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Introduction:

Corrosion Assessment and

Management for ship-shapedoffshore structure

IHIAP Offshore Engineering Div

Mohamad Faeze

13TH Jan 2014 Ver 1.0

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Contents

1.1 Marine corrosion mechanism

1.2 Types of corrosion

1.3 Factors affecting corrosion

1.4 Corrosion management

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1.1 Marine Corrosion Mechanism

Electrochemical actions

Galvanicactions

Intergranular actions

Crevice

actions

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Electrochemical actions

Principal components

Anode: Copper metal

Cathode: Zinc metal

Chemical reactions:

Zn(s) -> Zn2+(aq) + 2e-

Cu2+(aq) + 2e- -> Cu(s)

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Galvanic actions

Two dissimilar metals, which are

immersed in electrolyte(i.e. sea

water), are connected directly or

by a metallic path

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Intergranular actions

Microscopic form of corrosion

caused by a potential difference

between grain bodies of themetal and grain boundaries i.e.

weld zones

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Crevice actions

Occur in a relatively in confined

space i.e. open joints, under

nuts, bolt heads, washer

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Erosion actions

Occur under high-velocity

seawater flows in bends and

elbows of pipes and at strike

plates

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Microbial actions

Corrosion aided by bacterial

activity in the sprinkler system

Galvanized coated piping maydelay the process of corrosion

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1.2 Types of Corrosion

Uniform

corrosionPitting

Grooving

Weld

metalcorrosion

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Uniform(general) Corrosion

Formed almost uniformly on the

surface.

Often represents important

mechanism of general corrosionon the back of the under-deck

steel plates of oil tankers and

FPSOs(usually due to uncoated or

the coating has failed)

Traditional corrosion margin is

meant to guard against uniformcorrosion(i.e. in trading tanker,

plate panel margin is 20%, while

entire deck is 10%)

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Pitting corrosion

Pitting is a localized form of

corrosion, typically occurs on

bottom plating, on other

horizontal surfaces, and at

structural locations that may trap

water.

Pit growth rates can be many

times that for general corrosion,

usually associated with acidic

environment at the pit bottom.

Pit repairs and plate renewals is

usually only visible options for

this cases.

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Grooving corrosion

Manifested as localized line

corrosion.

Occurs at structural intersections

where water or heavy moisturecollects or flows.

The effect may be exacerbated

by structural flexibility and result

in loss in scale.

Often observed on longitudinal

bulkheads in trading tankers

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Weld metal corrosion

Galvanic action of weld material

with the base metal that may

initially lead to pitting or

grooving.

Often to occur in hand welds

than machine welds.

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1.3 Factors effecting corrosion

Types of structural materials (e.g. steel, aluminium alloy)

Corrosion protection scheme (e.g. coating, anodes,impressed current cathodic protection)

Types of cargo or stored material (e.g. oil, seawater, wax

content, oxygen content, salinity, reactivity)Dry-wet cycles related to loading/unloading of cargo orstored material

Humidity

Temperature

Oxygen

Water velocity

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Areas of corrosion concern for ship-

shaped offshore unitsSegregated ballast spaces

Cargo only spaces

Cargo and/or clean ballast spaces

Cargo and/or dirty ballast spaces

Cargo and/or storm ballast spaces

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1.4 Corrosion management

1. Corrosionmargin addition

2. Coating

3. Cathodicprotection

4. Ballastwater

deoxygenation

5. Chemicalinhibitors

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1.4.1 Corrosion margin addition

The objectives are to reduce costs associated

with downtime for repairs and renewal of

corroded structural components

The value of corrosion wastage(depth) with

time can be predicted from tr=C1(T-Tc).

Tr : corrosion depth, mm

C1 : determined from statistical analysis of corrosion measurement dataT : exposure time(i.e. 25 years design service life)

Tc : constant parameter

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1.4.2 Coating

Surface

preparation

Types of

Coating

Selection

criteria of

coating

material

Methodologies

for coating-life

prediction

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Surface preparation

All structural steel must be blasted before coating

per ISO 8501 (Sa3) and,

Shop-primed with inorganic zinc primer (having

minimum dry film thickness of 20microns)

Sharp edges(from steel cutting) must be treated,

i.e. by grinding to convert them into reasonably

smooth arc with a min radius of at least 2mm toprevent from “pull-back” phenomenon

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Types of coating

Epoxy paints

•Coal tar EP offer good resistance to water soil and inorganic acids

•Polyamide-hardened more resistance to moisture but less resistance to acids

•Amine-hardened more resistance to chemical

Silicon paints

• Excellent water repellent

•Maximum temperature of 650deg celcius

•Poor chemical resistance

Zinc paints

•Used for galvanic protection

•Organic requires less surface preparation• Inorganic easier to topcoat, more heat resistance

•Used effectively in neutral and slightly alkaline solutions

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Oil-basedpaint

• Easy application, relatively inexpensive

•Permeable, recommended for mild atmospheric conditions

Alkyd paint

• Must baked to dry

• better corrosion resistance than oil-based paint

• Not suitable for resistance to chemicals

Urethanepaints

• Good resistance to abrasion

• Corrosion resistance approaches vinyl and epoxy paints

Vinyl paints

• Better corrosion resistance than oil and alkyd paints• Adherence and wetting often poor

• Good resistance to aqueous acids and alkalines; max temperature of 65deg celcius

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Selection criteria of Coating Material

• Adhesion of at least 140bar per ASTM D4541

Adhesive strength

• The ability of the ease with which water can work its way through the coating; smaller thepermeability, the better the coating durability

Permeability

• To keep top of the tank cool(if possible 55deg celcius) to keep the coatings become semiplastic;coatings with a high GTT

Glass transition temperature

• With modern catalyst-activated coating technology, solventless coatings are feasible and desirable

No solvent

• Aluminum or aluminum oxide filler is thought to be desirablethan hydroschopic filler because it mayattract water and weakening the coating

Right filler

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1.4.3 Cathodic Protection

For large areas of submerged steel, cathodicprotection is the most common form

Applied with coatings and acts as backup

Two types:Impressed current system

More reliable long-term protection

Requires the use of continuous external electrical power

Galvanic system

Uses aluminum, magnesium, zinc anodes that attached tosteel material exposed to seawater

For offshore platforms, pipelines, mooring chains, aluminumor aluminum-zinc anodes are often employed

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1.4.4 Ballast Water Deoxygenation

In ballast water tank, oxygen is an important

factor that promotes corrosion

Corrosion rate can be reduced by 90% whenoxygen levels are reduced and maintain below

0.5%

Various methods to deoxygenate; vacuum,

biological process, or help of inert type of gas

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1.4.5 Chemical inhibitors

For corrosion protection of closed systems

such as engines or boilers

Five types: Absorption

Hydrogen evolution “  poison” 

Scavengers

Oxidizers

Vapor phase inhibitors

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References

The tankship tromedy: The impending disasters intankers. Devanney,J.(2006)

Rules for building and classing steel vessels. ABS (2000)

Handbook of corrosion protection for steel structures inmarine environment. AISI(1981)

Corrosion prevention of tanks and holds. DNV(1999)

 ASM handbook: Corrosion. Korb, L.J(1989)

Newbuild FPSO corrosion protection– 

 A design andoperation planning guidelines. Macmillan, A., Fischer,K.P., Carlsen, H., and Goksoyr,O.(2004)