Compositions – Preservative agents – Anti-corrosion
Reexamination Certificate
2001-12-06
2003-06-03
Anthony, Joseph D. (Department: 1714)
Compositions
Preservative agents
Anti-corrosion
C252S400230, C252S088100, C252S387000, C422S012000, C422S015000
Reexamination Certificate
active
06572789
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a new class of phosphinic acid-based corrosion inhibitors, to methods of preparing the inhibitors and to use of the inhibitors to inhibit corrosion in ferrous metal aqueous systems.
BACKGROUND OF THE INVENTION
Ferrous metals, such as carbon steel, are one of the most commonly used structural materials used in industrial aqueous systems. It is well known that corrosion of the metal is one of the major problems in industrial aqueous systems having ferrous metal in contact with an aqueous solution. Loss of metals due to general corrosion leads to deterioration of the structural integrity of the system because of material strength reduction. It can also cause other problems elsewhere in the system, such as under-deposit corrosion, reduction of heat transfer efficiency or even blockage of the flow lines due to the transport and accumulation of corrosion products in places with low flow rates or geometric limitations.
Corrosion inhibitors can be used to inhibit the corrosion of ferrous metals in aqueous or water containing systems. These aqueous systems, include, but are not limited to, cooling water systems including open recirculating, closed, and once-through systems; systems used in petroleum production (e.g., well casing, transport pipelines, etc.) and refining, geothermal wells, and other oil field applications; boilers and boiler water systems or systems used in power generation, mineral process waters including mineral washing, flotation and benefaction; paper mill digesters, washers, bleach plants, white water systems and mill water systems; black liquor evaporators in the pulp industry; gas scrubbers and air washers; continuous casting processes in the metallurgical industry; air conditioning and refrigeration systems; building fire protection heating water, such as pasteurization water; water reclamation and purification systems; membrane filtration water systems; food processing streams and waste treatment systems as well as in clarifiers, liquid-solid applications, municipal sewage treatment systems; and industrial or municipal water distribution systems.
Localized corrosion such as pitting may pose even a greater threat to the normal operation of the system than general corrosion because such corrosion will occur intensely in isolated small areas and is much more difficult to detect and monitor than general corrosion. Localized corrosion may cause perforation quickly and suddenly without giving any easily detectable early warning. Obviously, these perforations may cause leaks that may require unscheduled shutdown of the industrial aqueous system. Sudden failure of equipment due to corrosion could also result in environmental damage and/or present a serious threat to the safety of plant operations.
Corrosion protection of ferrous metal in industrial aqueous systems is often achieved by adding a corrosion inhibitor. For example, many metallic ion corrosion inhibitors such as CrO
4
2−
, MoO
4
2−
, and Zn
2+
have been used alone or in combination in various chemical treatment formulations. These inhibitors, however, have been found to be toxic and detrimental to the environment and their use in open-recirculation cooling water systems is generally restricted. Inorganic phosphates such as orthophosphate and pyrophosphate are also widely used. The inorganic phosphates have been found to contribute to scale formation (e.g., calcium phosphate, iron phosphate and zinc phosphate salts) if used improperly.
In order to obtain satisfactory corrosion protection and scale control at the same time, a robust treatment program and frequent testing and monitoring to ensure conformance are often required. Due to changes in water chemistry (e.g., phosphates, pH, Ca
2+
, etc.) or operating conditions (e.g., temperature, flow rate, polymer dosages, etc.), these requirements may be difficult to fulfill, especially in systems with a long holding time index (e.g., >3 days).
“Holding time index” is a term used to define the half-life of an inert species such as K
+
added to an evaporative cooling system. Evaporative cooling systems with a long holding time index put great demand on treatment chemicals as these chemicals must remain stable and function properly over long periods of time.
Orthophosphate and pyrophosphate are often used together to provide optimal corrosion protection, especially against carbon steel pitting corrosion. Orthophosphate is generally considered as an anodic corrosion inhibitor. Pyrophosphate is considered as a cathodic corrosion inhibitor.
It is well known that the combined use of an anodic inhibitor and a cathodic inhibitor could provide substantial synergistic benefits for reducing both localized (i.e., pitting) and general corrosion. Unfortunately, pyrophosphate is not stable in cooling water systems as it reverts to orthophosphate via a hydrolysis process. The reversion rate depends on many factors including system holding time index, temperature, pH, metal ion concentrations and bacteria activity. Furthermore, the reversion rate in a system is generally not predictable. In order to maintain satisfactory corrosion protection performance, a certain level of pyrophosphate (e.g., >1.5 ppm p-PO
4
) has to be maintained in the system by frequent monitoring and activating product feed when the level is lower than the specified value. Although this approach can be successful, it has a number of major drawbacks.
The drawbacks include the fact that maintenance of pyrophosphate increases the dosage demand of polymer dispersant and poses an even greater threat of phosphate scale formation due to the presence of higher total inorganic phosphate level in the water, especially when “upsets” occur. Upsets in the context of the usage herein refer to unanticipated changes in the concentration of inorganic phosphate or sudden changes in pH, cycle of concentration and substantial increase of temperature due to non-steady state operations in cooling waters. Furthermore, in some systems with very long holding time index (HTI), maintaining a certain specified level of pyrophosphate is often impossible with an acceptable pyrophosphate feed dosage.
Some organic phosphonates, such as 2-phosphono-butane-1,2,4-tricarboxylic acid (PBTC), 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP), and aminotrimethylene-phosphonic acid (AMP) have also been used previously as corrosion inhibitors alone or in combination with other corrosion inhibitors in various chemical treatment formulations. The effectiveness of these phosphonate base treatments, however, is generally substantially lower than the treatments based on inorganic inhibitors.
Some hydroxycarboxylic acids such as gluconic acid, sacharic acid, citric acid, tartaric acid and lactobionic acid have also been used in some treatment formulations. The use of these acids, however, results in a major challenge to control microbiological growth because these hydroxycarboxylates are easily consumable nutrients for bacteria growth. In addition, their corrosion inhibition effectiveness is also much lower than the inorganic corrosion inhibitors. Therefore, they are typically used in low demand and easy to treat systems, such as some comfort cooling systems.
U.S. Pat. No. 4,606,890 discloses that 2-hydroxy-phosphonoacetic acid (HPA) can be used as a corrosion inhibitor in cooling water. HPA was found to be a much more effective corrosion inhibitor than HEDP and PBTC (See, A. Yeoman and A. Harris, Corrosion/86, paper no. 14, NACE (1986)). However, HPA is not halogen stable and it will revert to orthophosphate in the presence of halogen based biocides. Since bleach or NaOBr are the most widely used biocides in cooling water systems, the halogen instability of HPA limits its application potential and reduces its effectiveness. In addition, HPA is found to be a relatively ineffective CaCO
3
scale inhibitor.
In order to address some of the limitations of HPA, an organophosphonic acid mixture has been used by many as mild steel corrosion inhibitor in cooling water applications
Morris John D.
Reed Peter E.
Yang Bo
Anthony Joseph D.
Breininger Thomas M.
Martin Michael B.
Ondeo Nalco Company
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