Corrosion and corrosion protection in
marine environment
Anders Rosborg Black
Specialist, FROSI O Level I I I I nspector
• I ntroduction to atmospheric corrosion in marine
environment
• Paint and protective coating systems
• Why pre-treat?
• Repair and maintenance
• I nspection
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
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 impactAtmospheric corrosion of steel
I mpact from water and moisture may occur due to:
• Rainfall, snow
• Condensation due to changes in temperature
• I mpact 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
Corrosion, moisture retention
The significance of pollution/ salts on the steel surface
K Clean steel
Corrosivity categori
Examples of typical environments in a temperate climate (informative only)
Exterior
Mass loss per unit surface/ thickness loss ( after first year of exposure)
Low - carbon steel Zinc
Atmospheres with low level of pollution. Mostly rural areas.
> 10 – 200 > 1.3 – 25 > 0.7 – 5 > 0.1 – 0.7
C3
medium
Urban and industrial atmospheres, moderate sulphur dioxide pollution. Costal areas with low salinity.
Industrial areas and coastal areas with moderate salinity.
> 400 – 650
> 50 – 80 > 15 – 30 > 2.1 – 4.2
C5-I
very high (industrial)
Industrial areas with high humidity and aggressive atmosphere.
> 650 - 1500
> 80 – 200 > 30 - 60 > 4.2 – 8.4
C5-M: very
high (marine)
Coastal and offshore areas with high salinity. > 650 - 1500
> 80 - 200 > 30 - 60 > 4.2 – 8.4
DS/ EN I SO 12944-2 corrosivity categories
Refineries are placed in coastal areas. Depending
How to limit corrosion
• Correct material selection • Organic coatings - paint
• I norganic coatings – metallising/ galvanising • Cathodic protection
• Good surface preparation
• Regards to the corrosivity category
Why paint?
• Decoration
• I dentification/ I mage
• Warning/ Safety
• Camouflage
• Specific properties: Cleaning, anti-slippage
Resin – film formation
+
Chemically curing:
Wet paints - overview
• Physically drying:
• Acrylic: For items not in marine environments and for repairs.
• Chemically curing:
• Oxidatively curing alkyd: For items in non-aggressive environments (C1 = > C4)
• Epoxy: For items in aggressive environments
Wet protective coating system - C4
Physically drying
Chemically curing
Acrylic primer 80 µm Epoxy primer 180 µm
Acrylic intermediate coat 80 µm -
Acrylic topcoat 80 µm Polyurethane topcoat 60 µm
Wet protective coating system, C5-I
and C5-M
Physically drying
Chemically curing
- ZnEpoxy primer 60 µm
Epoxy intermediate coat 200 µm Polyurethane topcoat 60 µm
Paint systems for low-alloy carbon steel for corrosivity
categories C5-I and C5-M
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)
Why pre-treat?
Why pre-treat?
Avoid osmotic blistering:
Avoid flaking:
Contaminants
•
Salts
•
Oil
•
Grease
•
Dust
•
Mill scale
•
Rust
Preliminary pre-treatment
• Cleaning:
–Degreasing in order to remove oil, grease and salts
• Method:
– cleaning by water/ steam
– emulsion cleaning – alkali degreasing
Manual or mechanical pre-treatment
• Methods
– steel brush
– grinding
– cutting/ chipping
Mechanical pre-treatment – abrasive
blast cleaning
Effect:
I mpactAbrasion
E = ½ mv2 ~ speed (pressure) is
important!
Abrasive blasting, dry
Examples: Sandblasting
Mechanicaa pre-treatment -
sandblasting
Examples:
Sandblasting, w et:
Wet abrasive blastingDispersion abrasive blasting
Abrasives - types
Mineral ( natural) : quarts
olivine garnet
Mineral ( artificial) : Copper slag
Aluminium silicate I ron slag
Aluminium oxide - corundum ” sponge”
Metallic: steel, iron
aluminium
Organic: plastics
shells (nuts, corn)
Others: ice
carbon dioxide baking powder
Steel surface qualities
A
B
Cleanliness control according to I SO 8501- 1:
I SO 8501-1: Sa 2½
Pre-treatment
0 2 4 6 8 10 12
No cleaning
Sa 3 Flame cleaning
Sa 2½ Sa 2
Sa 1
St 3 mechanical brushing
St 3 mechanical grinding/ sanding? St 3 manual brushing
St 2 St 1
Pre- treatment extends the corrosion protection
I nsufficient pre-treatment
UHP high pressure cleaning
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
Surface treatment cost
• I nitial cost - investment
• Service cost - maintenance
Surface treatment 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.
The maintenance process
• 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
I dentify coating breakdown
Non destructive tests - Dry Film Thickness (DFT)
I dentify coating thickness
Destructive tests - Adhesion test (x-cut, pull off )
I dentify potential reduction in
adhesion/ cohesion
Laboratory analyses - FTI R or solvent dissolution test
Repair
First:
• Cleaning
Thereafter based on the old coatings condition:
• I ntact, but matt and discoloured surface, Ri 0:
Maintenance painting
• Surface with spot rust, up to Ri 3 (1% rust):
Spot repair
Repair
Why new paint w hen more than 1 % of the surface is corroded?
- I n 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
Cracked coating is ground and chamfered
Design should allow for maximum access
for maintenance and repair painting
C.G. Munger, NACE 1984.
I nspection 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 vapour
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 w ill occur after prolonged
environmental exposure ( general breakdow n)
Control during the construction phase
I n order to avoid costly coating repairs, it must be ensured that all application work during the construction phase is carried out in accordance with:
- I nternational standards
- The owner’s specific standards
- The guidelines described in the paint’s technical data sheet
Control
Bresle-test Relative humidity
Wet film thickness Dry film thickness
