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Nanostructured Coatings for Corrosion Protection of Aluminum Alloys
Abstract: The aging U.S.
Air Force legacy aircraft fleet (e.g., KC-135, C-130, F-15, etc.) has a
well-documented corrosion problem. Corrosion
protection of aluminum-skinned aircraft and development of improved
environmentally benign surface treatments for aluminum aerospace alloys
are high priority topics for U.S. Air Force and the aerospace industry. Current
coating systems for aircraft corrosion protection are based on a surface
treatment, primer, and topcoat. These
coating systems generally utilize a chromated conversion coating (CCC)
surface treatment (e.g., Alodine 1200S) and a solvent-borne
epoxy-polyamide primer that is pigmented with a strontium chromate
inhibitor. Chemical conversion
coatings on aluminum alloys are intended to perform dual functions – to
promote adhesion and to improve corrosion resistance of the subsequent
organic coating. While traditional chromate-based conversion coatings are
very efficient and offer many valuable coating properties, increasing
environmental regulations restrict the use of chromates due to the
toxicity of hexavalent chromium. The
U.S. Department of Health and Human Services released the 9th Report on
Carcinogens (January 2001) in which hexavalent chromium compounds,
including the most widely used chromate corrosion inhibitor, strontium
chromate, are listed as known human carcinogens.
In addition, OSHA and the FDA all have restrictive exposure limits
on hexavalent chromium compounds. OSHA
recently proposed new guidelines reducing the permissible exposure limit
from 52 to 1 mg/m3. Both environmental and
toxicity issues are compounded by the fact that these coating system are
replaced every 6-8 years on average. The
size of the aging legacy fleet of AA2024-T3 skinned aircraft is rather
large and the current trend is to greatly extend these aircraft projected
lifetimes, as much as four times the original estimate.
Given the toxic waste products and the environmental impact of the
de-paint/paint cycle, the Air Force has a specific need for a chromate
free coating systems that can provide long-term environmentally benign
corrosion protection. The Air
Force Research Laboratory is developing environmentally benign
alternatives to the traditional chromated aircraft coating for aircraft
corrosion protection, targeted to a 30+ year performance life cycle. Cleaning and surface
pretreatment are critical steps in protecting aerospace alloys from
corrosion. One promising
solution to this complex problem is based on the use of organofunctional
silanes and the principles of sol-gel chemistry.
Organosilanes are relatively environmentally friendly chemicals and
work well for improving adhesion at the metal/coating interface, providing
better corrosion protection. These
materials have been used to engineer nanostructured ceramic coatings for
corrosion protection of aluminum aerospace alloys. This advanced hybrid
approach is called the Self-assembled NanoPhase (SNAP) coating process. The
approach starts with hydrolysis and condensation reactions of silanes to
generate nanomaterials with epoxy functionalities in-situ. The
nanomaterials are then crosslinked using epoxy/amine chemistry to form a
thin film (1 µm). The hybrid
coatings made by SNAP process are essentially nanocomposite materials and
have been demonstrated as a viable alternative for chromate conversion
coatings providing exceptional corrosion resistance and long-term coating
performance. |