Transcript Presentation Title - Institute of Materials Finishing

Corrosion types
Pitting corrosion
Caused by selective ‘leaching’ of cathodic
particles in aluminum alloy.
Detrimental for strength of structural parts;
even small, superficial pits can reduce the
strength by 50%.
Galvanic corrosion
Most important type of corrosion according to our customers.
Prevention by insulating metals from each other; with a coating e.g.
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Corrosion protection of Aluminum
Process conditions during manufacturing.
By cladding (cathodic protection). Thickness ~ 5% of sheet thickness.
Pretreatment, for instance:
- Sol gel coating; silane-based AC131 BB e.g. (‘Boegel’)
- Alodine 1000-1200; treatment with chromic acid forming a thin layer (50-100
nm) containing Cr6+-ions
Anodizing
Thickness of layer 1 – 5 µm. Forming a stable layer of aluminum-oxide with pores.
Thickness and pore-size are dependant on type of acid used, voltage, temperature and
process time. Pores increase level of adhesion of a coating system, thus increasing the
level of corrosion protection.
Chromic acid anodized aluminum
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Corrosion protection of Aluminum
Use of a coating system. Protection of surface by an impermeable layer and protection in a
scratch/ damaged spot.
Impermeable for oxygen and water.
Leaching of inhibitors must be
possible for protection!
Leaching dependant on
1) Solubility inhibitor.
2) Binder matrix (polarity, crosslink ratio, etc).
3) Aging of system. (up to 40-50 years!!)
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(Chromate free) corrosion protection.
Scribe corrosion
-
Anti corrosive pigments
Organic corrosion inhibitors
Leaching of inhibitors
Protection of cathodic sites (Cu)
, irreversible complex with inhibitor.
- Passivation of anodic sites (Al).
Filiform, undercutting, extended corrosion
-
Adhesion (substrate surface roughness)
Flexibility
Hydrophobic system
Barrier function
Concerns:
 Balance of water solubility of the corrosion inhibitor to water sensitivity of the primer / system.
 Organic corrosion inhibitor may react with fuctional groups in coating resin.
 Corrosion inhibitor is trapped in highly cross-linked primer.
 Compatibility of corrosion inhibitors in water based primers.
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Developments in Aerospace structural coating
systems.
Chromic Acid Anodizing or Chromate Conversion Coating.
Low solids chromate primer (high Sr-chromate content).
Low solids PUR Topcoat.
Chromate free aluminium pretreatment (TSA , Sol gel technology)
HS chromate primer or WB chromate primer
HS Topcoat or WB Topcoat
Chromate free aluminium pretreatment (TSA, Sol gel, other?)
HS chromate free primer or WB chromate free primer
HS Topcoat or WB Topcoat
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Developments in OEM Aerospace Exterior
coating systems.
Airbus Old
Airbus Current
Airbus Future ?
Al. 2024 T3 Clad
Al. 2024 T3 Clad
Al. 2024 T3 Clad / Composite
Chromic acid anodizing Tartaric Sulphuric acid
anodizing
TSA or other?
Low solids (basic)
chromate primer
Low solids chromate
refresh primer
Low solids topcoat
HS or WB basic primer
chromate free
HS or WB chromate free
Refresh primer
High solids topcoat
Water based (basic)
chromate primer
HS chromate free
refresh primer
High solids topcoat
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Comparison of water based products
versus high solids.
VOC
Potlife
Drying
Application:
Water based
High solids
< 250 g/l.
350 – 420 g/l.
+
-
longer potlife and viscosity
is constant.
Short potlife and strong
viscosity increase.
+
-
Low solids content
(volume) at spraying
viscosity.
High solids content at
spraying viscosity.
Layer thickness control
+
-
Flow
+
-
± (high RH conditions)
+
Spraying conditions.
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Aerospace corrosion tests: NSS, AIE, Filiform.
Neutral Saltspray: ( ASTM B117, ISO 7253, BSS 7225)
5% NaCl solution sprayed every hour at 40°C
Alternating Emersion Immersion test (EN 3212)
cycle:
2 hrs immersion in 3% NaCl at 35°C
2 hrs emersion at 35°C / 80%RH
Regular refreshment of electrolyte solution
Filiform corrosion (BSS 7258, EN 3665)
Initiation : 1 hr above a 12 N HCl solution
Exposition : At 40°C / 80 % RH
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Corrosion requirements Structural coating systems.
AIMS 04.04.001 / 038:
Substrates: 2024T3 clad and unclad.
Pretreatments: CAA, CSP and TSA
Saltspray 3000 h.
No blistering and max. 1.25 mm extended corrosion.
AIE 1500 h.
No blistering and max. 1.25 mm extended corrosion.
Filiform corrosion (including topcoat), 960 h.
Max. 2 mm extended corrosion
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Structural primer Airbus chromate reduction.
Solids content at spraying
viscosity (volume)
Weight percentage of Strontium
chromate in the dry primer.
Conventional primer
34 %
24 %
High solids primer
59 %
27 %
Water based primer
32 %
11 %
Lower chromate content in the WB epoxy-amine primer, but still a good
corrosion protection according to AIMS 04.04.001 and 04.04.038.
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Corrosion requirements Exterior coating systems.
Corrosion hours Alloy
test
Pretreatment
Corrosion requirement.
AIE
Filiform
Filiform
1500
960
960
2024 T3 clad
2024 T3 clad
2024 T3 unclad
CSP
CAA
abraded, solvent cleaned
Max. 1.5 mm
Max. 2 mm
Max. 8 mm
Filiform
Filiform
Filiform
Saltspray
720
720
2000
3000
2024 T3 clad
7075 T6 clad
2024 T3 clad
2024 T3 clad
Alodine  Boegel
Alodine  Boegel
Alodine  Boegel
Alodine  Boegel
Max. 3.2 mm
Max. 8.9 mm
Max. 5 mm
Max. 3.2 mm #
AIMS 04.04.031/33
BMS 10-72
AMS 3095A
Filiform
1000
Saltspray 3000
2024 T3 clad + unclad abraded, solvent cleaned
2024 T3 clad + unclad abraded, solvent cleaned
Max. 3 mm
Max. 3 mm
# Max. 25 % of scribed area affected.
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