WO2001021854A2

WO2001021854A2 - Corrosion inhibitor

Info

Publication number: WO2001021854A2
Application number: PCT/ZA2000/000174
Authority: WO
Inventors: Schalk Willem Vorster; Frans Boudewijn Waanders; Andries Johannes Geldenhuys
Original assignee: Potchefstroom University For Christian Higher Education
Priority date: 1999-09-23
Filing date: 2000-09-19
Publication date: 2001-03-29
Also published as: WO2001021854A3; AU7887100A

Abstract

This invention provides a corrosion inhibitor for iron-based metals including a solution of a polysaccharide biopolymer, or a precursor or derivative thereof, in a solvent. The polysaccharide biopolymer is preferably chitosan, which is obtained from deacetylated chitin. This invention further provides a method of inhibiting corrosion of iron-based metals and a method of manufacturing a corrosion inhibitor including the steps of providing chitin; at least partially deacetylating the chitin to obtain chitosan; and dissolving the chitosan in a solvent. The corrosion inhibitor according to the invention provides a relatively cheap and environmentally friendly and non-toxic corrosion inhibitor suitable for protecting iron-based metals against corrosion damage by aqueous solutions, or to minimise such problems to an appreciable extent.

Description

CORROSION INHIBITOR

INTRODUCTION AND BACKGROUND TO THE INVENTION

The present invention relates to a corrosion inhibitor and method for the inhibition of corrosion of metals. More particularly but not exclusively, this invention relates to a corrosion inhibitor and method for inhibiting corrosion of iron-based based metals located in an acideous environment and to a method of manufacturing such corrosion inhibitor.

In this specification the term "iron-based metals" means iron metal and metal alloys containing iron therein, i.e. ferrous metals.

Iron-based metal alloys such as mild steel are commonly used in constructing apparatus for handling aqueous systems in which water circulates. These metals are subject to corrosion in such an environment, particularly when the environment is acideous. Iron-based metals are preferred over other metals on account of their superior strength.

It is known that various materials that are present in aqueous systems, especially systems using water derived from natural resources, attack iron based-metals. Typical devices in which the iron metal parts are subject to corrosion include evaporators, single and multi-pass heat exchangers, cooling towers, and associated equipment, pumps and the like. As the system water passes through or over the device, a portion of the system water evaporates causing a concentration of the dissolved materials present in the system. These materials approach or reach a concentration at which they may cause pitting and corrosion, which eventually requires replacement of the metal parts.

Various corrosion inhibitors have been used previously in an attempt to address this problem. For example, chromates and inorganic polyphosphates have been used in the past to inhibit the corrosion of metals which are brought into contact with water. A disadvantage of chromates is that they are highly toxic and, consequently, present handling and disposal problems. A disadvantage of polyphosphates is that they tend to hydrolyse to form orthophosphate, which in turn can create scale and sludge problems in aqueous systems. Moreover, where there is concern over eutrophication of receiving waters, excess phosphate compounds also present disposal problems. Borates, nitrates, and nitrites have also been used for corrosion inhibition. Although they can serve as nutrients at low concentrations, they present potential health concerns at high concentrations.

Much recent research has been concerned with the development of organic corrosion inhibitors for replacing the known inorganic inhibitors. Among the known organic inhibitors are numerous organic phosphonates. These compounds may generally be used without detrimental interference from other conventional water treatment additives. For example, UK Patent 2,112,370 discloses the inhibition of metallic corrosion, especially corrosion of ferrous metals, by using hydroxyphosphonoacetic acid (HPAA). The HPAA can be used alone or in conjunction with other compounds known to be useful in the treatment of aqueous systems, including various polymers and copolymers.

Polymeric agents have been used for various purposes in water treatment. US Patent No. 3,709,815 discloses use of certain polymers containing 2- acrylamido-2-methylpropane sulfonic acid (2-AMPSA) for boiler water treatment. US Patent No. 3,928,196 discloses a method of inhibiting scale formation in aqueous systems using certain copolymers of 2-acrylamido-2- methylpropyl sulfonic and acrylic acid. US Patent No. 4,588,517 discloses use of copolymers formed from acrylic acid or methacrylic acid derivatives in combination with 2-acrylamido-2-methylpropane sulfonic acid derivatives to increase corrosion inhibition of phosphates.

US Patent No. 6,083,416 discloses a corrosion inhibiting processes for refrigeration systems, which includes organic compounds that are added directly into refrigeration systems. Such compounds comprise nitrogen containing heterobicyclic organic compounds, thio carboxylic acids and organophosphates. US Patent No. 4,561 ,982 discloses a scale inhibitor for water and aqueous systems, the inhibitor comprising an oxidised polysacchande.

US Patents Nos. 4,585,560 and 4,603,006 disclose polysacchande containing compositions used for preventing the build up of scale. US Patent No. 5,486,582 discloses a polymer scale preventive process using a coating of chitosan salt. US Patents Nos. 5,055,230, 5,081 ,209, and 5,532,025 further disclose polymeric corrosion inhibiting compositions.

JP Patent No. 3103404 discloses an aquatic resin composition in the presence of amongst other things chitosan. It is used to provide anti- corrosive and bactericidal coatings for metals, inorganic, and organic materials. JP Patent No. 10255552 discloses a resin composition which is used to cover a wire for insulative purposes. JP Patent No. 11293149 discloses a process for the coating of aluminium with a soluble polymer containing 1 - 30 % chitin. It provides for corrosion resistance and makes the surface of the aluminium hydrophilic.

The above compositions all suffer from one disadvantage or the other. For example, some of them are difficult to manufacture and therefore relatively expensive. Others are hazardous to the environment or not suitable for inhibiting corrosion of iron-based metals. Another disadvantage of some of the above corrosion inhibitors is that they have to be applied directly on the metal surface in a layer. It is obviously difficult if not impossible to reach certain areas to apply such layer, particularly in a piping system.

OBJECT OF THE INVENTION It is therefore an object of the present invention to provide a corrosion inhibitor, a method of manufacturing a corrosion inhibitor, and a method for the inhibition of corrosion of iron-based metals with which the aforesaid disadvantages can be overcome or at least minimised.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a corrosion inhibitor for iron-based metals including a solution of a polysacchande biopolymer, or a precursor or derivative thereof, in a solvent.

Further according to the invention the polysacchande biopolymer is chitosan.

The chitosan may be obtained from deacetylated chitin.

The solution may include at least 0.00001 % chitosan on a weight percentage basis. Preferably the solution includes between 0.00001 % and 10 % chitosan on a weight percentage basis. Further preferably the solution includes between 0.001 % and 0.1 % chitosan on a weight percentage basis. More particularly the solution may include 0.016% chitosan on a weight percentage basis.

The solvent may comprise an acid. The solvent may be acetic acid.

The corrosion inhibitor may be in the form of an additive for acideous water.

According to a second aspect of the invention there is provided a method for inhibiting corrosion of an iron-based metal including the step of applying to such metal a solution of a polysacchande biopolymer, or a precursor or derivative thereof, in a solvent.

The step of applying the solution to the metal may include the step of diluting the solution and bringing the diluted solution into contact with the metal.

Preferably the step of diluting the solution includes the step of introducing the solution to acideous water.

The polysacchande biopolymer may be chitosan.

The chitosan may be obtained from deacetylated chitin. The solution, in use, may include at least 0.00001 % chitosan on a weight percentage basis.

Preferably the solution, in use, includes between 0.00001 % and 10 % chitosan on a weight percentage basis.

Further preferably the solution includes between 0.001 % and 0.1 % chitosan on a weight percentage basis. More particularly the solution may include 0.016% chitosan on a weight percentage basis.

The solvent may comprise an acid. Preferably the solvent is acetic acid.

According to a third aspect of the invention there is provided a method of manufacturing a corrosion inhibitor including the steps of providing chitin; at least partially deacetylating the chitin to obtain chitosan; and dissolving the chitosan in a solvent.

The solvent may comprise an acid. Preferably the acid comprises acetic acid.

The step of providing chitin may include the further steps of decalcificating the exoskeleton of an animal with aqueous hydrochloric acid to obtain crude chitin; washing the crude chitin with water; deproteinating the crude chitin with dilute sodium hydroxide; and extracting pigments with an organic solvent from the crude chitin.

The chitin may be deacetylated to form chitosan with 40%-50% sodium hydroxide at 110-115°C.

The method may include the further step of diluting the solution to obtain a diluted solution including between 0.00001 % and 1% chitosan on a weight percentage basis.

The diluted solution may include between 0.001 % and 0.1 % chitosan on a weight percentage basis. Preferably the diluted solution includes 0.016% chitosan on a weight percentage basis.

The invention will now be described further by way of non-limiting examples.

Chitin is found in the exoskeletons of insects, crabs, lobsters, prawns, krill, and the like. The fishing industry produces substantial amounts of waste from which chitin is relatively easily obtained by a method including the steps of: decalcificating the exoskeletons at ambient temperature using dilute aqueous hydrochloric acid to obtain crude chitin; washing the crude chitin extensively with water; deproteinating the crude chitin with dilute sodium hydroxide; and extracting pigments, such as carotenoids, from the chitin with appropriate organic solvents.

Chitosan is produced from the chitin by deacetylating the chitin with 40%- 50% sodium hydroxide at 110-115°C. A comprehensive and critical evaluation of chitin and chitosan processing was given by Muzzarelli {Chitin, Pergamon Press, 1977. ISBN 0-08-020367-1 ).

Chitosan can be represented by the simplified formula set out below, which must be seen as a single unit (or -mer) which is contained in a polymer chain. The chitosan polymer chains may contain up to several hundred thousand such -mers, bound together via an oxygen atom:

CH₂OH-(CH)-(CHOH)₂-CH(NH₂)-CHOH

L o J

Chitosan is an amine, a family of organic substances containing a nitrogen (N) atom to which hydrogen or other organic group may be attached. Deacetylated derivatives are polycationic and have a tendency to adsorb onto negatively charged metal surfaces, a situation that arises when chloride ions cover the metal surface. It is generally found that the inhibition efficiency of amines in chloride-free acid solutions is rather low. It was therefore envisaged that these naturally occurring materials could also act as corrosion inhibitors for iron - based metals in acid chloride solutions. A degree of acetylation remains after the above-described deacetylation treatment, as full deacetylation is usually avoided because it may lead to destruction of the polymer structure. The material referred to in this disclosure as chitosan therefore contains a number of mers, which have not been deacetylated. This will amount to the presence of the -CH₃OH-group in a number of -mers.

The applicant has now surprisingly found that improved corrosion inhibition of iron-based metals can be achieved by the use of relatively low concentrations of polysacchande biopolymers derived from chitin. The applicant has further found that such corrosion inhibitors are not only inexpensive but are also highly effective environmentally benign corrosion inhibitors for iron - based metals. In particular chitosan, a deacetylated (poly)-glucose amine derived from the chitin-containing exoskeletons of shellfish, has now been shown by the applicant to demonstrate an exceptionally high degree of inhibition of the corrosion of ferrous materials in contact with acid chloride-containing water and acid sulphate-containing water.

A 2% chitosan concentrated solution was prepared by dissolving an appropriate amount of chitosan powder in an aqueous acetic acid solution. This concentrated solution was diluted to different test dilutions in order to attain an optimum inhibitor concentration.

Practice of the invention will become further apparent from the following non- limiting examples:

Example 1

In order to gain a clearer picture of the inhibiting action of chitosan, its inhibition action in acid sulphate solutions and in acid chloride solutions were studied.

The chemical composition of mild steel used in this investigation is reported in Table 1. Before weighing, steel samples (nominally measuring 50 mm x 20 mm x 3 mm) were chemically polished in order to create a clean, homogeneous surface and to deburr the edges. After cleaning, care was taken with the handling the samples to prevent contamination of the surfaces with grease and/or chloride ions. Using nylon thread, the samples were freely suspended in appropriate solutions after weighing. The solutions were static and were open to the air. Experiments were carried out at ambient temperature. After exposure, the corrosion crusts were removed by means of a soft brush, dried and reweighed. Exposure times ranged from 25 to 150 h. The inhibitor effectiveness was calculated using the formula:

Inhibitor effectiveness η (%) = Corrosion rate without inhibitor - Corrosion

rate with inhibitor x 100/ Corrosion rate without inhibitor

Corrosion-time relationships were obtained by fitting the average mass losses recorded per unit area to the power law:

Δm = Atⁿ

where Δm is the mass loss per unit area (mg cm^"2) and t the exposure

time (h). A and n are constants.

The results are summarised in Table 2: Table 2: Corrosion of mild steel in various solutions, inhibited by CS4, TMC and DAH.

Note: CS4 = Crab chitosan (85% deacetylated, 2 wt. % solution in 4% acetic acid), TMC = tri-methyl chitosan (85% methylated), DAH = dodecyl amine hydrochloride.

It is generally assumed that organic amine inhibitors in acid chloride solutions become protonated. It is furthermore widely believed that the protonated amines then adsorb electrostatically onto halide-covered metal surfaces. It is thought that adsorbed chloride ions form oriented dipoles with their negative ends towards the solution, thereby facilitating the adsorption of organic cations. In this process the electron density at the amine nitrogen atom is thought to play a vital role.

From Table 2 it is clear that chitosan, even in small quantities, operates as a corrosion inhibitor in acid chloride and sulphate solutions. It is also clear that the inhibiting efficiency in chloride solutions is enhanced by the presence of small amounts of acetate ions.

Example 2 In order to gain further information, steel rod samples were machined from the material used in Example 1 and fixed to an electrode holder after mechanical polishing, using 1000 grade silicon carbide paper. A vitreous carbon rod and silver/silverchloride reference electrode were positioned in an EG&G Princeton Applied Research electrochemical cell in which the test solutions were held. This three-electrode assembly was then connected to an EG&G Princeton Applied Research Potentiostat model 273A, and Tafel plots obtained in a way well known to corrosion engineers and corrosion scientists, and described by Ayssar H. Nahle in Corrosion Prevention & Control, August 1998, pp124-130. The available computer software (SoftCor III) facilitates the computation of a corrosion current and eventually a corrosion rate, commonly expressed in mm penetration of the metal per year of exposure to the corroding fluid.

The above Figure is a graphic representation of the corrosion rate of mild steel in pH 2.5 solutions (containing 0.25 mol/dm³ sodium sulphate) as a function of the concentration of chitosan expressed in weight percentage. The effectiveness of the chitosan as an inhibitor can be calculated as explained in Example 1. At a concentration of 0.001 % chitosan it was found that the effectiveness was in excess of 69% and at 0.016% it was found to be 84%.

The corrosion inhibitor according to the invention therefore provides a relatively cheap and environmentally friendly and non-toxic corrosion inhibitor suitable for protecting iron - based metal structures against corrosion damage by acideous aqueous solutions, or to minimise such problems to an appreciable extent.

The applicant has surprisingly found that the addition of small amounts (as low as 0.00001 % on a weight percentage basis) of chitosan in solution reduce the corrosion of iron - based metals in acideous aqueous systems, and that concentrations as low as 0.0001 % inhibits corrosion substantially. It was also found that the optimum percentage chitosan in solution is 0.016% and that the corrosion inhibition effectivity drops off sharply at concentrations above 0.016%, due to precipitation of the chitosan.

The applicant has further found that it is relatively easy to apply and use the corrosion inhibitor according to the invention for inhibiting the corrosion of iron based metals, by simply introducing the chitosan solution to acideous waste water to obtain the above concentration of 0.016% chitosan in the waste water. Acideous waste water, which normally causes the corrosion, is therefore used as the carrier to bring the corrosion inhibitor in contact with the metal surface to be protected. It will be appreciated that surfaces that are usually difficult to reach with conventional corrosion inhibitors can be protected in this fashion with the corrosion inhibitor according to the invention. It will be appreciated that variations in detail are possible with a corrosion inhibitor and methods according to the invention for the inhibition of corrosion of iron-based metals without departing from the scope of the appended claims.

Claims

1. A corrosion inhibitor for iron-based metals including a solution of a polysaccharide biopolymer, or a precursor or derivative thereof, in a solvent.

2. A corrosion inhibitor according to claim 1 wherein the polysaccharide biopolymer is chitosan.

3. A corrosion inhibitor according to claim 2 wherein the chitosan is obtained from deacetylated chitin.

4. A corrosion inhibitor according to claim 2 or 3 wherein the solution includes at least 0.00001 % chitosan on a weight percentage basis.

5. A corrosion inhibitor according to claim 4 wherein the solution includes between 0.00001 % and 10 % chitosan on a weight percentage basis.

6. A corrosion inhibitor according to claim 5 wherein the solution includes between 0.001 % and 0.1 % chitosan on a weight percentage basis.

7. A corrosion inhibitor according to claim 6 wherein the solution includes 0.016% chitosan on a weight percentage basis.

8. A corrosion inhibitor according to any one of the preceding claims wherein the solvent comprises an acid.

9. A corrosion inhibitor according to claim 8 wherein the solvent is acetic acid.

10. A corrosion inhibitor according to any one of the previous claims which is in the form of an additive for acideous water.

11. A corrosion inhibitor for iron-based metals substantially as herein described and exemplified.

12. A method for inhibiting corrosion of an iron-based metal including the step of applying to such metal a solution of a polysaccharide biopolymer, or a precursor or derivative thereof, in a solvent.

13. A method according to claim 12 wherein the step of applying the solution to the metal includes the step of diluting the solution and bringing the diluted solution into contact with the metal.

14. A method according to claim 13 wherein the step of diluting the solution includes the step of introducing the solution to acideous water.

15. A method for inhibiting corrosion according to any one of claim 12 to 14 wherein the polysaccharide biopolymer is chitosan.

16. A method for inhibiting corrosion according to claim 15 wherein the chitosan is obtained from deacetylated chitin.

17. A method for inhibiting corrosion according to claim 15 or 16 wherein the solution, in use, includes at least 0.00001 % chitosan on a weight percentage basis.

18. A method for inhibiting corrosion according to claim 17 wherein the solution, in use, includes between 0.00001 % and 1 % chitosan on a weight percentage basis.

19. A method for inhibiting corrosion according to claim 18 wherein the solution includes between 0.001 % and 0.1 % chitosan on a weight percentage basis.

20. A method for inhibiting corrosion according to claim 19 wherein the solution includes 0.016% chitosan on a weight percentage basis.

21. A method for inhibiting corrosion according to any one of claims 12 to 20 wherein the solvent comprises an acid.

22. A method for inhibiting corrosion according to claim 21 wherein the solvent is acetic acid.

23. A method for inhibiting corrosion of iron-based metals substantially as herein described and exemplified.

24. A method of manufacturing a corrosion inhibitor including the steps of providing chitin; at least partially deacetylating the chitin to obtain chitosan; and dissolving the chitosan in a solvent.

25. A method of manufacturing a corrosion inhibitor according to claim

24 wherein the solvent comprises an acid.

26. A method of manufacturing a corrosion inhibitor according to claim

25 wherein the acid comprises acetic acid.

7. A method of manufacturing a corrosion inhibitor according to any one of claims 24 to 26 wherein the step of providing chitin includes the steps of decalcificating the exoskeleton of an animal with aqueous hydrochloric acid to obtain crude chitin; washing the crude chitin with water; deproteinating the crude chitin with dilute sodium hydroxide; and extracting pigments with an organic solvent from the crude chitin.

28. A method of manufacturing a corrosion inhibitor according to claim 27 wherein the chitin is deacetylated to form chitosan with 40%-50% sodium hydroxide at 110-115°C.

29. A method according to any one of claims 24 to 28 which includes the further step of diluting the solution to obtain a diluted solution including between 0.00001 % and 1 % chitosan on a weight percentage basis.

30. A method according to claim 29 wherein the diluted solution includes between 0.001 % and 0.1 % chitosan on a weight percentage basis.

31. A method for inhibiting corrosion according to claim 30 wherein the diluted solution includes 0.016% chitosan on a weight percentage basis.

2. A method of manufacturing a corrosion inhibitor substantially as herein described and exemplified.