Electrolysis: processes – compositions used therein – and methods – Electrolytic material treatment – Metal or metal alloy
Reexamination Certificate
2000-09-19
2002-03-19
Bell, Bruce F. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic material treatment
Metal or metal alloy
C205S725000, C205S726000, C205S727000, C205S740000, C205S733000, C204S196060, C204S196170, C204S196190, C204S196230, C204S196370, C204S196240
Reexamination Certificate
active
06358397
ABSTRACT:
BACKGROUND OF THE INVENTION
This application is directed to a system for the cathodic protection of reinforcing members referred to as “rebars” in conventionally reinforced concrete structures. Such rebars are produced from mild steel (also referred to as “black steel”) which has less than 1% carbon and less than 2% of alloying elements, combined. More particularly the invention teaches a method of providing cathodic protection which is immediately commenced on newly embedded rebars in reinforced and/or prestressed concrete structures, that is, structures such as bridges, buildings including power stations, marine structures such as docks, and roadways which are yet to be built.
In applications where the cost of corrosion-protected rebars is justified, they have been coated with a synthetic resinous layer, typically an epoxide resin, which serves as a barrier against any liquid, thus denying formation of an electrochemical cell on the surface of the rebar. Such protection is referred to as “barrier protection” and is sometimes also obtained by painting rebars with a wide array of paints. Alternatively, rebars have been galvanically protected by being hot-dipped in zinc. Another alternative is to provide a rebar with both galvanic protection, and barrier protection. For example, some paints contain a high concentration of conductive metal such as zinc powder, or, metal salts such as zinc chromate.
Galvanized and aluminized steel products are commonplace as is the use of aluminum as an anodic metal. It is recognized that a thin aluminum film less than 0.2 mm thick, by itself, has limited protective function as a sacrificial anode because there is insufficient aluminum metal to be sacrificed over a long period of time in the range from 20 to 50 years. It is also recognized that a thicker coating of aluminum in the range greater than 0.2 mm thick up to about 1 mm thick, will provide effective protection as a sacrificial anode provided the aluminum itself is not destroyed by corrosive forces of its environment. Such corrosive forces exist in freshly poured concrete which has a pH above 9, up to about pH 13, which pH remains above 9 for several years while the concrete is curing, typically up to about 5 years, after which carbonation of the concrete, and acidification due to sulfur trioxide, acidic water and other factors, start to lower the pH of the concrete.
U.S. Pat. No. 5,100,738 to Graf teaches coating a rebar of a “steel alloy customary in reinforcing steels” (col 1, lines 58-59) immediately after rolling, with a layer of aluminum or aluminum alloy (together referred to as the “Al-layer”), then coating the once-coated rebar with a layer of a synthetic resin (“first layer”). The stated purpose of the Al-layer is that it “ensures reliable corrosion protection, in particular even when cracks appear in the first layer when in use, i.e. in particular upon bending of the reinforcing steel. In such cracks the second layer of aluminum or of aluminum alloy is exposed so that, until the concrete of a concrete structural part in which the reinforcing steel is imbedded sets, this layer then reacts with the free lime of the concrete of the cement with the assistance of oxygen to form a calcium aluminate, which ensures particularly solid and tight fusion with the reinforcing steel, so that no cracks, etc. into which moisture can penetrate occur or remain between the reinforcing steel and concrete. The first layer protects the second layer against external stresses of a chemical and/or mechanical nature.” (see col 1, lines 21-36). This statement of how the Graf reinforcing member functions is reiterated at col 2, lines 27-48).
Since the function of the Al-layer is to provide the metal reactant for the free lime so as to form the calcium aluminate, there is no need for a thicker layer of Al metal than is required for the chemical reaction. Therefore Graf specifies that the Al-layer “is under 200 &mgr;m (micrometers)”, preferably “in the order of magnitude of about 20 to 25 &mgr;m”. Upon reaction the aluminum provides the calcium aluminate which “ensures particularly solid and tight fusion with the reinforcing steel” and concrete.
However, the desired reaction to form the calcium aluminate is not the only function of the Al coating because Graf states the Al-layer “contains zinc, while the percentage of aluminum is greater than 50% and, preferably, between about 55% and 70%.” and, that “the percentage of zinc is smaller than 50% and, preferably, between about 28% and 43%.” (see col 1, lines 44-49).
There is no teaching in Graf as to how the desired thin Al-layer is applied. However it is known that a layer of Al less than 200 &mgr;m thick conventionally applied on a rebar cannot be non-porous, and recognizing this, Graf also uses his Al-layer of Al-alloy to function as a sacrificial anode.
There is no indication as to whether the duty of the thin layer as a sacrificial anode is completed before the chemical reaction forming the calcium aluminate occurs because it is evident that if the calcium aluminate were to be formed first, there would be no protection from a sacrificial anode. Therefore one skilled in the art will appreciate that, depending upon the thickness of the metal layer, it will provide at least a measure of cathodic protection of the rebar, by virtue of the metal layer functioning as a sacrificial anode. Thus, with the Graf rebar protected with a coating of aluminum functioning both as a reactant and a sacrificial anode, it is clear that there is no reason to use such an anodically protected rebar as a cathode.
If there was no synthetic resinous layer overlying the very thin Al-layer on the rebars, then upon the rebars being embedded in freshly poured concrete, their entire surface would be transformed into a very thin calcium aluminate surface which presumably would not be expected to corrode. Since the calcium aluminate provides barrier protection, and there is no suggestion or reason to believe that the calcium aluminate layer provides any galvanic protection, it is evident that Graf did not believe such rebars could be used with an impressed current. Further, since Graf deliberately coated the Al-layer with an epoxy resin (stated only in claim
8
) which is known to be electrically non-conductive, it would not be reasonable to use such a rebar with an impressed cathodic current of practical magnitude.
Still further, it is known that where an insulating layer of resin is provided on a metal surface which is then cathodically protected with an impressed current, and a break, crack or fissure exists in the resin which exposes the metal surface within the fissure, the exposed metal surface is protected, but the metal surface proximately surrounding the fissure becomes corroded causing the resin immediately above the corroding surface to be lifted from the metal surface. This phenomenon is more fully explained in a reference text titled “Handbuch des Kathodischen Korrosionsshutzes, 1980 (164-173)” which in relevant part states “Specific damage to steels results when a coating of non-ferrous metal (such as aluminum) is overlaid with a coating of resin and used in a cathodically protected system in which there is a break in the resin coating and the environment penetrates the break. Such damage is referred to as cathodic detachment. The generation of hydrogen by electrochemical reaction leads to separation of the resin coating and destruction of the metal with a high rate of corrosion.”
Rather than forming a calcium aluminate and relying upon that for protection, this invention relies upon the discovery that a continuous and uninterrupted essentially non-porous thin aluminum oxide (Al-oxide) layer, or, hydrated Al-oxide (HAl-oxide) layer, less than 100 &mgr;m thick, typically in the range from 5 &mgr;m to 75 &mgr;m thick, on the surface of essentially pure aluminum coated on a rebar survives in freshly poured concrete having a pH above 9 and up to about 13, long enough to protect the Al metal until the concrete sets. Hereafter the term “combined Al-oxide layer” refers to a thin co
Bell Bruce F.
COR/SCI, Llc.
Renner , Otto, Boisselle & Sklar, LLP
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