Introduction of Corrosion Facts about Corrosion Causes of Corrosion Types of corrosion
Introduction to Corrosion Assessment and Management
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Transcript of Introduction to Corrosion Assessment and Management
8/13/2019 Introduction to Corrosion Assessment and Management
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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)