1. Corrosion types Galvanic corrosion
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.

2. 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 ; treatment with chromic acid forming a thin layer ( 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

3. 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 years!!)

4. (Chromate free) corrosion protection.
Scribe corrosion Filiform, undercutting, extended corrosion
Anti corrosive pigments - Adhesion (substrate surface roughness)
Organic corrosion inhibitors - Flexibility
Leaching of inhibitors - Hydrophobic system
Protection of cathodic sites (Cu) - Barrier function
, irreversible complex with inhibitor.
Passivation of anodic sites (Al).
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.

5. 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

6. Developments in OEM Aerospace Exterior coating systems.
Airbus Old
Airbus Current
Airbus Future ?
Al T3 Clad
Al T3 Clad / Composite
Chromic acid anodizing
Tartaric Sulphuric acid anodizing
TSA or other?
Low solids (basic)
chromate primer
Water based (basic)
HS or WB basic primer
chromate free
Low solids chromate
refresh primer
HS chromate free
HS or WB chromate free
Refresh primer
Low solids topcoat
High solids topcoat

7. Comparison of water based products versus high solids.
VOC
< 250 g/l.
350 – 420 g/l.
Potlife +
longer potlife and viscosity is constant. -
Short potlife and strong viscosity increase.
Drying
Application:
Layer thickness control
Flow
Spraying conditions.
Low solids content (volume) at spraying viscosity.
± (high RH conditions)
High solids content at spraying viscosity.

8. 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

9. Corrosion requirements Structural coating systems.
AIMS / 038:
Substrates: 2024T3 clad and unclad.
Pretreatments: CAA, CSP and TSA
Saltspray 3000 h.
No blistering and max mm extended corrosion.
AIE 1500 h.
Filiform corrosion (including topcoat), 960 h.
Max. 2 mm extended corrosion

10. 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 and

11. Corrosion requirements Exterior coating systems.
# Max. 25 % of scribed area affected.