidity and Alkalinity

– Electrochemical potential

• The main factors which influence the rate of corrosion are:

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Other Types of Corrosion: Cavitation

Blade Tip Cavitation

Sheet Cavitation

Navy Model Propeller 5236

Flow velocities at the tip are fastest so that pressure drop occurs at the tip first.

Large and stable region of cavitation covering the suction face of propeller.

Consequences of Cavitation

1) Low propeller efficiency (Thrust reduction)2) Propeller erosion (mechanical erosion as bubbles collapse, up to 180 ton/in² pressure)3) Vibration due to uneven loading 4) Cavitation noise due to impulsion by the bubble collapse

Propeller Cavitation

Propeller Cavitation

• Preventing Cavitation

- Remove fouling, nicks and scratch.- Increase or decrease the engine RPM smoothly to

avoid

an abrupt change in thrust.

-Keep appropriate pitch setting for controllable pitch

propeller- For submarines, diving to deeper depths will delay

or prevent cavitation as hydrostatic pressure

increases.

Propeller Cavitation

• Ventilation

- If a propeller or rudder operates too close to the water surface, surface air or exhaust gases are drawn into the propeller blade due to the localized low pressure around propeller. The prop “digs a hole” in the water.

- The load on the propeller is reduced by the mixing of air or exhaust gases into the water causing effects similar to those for cavitation.

- Ventilation often occurs in ships in a very light condition(small draft), in rough seas, or during hard turns.

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A Typical Onsite Repair on a 4 x Blade Propeller  

Propeller impact damage

inspected & recorded 

Propeller hub cleaned &

inspected for cracks  

Propeller blades cleaned &

inspected for cracks

Propeller repaired and final polish

applied

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Types of Corrosion: Microbiologically Induced Corrosion (MIC)

WHAT IS MIC?

MIC is corrosion initiated or accelerated by microorganisms.

MIC is caused by specific genera of bacteria which feed on nutrients and other elements found in Fresh and Salt water.

It is generally understood that microorganisms (bacteria and fungi)are found living in almost every aqueous (water)environment on earth, but this does not mean that all species are directly or indirectly corrosive to steel.Microorganisms require water to propagate (live)…

NO WATER……..NO MIC CORROSION !

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WHAT IS REQUIRED TO SUPPORT MIC (Microbiologically Influenced Corrosion)

Microorganisms are required to produce MIC.

Three requirements to produce microorganisms are:

Microorganisms require water to propagate.

Microorganisms require a food source to propagate.

Microorganisms require specific environments to propagate such as water temperature and stagnant conditions.

TWO OF THESE THREE ELEMENTS ARE CONTAINED IN RIVER WATER.

NO RIVER WATER….NO MIC CORROSION!

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MECHANISMS OF MIC

Planktonic BacteriaInclusion and Receptor SitesSessile BacteriaSynergistic Colony FormationNodules (Tubercles)Pit Propagation

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Aerobic Bacteria(Pseudomonas Type)

Anaerobic Bacteria(Clostridium Type)

BACTERIA TYPES

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MIC on Heating Oil Tank

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This photo illustrates an area that has been cleaned and shows minimal general corrosion, intact mill scale, and intact coating. It also demonstrates rust staining over the intact coating. There is no evidence of MIC.

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MINIMIZING MICRoutine inspectionsClean environmentDesign of the bargeBarrier SystemChemical Treatments Through Green

ChemistryMaintain the Coating SystemOther methods

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Corrosion Control

• There are two methods used for corrosion control on ships:

– Modifying the corrosive environment

• Inhibitors

• Cathodic Protection

– Excluding the corrosive environment

• Coatings

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Corrosion Control by Inhibitors

• Corrosion inhibitors are used in areas where the electrolyte solution is of a known and controllable quantity.

• On ships this occurs in onboard equipment (boilers, tanks, pipes).

• Anodic inhibitors work by migrating to the anode and react to form salts which act as a protective barrier. Examples are chromates, nitrites, phosphates and soluble oils.

• Cathodic inhibitors migrate to the cathode, and either inhibit oxygen absorption or hydrogen evolution. Examples are salt compounds of magnesium, zinc, nickel or arsenic.

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Corrosion Control by Cathodic Protection

• Sir Humphrey Davy and his associate Michael Faraday first suggested this as a method to protect ships hulls in 1824.

• Zinc protector plates were attached to copper sheathed hulls of Navy vessels to reduce copper corrosion. The first full vessel to be protected in this way was “Samarang”.

• The principle of Cathodic Protection is to convert all the anode areas to cathodes, by polarising them to the same electrical potential as the cathodes.

• There are two methods:– Sacrificial anodes– Application of an external electric current (ie an impressed current)

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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, 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.

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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, (eg.epoxy paints, chlorinated rubber paints, alkyd paints etc).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.

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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

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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, ie. brush, roller, conventional spray, airless spray.

• After application the solvent evaporates.

• Solvent therefore is a high cost waste material.

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Underwater anti-corrosion paints

• The outside of a ships hull is generally coated with an anticorrosive paint system and an antifouling paint

• For the purposes of this talk we assume that the antifouling paint has littleor no effect on the anticorrosive properties of the scheme.

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

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

– Physical barrier properties

– Ionic resistance

– Adhesion

– Chemical inhibition

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Foul Release coatings can increase the efficiency of propellers in the following ways:

– No fouling where the coating remains intact

– No In-water cleaning required

– Reduced cavitation-induced noise in certain instances .Trials are continuing at Newcastle University and on ships to understand and quantify these benefits further.

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Why use paints with Cathodic Protection (CP)?

• CP systems are capable of protecting structures without paint .

• CP will only work when the anode and the structure are joined by aconductive medium eg. sea water.

• Structures subject to cyclic wet/dry immersion, eg. splash zones or ballasttanks, will only be protected for part of the time.

• The costs involved in the installation and maintenance of a CP system aresignificantly reduced if a protective coating is applied.

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Why use paints with Cathodic Protection?

• All coatings are subject to degradation over their service lifetime

• Coatings become damaged due to the operational environment

• CP systems provide protection at sites of damage or holidays (whilst the structure is immersed)

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Summary

• Coatings are a mixture of binder, pigment, and solvent and help to protectagainst corrosion by reducing the diffusion of oxygen and the electrolytictransport.

• Paints and CP systems work well together providing care is taken with the totalsystem design.

• Paint choice will influence the cost of corrosion protection at both newconstruction and maintenance.

• The direct cost of coatings is ~2% of a new vessel. With the surfacepreparation and coating application costs this increases to 8~10%.

• Coating failures can be very costly so choice of the correct product is essential,from new.

Thank you for your attention

12/2011

Prepared on