Copper article with protective coating

A copper heat exchanger unit for operating in a harsh environment wherein the exposed surfaces of the unit are first provided with a black oxide layer and then electrocoated with a protective acrylic barrier.

BACKGROUND OF THE INVENTION 
This invention relates to protecting copper articles from a corrosive 
environment, and in particular, to protecting copper heat exchangers used 
in refrigerated cargo containers. 
Ocean going cargo ships now carry large containers on their open decks 
which serve to expand the ship's utility. Many of these containers are 
equipped with refrigeration systems so that they can store perishable 
goods for relatively long periods of time. The refrigeration systems, 
however, are exposed to salt, air and water which causes the exposed parts 
to corrode at an accelerated rate. Heat exchanger surfaces used in the 
refrigeration systems are particularly susceptible to salt air and salt 
water corrosion. 
In an effort to combat the harmful effects of salt, air and water, heat 
exchangers used in sea going containers are typically fabricated of 
copper. In addition, exposed surfaces of the heat exchangers have also 
been coated with various types of paints for added protection. These 
protective coatings have met with only with limited success for a number 
of reasons. First, most coating materials do not adhere well to copper and 
eventually the coating will flake away to expose the copper substrate. 
Secondly, the coating must be relatively thin so that it does not 
adversely effect the heat transfer characteristics of the heat exchanger. 
Most thin layer coatings, however, are extremely porous and thus will not 
establish an impenetrable protective barrier for the underlying copper. 
As will be described in greater detail below, the present invention will be 
explained with specific reference to providing a protective barrier for a 
copper heat exchanger. However, it should be evident to one skilled in the 
art that the invention is not limited to this specific application and can 
be used in connection with any copper article where the need exists to 
protect the article from a hostile environment or the like. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to protect copper 
articles from a corrosive environment. 
It is another object of the present invention to protect a copper heat 
exchanger from the harmful effects of salt air and water. 
Yet a further object of the present invention is to extend the life of 
refrigeration systems used in sea going cargo containers. 
A still further object of the present invention is to provide a protective 
overcoating for a copper heat exchanger that will not adversely effect its 
heat transfer properties, but yet will provide a relatively non-porous 
barrier to salt air and water. 
Another object of the present invention is to provide a protective coating 
for a copper article that has improved adhesion properties. 
These and other objects of the present invention relate broadly to a copper 
article, and more specifically, to a heat exchanger for use in a sea going 
refrigeration cargo container. The exposed surface of the articles are 
first treated to produce a black oxide layer over the exposed surfaces. An 
acrylic outer layer is then electro-coated over the oxide layer to provide 
a relatively thin yet non-porous barrier that exhibits extremely good 
adhesion and protective properties against corrosive substances.

DESCRIPTION OF THE INVENTION 
Referring initially to FIG. 1, there is shown a fin coil heat exchanger, 
generally referenced 10, of the type typically used in association with 
refrigerated cargo containers. The heat exchanger includes one or more 
flow circuits 12 for carrying a refrigerant through the heat exchanger 
unit. For the purposes of explanation, the unit illustrated in FIG. 1 
contains a single flow circuit 12 consisting of an inlet line 13 and an 
outlet line 14 which are connected at one end of the exchanger by means of 
a 90.degree. tube bend 15. It should be evident, however, that more 
circuits may be added to the unit depending upon demands of the system. 
The unit further includes a series of radially disposed plate-like 
elements 16--16 that are spaced apart along the length of the flow 
circuit. The elements are supported in assembly between a pair of end 
plates 17 and 18 to complete the assembly. 
As noted above, heat exchangers of this type that are exposed to a harsh or 
corrosive environment are generally fabricated of copper because of its 
good heat transfer properties and resistance to corrosion. Nevertheless, 
these copper units can and will be adversely effected when exposed to salt 
air and water for extended periods of time. Attempts have been made with 
varying degrees of success to coat these copper units with various 
material in an effort to extend the useful life of the unit. These coating 
materials oftentimes reduce the heat transfer capacity of the unit, 
exhibit poor adhesion properties and fail to penetrate into all areas of 
the unit that might be exposed to a hostile environment. 
As will be explained in greater detail below, the exposed outer surfaces of 
the present copper heat exchanger are first pretreated to establish a 
black oxide coating thereon that creates a strong boundary layer much like 
that produced when aluminum is anodized. A thin acrylic overcoating is 
then electro-coated over the black oxide boundary layer to provide a 
strongly adhering protective barrier for extending the useful life of a 
unit exposed to a hostile environment. It has been found that this 
combination exhibits unexpected synergistic results and does not degrade 
the heat transfer properties of the unit. Additionally, the protective 
barrier is capable of penetrating deeply into remote, difficult to access 
areas, thus preventing early failure. 
Turning now to FIG. 2, there is shown a flow process diagram depicting the 
process steps involved in producing a uniform protective barrier over the 
entire outer surface of the copper heat exchanger. Initially, the two open 
ends of the flow circuit are closed by suitable plugs (not shown) and the 
exchanger is immersed in an alkaline bath 30 containing a strong base 
cleaner such as MI Clean 17 manufactured by Mitchell Bradford 
International, which is a division of Hubbard-Hall, Inc. of Waterbury, 
Conn. The bath contains a 4-7% concentration of MI Clean 17 in water and 
the solution is heated to a temperature of about 180.degree. F. The 
alkaline bath may also contain a 4 to 7% solution of sodium silicate in 
water. The heat exchanger is allowed to remain in the bath for about 5 to 
10 minutes to thoroughly clean and degrease all exposed surfaces of the 
unit. 
Upon removal from the alkaline bath, the unit is bathed in a cold water 
rinse 32 for about one minute or a period of time which is sufficient to 
remove the alkaline wash from the outer surface of the exchanger. The term 
cold water rinse as herein used refers to one in which the rinse water is 
at or about an ambient temperature. 
The rinsed heat exchanger is then placed in a second acidic cleansing bath 
34 for about 4 to 5 minutes to remove surface oxidations. The bath, held 
at an ambient temperature, contains about 10% concentration of Scone M-E 
Acid Brite 50 (also supplied by Hubbard-Hall, Inc.) in water. Acid Brite 
50 contains about 20% by weight hydrochloric acid, 11% by weight 
phosphoric acid and 10% by weight sulfuric acid along with other 
non-acidic materials which combine to thoroughly ride the outer surfaces 
of the heat exchanger of unwanted oxides. 
The unit, upon removal from the acid cleaning bath, is immediately placed 
in a cold water rinse 36 for about one minute or more to remove all trace 
of the acid bath from the outer surfaces of the unit. 
The twice cleaned and rinsed part is now immersed in an oxidizing bath 38. 
The bath contains an oxidizing solution containing equal parts sodium 
hydroxide and sodium chloride in water. A concentration of about two 
pounds of oxidizer to a gallon of water is used. The oxidizer is 
commercially available from Hubbard-Hall, Inc. and is sold under the 
tradename Black Magic CB. The unit is allowed to remain in the bath for 
between 5 and 10 minutes at a bath temperature of about 180.degree. 
F.-210.degree. F. until all exposed surfaces of the copper are thoroughly 
coated with a deep black colored oxide film. 
The oxidation process is quickly terminated by rinsing the unit in cold 
water for two to three minutes and then in hot water that is heated to 
about 120.degree. F. for about ten or eleven minutes. The unit is given a 
final rinse for about one to two minutes in deionized water at ambient 
temperature and allowed to dry. These rinses are depict at 40-42 in FIG. 
2. 
Upon drying, the unit is coated with an acrylic paint using commercially 
available coating equipment 44. The paint is available from Pittsburgh 
Plate Glass Industries, Inc. of Springdale, Pa. and is sold under the 
tradename Powercron 810-611 or Powercron 830-611. The oxidized unit is 
immersed in a bath of acrylic paint and an electrical current of 
.sup..about. 234 amps and 200 volts applied to the unit. The unit. is held 
in the bath for between nine and ten minutes to insure that all exposed 
and oxidized surfaces of the unit are fully covered with the acrylic 
overcoat to a thickness of between 0.0005 to 0.0010 inches. The unit is 
then removed from the bath and the paint Cured in an oven 48 for thirty 
minutes at 375.degree. F. 
Copper parts that were oxidized and coated by the method described above 
were tested to determine the parts' ability to resist corrosion. The AC 
impedance of each coated part was first measured and recorded. The average 
impedance of the parts was found to be about 8.times.10.sup.9 ohms per 
square centimeter and the average thickness of the acrylic coating was 
about 0.0007 inches. The parts were then exposed to steam spray for a 
period of about 48 hours and a second impedance measurement was then 
taken. The average impedance of the parts exposed to the steam was found 
to be about 7.times.10.sup.8 ohms per square centimeter. Clearly these 
tests showed that the acrylic coating was relatively less porous than 
similar coating presently in use and thus provided an improved protective 
barrier against corrosion. Further tests also showed that the coating 
exhibited improved adhesive properties and resistivity to ultraviolet 
radiation when compared to presently employed coatings. 
Although the present invention has been described with specific reference 
to a copper heat exchanger, it should be evident to one skilled in the art 
that the invention has wider applications and can be employed in 
conjunction with any type of copper article or part that may require 
extended protection from a hostile environment. 
While this invention has been explained with reference to the structure 
disclosed herein, it is not confined to the details set forth and this 
invention is intended to cover any modifications and changes as may come 
within the scope of the following claims: