Corrosion and corrosion protection in
marine environment
Anders Rosborg Black
Specialist, FROSIO Level III Inspector
FORCE Technology, Denmark
Outline
• Introduction to atmospheric corrosion in marine
environment
• Paint and protective coating systems
• Why pre-treat?
• Repair and maintenance
• Inspection
Corrosion – a major cost
Estimated costs: 3-4 % of GDP
1/3 can be prevented by use of
existing knowledge
1/3 can be prevented by
research/development
1/3 is too expensive to prevent
Atmospheric corrosion of steel
What affects the atmospheric corrosion rate of steel?:
•
Moisture (time of wetness)
•
Oxygen, temperature
•
Air pollution, industrial atmosphere
(SO2, NOx, sooth)
•
Chlorides, marine atmosphere
•
Dirt, hygroscopic salts
•
Galvanic impact
Atmospheric corrosion of steel
Impact from water and moisture may occur due to:
•
Rainfall, snow
•
Condensation due to changes in temperature
•
Impact from wet materials (wet wood, cardboard etc.)
•
Absorption of water:
a) Moisture retaining and corrosive salts on the steel surface
b) Pollution (gaseous) in the air
•
Design (crevices, debris traps etc)
Corrosion, moisture retention
Korrosionshastighed
Corrosion rate
The significance of pollution/salts on the steel surface
CaCl2
NaCl
Clean steel
Ren
surface
ståloverflade
0%
Relative
Relativ
humidity
luftfugtighed
25%
50%
75%
100%
DS/EN ISO 12944-2 corrosivity categories
Corrosivity
categori
Examples of typical environments in a
temperate climate (informative only)
Exterior
C1
very low
C2
low
C3
medium
C4
high
C5-I
very high
(industrial)
C5-M: very
high (marine)
Atmospheres with low level of pollution. Mostly
rural areas.
Urban and industrial atmospheres, moderate
sulphur dioxide pollution. Costal areas with low
salinity.
Industrial areas and coastal areas with
moderate salinity.
Mass loss per unit surface/thickness loss
(after first year of exposure)
Low-carbon steel
Zinc
Mass loss
g/m2
Thickness
loss
µm
Mass loss
g/m2
Thickness
loss
µm
≤ 10
≤ 1.3
≤ 0.7
≤ 0.1
>0.7 – 5
>0.1 – 0.7
>10 – 200 >1.3 – 25
>200 –
400
>25 - 50
>5 – 15
>0.7 – 2.1
>400 –
650
>50 – 80
>15 – 30
>2.1 – 4.2
Industrial areas with high humidity and
aggressive atmosphere.
>650 1500
>80 – 200
>30 - 60
>4.2 – 8.4
Coastal and offshore areas with high salinity.
>650 1500
>80 - 200
>30 - 60
>4.2 – 8.4
Refineries are placed in coastal areas. Depending
on the location the corrosivity category may be C4 or C5.
How to limit corrosion
•
•
•
•
•
•
•
Correct material selection
Organic coatings - paint
Inorganic coatings – metallising/galvanising
Cathodic protection
Good surface preparation
Regards to the corrosivity category
Other measures
Paint and protective coating systems
Why paint?
•
•
•
•
Decoration
Identification/Image
Warning/Safety
Camouflage
•
Specific properties: Cleaning, anti-slippage
• Protect
Resin – film formation
Chemically curing:
+
Physically drying:
Wet paints - overview
•
•
Physically drying:
•
Chemically curing:
•
•
•
Oxidatively curing alkyd: For items in non-aggressive environments (C1 => C4)
Epoxy: For items in aggressive environments
Polyurethane: For items for which colour fastness is important
Acrylic: For items not in marine environments and for repairs.
Wet protective coating system - C4
Corrosivity category C4
Physically drying
Acrylic primer
Acrylic intermediate coat
Acrylic topcoat
Chemically curing
80 µm Epoxy primer
80 µm 80 µm Polyurethane topcoat
180 µm
60 µm
Wet protective coating system, C5-I
and C5-M
Corrosivity categories C5-I and C5-M
Physically drying
-
Chemically curing
ZnEpoxy primer
Epoxy intermediate coat
Polyurethane topcoat
60 µm
200 µm
60 µm
Paint systems for low-alloy carbon steel for corrosivity
categories C5-I and C5-M
DS/EN ISO 12944-5,
table A.5
Thermal spraying (metallisation)
•
Methods
– Flame spraying (powder/wire)
– Arc spraying
•
Metals for corrosion protection
– Zinc
– Zinc/aluminium (85/15)
– (Aluminium)
•
Application
– As corrosion protection alone (100, 150, 200 µm), e.g. as CUI
– As corrosion protection for joints in hot-dip galvanised constructions
– As “primer” in a protective coating system (40-60 µm)
– Used for bridges and wind turbines as part of a “duplex system”
Why pre-treat?
Surface contamination must be removed
Why pre-treat?
Avoid osmotic blistering:
Avoid flaking:
Create a surface profile:
Contaminants
• Salts
• Oil
• Grease
• Dust
• Mill scale
• Rust
• Old coating
Preliminary pre-treatment
• Cleaning:
–Degreasing in order to remove oil,
grease and salts
• Method:
–
–
–
–
cleaning by water/steam
emulsion cleaning
alkali degreasing
solvent cleaning
Manual or mechanical pre-treatment
• Methods
– steel brush
– grinding
– cutting/chipping
”Almet” - files
”Bristle Blaster”
Mechanical pre-treatment – abrasive
blast cleaning
Abrasive blasting, dry
Effect:
Examples:
Sandblasting
Centrifugal blasting
Vacuum-sandblasting
Impact
Abrasion
E = ½ mv2 ~ speed (pressure) is
important!
Mechanical pretreatment- abrasive blasting
Mechanicaa pre-treatment sandblasting
Examples:
Sandblasting, wet:
Wet abrasive blasting
Dispersion abrasive blasting
Sandblasting with liquid under pressure
Abrasives - types
Mineral (natural):
quarts
olivine
garnet
Mineral (artificial):
Copper slag
Aluminium silicate
Iron slag
Aluminium oxide - corundum
”sponge”
Metallic:
steel, iron
aluminium
Organic:
plastics
shells (nuts, corn)
Others:
ice
carbon dioxide
baking powder
(sodium hydrogen carbonate)
water
Steel surface qualities
A
B
C
D
Sandblasting - cleanliness
Cleanliness control according to ISO 8501-1:
ISO 8501-1: Sa 2½
Pre-treatment
Pre-treatment extends the corrosion protection
No cleaning
Flame cleaning
St 1
St 2
St 3 manual brushing
St 3 mechanical brushing
St 3 mechanical grinding/sanding?
Sa 1
0
2
4
6
Sa 2
8
Sa 2½
10
Sa 3
12 Years
Insufficient pre-treatment
• Painting on mill scale after 3 years in C4 environment:
UHP high pressure cleaning
Control: Surface treatment according to ISO 8501-4
Why pre-treat?
• Remove salts and contamination
• Remove rust and mill scale
• Create a surface profile (roughness)
•
- in order to prepare the item for surface
treatment
Repair and maintenance
Surface treatment cost
•
Initial cost - investment
•
Service cost - maintenance
•
Lifetime cost – total cost
Surface treatment cost
Difference in lifetime cost
6 x maintenance
3 x maintenance
The maintenance process
The condition of the coating on large constructions should always be
inspected and reported on a regular basis.
Based on the aggressiveness of the environment the inspection interval
may be 1-5 years.
The maintenance process
In general the maintenance process includes:
•
Condition assessment
•
Reporting, choice of method for repair
•
Surface treatment: Pre-treatment and application
•
Control
Condition assessment
Condition assessment may include:
Method
Technique
Purpose
Visual inspection
- Degree of blistering,
rusting, cracking,
flaking, chalking
Identify coating
breakdown
Non destructive tests
- Dry Film Thickness
(DFT)
Identify coating
thickness
Destructive tests
- Adhesion test (x-cut,
pull off)
Identify potential
reduction in
adhesion/cohesion
Laboratory analyses
- FTIR or solvent
dissolution test
Identify coating type if
unknown
Condition assessment
ISO 4628-3:
Rust grade, corroded area, %
Ri 0
0
Ri 1
0,05
Ri 2
0,5
Ri 3
1
Ri 4
8
Ri 5
40/50
Ri 3 – 1% corroded
Repair
First:
• Cleaning
Thereafter based on the old coatings condition:
•
Intact, but matt and discoloured surface, Ri 0: Maintenance painting
•
Surface with spot rust, up to Ri 3 (1% rust): Spot repair
•
Corroded surface, Ri 4 and Ri 5: New painting
Repair
Why new paint when more than 1 % of the surface is corroded?
-
In general a failure rate of >15-20 % may be uneconomical to spot
repair
-
The percentage is based on the percentage area that will receive new
coating, not the area of visible failure
-
E.g. a small defect, 10x10mm may end up as a repair of perhaps 400500 mm in diameter if done correctly
Spot blasting and grinding
Cracked coating is ground and chamfered
Spot repair
Design should allow for maximum access
for maintenance and repair painting
C.G. Munger, NACE 1984.
See: DS/EN ISO 12944-3 Design considerations
Inspection is crucial
• The coastal location exposes the structures to heavy stresses and a
severely corrosive environment.
• Coatings must withstand and protect against humidity with high
salinity, reflecting UV light (as well as tidal and wave actions).
Water, (salts)
Water vapour
Sunlight – UV radiation
Coating defects
Coating defects can be introduced in every step of the coating
process during fabrication:
• Steel quality / surface preparation
• Application (+ climatic conditions)
• Quality of the paint
• Handling
Furthermore defects will occur after prolonged
environmental exposure (general breakdown)
Control during the construction phase
In order to avoid costly coating repairs, it must be ensured that all
application work during the construction phase is carried out in
accordance with:
- International standards
- The owner’s specific standards
- The guidelines described in the paint’s technical data sheet
Many costly failures can be prevented by paying thorough attention to
the complete painting process - going from the specification stage
over the application process and to the final coating inspection.
Control
Relative humidity
Dry film thickness
Bresle-test
Wet film thickness
Adhesion