CA1118589A - Method of corrosion inhibition in aqueous mediums - Google Patents

Abstract

Abstract of Disclosure A method for reducing corrosion of metal surfaces is disclosed according to which a specific corrosion inhibitor is utilized. The inhibitor comprises water-soluble zinc compound and copolymer of acrylic acid and hydroxylated lower alkyl acrylate. Water-soluble orthophosphate can also be used.

Description

METHOD OF CORROSION INHIBITION IN
AQUEOUS MEDIUMS

Technical Field .
The present invention is related to zinc-containing 5 corrosion inhibitor treatments. The ability of zinc to inhibit the corrosion of ferrous metals is, indeed, well known. Accord-ingly, soluble zinc salts are vital ingredients of many corrosion treatment programs. For example, U.S. 4,089,796 to Harris et al discloses a corrosion inhibiting composition comprising zinc and hydrolyzed polymaleic anhydride or soluble salt thereof. Other exemplary patents disclosing such zinc-containing treatments are U.S. 3,432,428 to Wirth et al and U.S. 4,120,~55 to Crambes et al.

An art-recognized major problem encountered with zinc-containing treatments, particularly in cooling water, is the un-controlled precipitation of zinc salts; because, to be effective,the zinc must reach the surfaces to be protected in a soluble form.
For example, the use of orthophosphate in combination with zinc as a cooling water treatment is well known as evidenced by U.S.

2,900,222 to Kahler et al. The orthophosphate can be provided âS
an actual addition, or as a reversion product from any one of co~nplex inorganic phosphate, organic phosphate or organic phos-phonate. When orthophosphate and zinc are both present in the ~, ~
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water, zinc phosphate precipitation becomes a concern. Whether or not orthophosphate is present, the zinc could precipitate in other forms, for example, as zinc hydroxide or zinc silicate. The solubility of the various salts, that is, the retention of the respective salt constituents in ionic form, depends on such factors as water temperature and pH and ion concentrations. Wirth et al states that although water temperatures can vary from 32~ to 200F, lower temperatures of 32 to 80F are preferred because "zinc tends to remain in solution better in cooler waters." This patent further states that alkaline waters, particularly above about pH 7.5, are relatively undesirable because "the dissolved zinc tends to deposit out or drop out much more rapidly in alkaline water." Similarly, Crambes et al points out that zinc salts are unstable in neutral or alkaline water and will precipitate with phosphates. Thus, if any of these conditions are present, the aqueous medium becomes prone to zinc precipitation. ~ecause of the formation of this zinc scale, many of the surfaces in contact with the aqueous medium will foul and the amount of effective (soluble) corrosion inhibitor present in the aqueous medium can be significantly reduced.

Although the present invention is considered to have general applicability to any aqueous system where zinc precipitation is a problem, it is particularly useful in cooling water systems.
Accordingly, the invention will hereinafter be described as it re-lates to cooling water systems.

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~~ r Description of the Invention There has existed for a long time the need for a zinc-containing corrosion inhibitor treatment which overcomes the above-noted problems, and the present invention is considered to fulfill that need.

According to the present invention, a corrosion in-hibitor treatment for metal surfaces exposed to an aqueous medium comprises (i) water-soluble zinc compound and (ii) a particular type of water-soluble polymer composed essentially of moieties derived from acrylic acid or derivatives thereof and hydroxylated lower alkyl acrylate moieties (HAA). The treatment could addi-tionally comprise (iii) orthophosphate. It was discovered that, although the polymer demonstrated no significant activity alone as a corrosion inhibitor, when it was combined with a zinc-contain-ing treatment the various ionic constituents of the treatment wereunexpectedly retained in their soluble form and a corresponding increase in corrosion inhibiting activity was observed. The present invention is, accordingly, also considered to be related to a method for inhibiting the formation of zinc scale in an aqueous medium.

The Polymer The polymers according to the present invention are those effective for the purpose which contain essentially moieties derived from an acrylic acid compound (AA), i.e., r~

R
--CH2 -- C --_ ( I ) Cl = O
Rl where R is hydrogen or a lower alkyl of from 1 to 3 carbon atoms and R1 = OH, NH2 or OM, where M is a water-soluble cation, e.g., NH4, alkali metal (K, Na), etc; and moieties of an hydroxylated lower alkyl (C2-C6) acrylate (HAA) as represented, for example, by the formula:
l3 --CH2 --C _ C = O (II) O

where R3 is H or lower alkyl of from 1 to 3 carbon atoms, and R2 is a lower alkyl having from about 2 to 6 carbon atoms.

In terms of mole ratios, the polymers are considered, most broadly, to have a mole ratio of AA:HAA of from about 1:4 to 36:1. This mole ratio is preferably about 1:1 to 11:1, and most preferably about 1:1 to 5:1. The only criteria that is considered to be of importance with respect to mole ratios is that it is desirable to have a copolymer which is water-soluble. As the proportion of hydroxylated alkyl acrylate moieties increases, the solubility of the copolymer decreases. It is noted that, from an efficacy point of view, the polymers having a mole ratio of AA:HAA
of l:1 to 5:1 were considered the best.

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The polymers could have a molecular weight of from about 1,000 to about 50,000 with from about 2,000 to about 6,000 being preferred.

The polymers utilized in accordance with the invention can be prepared by vinyl addition polymerization or by treatment of an acrylic acid or salt polymer. More specifically, acrylic acid or derivatives thereof or their water soluble salts, e.g., sodium, pot-assium, ammonium, etc. can be copolymerized with the hydroxy alkyl acrylate under standard copolymerization conditions utilizing free radical initiators such as benzoyl peroxide, azobisisobutyronitrile or redox initiators such as ferrous sulfate and ammonium persulfate.
The molecular weights of the resulting copolymer can be controlled utilizing standard chain control agents such as secondary alcohols (isopropanol), mercaptans, halocarbons, etc. Copolymers falling within the scope of the invention are commercially available from, for example, National Starch Company.

The hydroxy alkyl acrylate can be prepared by the addition reaction between the acrylic acid or its derivatives or water solu-ble salts and the oxide of the alkyl derivative desired. For ex-ample, the preferred monomer of the present invention is the propylderivative. Accordingly, to obtain the hydroxylated monomer, acry-lic acid is reacted with propylene oxide to provide the hydroxy-propyl acrylate monomer.

The polymers of the invention may also be prepared by re-acting a polyacrylic acid or derivatives thereof with an appropriateamount of an alkylene oxide having from 2 to 6 carbon atoms such as ethylene oxide, propylene oxide and the like. The reaction takes - ' 5~

place at the COOH or COM group of the moieties to provide the hydro-xylated alkyl acrylate moiety.

The polymer prepared either by copolymerization of M with hydroxy propyl acrylate (HPA) or reaction of M with propylene oxide would be composed of units or moieties having the structural formulas:

; CH2 - C ~ _ and _ - CH2 - C - __ Cl = O IC= O

_ CH3 where M is as earlier defined.

The Zinc Compounds Illustrative water-soluble zinc compounds which are considered to be suitable for use in accordance with the present invention are zinc oxide, zinc acetate, zinc chloride, zinc formate, zinc nitrate, zinc sulphate, zinc borate, etc.

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The Orthophosphate Compounds As already noted above, the treatment could further com-prise orthophosphate. Indeed, the use of zinc and orthophosphate ;
together as a corrosion inhibition treatment is well known. It has also already been noted that the orthophosphate could be provided asan actual addition product, e.g., sodium orthophosphate, or as a precursor compound such as complex inorganic phosphates, organic phosphates or organic phosphonates which revert to orthophosphate in the water.

Illustrative examples of orthophosphate as an actual addi-tion are monosodium phosphate and monopotassium phosphate. Any ;
other water-soluble orthophosphate or phosphoric acid would also be considered to be suitable.

The complex inorganic phosphates are exemplified by sodium pyrophosphate, sodium tripolyphosphate, sodium tetraphosphate, sodium septaphosphate, sodium decaphosphate and sodium hexameta-phosphate. Either the corresponding potassium or ammonium salts or the corresponding molecularly dehydrated phosphoric acids such as metaphosphoric acid or pyrophosphoric acid are considered to be suitable.
.
The organic phosphonates are exemplified by aminotri-methylene phosphonic acid, hydroxyethylidene diphosphonic acid and the water-soluble salts thereof.

Organic phosphates are exemplified in U.S. 3,510,436.

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The amount of each constituent added to the cooling water ;;
will, of course, be an effective amount for the purpose and will depend on such factors as the nature and severity of the corrosion problem bein~ treated, the temperature and pH of the cooling water and the type and amount of precipitation-prone ions present in the water.

In terms of active zinc ion, as little as about 0.5 parts of zinc per million parts (ppm) of cooling water are believed to be effective in certain instances, with about 2 ppm being preferred.
Based on economic considerations, the amount of zinc ion added could be as high as about 25 ppm, with about 10 ppm representing the pre-ferred maximum.

In terms of active polymer, as little as about 0.5 ppm polymer is considered to be effective, while about 2 ppm is the preferred minimum. Based on economic considerations, the polymer cou1d be fed in amounts as high as about 200 ppm, with about 50 ppm representing the preferred maximum.

In terms of active product added, the orthophosphate or precursor compound thereof could be fed in an amount as low as about 1 ppm, with about 2 ppm representing the preferred minimum. Based on economic considerations, the maximum amount is considered to be about 200 ppm. However, about 50 ppm is considered to be the pre-ferred maximum.

Methods for feeding corrosion inhibitors to cooling water are well known in the art such that details thereof are not consid-. .
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ered necessary. However, due to rather severe stability problems experienced when the polymer was stored at high concentrations with the remaining components, a two or three-barrel treatment is recom-mended.

The cooling water to be treated preferably will have a pH
of about 6.5 to about 9.5. Since zinc precipitation problems most commonly occur at pH's above about 7.5, the most preferred pH range is from about 7.5 to about 9.5.

EXAMPLES

Illustration of Zinc Precipitation Problem Example 1 As noted above, an art-recognized major problem encoun-tered with zinc-containing treatments, particularly in cooling water, is the uncontrolled precipitation of zinc salts from the water. Even in the absence of orthophosphate in the water, the zlnc can form precipitates such as zinc hydroxide.

This point is illustrated by the zinc-solubility results of several tests conducted in water containing no orthophosphate.
The tests were conducted, inter alia, to determine the solubility of zinc in the test water as a function of pH.

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Solution A: 1,000 ppm Zn~ obtained from 0.27 gram Zn S04~H20/100 ml SCW7: 170 ppm Ca as CaC03, 110 ppm Mg as CaC03 15 ppm SiO2 .

The tests were conducted using the following procedure:

1. Prepare SCW7 (detailed in Example 5 below) and adjust its pH to 4 with concentrated HCl.

2. To 2,000 ml of the above solution, add the required amount of Solution A with stirring.

3. Add 100 ml of the solution from step 2 to a bottle and agitate.

4. Slowly adjust the pH to the desired value with dilute NaOH solution and record pH.

5. Place the samples in an oven at the required temper-rature for 24 hours, after which time, filter through a 0.2 micron mi111pore filter.

6. Analyze the filtrate for soluble zinc and record final pH.

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The results of these tests are reported below in Tables 1 and 2 in terms of soluble zinc (ppm) remaining after 24 hours at various final pH values.

5 ZINC PP~ECIPITATION AS FUNCTION OF pH

Conditions: Initial Zinc = 2 ppm as Zn++
T = 120F
Time = 24 hours pH soluble zinc (ppm)

7.62 0.8 7~70 0,5 7.92 0.5

8.12 0.2 8.21 0.1 8.35 0.2 8.42 0.1 8.76 0.0 ZINC PRECIPITATION AS FUNCTION OF pH

Conditions: Initial Zinc = 10 ppm as Zn++
T = 120F
Time = 24 hours pH soluble zinc (ppm) 7.15 8.2 7.25 7.8 7.36 8.2 7.46 8.0 7.50 7.8 7.56 5.4 7.67 1.8 7.70 1.6 8.05 0.4 8.10 0.2 8.22 0.4 8.65 0.2

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Example 2 The problem of zinc precipitation in cooling water is further illustrated by the zinc-solubility results of additional tests similar to those in Example 1, but conducted in water con-taining both zinc ions and orthophosphate ions.

The following aqueous test solutions were first prepared: :
Solution A: 1,000 ppm P04~3, obtained from 0.400 gram Na3P04 12H20/100 ml Solution B: 1,000 ppm Zn+2, obtained from 100.27 grarn ZnS04 H20/100 ml SCW7: Same as Example 1 The following procedure was used:

1. Prepare SCW7 and adiust its pH to 4 with HCl solution.

152. To 2,000 ml of the above solution, add the appro-priate amount of Solution A, followed by the appropriate amount of Solution B with agitation.

3. Add 100 ml of the solution from step 2 to a bottle and adjust the pH to 7.5 with dilute NaOH with 20 agitation.

~118589 4. Place the samples in an oven for 24 hours at the appropriate temperature.

5. After the 24 hour period, filter the solution through a 0.2 micron millipore filter.

6. Analyze the filtrate for zn+2 and P04~3.

The results of these tests are reported below in Tables 3 ~-and 4 in terms of ppm soluble zinc and ppm soluble phosphate remain-ing after 24 hours at various final pH values.

ZINC PRECIPITATION AS FUNCTION OF pH

Conditions: Initial Zinc = 5 ppm as Zn++
Initial o-P04 = 10 ppm T = 120F
Time = 24 hours pH soluble zinc (ppm) soluble P04 (ppm) 7.0 3 6 7.5 0.5 5 ~3,0 0.1 3.7 8.5 0.1 2.4 9.0 0.1 1.2 .' ZINC PRECIPITATION AS FUNCTION OF pH

Conditions: Initial Zinc = 10 ppm as Zn++
Initial o-P04 = 5 ppm T = 120F
Time = 24 hours pH soluble zinc (ppm) soluble P04 (ppm~

7.5 3.3 0.8 8.0 0.2 0.7

10 9.0 0.1 0.1 `-EFFICACY IN RETAINING SOLUBLE ZINC-CONTAINING TREATMENTS

Example 3 A series of tests were conducted to determine the efficacy of various materials in retaining zinc-containing corrosion inhib-ition treatments in a soluble form. After all, the corrosion in-hibition efficacy of such treatments will, for the most part, depend on the constituents remaining soluble.

The test water contained both zinc and orthophosphate ions, and the test prGcedures were the same as in Example 2 but for a few different steps as follows:

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1. Solution C comprising 1,000 ppm of active treatment was also used.

3. Add 100 ml of solution from step 2 to a bottle, add the appropriate quantity of treatment solution (1 ml = 10 ppm), and adjust pH to appropriate value with dilute NaOH with agitation.

The results of the tests were calculated in terms of % in-crease in retention of soluble zinc ions and soluble phosphate ions vs. an untreated solution using the following equation:
% Retention =

100 x Sol.PO4 at T=24 hrs-Sol. PO4 in control at T=24 hrs.
Sol. PO4 in control at T=O hrs-Sol. PO4 in control at T=24 hrs., where Sol. PG4 = soluble PO4 in ppm. Of course, a similar equation was used for zinc calculations.

The results of these tests are reported below in Tables 5 and 6~In addition to testing various M /HPA copolymers in accordance with the present invention, various commercial polyacrylic acids (PAA) were also tested.

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The results of these tests are reported below in Tables 7-13 in terms of ppm soluble zinc retained in solution. For pur-poses of comparison with untreated test solution, Table 7 should be compared with the results of Table 1 and Tables 8-13 should be compared with the results of Table 2.

Visual comparisons of Table 7 with Table 1 and Tables 8-13 with Table 2 are provided in the accompanying drawing.

In Fig. 1 are presented a series of graphs which con-tain comparisons of Table 7 with Table 1 in terms of soluble zinc remaining in solution after 24 hours vs. pH of the test water. As can be seen from the figure, the lowermost graph represents a no treatment test wherein the zinc readily precipitates. In compari-son, the higher graphs represent various test solutions to which have been added the noted AA/HPA polymers. The polymers were all considered to be efficacious in retaining soluble zinc in solution.

Remaining Figs. 2-7 provide visua7 comparisons of respec-tive ones of Tables 8-13 with Table 2. Fig. 2 compares Table 8, , ..~

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: -Fig. 3 compares Table 9, Fig. 4 compares Table 10, Fig. 5 comparesTable 11, Fig. 6 compares Table 12, and Fig. 7 compares Table 13, all with Table 2 in terms of plots of soluble zinc remaining in solution after 24 hours vs. pH at various indicated treatment levels. The line marked "No Treatment" in each figure represents the results of Table 2.

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8~!39 Conditions: Initial Zinc = 2 ppm as Zn++
T = 120~
Time = 24 hours Treatment = AA/HPA
Dosage = 5 ppm actives Mole RatioMolecular Water Soluble Zinc AA:HPA Weight pH Retained (ppm) 101.8:1 6,000 8.27 1.8 " " 8.42 1.7 " " 8.78 1.8 " " 8.86 1.7 " " 9.10 0.9 15g.9:1 6,000-10,000 7.59 1.7 " " 7.85 1.7 " " 7.95 1.7 " " 8.00 1.8 " " 8.45 1.6 " " 8.72 1.6 " " 8.94 1.5 3:1 6,000 7.50 1.8 " " 7.85 1.8 " " 8.21 1.9 " " 8.54 1.8 " " 8.67 1.6 " " 8.94 1.5 " " 9.06 1.2 - ~ .

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TABLE 7 ( Conti nued) .
Mole RatioMolecular Water Soluble Zinc AA-HPA Weight pH Retained (ppm) 9.9:1 1,000-2,000 7.68 1.9 " " 8.06 1.8 " " 8.21 1.7 " " 8.38 1.8 " " 8.51 1.5 .
" " 8.88 1.7 1019.8:1 2,000-6,000 7.79 1.7 " " 7.92 1.8 :~
" " 8.25 1.4 " " 8.50 1.4 " " 8.67 1.1 " " 8.92 0.7 36:1 2,000-6,000 7.84 1.7 " " 8.05 1.8 " " 8.65 1.7 " " 8.~8 1.6 " " 8.95 1.6 . .

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Conditions: Initial Iinc = 10 ppm as Zn++
T = 120F
Time = 24 hours Treatment = AA/HPA, Mole Ratio AA:HPA = 1.8:1, Molecular '~eight = 6,000 :

WaterTreatment Dosage Soluble Zinc pH (ppm actives) Retained (ppm) 107.50 5 8.6 7.60 5 10.0 7.75 5 5.4 7 79 5 6.6 7.92 5 5.0 157.96 5 4.4 8.26 5 0.2 8.30 5 0.6 8.42 5 0.4 8.48 5 1.6 208.60 5 0.8 7.61 10 8.0 7.70 10 8.4 7.90 10 8.0 , , , , ~

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TABLE 8 ~Continued) -WaterTreatment Dosage Soluble Zinc pH (ppm actives) Retained (ppm) 8.00 10 8.2 58.27 10 8.4 8.75 10 8.0 8.90 10 8.6 9.02 10 7.4 9.18 10 6.8 109.23 10 3.4 8.82 20 9.4 8.88 20 9.8 9.13 20 10.0 - , -' . ' :

Conditions: Initial Zinc = 10 ppm as Zn++
T = 120F
Time = 24 hours Treatment = AA/HPA, Mole Ratio AA:HPA = 3:1, Molecular Weight = 6,000 -WaterTreatment Dosage Soluble Zinc pH (ppm actives) Retained (ppm) 107.27 5 10 7.54 5 10 7.77 5 10 8.02 5 8.4 8.08 5 8.0 158.20 5 0.8 8.37 5 2.0 8.45 5 0.8 8.55 5 0.0 7.62 10 7.8 207.90 10 7.6 8.00 10 8.0 8.29 10 7.0 8.34 10 8.4 , .
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TABLE 9 ~Continued) WaterTreatment Dosage Soluble Zinc DH (ppm actives) Retained (ppm~

8.41 10 7.4 5 8.56 10 7.2 8.60 10 8.4 8.97 1~ 6.6 9.14 10 7.0 9.30 10 4.8 108.58 20 9.0 "
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Conditions: Initial Zinc = 10 ppm as Zn++
T = 120nF
Time = 24 hours Treatment = AA/HPA, Mole Ratio AA:HPA = 9.9:1, Molecular Weight = 1,000-2,000 WaterTreatment Dosage Soluble Zinc pH (ppm actives) Retained (ppm) ~-107.60 5 8.6 7.72 5 8.0 7.80 5 6.6 7.93 5 5.0 8.04 5 3.2 158.27 5 1.2 8.35 5 1.0 7.53 10 9.0 7.75 10 10.0 7.97 10 10.0 208.15 10 9.4 8.30 10 6.6 ~.48 10 8.4 8.65 10 6.8 :

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~8589 TABLE 10 (Continued) Water Treatment Dosage Soluble Zinc pH (ppm actives) Retained (ppm) 8.72 10 5.6 58.92 10 6.0 9.10 10 5.0 9.25 10 4.4 8.65 20 7.6 8.90 20 7.2 109.10 20 7.6 9.30 20 3.2 , TABLE ll Conditions: Initial Zinc = 10 ppm as Zn++
T = 120F
Time = 24 hours Treatment = AA/HPA, Mole Ratio AA:HPA = 9.
Molecular Weight = 6,000-10,000 WaterTreatment Dosage Soluble Zinc pH (ppm actives) Retained (ppm) 107.50 5 7.6 7.68 5 8.2 7.7~ 5 2.4 7.80 5 2.0 7.86 5 2.8 157.88 5 0.8 8.20 5 0.6 8.27 5 0.2 8.58 5 0.2 8.10 10 8.4 208.13 10 8.4 8.20 10 8.2 8.25 10 8.6 8.50 10 7.2 `~

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TABLE 11 (Continued) WaterTreatment Dosage Soluble Zinc pH (ppm actives) Retained (ppm) 8.74 10 6.0 5 9.01 10 7.4 9.14 10 7.6 9.32 10 3.8 ., .

1~85~39 Conditions: Initial Zinc = 10 ppm as Zn++
T = 120F
Time = 24 hours Treatment = AA/HPA, Mole Ratio AA:HPA = 19.8:1, Molecular Weight = 2,000-6,000 WaterTreatment Dosage Soluble Zinc pH(ppm actives) Retained (ppm) 107.80 10 2.4 .
7.83 10 2.4 7.95 10 1.2 8.08 10 1.0 8.12 10 1.2 158.25 10 1.2 8.35 10 1.4 8.15 20 4.6 8.35 20 4.8 8.52 20 5.2 208.78 20 4.0 8.62 30 8.4 8.83 30 8.6 8.88 30 8.2 8.95 30 8.4 9.02 30 7.4 9.11 30 5.2 ~ ~ .

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Conditions: Initial Zinc = 10 ppm as Zn++
T = 120F
Time = 24 hours Treatment = AA/HPA, Mole Ratio AA:HPA = 36:1, Molecular Wei ght = 2,000-6,000 WaterTreatment Dosage Soluble Zinc pH(ppm actives) Retained ~ppm) 107.75 5 5.8 7,79 5 2.6 8.02 5 1.0 8.34 5 0.2 8.66 5 o,o 158.27 10 7.0 8.40 10 5.8 8.50 10 7.6 8.62 10 6.6 8.87 10 6.6-208.90 10 7.2 9.03 10 7.8 9.40 10 5.2 8.79 20 9.0 ~,, ,, ': , ~ `""'' ':

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TABLE 13 (Continued) .
Water Treatment Dosage Soluble Zinc pH (ppm actives) Retained (ppm) 8.95 20 8.6 5 9.05 20 8.6 9.11 20 7.0 9.21 20 7.8 9.23 20 7.2 EFFICACY AS CORROSION INHIBITOR

Example 5 Having already demonstrated both the zinc precipitation problem related to zinc-containing corrosion inhib;tor treatments in aqueous mediums and the resolution of this problem by combining the treatment with AA/HAA polymer, the following test results are presented to demonstrate, from a corrosion inhibition point of view, the benefits of the combined treatments.

The tests were each conducted with two non-pretreated low carbon steel coupons which were immersed and rotated in aerated synthetic cooling water for a 3 or 4 day period. The water was ad-~usted to the desired pH and readjusted after one day if necessary;

no further adjustments were made. Water temperature was 120F.Rotational speed was maintained to give a water velocity of 1.3 feet per second past the coupons. The total volume of water was 17 liters. Cooling water was manufactured to give the following conditions:

SCW7 (pH=7) SCWg (pH=8) ppm Ca as CaC03 170 170 ppm Mg as CaC03 110 110 ,~
ppm SiO2 15 15 ppm Na2C03 100 Corrosion rate measurement was determined by weight loss measurement. Prior to immersion, coupons were scrubbed with a mix-ture of trisodium phosphate-pumice, rinsed with water, rinsed with isopropyl alcohol and then air dried. ~eight measurement to the nearest milligram was made. At the end of one day, a weighed coupon was removed and cleaned. Cleaning consisted of immersion into a 50 solution of HCl for approximately 20 seconds, rinsing with tap water, scrubbing with a mixture of trisodium-pumice until clean, then rinsing with tap water and isopropyl alcohol. When dry, a second weight measurement to the nearest milligram was made. At the terminatlon of the tests, the remaining coupon was removed, cleaned and weighed.

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Corrosion rates were computed by differential weight loss according to the following equation:

Nth Day Weight Loss - 1st Day Weight Loss Corrosion Rate = N-1 where N = 3 or 4.

The cooling water was prepared by first preparing the following stock solutions:

Solution A - 212.4 g CaCl2 2H20/l Solution B - 229.9 9 MgS04~7H20/l Solution C - 25.5 9 NaSiO3~9H20/l Solution D - 85 9 Na2C03/l Treatment Solutions - 1.7% solutions (1.7 9/100 ml) Then, these solutions were combined using the following order of addition:

1. To 17 l of de-ionized water add~ with stirring, (a) 20 ml of Solution A, (b) 20 ml of Solution B and (c) 20 ml of Solution C.

2. Adjust pH to 6.

3. With stirring add treatment (except Zn+2).

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4. Add o-P04 Solution (if used).

5. Adjust pH to 7.0 if necessary.

6. Add zn+2 Solution (if used).

7. (a~ For SCW7 adjust pH to 7Ø

5(b) For SCWg add 20 ml of Solution D and adiust pH to 8Ø

The results of these tests are reported below in Table 14 in terms of corrosion rates in mils per year (mpy).

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~8~t~9 While the comparative test results were not so pronounced at pH = 7, the comparative results at pH = 8 were considered to be rather dramatic. Even though the AA/HPA polymer alone demonstrated little, if any, efficacy as a corrosion inhibitor, when combined with the zinc-containing treatments, the combined treatments demonstrated significantly enhanced results as corrosion inhibitors.
For example, at pH = 8, the corrosion inhibition efficacy of 30 ppm active polymer alone (86 mpy) and 10 ppm zn+2 alone (84 mpy) appeared to be non-existent as compared to the untreated system (82 mpy); however, when only 5 ppm polymer were combined with only 5 ppm Zn+2, the corrosion rate decreased to 13.6 mpy.

Having thus described the invention what is claimed is:

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Claims (41)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method for reducing the amount of corrosion of metal surfaces in contact with an aqueous medium prone to zinc precipitation comprising adding to said aqueous medium an effective amount for the purpose of effective:
(i) water-soluble zinc compound, and (ii) water-soluble polymer comprising moieties derived from acrylic acid or water-soluble salt thereof and moieties of hydroxylated lower alkyl acryiate, wherein the moieties of said polymer have the following formulas:

wherein R is hydrogen or a lower alkyl of from l to 3 carbon atoms; R1 is OH, NH2 or OM where M is a water-soluble cation; R2 is a lower alkyl of from about 2 to 6 carbon atoms, R3 is H or lower alkyl of from 1 to 3 carbon atoms and the mole ratio of x:y is 1:4 to 36:1, wherein cold water-soluble polymer retains a corrosion inhibiting amount of said zinc compound in soluble form in said aqueous medium and prevents uncontrolled precipitation of zinc salt therefrom.

2. A method according to claim 1, wherein said zinc compound is added in an amount sufficient to provide from about 0.5 to about 25 parts of zinc ion per million parts of aqueous medium, and wherein said polymer is added in an amount of from about 0.5 to about 200 parts of polymer per million parts of aqueous medium.

3. A method according to claim 2, wherein said zinc compound is added in an amount sufficient to provide from about 2 to about 10 parts of zinc ion per million parts of aqueous medium, and wherein said polymer is added in an amount of from about 2 to about 50 parts of polymer per million parts of aqueous medium.

4. A method according to claim 1, wherein said mole ratio of x:y is 1:1 to 11:1.

5. A method according to claim l, wherein said polymer has a molecular weight of from about l,000 to about 50,000.

6. A method according to claim 5, wherein said polymer has a molecular weight of from about 2,000 to about 6,000.

7. A method according to claim 1, wherein said mole ratio of x:y is 1:1 to 5:1.

8. A method according to claim 7, wherein said polymer has a molecular weight of from about 2,000 to about 6,000.

9. A method according to claim 8, wherein said zinc compound is added in an amount sufficient to provide from about 0.5 to about 10 parts of zinc ion per million parts of aqueous medium, and wherein said polymer is added in an amount of from about 0.5 to about 50 parts per million.

10. A method according to claim 9, wherein said aqueous medium is cooling water.

11. A method according to claim 10 wherein said polymer is a copolymer of acrylic acid or water-soluble salt thereof and 2-hydroxypropyl acrylate,

12. A method according to claim l, wherein said aqueous medium has a pH of from about 6.5 to about 9.5.

13. A method according to claim 10, wherein said aqueous medium has a pH of from about 6.5 to about 9.5.

14. A method of inhibiting the formation of zinc scale in an aqueous medium containing zinc ions under scale forming conditions, which method comprises adding to said aqueous medium an effective amount for the purpose of effective water-soluble polymer comprising moieties derived from acrylic acid or water-soluble salt thereof and moieties of an hydroxylated lower alkyl acrylate, wherein the moieties of the polymer have the following formulas:

wherein R is hydrogen or a lower alkyl of from 1 to 3 carbon atoms; R1 is OH, NH2 or OM where M is a water-soluble cation; R2 is a lower alkyl of from about 2 to 6 carbon atoms, R3 is H or lower alkyl of from 1 to 3 carbon atoms and the mole ratio of x:y is 1:4 to 36:1, wherein said water-soluble polymer retains a zinc scale inhibiting amount of said zinc ions in soluble form in said aqueous medium and prevents uncontrolled precipitation of zinc salt therefrom.

15. A method according to claim 14, wherein said polymer has a molecular weight of from about 1,000 to 50,000 and is added in an amount of from about 0.5 to about 200 parts per million parts of aqueous medium.

16. A method according to claim 15, wherein said aqueous medium is cooling water.

17, A method according to claim 16, wherein said aqueous medium contains phosphate ions which have been added as a treatment.

18. A method according to claim 15, wherein said mole ratio is 1:1 to 11:1.

19. A method according to claim 18, wherein said mole ratio is 1:1 to 5:1.

20. A method according to claim 14, wherein said zinc scale comprises at least one member selected from the group consisting of zinc hydroxide and zinc phosphate.

21. A method for reducing the amount of corrosion of metal surfaces in contact with an aqueous medium prone to zinc precipitation comprising adding to said aqueous medium an effective amount for the purpose of effective:
(i) water-soluble zinc compound, (ii) water-soluble orthophosphate or precursor thereof, and (iii) water-soluble polymer comprising moieties derived from acrylic acid or water-soluble salt thereof and moieties of hydroxylated lower alkyl acrylate, wherein the moieties of the polymer have the following formulas:

wherein R 16 hydrogen or a lower alkyl of from 1 to 3 carbon atoms; R1 is OH, NH2 or OM where M is a water-soluble cation; R2 is a lower alkyl of from about 2 to 6 carbon atoms; R3 is H or lower alkyl of from 1 to 3 carbon atoms and the mole ratio of x:y is 1:4 to 36:1, wherein cold water-soluble polymer retains a corrosion inhibiting amount of said zinc compound in soluble form in said aqueous medium and prevents uncontrolled precipitation of zinc salt therefrom.

22. A method according to claim 21, wherein said zinc compound is added in an amount sufficient to provide from about 0.5 to about 25 parts of zinc ion per million parts of aqueous medium, wherein said polymer is added in an amount of from about 0.5 to about 200 parts of polymer per million parts of aqueous medium, and wherein said orthophosphate or precursor thereof is added in an amount of from about l to about 200 parts per million.

23. A method according to claim 22, wherein said zinc compound is added in an amount sufficient to provide from about 2 to about 10 parts of zinc ion per million parts of aqueous medium, wherein said polymer is added in an amount of from about 2 to about 50 parts of polymer per million parts of aqueous medium, and wherein said orthophosphate or precursor thereof is added in an amount of from about 2 to about 50 parts per million.

24. A method according to claim 21, wherein said mole ratio of x:y is 1:1 to 11:1.

25. A method according to claim 21, wherein said polymer has a molecular weight of from about 1,000 to about 50,000.

26. A method according to claim 25, wherein said polymer has a molecular weight of from about 2,000 to about 6,000.

27. A method according to claim 21, wherein said mole ratio of x:y is 1:1 to 5:1.

28. A method according to claim 27, wherein said polymer has a molecular weight of from about 2,000 to about 6,000.

29, A method according to claim 28, wherein said zinc compound is added in an amount sufficient to provide from about 2 to about 10 parts of zinc ion per million parts of aqueous medium, wherein said polymer is added in an amount of from about 2 to about 50 parts per million, and wherein said orthophosphate or precursor thereof is added in an amount of from about 2 to about 50 parts per million.

30. A method according to claim 29, wherein said aqueous medium is cooling water.

31. A method according to claim 30, wherein said polymer is a copolymer of acrylic acid or water-soluble salt thereof and 2-hydroxypropyl acrylate.

32. A method according to claim 21, wherein said aqueous medium has a pH of from about 6.5 to about 9.5.

33. A method according to claim 31, wherein said aqueous medium has a pH of from about 6.5 to about 9.5.

34. A method for reducing the amount of corrosion of metal surfaces in contact with cooling water prone to zinc precipitation comprising adding to said cooling water an effective amount for the purpose of effective:
(i) water-soluble zinc compound, and (ii) water-soluble polymer comprising moieties derived from acrylic acid or water-soluble salt thereof and moieties of hydroxylated lower alkyl acrylate, wherein the moieties of said polymer have the following formulas:

wherein R is hydrogen or a lower alkyl of from 1 to 3 carbon atoms; R1 is OH, NH2 or OM where M is water-soluble cation; R2 is a lower alkyl of from about 2 to 6 carbon atoms; R3 is H or lower alkyl of from 1 to 3 carbon atoms and the mole ratio of x:y is 1:4 to 36:1, wherein said water-soluble polymer retains a corrosion inhibiting amount of said zinc compound in soluble form in said cooling water and prevents uncontrolled precipitation of zinc salt therefrom.

35. A method according to claim 34, wherein said zinc compound is added in an amount sufficient to provide from about 0.5 to about 25 parts of zinc ion per million parts of cooling water, and wherein said polymer is added in an amount of from about 0.5 to about 200 parts of polymer per million parts of cooling water.

36. A method according to claim 35, wherein said mole ratio of x:y is 1:1 to 11:1.

37. A method according to claim 36, wherein said polymer has a molecular weight of from about 1,000 to about 50,000.

38. A method according to claim 37, wherein said mole ratio of x:y is 1:1 to 5:1.

39. A method according to claim 38, wherein said polymer has a molecular weight of about 2,000 to about 6,000.

40. A method according to claim 39, wherein said polymer is a copolymer of acrylic acid or water-soluble salt thereof and 2-hydroxypropyl acrylate.

41. A method according to claim 40, wherein said cooling water has a pH of from about 6.5 to about 9.5.