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Paint SchoolJPS-E / Corrosion / 1

Corrosion

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Definition of Corrosion

Corrosion is a reaction between

Materialand

Surrounding environment

under formation of corrosion products

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Production and Degradation of Steel

Plates, pipes,profiles, etc.

Raw materialIron ore Rust

Reaction between the material andthe surrounding environment takes place

The presence of water / humidity and Oxygenis a pre-requisite for corrosion of steel

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Manufacturing / Refining Brings theMaterial to a Higher Energy Level

The resistance to the oxidation (corrosion) varies fordifferent materials

When brought in contact with an electrolyte (aqueoussolution that conducts electricity) some metals easilydissolve into ions. These are called Base metals

Noble metals have a very strong resistance to corrosion

and do not react to form ions unless exposed to veryaggressive chemicals

Oxidation of metals is a natural process

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How is Rust Formed ?

Cathode:O2+ 2 H2O + e 4 OH

Anode:Fe Fe 2++ e

Water, Iron ions and Hydroxyl ions combineto give rust : Fe (OH)2 , Fe2O3. n H2O

Water

Fe (OH)2

OH Fe 2+

e Cathode Anode

Steel

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Freely Corroding Steel

Steel plate

O2 + H2O + 2e 2OH O2 + H2O + 2e 2OH

Cathode

2e-

2e-

2e-

2e-

Anode

Cathode

Sea water (electrolyte)

Fe 2+-- - -

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How Fast Will A Metal Corrode ?

The speed of a corrosion process depends on:

The properties of the material

The surrounding environment

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How Fast Will A Metal Corrode ?

Type of material

Type of electrolyte / surrounding environment

Service conditions Contaminants aggravating the conditions

Corrosion protection

A general answer can not be given.In most cases the corrosion speed depend on:

In some cases there are threshold limits: Below a certain limit there is no corrosion Above a certain limit rapid corrosion will develop

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Some materials CorrodeFaster Than Others

Electrochemical potential (Noble / base metal)

Formation of protective oxide films on thesurface of the base material

Conductivity of electrolyte

Presence of aggressive ions

Service conditions

The corrosion speed depends on:

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Aluminium Zinc Steel

ProtectiveIron oxide

In a strong alkaline environment Aluminium and Zincwill corrode rapidly, while steel will be passive

Corrosion Speed differ WhenExposed to the Same Environment

pH > 10

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Corrosion has a deferent rates

Fresh WaterSea Water

Start The Video

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Pre-requisites For Corrosion

A Cathode: The noble metal / alloy (or part ofmetal)

An Anode: The less noble metal / alloy

An electrical connectionbetween the two metals.Conducting electrical current (by electrons)

An electrolyte: Conducting electrical current (byions)

A galvanic cell consists of:

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A Galvanic Cell

In seawater, a calcareous deposit is formed on the steel surface

2 e

Zn = Zn + 2 e

Cathode:Steel Anode:

ZincO2

O2 + H2 O + 2e = 2OH

2+ -

- -

-

-

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Electrochemical Corrosion

Start The Video

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Energy Metal/alloy Potential Corrosion(volts) *

Least energyrequired

for refining

High energyrequired

for refining

GoldSilverTitanium

Stainless steel (316, active)Ni-Al- BronzeCopperCarbon steelAluminium (pure)Zinc (anode alloy)Aluminium (anode alloy)Magnesium (anode alloy

+0,500- 0,205- 0,225

- 0,235- 0,380- 0,435- 0,600- 0,800- 1,080- 1,140- 1,550

Least corrosive

Very corrosive

*) Potential in seawater measured versus aCopper / Copper Sulphate reference electrode

Galvanic Series in Sea Water

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What Kind of Information Can BeObtained From The Galvanic Series ?

The difference in Galvanic Potential will tell which of the twomaterials that will corrode when connected

The metal with the more negative potential will corrodeThe metal with the more positive potential will be protected

Corrosion speed depends on the difference in potentialbetween the metals

The greater the potential difference, the greater the corrosion.Rule of thumb: A potential difference less than

50 mV will cause no additional corrosion

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Rapid corrosion

General corrosion

Some corrosion

100% Cathodic protection

Overprotection

Possible coating damage

+ 0.50

+0.25

0

-0.3

-0.55

-0.80

-1.05

-1.45

Steel in SeawaterPotential versus Zinc & Ag/Ag C1 reference electrodes

Increasing polarisation

Potential, voltAg / Ag CL

Potential, voltZinc

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How To Measure the CorrosionPotential Of A Structure

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Elements Influencing theCorrosion Speed of Metals

Temperature

Salinity

Oxygen contentWater velocity

Acidity (See below)

Type of electrolyte ( e.g. cargo or chemicals)

Content of contaminants / pollution thatpromotes corrosion

Micro-organisms.

Submerged materials

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0

0.2

0.4

0.6

0.8

1.0

1.2

0 1 2 3

SEAWATER VELOCITY, (m/s)

Corrosion Speed For Carbon Steel asFunction of Flow Velocity

Corrosion rate (mm/year)

4

Source: F.L. La Que, page 134

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0

0.1

0.2

0.3

0.4

0.5

0.6

0 20 40 60

Temperature

a) Steel surface

b) Sea Water

Corrosion Speed for Carbon SteelExposed To Seawater

ab

Corrosion rate (mm/year)

Temperature, C

The corrosion speed will increase with increasing steel temperature,but decrease with increasing seawater temperature

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The pH of the SolutionWill Affect the Corrosion Speed

Steel Corrosion increases at lower pH. (Acidic solutions,

pH 6 lower)

Strong alkaline solutions prevent corrosion(pH 10 higher)

Zinc and Aluminium

Slow corrosion at close to neutral solutions Heavy corrosion in acidic and alkaline solutions

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01

2

3

4

5

6

7

8

9

10

11

12

13

14

Neutral

Acidic

Alkaline

pH-scale

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Parameters Influencing The CorrosionSpeed. Atmospheric Corrosion

Humidity

Temperature

Concentration of salts Amount of air pollution,

including acid rain, soot and dust particles

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The corrosion rate for steel willnormally increase with

Increasing temperature

Increasing humidity

Temperature and HumidityAffects the Corrosion Rate

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Atmospheric CorrosionCorrosion Rate Depends on Humidity

Relative Humidity, %

Corrosion rate

0 20 40 60 80 100

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Welding is an Initiator for Corrosion

Metallurgical conditions

Cracks

Remaining stresses

Hydrogen absorption

Poor cleaning

May give phases withdifferent electrochemical

potentials: Galvanic corrosion

Initiation of crevice corrosion

Stress corrosion cracking

Hydrogen embrittlement

Osmotic blistering, crevicecorrosion

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How to Preventor Reduce Corrosion

1. Improve the construction Design

Position

3. Use corrosion protection

Paint

Cathodic protection

Metallic coatings

Erosion resistant coatings

Rubber or plastic

Anodic protection

4. Change the environment

Inhibitors

Remove Oxygen

Reduce water flowvelocity

Change pH Change temperature

Reduce relativehumidity, % RH

2. Make the optimalselection of materials

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Corrosion Protection

There are many ways of preventing corrosion Here: Focus on Paints and Cathodic Protection (CP)

For submerged structures our

general recommendation is:

Use Paint as the primary corrosion protection systemBack up the paint system by a CP system.

CP will protect weak points and damaged areasof the paint film and therebyprolong the lifetime of the structure

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Sacrificial Anode

Start The Video

C i P i B P i

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Corrosion Protection By PaintsPrinciples

The barrier effect.Builds a barrier that prevents seawater or other corrosiveagents from coming into contact with the substrate.

Example: Epoxy paints.

The cathodic effect.

The paint itself acts as a sacrificial anode (Galvanic effect).

Example: Zinc-rich paints.

Inhibiting / passivating effect

The paint passivates the surface of the steel.

Example: Paints incorporating rust preventing pigments.(Red lead, Chromate's and Phosphates.

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Cut the Current by Paint

Start The Video

P i t C B U d T R d

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Paint Can Be Used To ReduceGalvanic Corrosion

Two different materials are in electricalcontact

They are both exposed to the same,

corrosive environment

Protective Measure

Perform proper pre-treatment andpaint the Cathode

Never paint the ANODE only

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Paint & Cathodic Protection

Start The Video

A t f i P i t d CP

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Apart from using Paint and CP:How to Protect Against Corrosion ?

Good design

Avoid corrosion traps

Improved accessibility - maintenance

Proper materials selection

Insulate between dissimilar materials

Change the surrounding environment

Remove water / humidity Apply metallic coatings

Use corrosion inhibitors (closed systems)

Corrosion protection can be achieved in many ways

S t With Fl i W t M t B

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Systems With Flowing Water Must BeDesigned To Avoid Turbulent Flow

Erosion corrosion will develop

at sharp bends and intrusions

A id St t W t At

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x

Unfortunate Better Best solution

x

x

Avoid Stagnant Water AtBottoms Of Tanks and Containers

A id l i h D / H idi

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Avoid solutions where Dust / HumidityCollects

Unfortunate Better Best solution

Humidity is trapped

M t i l S l ti M i t

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Material Selection - MaintenancePhilosophy

Resistant material that will last the total designlife (No maintenance)

Non-resistant material. (Will be replaced whencorroded)

Material with fairly good resistance. Corrosionprotection system must be applied

When selecting materialsthere are basically three options:

Corrosion

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CorrosionProperties of the Materials

All materials have their strong sides

But, they also have their weak points

Knowledge is required for selecting the

correct material for a given application

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Corrosion Inhibitors

Corrosion inhibitors are chemicals mostlyadded to closed systems

Inhibitors will, even at low concentrations,

reduce the dissolution (corrosion) of themetal

They are divided into groups, often in

accordance with their protection mechanism

Various Ways Of Classifying

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Various Ways Of ClassifyingCorrosion Inhibitors

A. Organic inhibitors

B. Inorganic inhibitors

I. Anodic

II. Cathodic

III. Dual:

Anodic and cathodic

1. Forming thick protective films2. Reacts with the metal to form a

protective film on the surface3. Adsorption on the metal surface

Inhibitors

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InhibitorsClassified According to Mechanism

I. Anodic inhibitors:

Reduces the anodic dissolution of the metal

II. Cathodic inhibitors:

Reduces the cathodic reaction

III. Dual inhibitors:

Reduces both the anodic and the cathodic reaction

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What about Rust Converters ?

Two important properties:

Some convert red rust to black rust (magnetite)

Some react with iron / rust to form inert reactionproducts

Contaminant containing chromate and / or Phosphatewill remain on the surface. Thorough steel brushing

is required

NB! Water soluble salts will not be removed !

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Types of Corrosion

1. Uniform corrosion2. Galvanic corrosion

3. Selective (preferential) corrosion

4. Pitting corrosion

5. Crevice corrosion6. Micro biological corrosion (bacteria)

7. Corrosion fatigue

8. Stress corrosion cracking

9. Erosion corrosion

10. Cavitation

11. Stray current corrosion

Most Frequently Occurring

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Most Frequently OccurringTypes of Corrosion

On carbon steel

Uniform corrosion

Uneven corrosion (deep pits)

Galvanic corrosion


Most Frequently Occurring

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Most Frequently OccurringTypes of Corrosion

Stainless steels

Crevice corrosion

Pitting corrosion


Titanium Hydrogen embrittlement Fatigue

Aluminium

Pitting corrosionGalvanic corrosion

Copper based alloys

Erosion corrosion

Costs involved in Material Selection

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Costs involved in Material Selectionand Corrosion Protection

1. MATERIAL COSTS

Material itself

Pre-fabrication

Dimensions Components

Availability

Volume

2. INSTALLATION

3. CORROSIONPROTECTION REQUIRED

4. THERMAL INSULATIONREQUIRED

5. EXTENT OF

MAINTENANCE

6. DESIGN LIFE

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Uniform Corrosion

A steel surface consists of noble and less noble areasThis can be looked upon as small galvanic cellsThe anodic parts will corrode

++

++

-

-

-

+

-

-+

-

+

+-+

Rust

+

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Uniform Corrosion

Steel

Corroded area

Original thicknessThickness after corrosion

A corrosion attack that is uniformlydistributed over the entire surface

Welds may Corrode Rapidly If Not

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Welds may Corrode Rapidly If NotThe Correct Weld Material is Used

NOTEThe weld material (filler) must alwaysbe more noble than the base material

Corrosion attack on weldFiller is less noble than the steel

Cathode Anode Cathode

Electrolyte

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Galvanic Corrosion

Important parameters:Anode - Cathode area ratio.Electrolyte resistivity (Conductivity)Difference in galvanic potential

of the metals

Pre-requisites:Water (Electrolyte)Two metals with different potentialsElectrical contacts between the metals

e -

Fe ++

CathodeStainless steel

AnodeSteel

Galvanic corrosion is to a large extent

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Galvanic corrosion is to a large extentdetermined by the conductivity of the electrolyte

Anode Cathode

Corroded area

The corrosion attack decreases with increasingdistance from the cathode due to an increased ohmic resistance

Electrolyte (Seawater)

Galvanic Corrosion

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Galvanic Corrosion(or Bimetallic Corrosion)

Unpreferable area ratioSmall anode will corrode rapidly

Anode Cathode Anode

Electrolyte with low conductivityAttack close to cathodeAnode Cathode Anode

Cathode Anode Cathode

The severity of the attack depends on: Conductivity of the electrolyte Anode - Cathode area ratio

Electrolyte with good conductivity

Wide corrosion attack

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Galvanic Corrosion In A Pipe System

Sacrificial spool:Less noble materialthan CuNi

Will corrode instead ofthe expensive flange

Here the flange will beattacked from both sides

Galvanic Corrosion

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I = Current, mA

EU= Potential unnoble metalEE= Potential noble metal

RI = Resistance in spacer

Rv = Electrolyte resistance

Rm = Resistance of metal

Galvanic CorrosionCorresponding Electrical Circuit

I EU

RV

RI

RM

EE

Basic formula, Ohms law:U = RI

Galvanic Corrosion Can be Reduced

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Distance from coupling, m

Important parameters:

Type of metalsconnected

Area ratio

Surroundingenvironment

Galvanic Corrosion Can be Reducedby Using an Insulating Material.

1.50

1.00

0.50

0.00 0.30 0.80 1.30 1.80 2.30

WithoutinsulationWith insulating

spool piece, 2m

Corrosion rate of steel whenconnected to Stainless steel, mm / year.

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Steel with Mill Scale

Outdoor exposureThe mill scale cracks

Corrosion will develop on the steel

Mill scale is more noble than steel

Mill scale Corrosion

Steel Steel

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Pitting Corrosion on Stainless Steel

WaterSalts withaggressive ions Pitting corrosion

Oxide film

Stainless steel

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Pitting corrosion

Pitting corrosion is a localised attack on a materialnormally protected by a passive film

The passive film may be destroyed mechanically orby aggressive ions in an electrolyte

Severe corrosion may take place beneath the passive layer

Seen from above Cross section

Stainless steel

Passive layerPitting corrosion

Crevice Corrosion Occurs Under

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Crevice Corrosion Occurs UnderPaint Spillage or Plates

Stainless Steel

PaintPlate

Corroded areas

Ingress of seawater

Seawater

Crevice corrosion occurs in narrow gapswhere the oxygen concentration is lower thanon the freely exposed part of the material

E i C i

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Erosion Corrosion

Flow direction

Sharp corners creates turbulence, leading to erosion corrosion

Erosion corrosion on a plane surfaceCross section From above

C i F ti

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Corrosion Fatigue

No. of dynamic cycles

Load

No failureabove line

Fatigue only

Corrosion fatigue

Corrosion Fatigueis a combined effect of anaggressive environment and dynamic loads on a structure

St C t C i

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Stray Current Corrosion

- +Welding transformer

Quay

Corrosion

Stray current

To earth

Pontoon

Seawater

Welding on board

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Evaluation of Rust Grades, ISO 8501-1

Rust grade C Rust grade D

Rust grade A Rust grade B

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Paint School

JPS-E / Corrosion / 67

Tank hjrne, notcher

Tykk rust

Kanter

4 - 49

Severe Corrosion

Corrosion due to acombination of

Sharp edges Rough welds (not

grinded)

Missing stripe coating

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Paint School


Tank hjrne

Notcher

Kanter

Tykk rust

4 - 50

Severe corrosion in a tank

Corrosion due to acombination of

Sharp edges Rough welds (not

grinded)

Missing stripe coating

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Paint School


Rd overflate med gamle

gjennombrenningerReparasjonsarbeid drligutfrt

ruster igjennom

4 - 48

Corrosion due to Back Burning

Hot work on the backside of the paintedsurface

Premature rust due to:Insufficient pre-treatment and paint

application

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Paint School


Rust, sveis, grnn tank

0590 - 06

4226 - 57

Poor Pre-treatment and Application Technique

Premature rust onsharp edges, Weldseams and Weld beads

These areas shouldhave been grinded

Also: Spraying only inone direction: Createsshadows and inferiorcoating thickness

No stripe coating

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Paint School


Tank, rust, sveis

0590 - 29

4226 - 54

Corrosion of Welds in a Water Ballast Tank

Premature rust on welds

Rough weld seams.Should have beengrinded

Probably in combinationwith poor stripe coating

(Mud can be seen,particularly on the

bottom)

C i T Thi P i Fil

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Mler filmtykkelse

Rep. for tynt pfrt p enflekk

Rust gjennomslag

Corrosion. Too Thin Paint Film

Corrosion of steelon a flat area.

Paint film is toolow. This is verifiedby film thicknessmeasurements

Fl ki f P i t C i

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Tykk ls maling, hjrne

4 - 5

Flaking of Paint. Corrosion

The paint has beenapplied too thick

The film cracks and

breaks up. Corrosion hasdeveloped on the steelafter a short period of

time

A too thick film is just asdangerous as a too thin

C i Sh Ed d S W ld

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Konstruksjon, Alu. maling

rust i kant (skarp)

Rust i punkt sveis

mangelfullt stl ogmalingsarbeide.

Corrosion on Sharp Edge and Spot Welds

Early corrosion onsharp edge and ondiscontinuous welds

Paint has newly beenapplied

Exposed to wind andweather for

approximately 2 weeks

C i W ld S Shi H ll

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Skipsside gul/blGamle gjennombrenninger

Drlig rep. arbeid

Rust gjennomslag

4 - 47

Corrosion on Weld Seams on Ships Hull

Untreated or poortreated welds sufferingearly corrosion

Probably wire brushed

without washing withfresh water

In addition, the paintfilm is too thin

C i Old Shi H ll

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Skipsside i dokk

Meget rusten T/S - S/B

Stor jobb

Stor ruhet

Stort oljeforbruk

Corrosion on Old Ship Hull

Old vessel, ready forfull maintenance

Old paint system Suffering mechanicaldamages andexposed to wear and

tear

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Overflate med dype rust

tringerNyblst

Stor ruhet

Vanskelig male, har gttfor lenge

Pitted and Corroded Steel after Blast-cleaning

Corrosion has been goingon too long

The steel shows deeppits, sharp edges and is

very uneven Difficult for a paint

system to givesatisfactory protection

S t bl ti d P A li ti T h i

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Rustkanter T/S

0589 - 75

4226 - 73

Spot-blasting and Poor Application Technique.

Spot-blasted steel Loose paint edges

Application has beendone in only one

direction Corrosion has

developed shortlyafter application

Direction of application

C i S t t

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Vinduer

4226 - 39

0590 - 43

Corrosion on Superstructure

Intersection pointbetween the steelstructure and the rubberpacking is a difficultarea to paint

If not painted wellpremature rust develop

Bad construction

Bad stripe coating

Mill S l Pi

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Rr, delvis medgldeskall/rust viktig fjerne gl.sk. fr maling.

Kan ikke brstes vekk.

Bedre med lett rust enn g.l.sk.

Mill Scale on Pipes

Corrosion on the millscale

Rust and mill scalemust be removed prior

to paint application. Mill scale can be

removed by blast-cleaning

C i D t R il f Bl t l i

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Blseskade

0590 - 09

Corrosion Due to Recoil from Blast-cleaning

Blast-cleaning has beencarried out on theopposite side.

Recoil of grit has

damaged the intactpaint

This is easily done, butdifficult to see duringinspection

Corrosion developsafter a short period oftime

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Thank You !

2 Corrosion 1

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