1. Field of the Invention
This invention relates to the prevention of microbially induced corrosion, microbial fouling and ordinary electrochemical corrosion by the application of a coating of nickel-63 to the substrate. Specifically, this invention relates to prevention of corrosion of metallic surfaces by microbes.
2. Description of the Prior Art
Microbial corrosion, or microbiologically influenced corrosion (MIC), may be defined as metal loss caused, or accelerated, by microbial action at one or both of the two sites controlling electrochemical corrosion, namely the anode and the cathode. Microbial corrosion has been noted with cast iron, mild steel, stainless steel, aluminum and its alloys, and copper and its alloys. Concrete, wood and various plastics also undergo microbial degradation (by mechanisms that obviously fall outside the above definition for microbial corrosion). The destruction of metallic and non-metallic materials due to microbial action is a serious and costly problem for industrialized societies.
A number of microbes have been identified with degradative processes, including those listed below:
Cladosporium resinae--a fungus that corrodes aluminum fuel tanks.
Pseudomonas--a type of bacterium capable of reducing ferric iron to ferrous iron.
Thiobacillus ferrooxidans--a type of bacterium that oxidizes ferrous iron to ferric iron.
Thiobaccillus thiooxidans--a type of bacterium that produces sulfuric acid.
Gallionnella--a type of bacterium that can oxidize Fe.sup.+2 to Fe.sup.+3.
Nitrobacter--a type of bacterium which oxidizes NO.sub.2.sup.- to NO.sub.3.sup.-.
Nitrosomas--a type of bacterium which oxidizes NH.sub.3 to NO.sub.2.
Desulfovibrio, Desulfomonas, Desulfotomaculum--types of anaerobic, sulfate-reducing bacteria.
The mechanisms by which these microbes contribute to the corrosion of metals and degradation of non-metals are unique to the specific microbes, and many are not well understood. The most widely studied form of microbial attack involves the anaerobic sulfate-reducing bacteria (SRB).
The SRB are found in a variety of natural environments, including many types of soils and sediments, fresh, brackish and salt waters, natural hot springs, oil and gas wells, and sulfur deposits. They tolerate temperatures from &lt;5.degree. to 75.degree. C., a pH range of about 5 to 9.5, and hydrostatic pressures of at least 10.sup.5 kPa. They survive, but do not grow, under aerobic conditions.
Although there is not universal agreement about the mechanism by which SRB cause or accelerate metallic corrosion, it is believed that the bacteria bring about cathodic depolarization of the metal surface by removing hydrogen from cathodic sites in a sulfate-reducing reaction. The equations for this proposed mechanism are as follows:
(1) anodic reaction 4 FE.fwdarw.4 Fe.sup.+2 +8 e.sup.-
(2) electrolytic dissociation of water 8 H.sub.2 O.fwdarw.8 H.sup.+ +8 OH.sup.-
(3) cathodic reaction 8 H.sup.+ +8 e.sup.- .fwdarw.8 H.sub.2 O
(4) cathodic depolarization by SRB 8 H+SO.sub.4.sup.-2 .fwdarw.S.sup.-2 +4 H.sub.2 O
(5) corrosion product Fe.sup.+2 +S.sup.-2 .fwdarw.FeS
(6) corrosion product 3 FE.sup.+2 +6 OH.sup.31 .fwdarw.3 Fe (OH).sub.2
(7) overall reaction 4 Fe+SO.sub.4.sup.-2 +4 H.sub.2 O.fwdarw.3 Fe (OH).sub.2 +FeS+2 OH.sup.-
The prevention of microbially induced corrosion usually involves trying to prevent the occurrence, growth and metabolic activity of the microbes in the vicinity of the metal by the use of chemical reagents (biocides), cathodic protection, or protective coatings (e.g. an epoxy film).
U.S. Pat. No. 3,497,434 teaches a method for preventing fouling of a metal structure immersed in a marine environment by coating this metal structure with a metal toxic to marine organism. The metals toxic to marine organisms which are described in this patent are cadmium, tin, zinc and aluminum.
U.S. Pat. No. 4,123,338 describes a method of preventing fouling coupled with corrosion inhibition in water environments by the application of a coating of technetium 99 to the substrate.