Compositions – Preservative agents – Anti-corrosion
Reexamination Certificate
2001-12-10
2003-11-11
Anthony, Joseph D. (Department: 1714)
Compositions
Preservative agents
Anti-corrosion
C252S387000, C252S397000, C252S407000, C252S175000, C252S180000, C210S759000, C210S698000, C106S014110, C106S014240, C422S012000, C422S013000
Reexamination Certificate
active
06645400
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method of inhibiting corrosion as well as inhibiting scale and deposit formation resulting from the saturation of mineral salts, and buildup of corrosion byproducts. The invention generally relates to the inhibition of corrosion of metals in contact with an electrolyte and the reduction of insulating films owing to scale formation/deposition. The invention most particularly relates to the inhibition of corrosion on metals which experience active-passive transition by application of a hydrogen peroxide donor. Illustrative of such metals are steel(s), aluminum, titanium or mixtures thereof; however the instant invention contemplates the treatment of any metal which is capable of experiencing active/passive transitions when exposed to passivating agents.
BACKGROUND OF THE INVENTION
Corrosion of metals, particularly those metals found in cooling water circulating systems, and most particularly those in heat exchangers, are of critical concern.
In industrial cooling systems, water from rivers, lakes, ponds, wells, wastewater treatment plant effluent etc., is employed as the cooling media for heat exchangers. Such waters can contain a variety of either dissolved and/or suspended materials such as mineral salts, metals, organics, silt, mud etc.
The cooling water from a heat exchanger is typically passed through a cooling tower, spray pond or evaporative system prior to discharge or reuse. In such systems, cooling is achieved by evaporating a portion of the water passing through the system. Because of the evaporation that takes place during the cooling, both dissolved and suspended solids concentrate. The concentrating of various anionic ions such as chlorides and sulfates can increase the rate of corrosion of the metals making up the cooling system. This is especially true with the metals making up the heat exchangers that are experiencing higher temperatures.
Furthermore, contaminates such as hydrogen sulfide can also increase corrosion rates. Likewise, mineral salts, for example those of calcium and magnesium, can induce scaling of the heat exchanger. A scale common in cooling systems is calcium carbonate. Other scales or deposits such as calcium phosphate or iron can also inhibit heat transfer as well as induce under-deposit corrosion.
Deposit formation on heat exchangers seriously reduces heat transfer. Corrosion byproducts can form on the metal surface where a corrosion cell has formed. Deposits from metal oxides, silt, mud, microbiological activity, and process contamination can reduce the efficiency of heat transfer as well as increase corrosion.
Reducing the corrosion, scaling and deposition of heat exchangers and associated cooling system equipment is thus essential to the efficient and economical operation of a cooling water system. Excessive corrosion of the metallic surfaces can cause the premature failure of process equipment, necessitating down time for the replacement or repair of the equipment. Additionally, the buildup of corrosion products on the heat transfer surfaces impedes water flow and reduces heat transfer efficiency thereby limiting production or requiring down time for cleaning.
Aspects of Corrosion
In order for corrosion to occur, a corrosion cell must form. The corrosion cell consist of two half cells, the cathode, and the anode.
The cathode is defined as the point where the reduction of a reducible substance takes place. In waters where the pH is greater than 4.2 (like that of a cooling water system), the primary reducible substance is oxygen. The steps involved with the cathode include: oxygen diffusion to the metal surface, adsorb by either physical or chemical adsorption, electron transfer, rearrangement with other adsorbed materials (i.e. water and electrons with subsequent formation of hydroxide ions), de-sorption of the newly formed byproduct (hydroxide), and diffusion into the bulk meter. With increased concentration of hydroxide ions, oxygen diffusion and adsorption rates are reduced.
The anode is defined as the point where dissolution of metal ions occurs. The dissolution of metal ions at the anode is a chemical process. The reaction forms ferrous hydroxide. Initially, the potential at the anode is low; however with time, the electrical potential at the anode increases. The increased potential is the result of the increased concentration of metal ions (result of dissolution) in the immediate vicinity of the anode. The increased concentration of metal ions induces the formation of a Metal Ion Concentration Cell, as well as the reduction of oxygen. The increasing concentration of cationic charged ions at the anode increases the electrical potential of the anode.
General corrosion is defined as a state where the potential of the cathode decreases with time while the potential at the anode increases. At some point, the potentials of the cathode and anode shift or find neighboring electrodes of stronger or weaker potential. This shifting or jumping is the result of the mechanisms already described. As the hydroxide concentration at the cathode increases, oxygen adsorption decreases, and the cathodes potential goes down. At the anode, where the concentration of cationic ions increases, the demand for electrons increases, so the potential goes up. This process of electrode reversal continues across the surface of the metal resulting in a uniform loss of metal.
Pitting corrosion refers to a condition where the potential surrounding the anode is high (cathodic) and electron flow is not distributed across many anodes, therefore the electron comes from a local anode. Pitting corrosion is of great concern because of the high loss of metal from a localized area. At a metal loss rate of several mils per year (MPY), general corrosion would take many decades of continued corrosion before resulting in failure of the part, e.g. of a heat exchanger. However, in pitting corrosion, the electron flow and subsequent metal loss is from a localized area. Pitting corrosion often results in equipment failure long before reaching the expected life of the equipment, e.g. the heat exchanger.
Pitting corrosion occurs when the cathodic surface has been depolarized. Chlorides for example, compete for the metal surface with the oxygen donor. When a chloride ion is adsorbed at the metal surface, it prevents the oxygen from reaching the surface. The potential at the site is reduced, and the area becomes anodic. This induces a high flux of electrons to flow from the localized site to the surrounding cathode.
Passivation can be defined as the loss of chemical reactivity exhibited by certain metals and alloys under specific environmental conditions. The onset of passivation is associated with the formation of an oxide layer that is resistant to further oxidation.
The mechanism of passivation, as it relates to ferrous metal surfaces, involves the dissolution of metal ions, followed by formation of a ferrous hydroxide layer, followed by conversion to an insoluble ferric oxide by reaction with oxygen. Analysis of passive films indicates a layered structure with an outer layer of gamma iron oxide and an inner layer of magnetite.
Passivation occurs when we have established sufficient oxidation potential. At low oxidation potential, insufficient concentrations of oxidizer exist to establish a homogenous oxide layer; distinct anodes and cathodes exist. When sufficient oxidizer is present, the electrical potential of the entire surface is increased. The concentration of oxidizer is sufficient to induce the “flash” formation of an oxide layer. In other words, the oxidizer concentration is sufficiently high to react with the ferrous hydroxide across the entire metal surface. With increased oxidation potential, the current density increases. The current density required to induce passivation is called the critical current. At yet higher oxidizer concentrations, aggressive attack of the oxide layer occurs, and the corrosion rates increase; this is termed the transpassive region.
Passivation is a process requiring oxygen. Therefore, inhibitors t
Anthony Joseph D.
Lowrie, Lando & Anastasi
United States Filter Corporation
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