Patent Application
20110056516
The present invention relates generally to a process for the use of a modified sulfuric acid agent for the purpose of cleaning, deoxidizing, descaling, polishing, and/or brightening surfaces. These modified sulfuric acid agents prevent the attack on metal equipment used in their application and reduce or eliminate the release of hazardous or harmful metals to industrial waste water streams and prevent detrimental environmental release of harmful metals such as chromium, nickel and manganese used as alloying metals in so-called corrosion resistant alloys. In addition, the modified sulfuric acid agents are not aggressive to human skin and provide a greater safety profile during transport, storage and use when evaluated against standard sulfuric acid solutions.
Acid cleaning solutions are used in many applications for the cleaning of parts. The parts can be composed of metal, plastics or inorganic components. Examples that require acid cleaning are steel surfaces, coated steel, aluminum parts, aluminum and tin-coated steel cans, plastic and ceramic parts.
The acid cleaning solution is employed for the removal of undesirable components on the surface of the article and may also be used to obtain certain desirable properties of the article surface. Examples of desirable surface properties generated by acid cleaning are:
1. Appearance
2. Hydrophilic surface
3. Increased surface area and/or roughness
4. Improved adhesion of surface to subsequently applied materials
5. Better surface wetting by solvents and other applied materials
This patent should not be limited by the examples given in this application and the examples provided are given for an improved understanding of this patent.
The acid cleaning solution can be composed of a single acid, a mixture of acids for achieving the desired surface properties, surface-active agents, corrosion inhibitors and other synergetically useful components for surface modification.
In order to contain the acid cleaning solution, various containers or “treatment tanks” are employed. Since acid solutions in general aggressively attack many metal surfaces such as mild steels and aluminum, plastic tanks or tanks lined with plastic components or films are used on occasion to contain the aggressive acid cleaning solutions. However, the acid cleaners generally clean more completely and give more uniform results when operated at higher temperatures and/or higher concentrations of the acid components. The plastic tanks, whether composed of thermoplastic or thermoset polymeric materials, are not stable to these more aggressive conditions.
Due to these temperature and/or higher acid conditions, metal equipment is generally used to contain, pump and when desirable, spray the articles in order to generate a clean and desirable article surface.
Since acids will attack and dissolve metal, the treatment tanks used to contain, pump and transfer acid cleaners are generally constructed of metal alloys that are resistant to attack by the acids and other aggressive components of the acid cleaning solutions. The materials of construction of these resistant tanks can be hastalloy, stainless steels such as 302, 304 or 316 stainless and the like. In many cases, the equipment and tanks are composed of S.S. 316 alloy since this alloy gives excellent acid resistance and achieves these desirable properties at an acceptable cost.
Although these alloys are resistant to acid attack, a small amount of metal that comes in direct contact with the acid cleaning solution will dissolve and over a period of time, will render the equipment unsuitable for containing, spraying, transferring or pumping the acid solution. If the equipment is constructed of alloying metals such as chromium, nickel, copper, manganese and the like, these metals will end up in the waste stream from the industrial facility. Since low levels of metal ions in wastewater are difficult to completely remove, detrimental effects on the environment, aquatic life forms or on human health can result. It would be desirable to limit and control the removal of these metals from the cleaning equipment. This patent describes a unique acid solution that achieves these desired goals.
Compositions and methods of cleaning aluminum cans are disclosed in U.S. Pat. No. 3,969,135 issued Jul. 13, 1976 to Peter F. King et. al., U.S. Pat. No. 4,009,115 issued Feb. 22, 1977 to Robert Eric Binns, U.S. Pat. No. 4,668,421 issued Aug. 6, 1985 to David Y. Dollman and U.S. Pat. No. 4,111,722 issued Sep. 5, 1978 to Gary A. Reghi and Samuel T. Farina. These patents and examples therein are incorporated herein.
The present invention is a unique acid solution that contains components that limit or eliminate the removal of undesirable metals from the equipment used to contain, transfer, spray and pump these aggressive acid cleaners. The solution disclosed herein and method for employing the same prevent the attack on metal equipment while reducing or eliminate the release of hazardous or harmful metals as industrial waste. In addition, the solution disclosed herein and method for employing the same are generally not aggressive to human skin.
Sulfuric acid is one of the most widely purchased commodities in the world and is widely used in many diverse industries. A primary area of use for sulfuric acid is within the fields of metal working and remedial metal treatment. The metal working field is very broad and includes diverse areas such as steel mill metal rolling lines from the smelting stage to the final steps of coating finished metal components with paints or other coatings or polishing of metal surface itself. Further areas of the metal industries include foundry and metal casting processes in which numerous process steps employ acid cleaning to achieve desirable properties for end use products. One main use of sulfuric acid in industry is the cleaning of can containers for food, beverage and other articles.
In addition to metal applications, plastic and ceramic surfaces also require cleaning to prepare the surface for subsequent operations. The acid cleaning solutions are an excellent media for the removal of impurities such as oxidation products, organic and inorganic contaminants and soils and various scales such as laser cut scale, plasma cut scale, and/or weld splatter or weld scale that needs to be removed from metal, plastic and other surfaces prior to further processing of these components into finished industrial parts. The removal of these impurities are of particular importance for metal surfaces since the surface may be subsequently conversion coated, anodized, and/or painted and without complete removal of impurities, the function and appearance of the metal surface would be negatively effected. Plastic surfaces also have surface contamination that may interfere with the application of paints and acid solutions provide the best media for removal and activation of the surface for painting.
Although sulfuric acid solutions provide an excellent solvent media for the cleaning applications described above, they have the disadvantage of being too aggressive to metal surfaces and even when stainless steel equipment is employed to contain, transport and spray these aggressive solutions, some attack on the metal surface results that, over time, damages the equipment and can result in release of hazardous metal ions into the industrial waste water and/or into the environment. Sulfuric acid is also known to be aggressive to human skin, particularly at higher concentrations, and this property makes it an extremely dangerous compound to transport, store and handle from a safety standpoint. Therefore, other methods of surface cleaning and alternate acids have been used in many applications in order to circumvent these disadvantages.
Two methods have been used to replace sulfuric acid and provide cleaning methods that are less aggressive to equipment, less toxic to humans and friendlier to the environment. The first method employs mechanical removal of unwanted surface items and has typically involved grinding, medium blasting, and sanding. Mechanical removal is labor intensive, expensive and inexact. In cases were close tolerances are required on a finished part, mechanical removal often results in finished parts which are out of tolerance and unusable without expensive rework of that part. Frequently, the part must be scraped when the tolerance is too great.
The second approach, mild acids are used in cleaning applications. Acids such as phosphoric or organic acids such as citric acid have replaced the more aggressive sulfuric acid. Unfortunately, use of these relatively mild acids requires increased production times, elevation of the acid solution temperatures, or increased acid concentrations or some combination of increased temperature and/or processing time. All of these requirements significantly increase the costs of cleaning an article or manufacturing operation.
There are definite environmental drawbacks for employing these milder acids for industrial cleaning. Phosphoric acid has come under increased EPA regulation and is being regulated since release into the environment can result in increased algae growth in lakes and streams. Citric and other organic acids are known to chelate or bond metal ions and make it costly and difficult to remove chelated metal ions such as chromium, nickel, manganese from industrial waste water. To prevent environmental release of these detrimental metal ions, it would be of advantage to find a method of using sulfuric acid or other acids for industrial acid cleaning purposes.
In an attempt to use other inorganic acids both nitric acid and hydrochloric acids have been used in industrial cleaning. Using nitric acid always presents the risk of generating and releasing poisonous nitrous and nitric oxide gases. This detriment limits the use of nitric acid for industrial applications to only those plant locations and applications that can properly control or limit nitric acid decomposition. Although hydrochloric acid finds more widespread use in metal cleaning operations, the acid is very volatile and the hydrogen chloride gas that is readily released from aqueous hydrochloric acid readily rusts industrial buildings, equipment and attacks the lungs of workers that inadvertently come in contact with the gaseous hydrochloric acid.
To overcome these difficulties with the use of mild acids for cleaning and also to avoid the use of hazardous acids such as hydrochloric and nitric acids, in the present invention a modified sulfuric acid is used as one component in an acid cleaning solution. To further enhance the modified sulfuric acid various additional compounds may be added as accelerators, inhibitors, and/or buffers and/or surfactants, to mitigate, control, eliminate acid attack (etch) on metals and/or enhance the removal of soils, scales, metal tarnish, or oxidation present on the metal surface, or the improvement of a final finish on the metal such as with anodizing processes. By the use of a modified sulfuric acid, the attack on the tanks, transfer pipes, spray nozzles and pumping equipment is greatly reduced or eliminated. By mitigating the attack on equipment, the release of undesirable and/or toxic metals to wastewater, city sewers, or the environment is reduced or eliminated. The modified sulfuric acid is formulated by combining sulfuric acid with water and with amines or amine salts. Although many amines will meet the requirements of this invention, a preferred amine or amine salt is an ammonia and/or sulfate salt of ammonia. Ammonia in many cases is preferred due to its cost and/or performance profile when compared to other additions to sulfuric acid to prevent attack on equipment used for its containment, application and transfer during the cleaning process.
Other and further objects of the invention, together with the features of novelty appurtenant thereto, will appear in the course of the following description.
This invention is directed to an environmentally friendly process for treating metal making use of a formulation which includes sulfuric acid, water and one or more amines or sulfate salts of amines. Sulfuric acid will attack metal surfaces and does so more vigorously at higher concentrations and at elevated temperatures. Even so called inert alloys, such as stainless steel will be attacked by sulfuric acid of the appropriate concentration and temperatures. Addition of amines or amine salts will greatly reduce or eliminate the attack on stainless steel surfaces and will therefore, prevent metals present in these alloys from contaminating wastewater and subsequently the environment.
This inhibitive effect of amines and amine salts can be seen in Table 1 in which sulfuric acid corrosivity is compared with modified sulfuric acid. In Table 1, the sulfuric acid is modified by the addition of ammonia to the acid. When ammonia is added to sulfuric acid, in general, ammonium hydrogen sulfate is formed. It has also been found that ammonium sulfate or ammonium hydrogen sulfate addition to sulfuric acid will have the same inhibitive effect as the addition of ammonia since in solution the same chemical composition will be formed.
Not only does the ammonia modification of sulfuric acid protect the stainless steel but as can be seen from Table 2 below, less chromium and nickel is removed when compared to unmodified sulfuric acid. This much lower corrosion of chromium and nickel from industrial equipment shows that ammonia or other amines modify in such a way that more hazardous and toxic metals are protected from dissolving into the acid and resulting in much reduced environmental pollution.
The amount of amine that is beneficial for protecting industrial equipment can vary over a wide range as outlined below in Table 3.
As outlined in Table 3, a composition of one preferred embodiment contains ammonia as the corrosion inhibiting amine due to both its effectiveness as an inhibitor, ease of use and excellent cost profile. The reasonable cost is also due to ammonia's lower molecular weight of any amine that can be used in the formulation.
As is known by those skilled in the art, the addition of ammonia or any amine to a strong acid solution will generate amine salts of that acid. Therefore the addition of an amine to sulfuric acid will result in the following:
R1R2R3N+H2SO4----->[R1R2R3N]+—H+HSO4−H2SO4
Where R1, R2, R3=H, an alkyl or aryl group or can be a cyclic compound.
Depending on the amount of acid present and the amount of amine added to the acid, some or a significant amount of the diammonium sulfate can also be present as follows: [R1R2R3N—H]2+SO4−2 where R1, R2, R3=H, an alkyl or aryl group or a cyclic compound
In another preferred embodiment, anhydrous ammonia is used as the inhibiting amine in the sulfuric acid. The ammonia can be added or bubbled into the sulfuric acid to generate the salts as outlined above. Also, amine salts can be used instead of anhydrous ammonia. Therefore, when ammonia is the preferred inhibitor, the same results will be obtained by adding either or both of the following ammonium salts to the sulfuric acid.
[NH4]2SO4 and/or [NH4]HSO4
In any case, when ammonia or ammonium salts are added to sulfuric acid, the then modified sulfuric acid has been found effective in reducing the extent to which sulfuric acid attacks metals that are undergoing treatment. At the same time, the addition of ammonia or ammonium salts do not detract from the effectiveness of the sulfuric acid in removing oxidation and other impurities from surfaces undergoing a cleaning operation.
A mixture of one or more amines can be incorporated into the modified sulfuric acid and such a formulation will also give outstanding corrosion resistance to stainless steel equipment used to contain, transfer, spray or pump the acid cleaning solution. In some applications, a mixture of amines may also be preferred since various alkyl or aryl amines and polymers containing amine functionality will give excellent corrosion resistance not only to stainless steel tanks and surfaces but will also protect base metals such as hot rolled and cold rolled steel from excessive attack from the sulfuric acid cleaning solution.
Non-limiting and illustrative examples of amines and amine salts other than ammonia that can be used as a corrosion inhibiting agent in sulfuric acid are as follows:
The amine moiety that can bond to or chelate and protect the stainless steel surface exposed to a sulfuric acid solution can also be present in a polymeric structure. The polymers containing useful amine moieties for this invention can be made by many means known to those skilled in the art. Examples of such methods are condensation polymerization, radical polymerization, grafting onto existing polymer structures, ionic polymerization, Mannich reactions and the like.
Useful polymeric structures can incorporate the amine in the monomer, oligomer and/or polymer chain:
[—R1R2C—CR3R4—NR5—]N
Where R1, R2, R3, R4, and R5=hydrogen, alkyl, aryl, or cyclic group (s). And were N=1, 2, 3 or any value as long as the polymer is soluble and of a useful viscosity for its application as a corrosion inhibitor.
As is known to those skilled in the art, many other methods exist where an amine can be incorporated into a polymer structure. In an alternate embodiment, the polymer can have pendant amine groups that contain amine moieties as in:
Where R1, R2, R3, R4, and R5=hydrogen, alkyl, aryl, or cyclic structures were possible. And where N=1, 2, 3 or any value as long as the polymer is soluble and of a useful viscosity for its application as a corrosion inhibitor.
Many other examples are known, such as polyvinyl pyridine and the like that can have pendant amine groups either aryl or alkyl or mixtures thereof and should be included in this patent as being useful for the purposes thereof.
Also, numerous commercial products are available that can be added with ammonia and other amines that will protect stainless steels as well as other metal surfaces such as cold and hot rolled steels, cast parts, chromium steels and the like. These products, although known by their commercial names, are also known to contain amine moieties useful for the purposes of this invention. Not limited to, but given by way of example, are Rodine® products for acid solutions produced by Henkel Surface Technologies of Madison Hts, Mich. and Activol® products produced by Harry Miller Corporation of Philadelphia, Pa.
Modified sulfuric acid of this invention can be used alone as the sole acid for metal cleaning solutions or can be employed in combination with other acids. Examples of acids that can be combined with modified sulfuric acid are organic acids such as acetic, citric, tartaric, maleic and the like as well as inorganic acids such as hydrofluoric, nitric, phosphoric and/or hydrochloric acid. These acids are given by way of example and the acids that can be employed for cleaning of industrial components are not limited to this brief list provided here.
One significant use of the modified sulfuric acid is for the cleaning and removal of lubricants and aluminum “fines” generated in the drawing of aluminum cans used as containers primarily for beverage and food applications. In these applications, the sulfuric acid is only one component of the acid cleaning formulation and hydrofluoric, nitric and/or phosphoric acid can be added to the sulfuric acid to aid in the cleaning application. Hydrofluoric acid is the most common acid that is added and various surface active agents or surfactants are also commonly used to aid the removal of drawing lubricants.
In both the transportation of cleaning solutions as well as in there use in industrial installations, the aggressive nature of the acid solutions requires the use of stainless steel drums for safe transport and also stainless steel tanks, piping, spray nozzles and pumping equipment. The modified sulfuric acid of this invention protects this equipment from attack and extends the life of the shipping drums and industrial equipment. In addition, since there results considerable reduction in the removal of metal from both the shipping containers and the application equipment, the possibility of hazardous and environmentally harmful metals being released into the environment is greatly reduced or may be completely eliminated.
A typical can cleaning product may contain from about 25 to about 33% sulfuric acid, from about 8 to about 13% surfactant and from about 0.5 to about 1.0% hydrofluoric acid. This “concentrate” requires shipment in stainless steel equipment and is hazardous and corrosive to humans. Through the use of the modified sulfuric acid of this invention in the “concentrate” formulation, the life of the stainless steel shipping drums is greatly extended due to less attack on the metal surface by the corrosive acid. In addition and surprisingly, the modified sulfuric acid also protects human skin and it has been shown that the corrosiveness to human tissue is reduced, making the handling and transfer of the acid cleaning solution much less hazardous.
The sulfuric acid cleaning solution is diluted when used in the industrial facilities for cleaning of aluminum containers. Aluminum cans are covered lubricants required during the metal drawing operation and aluminum “fines” or small aluminum particles are also formed during the drawing process and must be removed by acid cleaning. These contaminants are removed by dilute sulfuric acid through a two step operation:
(1) The acid cleaning solution is sprayed onto the exterior and interior of the aluminum can for 40 to 60 seconds at a temperature from 110° F. to 150° F. The composition of this diluted cleaning solution may contain from 3% to 6% sulfuric acid, 0.5% to 3% surfactant and 10 to 30 ppm of hydrofluoric acid.
(2) The excess acid cleaning solution is rinsed with cold water for from 20 to 40 seconds before being dried in an oven.
All industrial equipment that comes into contact with this acid cleaning solution must be constructed of 316 stainless steel. As discussed above, through the use of the modified sulfuric acid of this invention, this industrial equipment is protected and little or no release of hazardous metals such as chromium, nickel or manganese salts are released into the wastewater stream or subsequently into the environment. An important and added advantage of employing the modified sulfuric acid in formulating can coating acid cleaners is the greater safety in the shipment and transfer of this acid, since it has been shown to be much less aggressive to human skin when compared with sulfuric acid that does not contain ammonia or an amine corrosion inhibiting additive.
The advantages provided in protecting both equipment and personnel in handling and shipping the modified sulfuric acid of this invention can be used to advantage in the cleaning of stainless steels to prevent the attack and removal of hazardous metal from the stainless steel. Stainless steel is used in industrial parts such as automotive mufflers, “chrome” trim for both automobiles and appliances, stainless wire, stainless steel ball bearings for industrial and ball point pen tips, and for appliance fascia such as refrigerators, freezers, dish washers, stoves and wine storage cabinets. This provides a short list were stainless steel is used in large quantities. In all applications, the use of modified sulfuric acid for surface cleaning and surface preparation prepares the surface for subsequent coatings or processes. In all these applications, modified sulfuric acid is of considerable advantage both for cleaning and for protecting workers employing the product.
Multiple tests to determine the effects of the modified sulfuric acid for scale removal and metal loss characteristics have been conducted on commercially available metal parts. Commercially available metal test panels were also used to validate performance and/or eliminate/minimize observation variances that could occur because of production variables present in the tested metal parts.
The first test that will be presented by way of example, is a comparison between sulfuric acid and the modified sulfuric acid of this invention in protecting S.S. 304, S.S. 316 and hot rolled steel (HRS). The steps or procedure for acid cleaning is shown in Table 4A below. It should be noted that the modified sulfuric acid was used at about 16% while the “normal” sulfuric acid was used at about 10% concentration.
It can be clearly seen from Tables 4A and 4B that the modified sulfuric acid of this invention protects 304 SS and 316 SS to a much greater extent than normal sulfuric acid even though the modified sulfuric acid was employed at higher and more aggressive concentration. Also, HRS was attacked at a much lower rate. This is a non-limiting example which clearly demonstrates that this invention can protect various stainless steels and therefore reduce contamination of wastewater and release of toxic metals to the environment.
In addition to protecting stainless steel, the modified sulfuric acid of this invention when used in a cleaning or deoxidizing operation, must show that the acid cleaner of this invention can perform the required cleaning or removal of scale, oxide products and/or laser scale in an industrial application. As illustrated below by Table 5, excellent results were obtained using the modified sulfuric acid in scale and laser scale removal even when the modified sulfuric acid was used at one-half and one-sixth the concentration.
In a more extensive evaluation of the modified sulfuric acid of this invention against three commercially available acid deoxidizing products was conducted. In this test, the mild steel metal was first cleaned by using a highly alkaline cleaner to remove all oil and other contaminants. After a second alkaline cleaning and extensive water rinsing, the metal parts, which contained both laser scale as well as normal weld scale, were acid cleaned or deoxidized. The terms acid cleaning and deoxidizing are used interchangeably in the industry and in this patent application. The three “deoxidizers” used in the comparison were Deoxidizer#1, Deoxidizer #2 and a competitive Ammoniated Acid Deoxidizer Product (a control Deoxidizer was also employed for comparison). These results are summarized in Table 6 below.
It can be seen from these results in Table 6 that the modified sulfuric acid, in all cases, is equal to the commercial products, and in most cases, is superior in laser and weld scale removal. It must also be remembered that the modified sulfuric acid has the additional advantage of being safer to handle and much less corrosive to the stainless steel drums used for transport as well as the equipment used in the cleaning, pumping, spraying and transfer operations in the industrial plants.
Additional tests were conducted to determine the advantages of the modified sulfuric acid of the present invention in the areas of remedial metal treatment and mineral deposit removal. The tests were conducted comparing the inventive modified sulfuric acid as an oxidation and/or mineral deposit removal agent for brass, chrome, copper, and ceramic tile compared to other commercially available products. The results of these tests are approximate and are as follows in Tables 7A and 7B below and are based on visual observation.
As can be seen from Tables 6 through 8, the modified sulfuric acid of this invention not only gives outstanding results when used as a acid cleaning agent and deoxidizer but also shows equal or superior results when used as a metal polish, to remove scale from metal and piping and for the cleaning of deposits on ceramic tile and non-metal parts.
Another added advantage of preventing the removal of excess metal from the parts and the greater safety advantage of the product makes it the acid of choice for these applications.
Product safety is an important characteristic of any commercial product. This invention imparts an element of safety to users and their processing equipment in two ways. First reduction of metal loss extends the life of process equipment. Second, commercial sulfuric acid may destroy the epidermis and penetrate some distance into the skin and subcutaneous tissue and cause ulceration of the skin. On the other hand, the modified sulfuric acid of the present invention has been shown to be comparatively safe in relevant tests of dermal irritation using New Zealand white rabbits.
Primary Dermal Irritation/Corrosion Studies of the inventive product were conducted by Corning Hazelton, Inc. utilizing the Draize Technique. The concentration of the product was 10% and the pH was 0.40. The primary dermal irritation potential was evaluated on three rabbits under a four (4) hour semi occluded condition.
The rest material produced very slight to well-defined erythema and very slight to slight edema reactions. No other dermal irritation was observed. All irritation cleared by the end of a 96 hour observation period.
The average of the individual index scores (the total of erythema and edema scores at 4, 24, 48, and 72 hours divided by 4 was 1.8 (considered to be slightly irritating under the Draize Technique). All procedures used in this study were in compliance with the Animal Welfare Act Regulations. The dose, method, frequency and duration of the administration utilized in the study were chosen based on the requirements of the appropriate regulatory test guidelines.
These tests indicate that in concentrations of 10%, the inventive product demonstrates low dermal toxicity to this hypersensitive creature. While at full strength, the inventive sulfuric acid product demonstrated significant irritation to the rabbits, it can logically be assumed that because human skin is less sensitive than the skin of white rabbits, stronger concentrations than the 10% level can be safely tolerated by humans. Data in Tables 2-5 indicate that the inventive product performs well in the metal working industry for the intended purposed at a concentration of approximately 6.5%, thus demonstrating that the inventive modified sulfuric acid is reasonably safe compared to commodity sulfuric acid which causes significant dermal damage at these concentrations.
Because of this safety factor, it is possible to ship products for the uses claimed, which have been formulated using ammoniated compounds and which have inherent safety characteristics, at freight rates advantageous over other sulfuric acid formulations and other organic and inorganic products. Under OSHA (the Occupational Safety and Health Administration) and DOT (Department of Transportation) data and classifications, many acids must be shipped under “hazardous” shipping classes which carry considerably higher shipping rates than non-hazardous products. Because the inventive sulfuric acid product exhibits inherent safety at least up to certain threshold concentrations, concentrations at or below this threshold could logically qualify for shipment at non-hazardous rates.
Further, the safety imparted by the modified sulfuric acid of the present invention makes it possible to include it within consumer products intended to be used as tub and tile cleaners, toilet bowl cleaners, or metal cleaners or polishes where commercial sulfuric acid would not be safe to use.
The deposition of “scale” on the operating surfaces of boilers or cooling water systems (towers) and within the plumbing which comprises these systems greatly decreases their efficiency. Accordingly, energy costs for the users of such systems increase as system efficiency decreases. Due to the characteristics of the modified sulfuric acid of the present invention it is suitable for use both to remove deposited water scales or, with its inclusion within the water within the boiler or water cooling system, to reduce, mitigate, or control water scale deposits within these systems.
From the foregoing it will be seen that this invention is one well adapted to attain all ends and objects hereinabove set forth together with the other advantages which are obvious and which are inherent to the structure.
It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of the claims.
Since many possible embodiments may be made of the invention without departing from the scope thereof, it is to be understood that all matter herein set forth is to be interpreted as illustrative, and not in a limiting sense.
This application is a continuation-in-part of and claims priority to U.S. Application Ser. No. 12/335,072, filed Dec. 15, 2008 and entitled PROCESS FOR SURFACE TREATMENT OF METALS, which is a continuation-in-part and claims priority to U.S. application Ser. No. 11/679,997, filed Feb. 28, 2007 and is entitled PROCESS FOR SURFACE TREATMENT OF METALS, both documents are hereby incorporated by reference herein to the extent permitted by law.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12335072 | Dec 2008 | US |
| Child | 12948383 | US | |
| Parent | 11679997 | Feb 2007 | US |
| Child | 12335072 | US |
