NASA and other space organizations face the
difficult challenge of protecting launch pad structures from corrosion.
Thin-gauge stainless-steel and aluminum structures, such as protective
bellows around drive mechanisms, flex repeatedly and thus require highly
flexible and adherent coatings. The aerospace industry has traditionally
used paints having high volatile organic compound (VOC) content for protecting
vehicles and support structures. Flexible paints employ highly solvated
rubber binder resins that render the products highly volatile and difficult
to apply by spraying. Silicone-based paints are formulated to yield temperature-
and weather-resistant coatings that prevent corrosion by forming effective
electrolyte barriers. However, silicones are normally delivered from organic
solvents and exhibit poor adhesion to unprimed metals.
Waterborne elastomeric anticorrosion coatings are being developed
for the corrosion protection of metals such as aluminum and stainless
steel in
corrosive environments. These coatings consist of aqueous dispersions
of silicone resins, stabilized with polymeric surfactants and
pigmented with
nontoxic anticorrosive additives. The latter silicone-modified polymers
yield emulsions that adhere the coating to metal surfaces. By forming
a topcoat-bound primer layer in situ, low-VOC coatings having
simple application
properties can be formulated. The ultimate goal in developing the coatings
is to provide an effective, environmentally sound method for protecting
the surfaces of aluminum and stainless steel without introducing additional
pretreatment and priming steps.
A formulation of a VOC-compliant primerless silicone coating for
corrosion control was characterized by Electrochemical Impedance
Spectroscopy
(EIS) and open-circuit potential measurements. The test samples used
in this
investigation consisted of panels of 2024-T3 aluminum, 316 stainless
steel, and cold-rolled steel 1010 coated on one side with the experimental
primerless
silicone coating. Panels of the bare alloys, as well as an aluminum
panel and a 304 stainless-steel panel coated with Aerocoat 7 (AR-7),
were also
tested. AR-7 was used as a control coating because of its excellent
corrosion protection performance during 18 months of exposure at
the Kennedy Space
Center Beach Corrosion Test Site. Each sample was placed in an electrochemical
cell and studied at various immersion times for up to 1 week in an
electrolyte solution that consisted of aerated 3.5-percent salt (weight-by-weight)
(NaCl). The cell is designed to expose a circular area of 1 square
centimeter
to the electrolyte.
Open-circuit potential, as well as impedance measurements and visual
observations, indicated that the newly developed primerless silicone
coating provided
effective corrosion protection of 316 stainless steel but was ineffective
on aluminum 2024 T3 and cold-rolled steel. The failure was greater
in the case of the cold-rolled steel. It was also determined that
AR-7 provides
a better degree of corrosion protection on aluminum 2024-T3 than
on 304 stainless steel.
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Figure 1. Open-Circuit Potential as a Function of Immersion Time in
3.5% NaCl for Silicone Coating on 2024-T3 Aluminum
Figure 2. Bode Magnitude Plots for Silicone-Coated Carbon Steel
at Different
Immersion Times in 3.5% NaCl
Contacts:
Dr. L.M. Calle (Luz.Calle-1@ksc.nasa.gov),
YA-C2-T, (321) 867-3278; and L.G. MacDowell, YA-C2-T, (321) 867-4550
Participating Organization: Dynacs Inc. (R.D. Vinje)
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