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.

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

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!!)

(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.

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

Developments in OEM Aerospace Exterior coating systems. Airbus Old Airbus Current Airbus Future ? 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) 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

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.

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

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. Filiform corrosion (including topcoat), 960 h. Max. 2 mm extended corrosion

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.

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