U.S. patent application number 11/330857 was filed with the patent office on 2007-07-12 for corrosion inhibitors in polyurea coatings.
Invention is credited to Aihua A. Luo, Yar-Ming Wang.
Application Number | 20070158616 11/330857 |
Document ID | / |
Family ID | 38231918 |
Filed Date | 2007-07-12 |
United States Patent
Application |
20070158616 |
Kind Code |
A1 |
Luo; Aihua A. ; et
al. |
July 12, 2007 |
Corrosion inhibitors in polyurea coatings
Abstract
Durable elastomeric polyurea coatings can be applied to metal
surfaces subjected to impacts and environmental corrosion.
Corrosion resistance of the coatings, especially on magnesium alloy
surfaces, is improved by incorporation of particulate inhibitor
additives in the liquid polyurea precursor coating material.
Examples of inhibitors include potassium dichromate, sodium
dichromate, potassium permanganate, vanadium pentoxide, phosphorus
anhydride, cerium oxide, lanthanum oxide, calcium oxide and sodium
oxide.
Inventors: |
Luo; Aihua A.; (Troy,
MI) ; Wang; Yar-Ming; (Rochester Hills, MI) |
Correspondence
Address: |
GENERAL MOTORS CORPORATION;LEGAL STAFF
MAIL CODE 482-C23-B21
P O BOX 300
DETROIT
MI
48265-3000
US
|
Family ID: |
38231918 |
Appl. No.: |
11/330857 |
Filed: |
January 12, 2006 |
Current U.S.
Class: |
252/387 |
Current CPC
Class: |
C09D 5/084 20130101 |
Class at
Publication: |
252/387 |
International
Class: |
C23F 11/00 20060101
C23F011/00 |
Claims
1. A coating material for a magnesium alloy surface of an article,
the coating material comprising: liquid precursors of an
elastomeric polyurea formulated for spray coating or immersion
coating of the magnesium alloy surface, the precursors containing
particles of a corrosion inhibitor for the magnesium alloy.
2. A coating material for a magnesium alloy surface as recited in
claim 1 in which the corrosion inhibitor is at least one of
potassium dichromate, sodium dichromate, potassium permanganate,
vanadium pentoxide, and phosphorus anhydride.
3. A coating material for a magnesium alloy surface as recited in
claim 1 in which the corrosion inhibitor is at least one of cerium
oxide and lanthanum oxide.
4. A coating material for a magnesium alloy surface as recited in
claim 1 in which the corrosion inhibitor is at least one of calcium
oxide and sodium oxide.
5. A coating material for a magnesium alloy surface as recited in
claim 1 in which the polyurea comprises the 4, 4' isomer and 2, 4'
isomer of methylene bisphenyl isocyanate.
6. A component for an automotive vehicle, the component comprising
a magnesium alloy surface with a coating of an elastomeric
polyurea, the elastomeric polyurea comprising particles of a
corrosion inhibitor for the magnesium alloy in an amount up to
about one percent by weight of the polyurea.
7. A component for an automotive vehicle as recited in claim 6 in
which the corrosion inhibitor is at least one of potassium
dichromate, sodium dichromate, potassium permanganate, vanadium
pentoxide, and phosphorus anhydride.
8. A component for an automotive vehicle as recited in claim 6 in
which the corrosion inhibitor is at least one of cerium oxide and
lanthanum oxide.
9. A component for an automotive vehicle in which the corrosion
inhibitor is at least one of calcium oxide and sodium oxide.
10. A component for an automotive vehicle in which the polyurea
comprises the 4, 4' isomer and 2, 4' isomer of methylene bisphenyl
isocyanate.
Description
TECHNICAL FIELD
[0001] This invention pertains to corrosion resistant polymer
coatings for metal surfaces. More specifically, this invention
pertains to the use of corrosion inhibitors in elastomeric polyurea
coatings for magnesium alloy surfaces.
BACKGROUND OF THE INVENTION
[0002] Light weight cast and wrought magnesium alloys have been
developed for automotive applications. In some such applications
they are exposed to conditions in which they undergo galvanic
corrosion. For example, contact of the magnesium part with water
and salt and/or with dissimilar metals leads to the corrosion of
the affected magnesium alloy surface. Many automotive chassis and
body components require protection from galvanic corrosion.
[0003] Polymer coatings have been used on metallic parts to provide
scratch-resistance, chip-resistance and resistance to corrosion.
Epoxy resins and polyurea resins have been developed for individual
usage and in combination for these applications. Commercially
available polyurea resins in precursor formulations can be applied
to, for example, a truck bed by spraying or dip-coating, and
rapidly cured. Also, epoxy resins in sprayable powder form are
available as metal surface coatings. But sprayable epoxy powder
resins are quite expensive, and polyurea resins have not provided
sufficient corrosion resistance.
[0004] It is an object of this invention to provide elastomeric
polyurea resin formulations with corrosion inhibitors for metal
surfaces and especially magnesium alloy surfaces.
SUMMARY OF THE INVENTION
[0005] In the context of this invention, the term "polyurea" refers
to polymerization products of the reaction of a suitable
poly-functional isocyanate monomer(s) with a suitable
polyfunctional amine monomer(s). The polyfunctional monomers
provide many urea links [--NH--(CO)--NH--] between the isocyanate
and amine moieties of the polymer chains, and between polymer
chains. Such materials are known and commercially available. They
typically combine impact resistance, heat resistance up to
150.degree. C., and insensitivity to moisture, all desirable
properties for components of automotive vehicles and the like. A
mixture of monomers or prepolymers is formulated for spray coating
or immersion coating onto a metal substrate such as a magnesium
alloy casting or formed panel. Catalysts and chain extenders may be
incorporated to provide for a suitable cure rate of the primary and
secondary amine precursors and isocyanate precursors, or prepolymer
precursors, to allow leveling of the sprayed coating and adhesion
to the cleaned workpiece surface before the cure is completed.
Coextensive polyurea coatings applied on metal surface provide
effective and durable protection for the underlying metal. However,
truck beds and the like are subjected to severe wear and tear and
the coatings are chipped or scratched to expose the underlying
metal to corrosive media such as salt water. Exposed magnesium
alloy body components are susceptible to corrosion in these
circumstances.
[0006] In accordance with this invention, particulate corrosion
inhibitors are mixed with the coatable polyurea precursor
formulation. The inhibitor particles are used in sufficient
abundance in the matrix of the cured elastomer to inhibit corrosion
of an underlying magnesium alloy surface. The corrosion inhibitors
can be identified and selected by suitable testing procedures with
a magnesium alloy selected for a particular application. In general
metal oxides and metal oxide containing compounds are suitable
corrosion inhibitors. Examples of inhibitors to impede electrolytic
corrosion include potassium dichromate, sodium dichromate,
potassium permanganate, vanadium pentoxide, and phosphorus
anhydride. Examples of inhibitors that provide barrier-film type
corrosion protection when incorporated in the polyurea include
cerium oxide and lanthanum oxide. And examples of inhibitors for
inclusion in a polyurea that help maintain an alkaline environment
for magnesium passivation include calcium oxide and sodium oxide.
These corrosion inhibiting additives may be used alone or in
combinations in the polyurea for protection of different magnesium
alloys used in different parts subjected to different corrosion
inducing environments. Inhibitor additions in the amount of about
one-tenth weight percent to about one weight percent of the polymer
coating material are usually suitable. The amount of inhibitor
addition is a balance between obtaining useful corrosion inhibition
without compromising the bonding strength of the coating to the
underlying metal or polymer surface. The particle sizes of the
inhibitors are suitably in the range of about five to about fifty
micrometers or as necessary to obtain a desired smoothness of the
polyurea coating.
[0007] The mixture of inhibitor particles and polyurea precursors
are suitably applied to the surface of a magnesium alloy workpiece
and cured to a durable scratch and corrosion resistant finish.
Typical coating thicknesses are in the range of about one-half
millimeter to about two millimeters. The material may be applied as
a primary coating on the magnesium surface, or over an earlier
applied coating, such as a conversion coating or an epoxy powder
coating.
[0008] Other objects and advantages of the invention will be come
apparent from a description of preferred embodiments.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0009] The inhibitor-containing polyurea coating compositions of
this invention can provide beneficial protection to many different
metal workpieces but the practice of the invention is particularly
applicable to articles of manufacture for automotive vehicles based
on magnesium alloys because of their tendency to corrode in their
environment.
[0010] Commercial magnesium alloy systems adapted for sand and
permanent mold castings include magnesium-aluminum-manganese (AM),
magnesium-aluminum-zinc (AZ), magnesium-rare earth-zirconium (EK,
EZ, and ZE), magnesium-zinc-zirconium (ZK), and
magnesium-thorium-zirconium (HK, HZ, and ZH). AZ and AM alloys are
also used in high-pressure die casting applications. Other die cast
alloys include magnesium-aluminum-silicon (AS)
magnesium-aluminum-strontium (AJ), magnesium-aluminum-rare earth
(AE). AM50 is a representative magnesium-based casting alloy of
typical nominal composition, by weight, of 5% aluminum, 0.4%
manganese, and the balance magnesium. Compositional specifications,
temper specifications, and physical properties for alloy members of
these systems are available from commercial sources and technical
references. Wrought magnesium alloys, produced as bars, billets,
shapes, wire, sheet, plate, and forgings, are often made using
members of the AZ system, such as AZ31B,C having a typical nominal
composition, by weight, of 3.0% aluminum, 0.3% manganese, 1% zinc,
and the balance magnesium. Extruded bars, rods, tubes, and the
like, may be made of magnesium alloy systems such as AZ80A and
ZK60A. The practice of the invention is applicable to the families
of magnesium alloys.
[0011] Spray polyurea coatings are applied to metal surfaces
(ferrous alloys, aluminum alloys, and magnesium alloys) on
commercial and personal vehicles. Typical coating applications
include truck bedliners, quarter panels, cab floor liners, and
trailer liners. Prepolymer precursors of the coatings are
formulated by commercial suppliers for spray application or coating
by immersion of the parts in a bath. The original monomeric
constituents often include aromatic diisocyanates (and higher
functionality isocyanates) and suitable primary and secondary
diamines and triamines. Mixtures of the 4, 4 ' isomer and 2, 4'
isomer of methylene bisphenyl isocyanate (MDI) may be used to
adjust curing rate. In the case of polyurea spray coatings, a
liquid volume containing the diisocyanate is prepared separately
from the polyamine-containing volume and the two streams are mixed
in suitable proportion in a high pressure spray gun for application
on a workpiece surface. In accordance with this invention, one or
more corrosion inhibitors are incorporated into the liquid
prepolymer formulation(s), especially for coatings on magnesium
alloy workpieces.
[0012] Examples of inhibitors to impede electrolytic corrosion
include potassium dichromate, sodium dichromate, potassium
permanganate, vanadium pentoxide, and phosphorus anhydride.
Examples of inhibitors that provide barrier-film type corrosion
protection when incorporated in the polyurea include cerium oxide
and lanthanum oxide. And examples of inhibitors for inclusion in a
polyurea that help maintain an alkaline environment for magnesium
passivation include calcium oxide and sodium oxide. The selection
of one or more inhibitors for a particular cast or formed magnesium
alloy is assisted by preliminary testing of a candidate inhibitor
with the specific alloy. Polyurea coatings applied on metal
surfaces like pickup truck beds provide an effective and durable
protection for the underlying metal. However, once the coating is
damaged, by scraping, scribing or the like, exposed metal
(especially a magnesium alloy) is susceptible to corrosion. Once
exposed to salt water or other corrosive media, inhibitor particles
included in the polyurea coating release species (e.g., ions) to
interact with the metal to form a stable barrier layer, a passive
film, or an alkaline environment to mitigate corrosion.
[0013] Automotive vehicles are frequently exposed in normal usage
to aqueous salt solutions, and immersion of a candidate magnesium
alloy casting or panel in such a solution provides a test for
selection of an inhibitor material with respect to this type of
corrosion. For example, a cold rolled panel of AM50 alloy was
immersed in a five weight percent aqueous sodium chloride solution
at room temperature in a vessel in which the rate of evolution of
hydrogen gas could be measured. In the absence of any inhibitor the
solution did slowly react with the panel and a rate of hydrogen
release of about 31 liters per hour per square centimeter of panel
surface (L/hr/cm.sup.2) was measured. This hydrogen release rate
was used as a control value for comparison with the effect of
candidate corrosion inhibitors by this corrosion mechanism.
[0014] Four additional solutions, each of five weight percent
aqueous sodium chloride were prepared containing, respectively, one
gram per liter (1 g/L) CrO.sub.3, 1 g/L La.sub.2O.sub.3, 1 g/L
Ca(OH).sub.2, and 1 g/L K.sub.2Cr.sub.2O.sub.7. An AM 50 test panel
was immersed in each of the four solutions and the corresponding
hydrogen release rates measured. The hydrogen release rates for the
candidate inhibitor additives were as follows: CrO.sub.3, 45
L/hr/cm.sup.2; La.sub.2O.sub.3, 20 L/hr/cm.sup.2; Ca(OH).sub.2, 6
L/hr/cm.sup.2; and K.sub.2Cr.sub.2O.sub.7, 5 L/hr/cm.sup.2. In
these tests the CrO.sub.3 was ineffective in inhibiting corrosive
attack by aqueous sodium chloride on the AM 50 panel. But
La.sub.2O.sub.3, Ca(OH).sub.2 and K.sub.2Cr.sub.2O.sub.7 did
inhibit corrosive salt attach on the magnesium alloy in salt
solution.
[0015] Following the above aqueous sodium chloride tests on bare
AM50 panels, three batches of a commercial polyurea spray
formulation were prepared to contain, respectively one weight
percent each of, La.sub.2O.sub.3, Ca(OH).sub.2 and
K.sub.2Cr.sub.2O.sub.7 as inhibitor materials. AM50 panels were
spray coated to a thickness of about one to two millimeters with
inhibitor additive-containing polyurea.
[0016] Two Huntsman Corporation polyurea precursor liquids at about
160.degree. F. were combined for spraying in a Gusmer high pressure
gun-type, spray machine operating at about 2000 psi. The combined
polyurea precursor streams were sprayed onto the panel surfaces to
a thickness of about one to two millimeters. One precursor liquid
volume contained the MDI mixture (Suprasec 9520) and the second
precursor mixture contained amine terminated polyols, chain
extenders and one of the inhibitor additives identified above. The
spray polyurea elastomer coatings quickly gelled on the surfaces of
the panels and were dry within about ten seconds.
[0017] Some of the coated panels were then scribed through the
coating to expose the metal surface to initiate a situation of
paint damage. Coated and scribed panels were than tested in a
cyclic corrosion chamber using a standard test procedure, GM 9540P.
This accelerated corrosion test comprises three eight hour steps
(or shifts) per day which may be repeated over many days. In the
first eight hour shift (called Dry Soak), the coated and scribed
panels are exposed to an air atmosphere at 30% relative humidity at
140.degree. F. In the second shift, the panels at ambient
temperature are subjected to four successive 1.25% salt solution
mist sprays. The successive sprays are separated by at least one
hour in the eight hour shift. In the third eight hour shift, the
salt mist-coated panels are held in air at 100% relative humidity
(Wet Soak) at about 120.degree. F. This 24-hour process constitutes
one test cycle. This testing of the scribed coated panels was
repeated over thirty days. No observable corrosion occurred in any
of the scribed samples after 30 days (4 years field
equivalent).
[0018] While chromates and dichromates are useful in many polyurea
coating applications to provide corrosion resistance to magnesium
alloy components it is recognized that there may be some release of
chromium to the environment. For this reason it may be preferred to
use the alkaline metal oxide inhibitors, such as the oxides of
calcium, sodium, lanthanum or cerium, where these oxide inhibitors
or their equivalents provide suitable inhibition of corrosion.
[0019] The practice of the invention has been illustrated by
examples of preferred embodiments, but the scope of the invention
in not limited to the specific examples.
* * * * *