Polymer-zinc corrosion inhibiting method

Abstract

Use of low molecular weight polymers and zinc to inhibit the corrosion of metals by oxygen-bearing waters.

Description

BACKGROUND OF THE INVENTION

This invention relates to the inhibition of corrosion in water systems which utilize oxygen-bearing waters.

More particularly, this invention relates to the use of compositions comprising low molecular weight polymers and zinc to inhibit the corrosion of metals in water systems which contain oxygen-bearing waters.

Oxygen corrosion is, of course, a serious problem in any metal-containing water system. The corrosion of iron and steel is of principal concern because of their extensive use in many types of water systems. Copper and its alloys, aluminum and its alloys, and galvanized steel are also used in water systems and are subject to corrosion. We have discovered novel corrosion inhibitors which will inhibit oxygen corrosion in water systems containing such metals.

SUMMARY OF THE INVENTION

We have found that compositions comprising low molecular weight polymers and zinc are effective corrosion inhibitors. Suitable polymers include water-soluble salts of acrylic acid, acrylates, methacrylates, unhydrolyzed or partially hydrolyzed acrylamides, and acrylamidomethyl propane sulfonates. The polymers may be homo-, co-, or ter- polymers of any of the aforementioned polymers and may have a molecular weight of from about 500 to about 10,000. The preferred molecular weight, however, is about 1,000. Suitable water-soluble salts include the alkali metal, alkaline earth metal, zinc, cobalt, ammonium or amino and lower alkanol amine salts.

The zinc ion may be supplied in many ways. For example, the zinc ion may be added by utilizing a water-soluble zinc salt, such as, zinc chloride, zinc acetate, zinc nitrate, and zinc sulfate, which forms zinc ions in aqueous solution. The zinc ion may also be supplied by adding zinc just to a solution of the polymer.

Our corrosion-inhibiting compositions can contain a ratio of polymer to zinc of from about 1:3 to about 5:1 by weight. The preferred ratio, however, is from about 3:1 to 4:1 by weight. These compositions will effectively inhibit corrosion of metals when maintained in a water system at a concentration of at least about 1 ppm at the above ratios and, preferably, about 5 to 50 ppm. Maximum concentrations are determined by the economic considerations of the particular application.

Compounds such as benzotriazole or mercaptobenzothiazole may be added to the final formulation in varying amounts to improve its usefulness in a wider variety of industrial applications where both steel and copper or its alloys are present in the same system.

The following table illustrates the effectiveness of polyacrylic acid (molecular weight about 1,000) and a partially hydrolyzed polyacrylamide (molecular weight about 7,000) when used with and without zinc at the dosages and pH indicated.

The test procedure used was a standard 5-day continuous immersion, mild agitation test carried out in synthetic Pittsburgh tap water for 5 days (95.degree.F.). Steel corrosion rates are noted in the following table:

TABLE 1 ______________________________________ Weight/Loss Corrosion Rate Data for Polymers of Polyacrylic Acid and Partially Hydrolyzed Polyacrylamide With and Without Zinc Inhibitor Partially Hydrolyzed Polyacrylic Polyacrylamide Corrosion Acid (ppm) Zn.sup.+.sup.+ Rate (mdd) pH ______________________________________ 0 0 0 200 7.2-7.4 0 10 0 143 6.6-7.0 0 10 10 14 6.2-7.0 30 0 0 152 6.1-6.1 30 0 10 23 -- ______________________________________

The following table illustrates the influence of the ratio of polymer to zinc on several corrosion-inhibiting compositions of this invention. These tests were run in synthetic Pittsburgh water. Steel electrodes were used in polarization test cells with the initial pH at 7.0. Inhibitor concentrations were calculated on the basis of active material. The amount of corrosion that had taken place was determined from the current density at the intersection of an extrapolation of the so-called "Tafel" portion of the anodic polarization curve with the equilibrium or "mixed" potential value, usually referred to as the corrosion potential, "E.sub.corr ". Application of Faraday's Law allows a computation of a direct mathematical relationship between the current density at E.sub.corr, expressed in amperes per square centimeter and a more useful corrosion rate expression such as milligrams of steel consumed per square decimeter of surface per day (m.d.d.) and mils per year (m.p.y.). This relationship is such that a current density value of 4.0.times.10.sup..sup.-7 amperes/cm.sup.2 .times.1.0 mg/dm.sup.2 /day. Further, the m.p.y. value is calculated from the usual formula: m.p.y..times.m.d.d..times.1.44/density, using a density value of 7.87 g/cm.sup.3 for steel. The corrosion rate for steel in this water without inhibitor is 100 mdd. This control value applies to both Tables 2 to 3.

TABLE 2 __________________________________________________________________________ Polarization Corrosion Rate Data to Illustrate the Influence of Various Polymer to Zinc Weight Ratios (mdd) Weight Ratio Polymer to Zinc .fwdarw. 1 : 2 1 : 1 2 : 1 3 : 1 4 : 1 5 : __________________________________________________________________________ 1 Polymer Dosage (mg/l) .fwdarw. 10 30 100 10 30 100 10 30 100 10 30 100 10 30 10 30 __________________________________________________________________________ Polymer Used Polyacrylic Acid 36 3 3 35 62 4 80 54 17 42 38 3 17 6 30 7 Sodium Polyacrylate 69 52 25 46 48 34 1 31 1 2 8 1 4 1 58 6 Partially Hydrolyzed Polyacrylamide 80 -- -- 43 -- -- 32 3 -- 67 3 -- 51 66 80 66 __________________________________________________________________________

The following table illustrates that the effectiveness of polymeric corrosion inhibitors decreases as the molecular weight increases.

TABLE 3 ______________________________________ Polarization Corrosion Rate Data for Sodium Polyacrylates Approximate Dosages (mg/l) Corrosion Rate Molecular Weight Polymer Zinc (mdd) ______________________________________ 900 30 10 8 900 100 33 1 2,500 30 0 73 2,500 30 10 70 2,500 100 30 50 5,000 30 0 74 5,000 30 10 48 5,000 100 30 61 10,000 30 0 67 10,000 30 10 76 10,000 100 0 53 10,000 100 30 56 ______________________________________

Claims

We claim:

1. A method of inhibiting the corrosion of metals in a water system comprising maintaining in the water of said system at least about 5 ppm of a composition comprising zinc and at least one low molecular weight polymer selected from the group consisting of polyacrylamide, poly acrylic acid and sodium polacrylate.

2. A method as in claim 1 wherein the polymer has a molecular weight of from about 500 to about 10,000.

3. A method as in claim 1 wherein the ratio of polymer to zinc is from about 1:3 to about 5:1 by weight.

4. A method as in claim 1 wherein the ratio of polymer to zinc is from about 3:1 to about 4:1.