1Akzo Nobel Aerospace Coatings

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.

2Akzo Nobel Aerospace Coatings

Corrosion protection of Aluminum

Process conditions during manufacturing.

Chromic acid anodized aluminum

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

AnodizingThickness 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.

3Akzo Nobel Aerospace Coatings

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!!)

4Akzo Nobel Aerospace Coatings

(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.

5Akzo Nobel Aerospace Coatings

Developments in Aerospace structural coating systems.

Chromate free aluminium pretreatment (TSA , Sol gel technology)

HS chromate primer or WB chromate primer

HS Topcoat or WB Topcoat

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, other?)

HS chromate free primer or WB chromate free primer

HS Topcoat or WB Topcoat

6Akzo Nobel Aerospace Coatings

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

Water based (basic)chromate primer

HS or WB basic primer chromate free

Low solids chromate refresh primer

HS chromate free refresh primer

HS or WB chromate free Refresh primer

Low solids topcoat High solids topcoat High solids topcoat

7Akzo Nobel Aerospace Coatings

Comparison of water based products versus high solids.

Water based 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.

-

-

+

8Akzo Nobel Aerospace Coatings

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°C2 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

9Akzo Nobel Aerospace Coatings

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

10Akzo Nobel Aerospace Coatings

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.

11Akzo Nobel Aerospace Coatings

Corrosion requirements Exterior coating systems.

Corrosion test

hours Alloy Pretreatment Corrosion requirement.

AIMS 04.04.031/33 AIE 1500 2024 T3 clad CSP Max. 1.5 mm Filiform 960 2024 T3 clad CAA Max. 2 mm Filiform 960 2024 T3 unclad abraded, solvent cleaned Max. 8 mm BMS 10-72 Filiform 720 2024 T3 clad Alodine Boegel Max. 3.2 mm Filiform 720 7075 T6 clad Alodine Boegel Max. 8.9 mm Filiform 2000 2024 T3 clad Alodine Boegel Max. 5 mm Saltspray 3000 2024 T3 clad Alodine Boegel Max. 3.2 mm # AMS 3095A Filiform 1000 2024 T3 clad + unclad abraded, solvent cleaned Max. 3 mm Saltspray 3000 2024 T3 clad + unclad abraded, solvent cleaned Max. 3 mm # Max. 25 % of scribed area affected.