Abstract:
A method is taught for protecting steel parts from corrosion due to exposure to a corrosive aqueous environment. The method comprises adding a first quantity of organic sulphur salts to the corrosive aqueous environment, wherein the first quantity of organic sulphur salts react with iron, formed from corrosion of the steel parts, to form a passive iron-sulphur barrier. A second quantity of organic sulphur salts is added to the aqueous medium over the life of the steel parts to maintain a predetermined sulphur concentration in the corrosive aqueous environment to maintain the passive barrier. Also taught is the use of organic sulphur salts in protecting steel parts from corrosion due to exposure to a corrosive aqueous environment.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to methods of preventing steel corrosion in aqueous environments using passive iron-sulphur layers formed on the steel surface. 
       BACKGROUND OF THE INVENTION 
       [0002]    Water can cause severe corrosion to steel or iron parts that are located in aqueous environments containing chlorides, acids or other corrosive components at high levels. Underground gas wells are particularly susceptible to such corrosion, due to the potentially harmful environment in which natural gas can be found. 
         [0003]    It is important to be able to protect to such steel parts in situ in both a simple and inexpensive manner. As well, due to the highly corrosive environment surrounding the steel parts, it is crucial that the protective means take effect quickly, preferably immediately, to avoid damage to the steel parts that will then require costly repair. 
         [0004]    Currently corrosion inhibiting chemicals are applied to such steel parts to reduce the affects of corrosion. Such inhibiting chemicals are typically surface active liquids, such as quaternary amines, that are often water insoluble. Due to their insolubility, these inhibitors must be applied with large quantities of interfacial chemicals such as alcohols, glycols, and other water soluble solvents, in order to introduce the inhibiting chemicals into the aqueous phase surrounding the steel parts. 
         [0005]    It is therefore desirable to find means for protecting submersed steel parts against corrosion that are effective, simply to implement and quick acting. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention thus provides a method of protecting steel parts from corrosion due to exposure to a corrosive aqueous environment. The method comprises adding a first quantity of organic sulphur salts to the corrosive aqueous environment, wherein the first quantity of organic sulphur salts react with iron, formed from corrosion of the steel parts, to form a passive iron-sulphur barrier. A second quantity of organic sulphur salts is added to the aqueous medium over the life of the steel parts to maintain a predetermined sulphur concentration in the corrosive aqueous environment to maintain the passive barrier. 
         [0007]    The present invention also relates to the use of organic sulphur salts in protecting steel parts from corrosion due to exposure to a corrosive aqueous environment. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The present invention is described in further detail herein, with reference to the following drawings, wherein: 
           [0009]      FIG. 1  is a pictorial diagram illustrating the mechanism behind one embodiment of the present invention; 
           [0010]      FIG. 2  is a schematic diagram illustrating one embodiment of the method of the present invention; and 
           [0011]      FIG. 3  is a graph showing the relationship between temperature of the aqueous environment (° C.) and time required for solution of various sizes of pellets of sodium isopropyl xanthate (seconds). 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0012]    The present invention provides a novel approach to corrosion protection by creating a passive iron-sulphur barrier on the steel to protect it. The barrier is created by adding sulphur to the corrosive aqueous environment of the steel parts, which then reacts with exposed iron resulting from any initial corrosion to form a passive iron-sulphur barrier. 
         [0013]    Solubility of the sulphur in the corrosive aqueous environment is important to allow for rapid forming of the passive iron-sulphur barrier. It is therefore important to find forms of sulphur that are safe, stable and quickly dissolvable in water. The present inventor has found that organic sulphur salts derived from carbon disulphide and thiophosphate salts are surprisingly effective forms of sulphur for the present application. For the purposes of the present invention, the phrase “organic sulphur salts” is used to describe and include all of the organic sulphur salts derived from carbon disulphide and thiophosphate salts of the present invention, which are described in further detail herein. Such salts are easily dissolvable in the corrosive aqueous environment and react quickly with the already-corroded iron to form the iron-sulphur layer. These salts are also advantageous in that they are safe to use and do not generate H 2 S as some other sulphur compounds are prone to do in acidic conditions. 
         [0014]    Examples of carbon disulphide derived salts are xanthates, thiocarbamates and thiocarbonates, all of which are solid organic salts that are water soluble and disperse as they move through the aqueous environment. These salts become evenly distributed to react with corroded iron from the steel to produce the passive iron-sulphur barrier. For static water applications, solid salts are preferred for optimal dispersion. For dynamic water applications, solutions of the salts are preferably used. 
         [0015]    Thiophosphate salts can be, for example, monothiophosphate or dithiophosphate salts with or without organic chains. Thiophosphate salts may be trisodium salts or disodium or monosodium salts with organic chains. They can be added as solids or as solutions. 
         [0016]    The organic sulphur salts derived from carbon disulphide used in the present invention are water soluble and have organic chains of preferably one to seven carbon groups (C1-C7). The inventor has found that these are most effective in delivering sulphur to the iron in the steel to produce the passive barrier. 
         [0017]    A preferred example of a xanthate for use with the present invention would be sodium isopropyl xanthate, as represented by formula (I) below: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    An example of a suitable thiocarbamate is sodium isopropyl thiocarbamate, shown in formula (II) below: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    Finally, an example of a suitable thiocarbonate is sodium thiocarbonate, as shown in formula (III) below: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0018]    Organic sulphur salts in the form of thiophosphate salts are also water soluble and include organic chains preferably having one to seven carbon groups (C1 to C7). These are also effective in delivering sulphur to the iron in the steel to produce the passive barrier. 
         [0019]    Examples of monothiophosphate salts are shown in formulas (IV) and (V) below: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    Examples of dithiophosphates are shown in formulas (VI) and (VII) below 
         [0000]    
       
                 
         
             
             
         
       
     
         [0020]    The mechanism of the reaction is that as iron corrodes from the steel, it reacts with the sulphur in the organic sulphur salt to form an iron-sulphur complex, which deposits on the steel. A pictorial illustration of the mechanism of the present invention is illustrated in  FIG. 1 , using sodium isopropyl xanthate as an example. In  FIG. 1 , iron corrodes on the surface (Fe 2+ , Fe 3+ ) and reacts with the xanthate, creating a barrier between the metal surface and the corroding species, illustrate, for example, by are acid (H + ) and chloride (Cl − ). 
         [0021]    The complex builds up to between 4 and 10 micrometers in thickness to form a passive barrier against corrosive agents in the aqueous environment. 
         [0022]    Organic sulphur salts derived from carbon disulphide and thiophosphate are highly advantageous in that they do not corrode steel or decompose to produce H 2 S, giving them a distinct advantage over other organic sulphur compounds that can form H 2 S when the pH of the environment is lowered. As is well known in the art, H 2 S is a highly undesirable and dangerous gas in working environments. 
         [0023]    A preferred method for carrying out the present invention is illustrated in  FIG. 2 . An initial treatment is undertaken where the organic sulphur salt is added to achieve a concentration of between 0.1% and 5% by weight of organic sulphur salt in the aqueous environment. The sulphur salts react with existing iron on the corroded steel surface and form the passive iron-sulphur barrier. 
         [0024]    Over time, the barrier is exposed to oxidation, causing a slight shrinkage of the barrier, leaving some of the steel surface unprotected. The rate of oxidation has been estimated to be about one third of the rate of corrosion of the steel parts in the corrosive aqueous environment. To overcome this shrinkage and to maintain sulphur content in the corrosive aqueous environment, a dosage of the organic sulphur salt is further added to the aqueous environment. The reaction kinetics of the sulphur in the organic sulphur salt and the iron ions as it corrodes from the steel is significantly faster than the formation of iron oxide (rust), hence, concentration of the sulphur salt in the aqueous environment medium can be maintained as low as 50 to 100 ppm and still effectively rejuvenate and maintain the barrier. 
         [0025]    The present inventor has found that integrity of the iron-sulphur barrier are not inhibited by water and gas flow rates commonly found in underground applications such as natural gas wells. 
         [0026]    The organic sulphur salts can be in the form of solid briquettes or pellets, preferably used for slow flowing or static corroding medium situations. Alternately the solid salts can optionally be dissolved into solution for injection into fast flowing streams. Organic or aqueous solvents can be utilized for preparing such solutions, dependent on what the fast flowing solution is. The salts themselves are soluble in both organic and aqueous solvents, as they have both organic and inorganic properties. Such solvents are well known in the art and would be clearly understood by a skilled artisan. 
         [0027]    Applying solid organic sulphur salts as water soluble briquettes or pellets is particularly preferred as this allows for the sulphur to be evenly distributed through the aqueous environment as the briquettes or pellets dissolve and sink. Solid briquettes and pellets have been found to be particularly effective in natural gas wells and for static aqueous solutions in tanks. As indicated above, a primary dose produces the passive iron-sulphur barrier while low doses over the life of the steel parts helps to maintain the passive barrier. 
         [0028]    Solutions of salts can be injected into a moving liquid stream via a pump or with a spray nozzle, or other well known means, for coating at several locations along the moving liquid stream. Solutions are particularly effective for protecting seawater pipelines, natural gas lines and oil pipelines. Solutions of salts can also be used on metal parts, using a process in which metal parts dipped into the salt solution to produce a rust resistant undercoating. This is particularly effective for protecting, for example, auto parts. 
         [0029]    It has been observed that the temperature of the corrosive aqueous environment increases dissolving rates of the organic sulphur salts logarithmically, up to approximately 60° C. This, in turn leads to an increase in the rate of formation the passive iron-sulphur barrier. For example, the relationship between temperature of the aqueous environment (° C.) and time required for dissolution of a various sizes of pellets of sodium isopropyl xanthate (seconds) is shown in  FIG. 3 . 
         [0030]    Through extensive experimentation, it has been noted that optimizing the size of the briquettes or pellets is dependant on temperature and depth of the aqueous environment. It is most desirable to have the briquettes or pellets reach near the bottom of the well before completely dissolving, thereby producing an even distribution of salts in the corrosive environment. For example, in the case of gas wells, briquettes or pellets having a diameter of between 15 mm and 25 mm have been found to be preferably for aqueous environments with a temperature of approximately 40° C. to 60° C. and a depth of approximately 1000 to 4000 meters. 
         [0031]    In additions to those application methods listed above, any number of methods can be envisioned for applying the organic sulphur salts into the aqueous environment and it would be obvious to a skilled person in the art that known application methods are encompassed and included in the present invention. In one embodiment, the briquettes, pellets or dissolved salt solution can be added directly from a drum through a funnel into the aqueous environment. 
         [0032]    Once the passive barrier is formed, water is pumped out of the aqueous environment and a low dosage of the sulphur salt is added to maintain a minimum sulphur content in the aqueous environment and rejuvenate the passive barrier from any shrinkage due to oxidation. Addition rates vary according to the removal rate of water from the aqueous environment, in order to maintain a 50 to 100 ppm concentration of organic sulphur salts in the corrosive aqueous environment. 
         [0033]    In a preferred embodiment of the invention, addition of other well treatment chemicals can also be carried out during the present process, by mixing these additional chemicals with the organic sulphur salt pellets, briquettes or solution. For example, anti-scaling agents such as sodium tripolyphosphate can be added. 
         [0034]    This detailed description is used to illustrate the prime embodiments of the present invention. It will be apparent to those skilled in the art that various modifications can be made in the present methods and use and that various alternative embodiments can be utilized. Therefore, it will be recognized that modifications can be made in the present invention without departing from the scope of the invention, which is limited only by the appended claims.