Method of Inhibiting Corrosion in Brine Solutions

Corrosion occurs when metals are oxidized to their respective ions or insoluble salts. For example, corrosion of metallic iron involves conversion to soluble iron An a +2 or +3 oxidation state or to formation of insoluble iron oxides and hydroxides. Metal loss from the solubilization of the iron can cause the structural integrity of the system to deteriorate over time. This can cause leakage between the water system and process streams. Also, the formation of insoluble salts in the corrosion process can produce a build-up of deposits which impede heat transfer and fluid flow.

Aqueous brine solutions are commonly used as refrigeration media. However, brine solutions have a significant corrosive effect on metals. Therefore, in order to reduce the extent of corrosion when brine solutions are used as refrigeration media, corrosion inhibitors typically are added to the brine solutions.

Chromate has traditionally been an extremely widely used and effective corrosion inhibitor. During recent years, however, the use of chromate has come under increasing scrutiny due to environmental concerns. Unfortunately, phosphates, which are generally excellent corrosion inhibitors, are undesirable in this application as well since they generally lead to unacceptable levels of scale formation. Similarly, nitrites may also not be used since they change the nature of corrosion from general to localized. The localized corrosion formed in presence of nitrite can cause perforation in a relatively short time.

Two corrosion inhibitors which have been used extensively in fresh water systems are gluconate and sorbitol. They have been used as corrosion inhibitors in fresh-water systems at levels up to 100 parts per million (ppm). It has never been suggested use gluconate or sorbitol An brine systems, since brines may produce a different type of corrosion (pitting or localized corrosion) than encountered in fresh-water systems.

Therefore, it would be highly desirable to develop a new method of inhibiting corrosion in brine solution systems which is environmentally acceptable, economical, and will inhibit localized corrosion or scale formation.

This invention relates generally to corrosion inhibitors, and more particularly, to a method of inhibiting corrosion in brine solution systems.

The method of the invention comprises treating an aqueous brine solution with a level of gluconate or sorbitol far above any heretofore used in any corrosion control application. The use of gluconate or sorbitol is environmentally acceptable, economically appealing, and does not lead to localized corrosion or scale formation.

In a preferred embodiment of the present invention, gluconate is added to an aqueous brine solution to inhibit localized corrosion and scale formation. The gluconate should be present in the range of about 200 to 15,000 ppm by weight of the overall solution, and preferably at a level of at least about 2000 ppm. In a particularly preferred embodiment, the gluconate is present at a level of about 4130 ppm by weight of the overall solution.

In an alternative embodiment of the present invention, an alkane polyol corrosion inhibitor, such as mannitol or sorbitol, is added to the aqueous brine solution. Sorbitol is the preferred alkane polyol corrosion inhibitor. The alkane polyol should be present in the range of about 200 to 15,000 ppm by weight of the overall solution, and preferably at a level of at least about 2000 ppm.

In a further alternative embodiment of the present invention, a combination of gluconate and an alkane polyol corrosion inhibitor is added to the aqueous brine solution. The gluconate and alkane polyol should each be present in the range of about 900 to 2500 ppm by weight of the overall solution.

Borax, a buffering agent, may be added to the aqueous brine solution as a corrosion inhibitor for mild or carbon steel. If borax is used, it should be present in the range of about 100 to 3,500 ppm by weight of the overall solution, preferably in the range of about 300 to 800 ppm and most preferably at a level of about 400 ppm.

Tolyltriazole, mercaptobenzothiozole, or benzotriazole may be added to the aqueous brine solution as a corrosion inhibitor for yellow metals. Tolyltriazole is preferred. If tolyltriazole is used, it should be present in the range of about 5 to 400 ppm by weight of the overall solution, preferably in the range of about 30 to 100 ppm and most preferably at a level of 40 ppm.

Brines from the group comprising aqueous solutions of CaCl.sub.2, CaBr.sub.2, KCl, KBr, LiCl, LiBr, MgCl.sub.2, MgBr.sub.2, NaCl and NaBr may be treated in accordance with the invention. In a preferred embodiment, CaCl.sub.2 brine is treated. CaCl.sub.2 should be present in the brine in the range of about 10 to 35% by weight of the overall solution, preferably, in the range of about 20 to 30% and most preferably at a level of about 25%.

In an alternative embodiment, LiCl brine is used. The LiCl should be present in the range of about 10 to 45% by weight of the overall solution, preferably, in the range of about 20 to 35% and most preferably at a level of about 25%.

The present invention performs most effectively when the aqueous brine solution is in the temperature range of about -20.degree. to 300.degree. F., preferably in the range of about -20.degree. F. to 140.degree. F. and most preferably at about 90.degree. F.

The pH for treating the aqueous brine solution in accordance with the present invention should be in the range of about 4 to 11, preferably in the range of about 5.5 to 9.5 and most preferably in the range of about 7 to 9.

The following example is intended to be illustrative of the present invention and to teach one of ordinary skill how to make and use the corrosion inhibitor composition. This example is not intended to limit the invention or its protection in any way.

25 to 30% CaCl.sub.2 and LiCl solutions were treated with 2000 to 4130 ppm gluconate, 2000 to 2100 ppm sorbitol, 300 to 480 ppm borax and 11 to 39 ppm tolyltriazole. These solutions were tested at a temperature of 120.degree. to 130.degree. F. and in the pH range of 5.9 to 9.2. Gravimetric analyses were conducted to determine the corrosion rates.

The test conditions are recorded in the table below. Typical corrosion rates in untreated brine systems are approximately 5 to 6 mpy for a 30% CaCl.sub.2 solution and about 6-8 mpy for a 20% CaCl .sub.2 solution. As illustrated in the table below, the corrosion rates for mild steel and copper achieved using the present invention were 1.5 to 1.7 mpy and 0.1 mpy respectively.

If the above procedure is followed using about 2000 ppm sorbitol, 300 to 400 ppm borax and 11 to 39 ppm tolyltriazole, but no gluconate, the corrosion rates for mild steel and copper would be satisfactorily reduced. The results, however, would not be as good as those obtained in the presence of gluconate.

While the present invention is described above in connection with preferred or illustrative embodiments, these embodiments are not intended to be exhaustive or limiting of the invention. Rather, the invention is intended to cover all alternatives, modifications and equivalents included within its spirit and scope, as defined by the appended claims.