Patent Publication Number: US-9416455-B2

Title: Protecting a metal surface from corrosion

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of U.S. Patent Application 61/857,066 filed Jul. 22, 2013 entitled PROTECTING A METAL SURFACE FROM CORROSION, the entirety of which is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     Exemplary embodiments of the present techniques relate to protecting a metal surface from corrosion through the use of a sacrificial anodic material. 
     BACKGROUND 
     This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present techniques. This description is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art. 
     Corrosion is defined as a chemical or electrochemical reaction between a material, usually a metal, and the environment that deteriorates the properties of the material. Metallic corrosion cost the industries of the United States an estimated $170 billion annually. Various industries that are affected by the detrimental effects of corrosion include electrical power plants, chemical processing plants, oil/gas production and refineries, water and wastewater management, among others. 
     In the oil and gas industry, the iron (Fe) in a steel pipe has a tendency to corrode in the presence of corrosive materials that are by-products of the hydrocarbon production, including oxygen (O 2 ), hydrogen sulfide (H 2 S), and carbon dioxide (CO 2 ). The corrosion process releases Fe 2+  ions and electrons which reduce the corrosive materials. The released Fe 2+  ions react with the products of the reduction to form corrosion by-products, such as iron(II) hydroxide (FeOH 2 ), iron sulfide (Fe 2 S 3 ), or iron carbonate (Fe 2 CO 3 ), among others, within the flow stream of the oil and gas. 
     Corrosion can be enhanced by the aqueous fluid that is inevitably produced alongside hydrocarbons during the production of crude oil and natural gas. Within the aqueous fluid, the natural occurrence of corroding agents alone, such as carbon dioxide (CO 2 ) and hydrogen sulfide (H 2 S), can lead to significant corrosion problems. Additionally, the CO 2  and H 2 S can combine with water to form carbonic acid (H 2 CO 3 ) and dissolved hydrogen sulfide (H 2 S), respectively. The formation of such acids further increases the rate of corrosion. For example, the formation of H 2 CO 3  can significantly lower the pH of water and increase corrosion formation resulting in pitting corrosion and possibly the formation of hairline cracks throughout the production system. 
     There are numerous types of corrosion which are usually classified by the cause of the material deterioration. Galvanic corrosion is a type of corrosion that can occur when metals or semi-metals having varying electrode potentials come into contact with each other through the use of an electrolytic material such as water. The electrolytic material provides a means for ion migration whereby ions of a less noble metal gravitates to a more noble metal. This movement causes the less noble or less stable metal to corrode more rapidly.  FIG. 1  is a galvanic corrosion chart  100 . The chart  100  contains the galvanic series ranks of metals and semi-metals according to their potential and determines the nobility of such materials. Metals that are less noble, or anodic, and that will corrode more easily are contained at the negative end  102  of the chart  100 . Conversely, metals that are more noble, or cathodic, and that are more resistant to corrosion are contained at the positive end  104  of the chart  100 . During galvanic corrosion, the anode metal will sacrifice its electrons resulting in decomposition. The cathode metal accepts the released electrons and is protected from corrosion. It should be noted that chart  100  is drawn up for metals and semi-metals in seawater. Therefore, while the relative position of the metals on chart  100  may change in other environments, it is the distance between the metals on chart  100  that indicates the risk for galvanic corrosion. Although galvanic corrosion may occur more quickly when metals of different type are in electrical contact, it will still occur in neat metals, due to the presence of more electropositive and electronegative sites. 
     An example of this is pitting corrosion. Pitting corrosion, or pitting, is a form of localized galvanic corrosion that leads to the creation of small holes in the metal. The driving power for pitting is the depassivation of a small area, which becomes anodic while an unknown, but possibly large area, becomes cathodic, which can lead to very localized galvanic corrosion. Pitting can be initiated by localized chemical or mechanical damage to a protective oxide film or to the metal, low dissolved oxygen concentrations, or high concentrations of contaminants in source water. Additionally, crevice corrosion is a form of localized pitting which takes place in narrow clearances or cervices on a surface of a metal where fluid has become stagnant. 
     Since preventing corrosion may be difficult in certain environments, one of the most economical solutions is to control the corrosion rate. There are various methods used to slow corrosion including chemical inhibition, coatings, or corrosion resistant alloys. Each of these methods has its own advantages and disadvantages with the cost to implement the method usually dictating which particular method to use. 
     Chemical inhibitors, such as neutralizers, film forming reagents, and non-nitrogen-based corrosion inhibitors, may be utilized to provide protection to a surface in contact with a flowing stream. The chemical inhibitor may be added to the flow stream and thereby deposits a thin film upon a surface of the system. The thin film facilitates the prevention of various reactions between corrosive compounds in the flow stream and that particular surface. Likewise, coating inhibitors may be painted or sprayed onto a surface to act as a barrier to inhibit contact between corrosive materials and the surface. Corrosion resistant alloys may also be used, including mixtures of various metals such as chrome, nickel, iron, copper, and cobalt, among others. Such metals in combination provide corrosion resistance more effectively than a surface composed of only one type of metal. 
     SUMMARY 
     An exemplary embodiment provides a method for protecting a metal surface from corrosion. The method includes injecting particles comprising a sacrificial anodic material into a fluid proximate to the metal surface. 
     Another exemplary embodiment provides a method for protecting a metal surface within a flow system from corrosion. The method includes providing sacrificial anodic particles and injecting the sacrificial anodic particles into a fluid stream within an injection manifold. The method also includes separating the sacrificial anodic particles from the fluid stream, recycling reusable sacrificial anodic particles, and re-injecting the reusable sacrificial anodic particles into the fluid stream. 
     Another exemplary embodiment provides a system for protecting a metal surface from corrosion. The system includes a sacrificial anodic material and an injection pump configured to inject the sacrificial anodic material into a fluid. The system also includes a separation system configured to remove the sacrificial anodic material from the fluid. The system also includes a recycling system configured to re-inject the sacrificial anodic material into the fluid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages of the present techniques are better understood by referring to the following detailed description and the attached drawings, in which: 
         FIG. 1  is a schematic of a galvanic chart used to determine the electrochemical potential of various metals and semi-metals; 
         FIG. 2(A)  is an illustration of a subsea natural gas and crude oil field where sacrificial anodes can be injected from corrosion; 
         FIG. 2(B)  is a block diagram of a system for injecting sacrificial anodes particles into an oil and gas production system; 
         FIG. 3  is a detailed illustration of sacrificial anodes particles in a suspension; 
         FIG. 4  is an illustration of sacrificial anode particles in a pipeline of an oil and gas production system; 
         FIG. 5  is a detailed illustration of sacrificial anode particles used as a sacrificial anode and as a passivation agent; and 
         FIG. 6  is a process flow diagram of a method for injecting a sacrificial anode material into a fluid. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description section, specific embodiments of the present techniques are described. However, to the extent that the following description is specific to a particular embodiment or a particular use of the present techniques, this is intended to be for exemplary purposes only and simply provides a description of the exemplary embodiments. Accordingly, the techniques are not limited to the specific embodiments described below, but rather, include all alternatives, modifications, and equivalents falling within the true spirit and scope of the appended claims. 
       FIG. 2(A)  is an illustration of a subsea hydrocarbon field  200  that may consist of various types of production equipment that is susceptible to corrosion. It should be noted that the present techniques are not limited to subsea fields, but may be used for the mitigation of plugging in the production or transportation of oil, natural gas, or any number of liquid or gaseous hydrocarbons from any number of sources. 
     As shown in  FIG. 2(A) , the hydrocarbon field  200  can have a number of wellheads  202  coupled to wells  204  that produce hydrocarbons from a formation (not shown). The wellheads  202  of  FIG. 2(A)  may be located on the ocean floor  206 . Each of the wells  204  may include single or multiple wellbores or branch wellbores. Each of the wellheads  202  can be coupled to a central pipeline  208  by gathering lines  210 . The central pipeline  208  may continue through the field  200 , coupling to further wellheads  202 , as indicated by reference number  212 . A flexible line  214  may couple the central pipeline  208  to a collection vessel  216  at the ocean surface  218 . The collection vessel  216  may, for example, be a floating processing station, such as a floating storage and offloading unit, that is anchored to the sea floor  206  by a number of tethers  220 . In land based embodiments, the collection vessel  216  may include a central collection and processing facility in an oil or gas field. The collection vessel  216  may have equipment for separation, water treatment, chemical treatment, and other processing techniques. 
     Throughout the hydrocarbon field  200 , corrosion can attack critical equipment including the wellheads, the wells, and pipelines, among other equipment composed of metal or alloys. The electrochemical reaction that defines corrosion begins with a chemical reaction involving the transfer of electrons. General equations, using the metal of iron (Fe) as an example, that detail an electrochemical reaction occurring during the formation of corrosion are shown below.
 
Fe→Fe 2+ +2 e   −   (1)
 
2 e   − +H 2 S→S 2− +H 2   (2)
 
2 e   − +2H 2 O→2(OH − )+H 2   (3)
 
Equation (1), which takes place at an anodic site, results in the oxidation of Fe to an ion, Fe 2+ , which has a valence charge of 2+, and in the release of electrons, 2 electrons. The 2 electrons of Equation (1) flow through the metal to a cathodic site. This type of electrochemical reaction is considered an anodic reaction since the Fe oxides. In Equations (2) and (3), the electrons react with a corrosive material, such as the H +  ion in H 2 S, H 2 O, or an acid. In either Equation (2) or (3), the electron reduces the H +  ion to hydrogen gas, H 2 . It should be noted that the value of the number of electrons, n, depends primarily on the nature of the metal.
 
     One method of reducing and preventing corrosion includes the use of a sacrificial anode inhibitor. The sacrificial anode can be comprised of a metal that is at a more negative position on the galvanic chart. Further, in some embodiments described herein, the sacrificial anode may be comprised of two or more metals, where one of the metals is less noble or corrodes more readily than the other metal and may be considered as the anode metal portion. A less noble metal is located on the negative end of the galvanic chart and releases its electrons. The other metal may be considered as the cathode metal portion. The cathode metal is less chemically active and corrodes at a slower rate than the anode metal. In these embodiments, corrosion of the less noble metal may help prevent corrosion by removing corrosive materials from the system before they can attack the surface being protected. Additionally, the sacrificial anode may act as a passivation agent by combining with a naturally-occurring corrosive agent, such as H 2 S, within a flow stream of a tubular construct. This results in the degradation of both the sacrificial anode and the generated H 2 S within the flow stream. Therefore, corrosive agent H 2 S may possibly be reduced or eliminated. 
     In some embodiments, the particles may settle on the surface, establishing an electrical contact with the metal being protected. The sacrificial anode may then oxidize, providing a source of electrons as the particles corrode. The electrons that are released from the sacrificial anode can flow through the metal, reducing corrosive agents and preventing corrosion in the local area or the entire surface of the metal. 
     In an embodiment, sacrificial anodes may be added as particles to mitigate the formation of corrosion. As shown in  FIG. 2(A) , the sacrificial anode particles can be transported via an injection line  224  to one or more injection points, such as at injection manifold  226 . Although the injection line  224  is shown as being independent of the flexible line  214 , the injection line  224  may be incorporated along with the flexible line  214  and other production, utility, and sensor lines into a single piping bundle. In various embodiments, the injection manifold  226  may be located on the flexible line  214 , the central pipeline  208 , the gathering lines  210 , or on any combinations thereof. 
     One or more static mixers  228  can be placed in the lines to assist in suspending and distributing the sacrificial anodes  236 , for example, in the central line  208  downstream of entry points  230  for each of the gathering lines  210 . The placement of the static mixers  228  is not limited to the central line  208 , as static mixers  228  may be placed in the flexible line  214 , the gathering lines  210 , the wellheads  202 , or even down the wells  204 . 
     In some embodiments, the amount of sacrificial anode particles used may be determined by analyzing or monitoring the reduction/oxidation (redox) potential of the produced fluids. The redox potential of the produced fluids brought up by the flexible line  214  may be monitored, for example, by an oxidation/reduction potential (ORP) analyzer  232  located at the collection vessel  216  or at any number of other points in the natural gas field  200 . The ORP analyzer  232  may determine the concentration of the sacrificial anode particles, the redox potential of the aqueous phase in the production fluid, and the like. The output from the ORP analyzer  232  may be used to control an addition system  234 , which may be used to adjust the amount of sacrificial anode particles  236 , sent to the injection manifold  226 . The facilities and arrangement of the equipment in the hydrocarbon field is not limited to that shown in  FIG. 2(A) , as any number of configurations may be used in embodiments. Further, the use of the sacrificial anode particles is not limited to offshore fields, but may be used in onshore fields, pipelines, or any other system needing convenient protection from degradation. 
       FIG. 2(B)  is a block diagram of a system  238  for injecting sacrificial anode particles  236  into a hydrocarbon production system  200 , such as discussed with respect to  FIG. 2(A) . In  FIG. 2(B) , sacrificial anodes  236  may be mixed to form an aqueous suspension in a holding tank  240  before injection into the injection manifold  226 . The aqueous suspension can be maintained by mixing, by the addition of thickening agents, by the addition of thixotropic agents, or any combinations thereof. From the holding tank  240 , an injection pump  242  may be used to pump the sacrificial anode particles  236  into the injection manifold  226  through the injection line  224 . A flexible line  214  can transport production fluids, including hydrocarbons, water, and the sacrificial anodes  236  to a separator  244 . The separator  244  may be included in the system  238  to separate the sacrificial anodes  236  from production fluids  246 . The separator  244  may include any number of technologies, such as magnetic or electromagnetic separation, filtration, flocculation, or other methods for separating solids from liquids. In some embodiments, the sacrificial anode particles  236  may be modified to facilitate their separation from the hydrocarbon. For example, the sacrificial anode particles  236  may include a ferromagnetic core or shell to allow magnetic separation to be used. Materials to facilitate such magnetic attraction may include iron, nickel, cobalt, gadolinium, various alloys, or any combinations thereof. 
     Any unspent sacrificial anode particles  248  may then be passed to a recycling system  250  to reclaim any reusable sacrificial anodes  252 . The reclaimed reusable sacrificial anode particles  252  may then be mixed into the suspension with a portion of fresh sacrificial anode particles  236  and reinjected into the injection line  224 . Any spent sacrificial anode particles  254 , along with precipitants formed from the degradation of the sacrificial anode particles  236 , may be sent to waste  256 . The facilities and arrangement of the equipment in the oil and gas production system is not limited to that shown in  FIG. 2(B) , as any number of configurations may be used in embodiments. 
       FIG. 3  is an illustration  300  depicting a suspension  302  of sacrificial anode particles consisting of fine separate particles  304 . For ease of injection into a flow line, the sacrificial anode particles  304  can be suspended in a carrier fluid  306 , such a gel or fluid, as shown in  FIG. 3 . The carrier fluid  306  can be aqueous based or water-soluble and can have a sufficient viscosity in order to suspend the sacrificial anode particles  304  within the carrier fluid  306  with little to no agitation. In some embodiments, the carrier fluid  306  can be a thickening agent such as polyethylene oxide, polyethylene glycol, ethylene glycol, among others. 
     In some embodiments, the sacrificial anode particles  304  may be composed of magnesium (Mg), zinc (Zn), aluminum (Al), or any combinations thereof. Each metal has its advantages and disadvantages. For instance, Mg has the most negative electropotential of the three metals and is more suitable for areas where the electrolyte resistivity is higher. This application is usually suited for on-shore pipelines and other buried structures. In some cases, the negative electrochemical potential of Mg may prove to be a disadvantage. For example, if the potential of the protected metal becomes too negative, hydrogen ions may evolve on the cathode surface leading to hydrogen embrittlement or to disbonding of a coating layer. In such situations, Zn sacrificial anode particles may be used. 
     Zn is generally used in salt water, where the resistivity is generally lower. Typical applications that may use Zn as an anode include off-shore pipelines, internal surfaces of storage tanks, and production platforms. Zn is considered a more reliable sacrificial anode than magnesium or aluminum due to its well-known corrosive resistance and its lower driving voltage is considered advantageous where there is a risk of hydrogen embrittlement. However, Zn may not be suitable for use at higher temperatures, as it tends to passivate. Al is lighter in weight and has a higher capacity than Mg or Zn, since it releases three electrons for each Al 3+  ion formed. However, due to, such properties as electrochemical capacity and consumption rate, Al may not be considered as reliable as Zn. Regardless, any one of the metals may be used, providing there is a difference in electrochemical potential between the metals. 
     As shown in  FIG. 3 , the sacrificial anode particles  304  may be composed essentially of Mg  308 , essentially of Zn  310 , or from particles of Mg  308  and Zn  310  in combination. In combination particles, Mg  308  is considered as the anode metal since it is more electropositive and undergoes oxidation more readily than Zn  310 . Therefore, in a combination sacrificial anode particle  304 , regions of Mg  308  can be formed on the surface of the sacrificial anode particle  302  while the core of the particle consists of Zn  310 . In this example, Mg  308  will be the first metal sacrificed since it is consumed or corrodes at a faster rate than Zn  310  in the presence of a corrosive agent or an electrolyte. 
     In order to facilitate formation of the suspension  302 , it is important that the sacrificial anode particles  304  should have a relatively small diameter. In some embodiments, the particles of the sacrificial anode  304  have a diameter preferably in the range of about 1 micrometer (μm) to about 100 μm. A smaller particle diameter supports better anti-corrosive protection due to an increase in the reaction surface area. Additionally, a smaller particle diameter minimizes damage resulting from the normal use or aging on the process equipment including erosion of metals. The details presented concerning the sacrificial anode particles is not limited to that shown in  FIG. 3 , as any number of configurations and properties may be used in embodiments. 
       FIG. 4  is a general illustration depicting sacrificial anode particles  402  that are injected into an injection manifold  404  and enter into a pipeline  406  of an oil and gas production system. Although the sacrificial anode particles  402  are suspended within the flow stream  408 , some of the particles  402  can settle out of suspension onto a surface of the pipeline  406 . In  FIG. 4 , the particles  402  that settle upon the pipeline  406  can form an electrical contact with surface and release electrons (e − ) into the metal of the pipeline  406 . As a result, the particles  402  are sacrificed instead of the metal of the pipeline  406 . This can effectively inhibit or eliminate a corrosive reaction from taking place on the pipeline  406  by transferring the reaction to the metallic surface of the particles  402 . The facilities and arrangement of the pipeline system is not limited to that shown in  FIG. 4  as any number of configurations, materials, and properties may be used in embodiments. 
       FIG. 5  is an enlarged illustration  500  of sacrificial anode particles  502 , consisting of Mg metal, Zn metal, or a combination of both Mg and Zn metals in the flow stream  504  of an oil and gas production system. Since it is a naturally occurring component of crude oil and natural gas, H 2 S  506  can often exist within the flow stream  504 . The H 2 S  506  is considered a corrosive agent and attacks material surfaces leading to material corrosion, degradation, cracking, or embrittlement, among others. The H 2 S  506  prefers to react with the metal of the pipeline  508  in the oil and gas system. By using sacrificial anode particles  502 , the H 2 S  506  may react with the metal of the sacrificial anode particles  502  instead of the metal of the pipeline  508  since the sacrificial anode particles  502  will be composed of a more electropositive metal than the pipeline  508 . In solution, this protects the metal pipeline  508  from corrosion by degrading the corrosive materials, e.g., H 2 S  506 . 
     When a sacrificial anode particles  502  is in contact with the surface, the particles  502  releases electrons (e − )  512  which pass into the pipeline  508  through a contact point with the pipeline  508 . The corrosive H 2 S  506  accepts the electrons  512 , forming hydrogen. Therefore, the sacrificial particles  502  corrode in the place of the pipeline  508 . The reaction between the H 2 S  506  and the metal of the particles  502  releases sulfur (S 2− ) ions, hydrogen gas (H 2 ), and metal ions. The S 2−  ions and the metal ions may form a metal sulfide compound which can precipitate and fall out of the flow stream  504 .  FIG. 5  also depicts a sacrificial anode particle  502  consisting of both Mg  510  and Zn  514  that may settle upon the pipeline  508 . As previously discussed, Mg  510  corrodes at a faster rate than Zn  514 . Therefore, the Mg  510  will degrade first and the Zn  514  will degrade thereafter. After the Mg  510  has released all of its electrons  512 , any H 2 S  506  remaining may then corrode the Zn  514 . Likewise, a particle  502  consisting entirely of Zn  514  may also settle upon the pipeline  508 . Since Zn  514  can be more electropositive than the metal of the pipeline  508 , the Zn  514  would release its electrons into the metal and become susceptible to corrosion by H 2 S  506 . In some embodiments, any combination of particle composition  502  may be utilized to inhibit corrosion. 
     Also shown in  FIG. 5 , the sacrificial anode particles  502  may as also act as a passivation agent while suspended within the flow stream  504 . Without the use of sacrificial anode particles  502 , the presence of H 2 S  506 , the corrosive and toxic by-product of hydrocarbon production, may result in sour gas. In  FIG. 5 , the electrons from a Zn portion  514  of a sacrificial anode particle  502  may sacrifice its electrons  516  to H 2 S  506  while suspended in the flow stream  504 . The reaction between the H 2 S  506  and the metal of the particle  502  releases S 2−  ions, H 2 , and Zn 2+  ions. The S 2−  ions and the Zn 2+  ions may form a zinc sulfide compound which may precipitate and fall out of the flow stream  504 . Therefore, the reaction of Zn electrons  516  released to H 2 S  506  protects the oil and gas system from toxic formation since H 2 S  506 , along with sacrificial anode particles  502 , may be degraded within the flow stream  504 . The facilities and arrangement of the pipeline system is not limited to that shown in  FIG. 5  as any number of configurations, materials, and properties may be used in embodiments. 
       FIG. 6  is a process flow diagram of a method for protecting a metal surface from corrosion in a flow system. The method  600  begins as at block  602  where sacrificial anodic material is provided as described with respect to  FIGS. 2(A) and 2(B) . At block  604 , the sacrificial anodic material is injected into a fluid stream as described with respect to  FIGS. 2(A) and 2(B) . At block  606 , the sacrificial anodic material is separated from the fluid stream as described with respect to  FIG. 2(B) . At block  608 , any unspent sacrificial anodic material is recycled as described with respect to  FIG. 2(B) . It should be noted that not all of the blocks of  FIG. 6  may be used or needed in every embodiment as any number of injection, separation and recycling techniques may be added or removed. 
     While the present techniques may be susceptible to various modifications and alternative forms, the embodiments discussed above have been shown only by way of example. However, it should again be understood that the techniques is not intended to be limited to the particular embodiments disclosed herein. Indeed, the present techniques include all alternatives, modifications, and equivalents falling within the true spirit and scope of the appended claims.