Chemical apparatus and process disinfecting – deodorizing – preser – Process disinfecting – preserving – deodorizing – or sterilizing – Maintaining environment nondestructive to metal
Reexamination Certificate
2000-06-30
2002-06-04
McKane, Elizabeth (Department: 1744)
Chemical apparatus and process disinfecting, deodorizing, preser
Process disinfecting, preserving, deodorizing, or sterilizing
Maintaining environment nondestructive to metal
C422S006000, C422S018000, C252S389620, C252S389300, C106S014440, C427S207100
Reexamination Certificate
active
06399021
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention in a broad aspect relates to inhibiting the corrosion of metals. The invention more particularly concerns compositions and methods of controlling the pH proximate a point of incipient corrosion of a metal at a pH value where the metal is passive to corrosion. The invention more especially concerns coating compositions for metals that provide pH control at a metal surface for passivation. The coating compositions are water vapor permeable and comprise a reservoir layer that is preferably of a water repellent but semipermeable carrier in which one or more particulate or liquid passivating materials are dispersed. The passivating materials preferably include one or more particulate or liquid pH buffers, but can also include other types of corrosion preventative materials, such as corrosion inhibitors. The invention can take the form of a single layer coating or of a coating formed by a plurality of layers.
2. Related Art
The corrosion of steel and other metal products continues to be a serious technical problem which has profound effects an the economy and the standard of living. It causes premature replacement of infrastructure, which in turn causes loss of natural resources, and gives rise to inferior roads and buildings. It also causes premature replacement of equipment and parts in industry and in boats and other marine vehicles, automobiles, and aircraft.
The process of corrosion requires several physical conditions. These conditions include a metallic path, an electrolyte, anode, cathode, and a potential difference between the anode and the cathode (tendency to corrode) A metallic path allows the transfer of electrons between the anode and cathode sites; this path is normally the substrate metal. The electrolyte is normally an aqueous solution around the substrate metal and contains ionic species capable of transferring charge between the anode and the cathode of the substrate metal. The anode is the location where the substrate metal corrodes and mass loss occurs. At the anode, metal atoms lose electrons and convert to metal ions which are drawn into the surrounding electrolyte. At the cathode, ionic species receive electrons from the substrate metal and convert them back to molecular form. Potential difference (or tendency to corrode) between the anode and cathode can result from many different conditions which include: variations in metal or alloy compositions; difference in amount of dissolved oxygen; presence of impurities on the substrate; ionic strength and/or constituents; temperature differences; etc.
Corrosion can be prevented, halted, or reduced by interrupting the transfer of electrons, by changing the chemistry at the anode or cathode, or by isolating the substrate from the electrolyte. Methods to achieve the prevention of corrosion include the use of barrier coatings or claddings, sacrificial coatings, corrosion inhibitors, cathode protection, and surface passivation. The barrier coatings or claddings include paints, organic coatings, ceramic and inorganic coatings, plastics, noble metal platings or claddings (such as nickel) and more.
Sacrificial coatings prevent corrosion by having a greater tendency to corrode than the metal they protect, thus converting the substrate metal to a more noble (non-corrosive) potential. Sacrificial coatings include zinc, aluminum, and magnesium metals and alloys applied as claddings, hot dip coatings, platings, or as fillers in primers and paints or other organic coatings. corrosion inhibitors change the surface chemistry at the interface between the metal substrate and the electrolyte solution. This interface barrier may be formed by oxidizing the anode surface, precipitating a film or barrier layer that limits diffusion of ionic species between the bulk electrolyte and the substrate surface, or by adsorbing compounds which impart a hydrophobic film to the substrate metal surface. Cathodic protection of a substrate surface may be achieved by converting the entire surface of the substrate metal to a cathode through the use of sacrificial anodes or impressed electrical current. Surface passivation involves importing an oxide film to the substrate metal surface, thus preventing or reducing the tendency of the substrate metal to develop anode and cathode sites. Metallic substrates develop passive surfaces in specific environments or when exposed to solutions with specific pH ranges. For example, steel and iron substrates are naturally passive when exposed to aqueous solutions that have a pH of 8.5 or above. Aluminum also has a naturally passive surface due to an oxide that forms a tightly adherent oxide film limiting further exposure of oxygen to the metal substrate. Passive films, however, can be attacked and compromised by certain ionic species. In the case of iron and iron alloy materials, the naturally passive surface can be compromised by chloride ions and hydrogen ions, among others.
The previously mentioned mechanisms of corrosion protection have various drawbacks. Barrier coatings can be expensive and offer very little protection against corrosion if they are compromised, damaged mechanically, or have insufficient coverage. Sacrificial coatings have the potential of creating embrittlement of high strength steels due to the creation of monatomic hydrogen by-products from the corrosion reaction. The coatings may also be rapidly used up under certain accelerated corrosion conditions. Corrosion inhibitors are often expensive and some have been shown to be environmentally unfriendly or toxic. Many of these are available only as liquids making them inappropriate for certain applications as they function best in certain concentration ranges. Cathodic protection can be an expensive protection means that requires skilled professionals for its design and application. It is more readily applicable to new structures, but is difficult and/or expensive to install on an existing structure. Surface passivation has been used relatively little because it requires control of the environment around the substrate metal surface.
In nature, stable materials exist at their lowest form of energy. Iron typically exists as iron oxide ore. Mankind spends a tremendous amount of money refining and adding energy to the iron ore to create steel and other iron products with the necessary properties for fabrication of metal products and for construction of roads, bridges, buildings, and the like. The natural response of such products to the environment is to return to their lowest, most stable energy state, i.e., the iron oxide state. This corrosion process is accelerated when the products are exposed to corrosive constituents in the environment. Large amounts of time and money are expended annually in the use of coating materials to inhibit such corrosion.
Eight types of corrosion are defined by the National Society of Corrosion Engineers, namely: (1) General; (2) Localized; (3) Galvanic; (4) Environmental Cracking; (5) Erosion-Corrosion Cavitation and Fretting; (6) Intergranular; (7) Dealloying; and (8) High Temperature. General corrosion results from open exposure to corrosive conditions. Localized corrosion affects smaller portions of the metal surface than general corrosion but the rate of penetration may be very fast. Crevice corrosion is a form of localized corrosion resulting from corrosive exposure in a shielded location where oxygen depletion occurs. The oxygen depletion results in the development of acidic conditions which accelerate the corrosive loss of base metal. Electromotive corrosion is an accelerated form of localized corrosion due to stray electrical currents passing through an active corrosion cell.
General corrosion, crevice corrosion and electromotive corrosion are typically the kinds of corrosion of primary concern with iron and steel products. However, this invention is not only applicable in inhibiting those kinds of corrosion, but also in inhibiting other types of corrosion.
Galvanic corrosion occurs when two metals with different potentials or tendencie
Dalton William M.
Heimann Robert L.
McGowan Nancy M.
Webb David R.
Boyer Michael K.
Elisha Technologies Co LLC
McKane Elizabeth
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