US3399160A - Organic coatings containing sacrificial anodes - Google Patents

Description

Aug. 27, 1968 w. P 3,399,160

ORGANIC COATINGS CONTAINING SACRIFICIAL ANODES Filed Oct. 10, 1967 F01. Y4L KAPYL PUL YE THEE BINDER I NVEN TOR. WO0DROW f. KEMP United States Patent 3,399,160 ORGANIC COATINGS CONTAINING SACRIFICIAL ANODES Woodrow E. Kemp, Pittsburgh, Pa., assignor to Koppel-s Company, Inc., a corporation of Delaware 7 "Continuation-impart of application Ser. No. 557,770, June 15, 1966. This application Oct. 10, 1967, Ser. No. 676,995 2 Claims. (Cl. 26031.2)

ABSTRACT OF THE DISCLOSURE- A corrosion-resistant coating systemfor ferrous metal surfaces comprising a sacrificial anode pigment and an organic binder having a low permeability by moisture and gases. The sacrificial anode pigment is most preferably zinc dust, but may also be magnesium or a binary alloy of the two metals. The organic binder is a polyhydroxyl polyalkaryl polyether. The coating'sys'tem is packaged in one container in a solvent used as a' vehicle to apply the coating. The absence of coreactive ingredients in the coating system now only allows the system to be unitarily packaged, but provides long shelf life as well.

Cross-reference to related applications This application is a continuation-in-part of application Ser. No. 557,770, filed June 15, 1966, as a continuation-in-part of application Ser. No. 218,859, filed Aug. 23, 1962, both now abandoned.

Background of the invention This invention relates generally to corrosion-resistant coating systems for ferrous metal surfaces, and more particularly, to a corrosion-resistant coating system utilizing sacrificial anode pigment. There are two general classes of corrosion reactions: those in which a direct combination takes place between the metal and the nonmetallic element and those in which the metal dissolves and replaces hydrogen in water in acids or another metal in solution. When metals corrode in the atmosphere, usually both processes are involved. Both types of corrosion are electrolytic in nature and depend on the operation of electrolytic cells on the metal surface.

In the reaction where the corroding metal replaces hydrogenor another metal in solution, two reactions take place simultaneously. The metal ions pass into solution and hydrogen ions pass out of solution to either form hydrogen gas, combine with atmospheric oxygen to form water, or combine with carbon dioxide to form carbonic acid. There are, therefore, two kinds of areas, one where the metal dissolves called the anode, and one where hydrogen ions are discharged or consumed by oxygen or carbon dioxide called the cathode. All metal surfaces contain a multiplicity of such areas due to the presence of impurities, oxides, physical stress of differences in crystal structure. The quantity of current which passes between such galvanic cells is proportional to the amount of metal which dissolves.

The rate at which corrosion of a metal proceeds depends in large part on secondary processes of diffusion wherein metal ions are removed from proximity to the anode and the rate-at which hydrogen is removed from the cathode. When hydrogen becomes concentrated at the cathode, i.e., is not removed as rapidly as it is formed,

what is known as cathodic polarization occurs and the corrosion rate may drop to virtually zero. A-nodic metal coatings such as zinc, magnesium, and magnesiumrzinc alloys on iron and steel protect the base metal by changing exposed areas of it from anodes to cathodes and in accomplishing this result, dissolution of the anode occurs.

Perhaps the best known and oldest cathodic protection system is the galvanized process wherein steel. is hotdipped into a molten zinc bath to provide a layer of zinc over the steel.

Obviously such a process, aside from its expense, has shortcomings when large objects are to be protected or surfaces are to be coated which cannot be placed into a bath such as, for example, a girder forming part of a bridge.

For these reasons, cathodic protection coating systems have been developed comprising finely ground anodic particles dispersed in binders, both organic and inorganic.

The inorganic compounds hitherto used have involved several components which were kept separate and were mixed immediately prior to use. For example, the zinc dust has been in one container, and the inorganic binder has been in another container. The contents of the containers were mixed immediately prior to the applying of the composition as a coating to the metal. Sometimes it has been necessary to then flush the coating with a still further component, such as an acid, to cure the coating. A further disadvantage of the coatings as heretofore known is that they have been sensitive to moisture until they were completely cured. Thus, were the coating to become wet before it was completely cured as, for example, by rain (and curing may take many hours), the entire coating must be removed; and the surface sand-blasted again and properly cleaned.

Organic binders, on the other hand, have the advantage of a rather quick cure and may be diluted with highly volatile solvents to hasten the drying. They act as physical barriers which exclude more or less effectively the corrosive elements of the environment from the metal surface. When an anodic metal (to iron or steel) such as zinc dust is dispersed in an organic vehicle in a sufficiently high concentration that metal to metal contact is achieved, the coating is called a zinc rich coating and if it is painted on iron or steel it will provide cathodic protection to the steel through self sacrifice in the same manner as if the iron or steel had been plated with the pure metal, i.e., galvanizing.

However, many organic coatings have been tried with very little success. V. L. Fl-ack in an article at pp. 36-44 in Materials Protection of March 1963, discusses seven organic coatings containing zinc as well as seven inorganic based zinc coatings and galvanized iron. His organic materials included linseed oil, chlorinated rubber, epoxy-ester, epoxy-amine, epoxy-polyamide and urethane. In his summary on page 44, he states: Inorganics performed best in salt water tests because they provided zinc without the coating blistering or the binder being attacked by the high pH generated at the coating surface.

In addition to the above comment applicable to all of the organic materials tested, the individual binders were all objected to based on one or more of the following: poor organic solvent resistance, lack of high temperature stability, brittleness, lack of adhesion, sensitivity to moisture and inability to accept high percentage of anode loadings.

These objections are not new. Rather they are notoriously well know to those skilled in the art. For this reason, no cathodic protective coating system using organic binders have found any significant use in industry prior to the present invention.

Summary of the invention Surprisingly, however, I have discovered a novel sacrificial anodic organic coating composition which overcomes all of the previous objections to the prior art compositions. The coating composition (as a composition ready for application) is stored in a single container unlike the inorganic systems and many of the prior art organic systems. The coating composition has indefinite storage life in a sealed container. The coating, when applied as a film to a metalsurface, however, dries to the touch within fifteen minutes and is insensitive to water within onehalf hour. Most important of all, the coating once applied to a ferrous metal substrate does not blister, is not affected by water and ordinary organic solvents, and provides protection against formation of rust even under salt conditions for an extended periodof time. I

In accordance with my invention, I provide a coating composition for the cathodic protection of a ferrous metal surface consisting essentially of a smooth, homogeneous mixture of 98 to 87% of a sacrificial anode selected from the class consisting of zinc dust, a binary zincmagnesium alloy and magnesium powder and 2 to 13% of a noncrosslinked polyhydroxyl polyalkaryl polyether binder of the formula:

4 Furthermore, the permeation of these binders not only results in premature exhaustion of the zinc and physical degradation of the binder, but the permeation negates the physical barrier eifect of the organic coating previously alluded to. The exhaustion of the anode material and degradation of the binder finally allow the gases to diffuse through the coating and corrode the base metal itself.

Brief description of the drawings FIG. 1 is an enlarged cross-sectional cutaway of the protective coating of this invention.

FIG. 2 is an enlarged cross-sectional cutaway illustrat ing a prior art coating. FIG. 3 illustrates the effect of anodic corrosion upon the binder. I Y

'Detailea' description As stated above, the invention comprises asacrificial anode and a polyhydroxyl polyalkaryl polyether binder.

The sacrificial anode component is in finely divided form. Zinc dust is commercially available as a metallic zinc pigment in powder form. In accordance with this invention, the fine grade of zinc dust is used, that is, zinc dust which has an average particle size of 2-3 microns. The binary-magnesium zinc alloy usable in this invention is described in great detail in United States Patent No. 2,877,126. Such binary alloy consists of magnesium generally in an amount of between and 15 weight percent and zinc in an amount between 70 and 85 weight percent. The alloy should be in a particulate form (by crushing, milling, grinding, etc.) to a powder where n: 10-30 Where Ar and Ar'=@ Where X =H or an alkyl group having from 1-9 carbon atoms, diluted with sufficient solvent to provide a mixture of the consistency that is desired for the mode of application to said surface.

The amazing results of my invention are due to my discovery that the prior art zinc rich organic coatings were short-lived because the organic binder was permeable by moisture, oxygen and carbon dioxide. Such organic anode coatings had a limited life because the zinc or magnesium surface area was so large and the difiusion rate of water, oxygen and carbon dioxide through the organic binder was so great that the zinc was consumed through chemical and electrolytic action within a relatively short period and was not available to protect the base metal, iron or steel.

Ideally, the zinc or magnesium metal powder should remain substantially encapsulated by the binder and unattacked by oxygen or carbon dioxide until an area of the iron or steel substrate is exposed by a scratch or mar when it can perform its protective function. The diffusion rate of water, oxygen or carbon dioxide through the organic binder in prior art resin systems has been too high to maintain the zinc metal in its virgin state and further the corrosion product formed from the zinc (Zn(OH) ZnCO is sufficiently voluminous that the coherence of the binder has also been mechanically destroyed, resulting in further loss of zinc metal.

whose size is less than 150 microns and preferably less than 50 microns. The magnesium powder may be the commercial grade magnesium and that is ground, milled, or otherwise pulverized to particulate particles having a size less than 150 microns and preferably less than 50 microns. It has been found that with the invention and using magnesium powder, a voltage of more than of the voltage obtained from magnesium anode with iron is obtained.

The organic binder component is a polyhydroxyl, polyalkaryl polyether. The polyether is commercially available under the trademark Phenoxy PKI-IH from the Union Carbide and Carbon Company. Such polyethers are formed by the reaction of a monohydric aromatic alcohol with an epoxide of high molecular weight. The addition of the phenol to the epoxide may be carried out in a conventional manner, for example, in the manner discussed by Shecter and Winstra in Industrial and Engineering Chemistry, volume 48, No. 1, January 1956, pages 89-91, by heating the epoxide with phenol, preferably with a base catalyst. It should be noted that the phenol reacts only with the epoxy end groups of the epoxide resin and that each phenol only reacts with one epoxy group. Thus, the polyether is not a highly crosslinked network as are the epoxy resins. Thus, the polyether may be diluted with a volatile solvent to desired viscosity and then retained in a single container indefinitely without set up and so long as the container is sealed preventing the volatiles from evaporating.

The monohydric aromatic alcohol may be an alkyl phenol, such as orthoor para-cresol and nonyl phenol.

The epoxide is preferably a diepoxide, such as that formed by the condensation of epichlorohydrin and-a bis-phenol. Such diepoxides are commercially available under the trademark Epon, and may be illustrated by the following formula:

where n: l0-30.

The polyhydroxyl polyalkaryl polyether is formed by reacting the above diepoxide with a phenol of the class described as illustrated by the following formula:

anode be too great, not enough binder will be present to hold the sacrificial anode to the ferrous metal surface. Similarly, too much solvent will thin the composition to Where Ar and Ar=@ Where X :11, or an alkyl group having from l-9 carbon atoms.

Examination of the above formula for the polyether usable in this invention readily discloses that the epoxy groups have been fully reacted with the phenol before the anodic material or any solvent ingredients are even added. Thus, there is no further curing or cross-linking which will occur and the compound is therefore storable in a single container with indefinite pot life.

Advantageously, there is added to the hot reaction mass resulting from the addition of the phenol to the epoxide, prior to removal of the addition product from the reactor, solvents such as methyl isobutyl ketone, Cellosolve acetate, ethyl acetate, and cyclohexanone to lower the viscosity. Sufiicient solvent is added at this time to reduce the solids content to between 40 and 50% of the solution. If the solvent is not added at this time, the reaction mass sets, upon cooling, to a glassy solid mass and is difficult to remove from the reaction vessel or to handle.

It has been found that coating films made in accordance with this invention are substantially nonresponsive to attack by common solvents.

Attack on the film can only be accomplished by a combination of the solvents made from ethers, acetates, and ketones. (This combination may be diluted thereafter with extenders such as ethylene glycol monoethyl ether acetate, methyl isobutyl carbinol, methyl ethyl ketone and toluene.) Such a mixture of solvent appears too infrequently, however, that the coatings can be deemed to be substantially impervious to solvents.

The range of material suitable for manufacturing the novel organic cathodic protective coating for ferrous metals in accordance with this invention is as follows (based on 100 parts of the sacrificial anode metal):

Solvents -90 the point that it will not develop adequate thickness on the ferrous metal surface. Too little solvent will cause great difliculty in the application of the composition to the metal. The solvent, of course, evaporates as the coating sets thereby providing a high ratio of sacrificial anode to binder in the final film remains on the ferrous metal surface.

It has been found that if the sacrificial anode tends to settle upon long-time standing of the composition, such settlement can be retarded or eliminated by the addition of a bodying agent, a typical example of which is a silica aerosol sold under the trademark Cab-O-Sil. Such bodying agents may be used to the extent of from 1 to 3 by weight of the final composition.

The coating film may be applied to the metal surface by any of the known methods, such as, brushing, painting, doctor blade and the like. Advantageously, however, the coating is applied by spraying. The coating film is preferably a one-coat film that is between 2 and 3 mils in thickness. It is desirable not to make the coating thicker than 5 mils. The coating may be overcoated by a conventional paint or coating. This feature is unusual because the coatings heretofore known required an alkali-resistant overcoat or were not amendable to overcoating.

In FIG. 1, the coating of my invention is illustrated. The coating provides a protective layer over the ferrous metal surface substantially impermeable by oxygen, CO or water vapor. Therefore, the only corrosion occurring is that of the exposed zinc particles on the surface of the coating. The gases and water vapor do not readily diffuse into the binder to attack and use up the anodic material or attack the base metal.

-In contrast, the prior art coating in FIG. 2 is allowing the oxygen, carbon dioxide, and water vapor to pass through the binder to attack and dissipate the anodic material at (a) the so formed corrosion products (as may also be seen in more detail in FIG. 3) are destroying the physical integrity of the binder at (b) causing the binder to physically part or peel from the ferrous metal substrate; and as a final result of the first two defects the gases at (c) are passing directly to the ferrous metal substrate surface and corroding the substrate.

The following examples by way of explanation and not of limitation will illustrate further the benefits, advantages, and novel features attained by this invention.

7 EXAMPLE I To an autoclave equipped with a reflux condenser, heater and agitator is added 100 parts of a diepoxide having the formula:

where 11:10-30.

34 parts of phenol, and 0.3 part of alpha methyl benzyldimethylamine (as a catalyst). The mixture is heated with agitation to a temperature of 105 to 115 C. and there maintained under agitation for a period of about four hours. The reaction mass is diluted with 200 parts of methyl isobutyl ketone and cooled to room temperature. The resulting solution contains about 40% by weight of the polyhydroxyl polyalkaryl polyether. Thereafter, 12 parts of the solution of polyether resin is added to a vessel equipped with an agitator along with 24 parts by weight of a solvent mixture comprised of 19 parts by weight of ethylene glycol monoethyl ether acetate, and parts by weight of toluene. The mixture is agitated until a clear solution results. Thereafter, 64 parts of zinc dust having an average particle size of 2-3 microns is added and the mixing continued until a smooth product is obtained. The viscosity is adjusted with additional solvent to provide the proper fluidity for spray application. The product formulated as above, when applied to a carbon steel surface and allowed to dry, is resistant to gasoline, jet fuel, aromatic solvents such as toluene or xylene, and ketones such as acetone. In contact with water or salt solutions, the coating generates acathodic current having a magnitude of approximately 0.5 volt or a minimum of 80% of that generated by a pure zinc sheet similarly coupled.

The coating was applied to the surface of sheet mild steel to give a resulting film of 2-3 mils in thickness. The amount of zinc present is then about one ounce per square foot. To compare the result of this coating with that obtained from a commercial galvanized coating, a sheet of galvanized steel was obtained having galvanized thereon zinc to the extent of about one ounce per square foot. Both surfaces were scratched to expose the steel, the width of the scratch being /32 f an inch. The scratched sheets were then exposed to a Standard ASTM Salt-Fog-Cabinet for 1,000 hours. The galvanized specimen exhibited gross rusting in the scratch mark, whereas the composition of this invention showed no rusting. In fact, the corrosion products resulting from the coating of this invention tended to heal the scratch mark by the deposition of a grayishwhite zinc oxide matrix thus preventing further electrical drain on the sacrificial anode metal. The sacrificial anode coating is therefore eminently suitable for the protection of iron and steel surfaces exposed to marine conditions, such as fuel tanks (tanker service), piling, and similar structures.

EXAMPLE II In a vessel equipped with an agitator is placed 12 parts by weight of solution of polyether resin made in accordance with the procedure of Example I and 30 parts by weight of a solvent mixture comprising 8 parts by weight of methylethyl ketone, 19 parts by weight of Cellosolve acetate, 4 parts by weight of toluene and 2 parts by weight of silica aerosol (Cab-O-Sil). The solution is stirred until a homogeneous mixture results. Thereafter, parts of zinc dust having an average particle size of 2-3 microns is added, and the mixing continued until a smooth product is obtained. The resulting composition provides an excellent cathodic protective coating for ferrous metal surfaces. The composition shows little tendency for settling of the zinc dust from the liquid.

EXAMPLE III The procedure of Example I is repeated except that 64 parts of magnesium powder having a particle size less than 150 microns is substituted for the zinc dust. The composition is applied to mild steel to provide a film having a thickness of 2 mils. The coating is found to generate l.0 volt or a minimum of 80% of the current generated by a pure magnesium anode coupled with a specimen of the mild steel. This composition hasthe advantage that the voltage generated is higher than the voltage generated by a coating of zinc dust and thus will throw the current farther and thereby protect a larger damaged or uncoated area of ferrous metal than will the compositions of Example I.

EXAMPLE IV The procedure of Example I is repeated except that a zinc-magnesium alloy zinc, 30% magnesium) having a particle size less than 150 microns is substituted for the zinc dust. This coating, when applied to a mild steel surface as a coating three .mils in thickness, generates a cathodic current higher than zinc dust of Example I but less than the magnesium powder of Example III. The alloy generates a voltage of approximately 0.7 volt. This potential is more than of the potential generated by the alloy when coupled with steel.

EXAMPLE V The procedure of Example I was repeated except that a mixture of 32 parts of zinc dust and 32 parts of magnesium alloy was used. The resulting coating, when applied to a mild steel surface, generated a voltage of approximately 0.6 volt.

EXAMPLE VI Resin H20 02 CO2 Polyhydroxyl polyalkaryl polyother 3.0 6 20 Polystyrene 167, 000 250 1, 252 Epoxy (amine cured).-. 400 133 Polyvinyl chloride. 66, 800 67 134 Chlorinated rubber 160, 000 50 148 These results clearly show the marked superiority of the binder used in the coating of this invention.

EXAMPLE VII To further compare the coating of this invention with the prior art a coating was made in accordance with Example I and applied to a steel plate as a coating of approxibinders and applied to steel plates as coatings of the same thickness. Scores of & width were then made on all mately 23 mils thickness. Other coatings were prepared using polystyrene, chlorinated rubber, alkyd, and epoxy plates and the plates exposed to the atmosphere at a test location near Wilmington, NC The plates were 80 feet from the ocean, and faced south. The plates were inclined at an angle of 45 to the horizontal.

The plates were constantly checked to determine the amount of time for rusting to occur over 25% of the plate area. The results are as follows.

Time for rust, over Binder: 25% of area, days Polystyrene 200 Chlorinated rubber 250 Alkyd 180 Epoxy 370 Polyhydroxyl polyalkaryl polyether over 1,825

EXAMPLE VIII Binder: 50% of area, days Silicate 500 Polystyrene 270 Chlorinated rubber 210 Epoxy 500 Polyhydroxyl polyalkaryl polyether over 1,500

protection to the steel plates while the polyhydroxy polyalkaryl polyether coating provided more than satisfactory protection over periods of time from 3 to 5 times as long as the next best material.

The foregoing has presented a novel organic coating rich in sacrificial anode. When applied to a ferrous metal substrates as a smooth, homogeneous coating the relatively impermeable coating protects the substrate from corrosion directly, and prevents dissipation of the anodic material by premature corrosion. The coating is resistant to weathering and to common solvents. A coating of 2 to 3 mils will generate a cathodic current equal to at least 80% of that generated by a pure metallic anode and will protect ferrous metal surface under adverse conditions for a period of many years before the sacrificial anode is consumed.

I claim:

1. A protective film for the cathodic protection of a ferrous metal surface, said film being characterized by a permeability to oxygen, carbon dioxide, or water vapor of less than 50 cubic centimeters per 1 mil thickness per 100 square inches area over a 24-hour period at 1 atmosphere of pressure, said film consisting essentially of a smooth, homogeneous mixture of:

(a) 98 to 87% of a sacrificial anode selected from the class consisting of zinc dust, a binary-zinc-magnesium alloy and magnesium powder; and

(b) 2 to 13% of a non cross-linked polyhydroxyl polyalkaryl polyether binder of the formula:

EXAMPLE 1X A test cell was constructed to illustrate the effect of oxygen, carbon dioxide and Water vapor upon the permeable prior art coatings and the relatively impermeable coating of this invention.

The test cell comprised a closed vessel having two gas inlet ports and a vent. About l-2 inches of water were placed in the bottom of the vessel and tubes leading from the inlet ports were extended down beneath the surface of the water to allow CO and oxygen to be respectively sparged into the vessel through the water. The ratio of the gases was about 20% O and 80% CO Test samples prepared in similar manner to Example VII were suspended within the test cell above the water. The samples had wide scribe lines scored through the coating. The samples were examined to determine the length of time to produce rust over 25 of the area of the plate. The results were as follows.

Time for rust, over Binder: 25 of area, days Polystyrene 21 Chlorinated rubber 27 Alkyd 14 Epoxy 85 Polyhydroxy polyalkaryl polyether 440 The above examples clearly show the difference between the coating of this invention and the prior art coatings. All of the prior art binders tested give short-lived wherein n is an integer having a value of 10-30;

and

X is hydrogen or an alkyl group from 1-9 carbon atoms;

said film being formed by diluting said mixture with from 29-90 parts solvent per parts of sacrificial anode to provide a mixture of a consistency desired for the mode of application to said surface.

2. The composition of claim 1 wherein said sacrificial anode is zinc dust.

References Cited ALLAN LIEBERMAN, Primary Examiner.

L. T. JACOBS, Assistant Examiner.

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