US3707450A - Coating for cathodically protected structures - Google Patents

Abstract

A CATHODICALLY PROTECTED METAL ARTICLE, SUCH AS A PIPELINE, TANK, OR VESSEL, IS COVERED WITH A FIRST COATING OF CHLORINATED RUBBER, AND A SECOND, OUTER COATING OF A BITUMINOUS MATERIAL WHEREIN EITHER OR BOTH COATINGS CONTAIN A DISPERSION OF NOT LESS THAN 5% BY WEIGHT OF CALCIUM CARBONATE. THE THUS COATED ARTICLE EXHIBITS IMPROVED CORROSION RESISTANCE.

Description

United States Patent O 3,707,450 COATING FOR CATHODICALLY PROTECTED STRUCTURES Herbert E. Townsend, Hellertown, Pa., assignor to Bethlehem Steel Corporation No Drawing. Filed Aug. 5, 1970, Ser. No. 61,475 Int. Cl. C23f 13/00 US. Cl. 204147 7 Claims ABSTRACT OF THE DISCLOSURE A cathodically protected metal article, such as a pipeline, tank, or vessel, is covered with a first coating of chlorinated rubber, and a second, outer coating of a bituminous material wherein either or both coatings contain a dispersion of not less than by weight of calcium carbonate. .The thus coated article exhibits improved corrosion resistance.

BACKGROUND OF THE INVENTION This invention relates to an improvement in corrosion resistant coatings for metal structures and particularly for cathodically protected structures.

Underground pipelines, such as those used for transcontinental transmission of gas, are protected throughout much of their length by cathodic protection. This protection may be effected by means of an impressed current or by use of sacrificial anodes.

Gas transmission lines are generally subjected to high stress (up to 72% of the nominal yield strength) owing to high pressure. This fact, coupled with corrosive, low resistivity (less than 10,000 ohm-centimeters) soils encountered in the course of long pipelines, places heavy demands on any cathodic protection system.

Cathodic protection is effective for pipelines without the use of external coating. However, the general practice is to coat the pipe with a relatively heavy coating of a bituminous material, usually with a thin primer coating adjacent to the metal substrate, in order to minimize the amount of current required for a given amount of protection.

While underground structures, such as pipelines and storage tanks, are usually freeof corrosion when a bituminous coating applied thereto remains intact, disbonding of the coating, i.e. the tendency of the coating to become separated from the metal substrate, is a constant problem, particularly in an area immediately adjacent to a coating defect. Disbonding increases the area of exposed metal surface. This means that, for a fixed amount of impressed cathodic current, the current density (current per unit area) decreases. Hence, the degree of cathodic protection, which is determined by the polarization of the metal-soil interface measured against a standard reference electrode, will also decrease. Alternatively, if the current is adjusted in order to maintain the same current density and degree of cathodic protection, greater amounts of current are required as the coating proceeds to disbond.

In some cases, organic compounds such as imidazolines, ethanolamines, sulfonic acids, etc., have been added to the coating in an attempt to retard disbonding.

Another approach to increasing the effectiveness of cathodic protection for underground structures has been to add calcium carbonate to the soil surrounding the structure in order to promote the deposition of a protective, calcareous deposit.

It .is an object of this invention to decrease the amount of disbonding between an underground cathodically protected metal structure and its protective coating.

It is another object to minimize the effects of any dis- 3,707,450 Patented Dec. 26, 1972 bonding which may occur in the coating of such structures.

It is a further object to reduce the amount of current required in a cathodic protection system for coated underground metal structures.

SUMMARY OF THE INVENTION I have found that underground metal pipelines and underground metal storage vessels, which are cathodically protected, can be coated with a material containing a calcareous inhibitor, whereby disbonding can be decreased substantially or eliminated, and the current necessary to effect cathodic protection to the metal over long periods of time can be reduced effectively as well.

Briefly, this invention comprises coating the surface of, for example, a metal pipeline with a relatively thin primer coating of chlorinated elastomer, such as rubber or synthetic rubber. The primer is coated with a bituminous material of the type preferably represented by coal tar. The outer, bituminous coating is impregnated with at least 5% calcium carbonate, while the primer, preferably, also contains about 5% or more of calcium carbonate.

In cathodically protected pipe bearing the coating of this invention, not only is the amount of disbonding reduced, but also, when disbonding does occur, corrosion protection can be maintained with a substantial reduction in protective current, as compared to prior art protective measures.

DETAILED DESCRIPTION Is one example by which my invention can be performed, lengths of steel pipe, of the type used in transmission of natural gas, are cleaned by shotblasting and then coated with a chlorinated rubber primer containing about 25% by weight of limestone (less than IOU-mesh, U.S. Std. Sieve Series). The primer coating is prepared by mixing calcium carbonate, in the form of limestone, to a commercial grade of chlorinated rubber primer solution, thus forming a suspension. Commercial chlorinated rubber primers, intended as prime-coats for coal tar enamel, are available in liquid form with the chlorinated rubber dissolved in an appropriate solvent such as xylene. The suspension, which contains from 50% to 55% solvent by weight, is applied to the outer surface of the pipe at a rate of one gallon of suspension for every 650 to 850 sq. ft. of pipe surface, resulting in a primer coating thickness, after evaporation of the solvent, of about 0.001 inch. Once the primer is dry and firmly set on the pipe surface, the pipe is ready for application of the bitumunous outer coating.

In preparing the outer coating, coal tar enamel-a mixture of topped coal tar, inert filler and, optionally, a plasticizeris heated to a freely flowing condition, and ground limestone (less than -mesh is incorporated with the hot tar in an amount equal to about 25% by weight. After the limestone has been thoroughly mixed with the molten coating the mixture is ready for application as the outer coating to the pipe. The hot tar enamellimestone mixture, at a temperature of about 450 F., is poured from a tank, downwardly onto a horizontal section of pipe which is rotating about its longitudinal axis, whereby the pipe is progressively completely covered with the mixture to a thickness of about 0.1 inch.

In general, control of the thickness of the coating is maintained by applying an overlap of felt, fiberglass or kraft paper, to squeeze the coating to the desired thickness. This overlap also gives protection to the coating during handling and installation of the pipe.

Coating materials of the type used for coating individual lengths of pipe are also used for coating any bare areas on the assembled pipeline, such as those areas existing where individual lengths of pipe have been joined by welding.

After the pipeline has been fabricated from coated sections and located in its pipeline trench, electrical connections are made for cathodic protection by impressed current in the manner well known in the art.

The anodes used in the impressed current protection system may be any conductive material such as copper, carbon, iron, etc. which will deteriorate slowly and provide long service. The source of impressed direct current may be, for example, a rectifier, generator or battery. In order to protect any pipe surface areas which may become exposed during use, a cathodic voltage of at least -0.85 volt with respect to a copper-copper sulfate reference electrode should be maintained at the metal surface.

A pipeline prepared in the manner of this invention will require less current to protect the pipe surface than is required of prior art cathodic protection methods. The reason for the reduction in impressed current is due to a decease in disbonding, and to the inhibitive action of the coating of the invention in promoting the formation of protective calcareous films on any exposed metal surfaces and in pores in the coating.

Reduction in current consumption by the practice of my invention results not only in lower operating cost than in prior cathodic protection circuits, but also reduces the amount of cathodically evolved hydrogen. Hydrogen can be a source of embrittlement if absorbed by pipe or tanks constructed of steels having a yield strength greater than 100,000 p.s.i.

This invention has particular application to gas transmission pipelines, as these lines are usually maintained under an internal pressure, and any corrosion which might Weaken the walls of the pipe to the point where rupture, and consequent explosion, could occur cannot be tolerated. Likewise, the invention is quite applicable to underground tanks, particularly those under pressure.

Various alternatives will present themselves by which satisfactory corrosion resistance can be obtained by application of this invention to underground structures.

For example, the coal tar enamel used for the outer coating may be plasticized, semi-plasticized or unplasticized, depending on the temperature conditions to which the structure is exposed. Plasticizing of the enamel is usually effected with ground coal. Bitumens other than coal tar are operative, for example, petroleum asphalt or natural asphalt can be used, although coal tar, in the form of coal tar enamel, has been found to be the most satisfactory.

Calcium carbonate in almost any form is satisfactory as the inhibitor in the coating, as long as the carbonate is of a degree of fineness adaptable for thorough and uniform mixing with the primer or outer coating. In this regard, ground limestone is quite suitable if of a fineness less than about lOO-mesh. The calcium carbonate may be added in the form of ground dolomite. Other alkaline earth metal carbonates, including barium and strontium, may be used alone or jointly. Any of these alternative inhibitor materials should be of a size less than 100-mesh in order to realize the greatest advantage from their use. The benefits of the invention may be obtained but to a lesser degree, with somewhat larger particle size inhibitor.

The maximum amount of calcium carbonate which can be used in primer or outer coating is that amount which permits the primer material or the bitumen to flow freely at the recommended application temperature. Thus, the upper limit of calcium carbonate which can be used effectively in this invention will vary, depending on the exact nature of the coating material.

Manufacturers recommended application temperature for unplasticized coal tar enamel ranges, generally, between 375 and 475 F., and for plasticized coal tar enamel, the application temperature is between 450 and 550 F.

In tests conducted on one commercialtype coal tar enamel, to establish permissible amounts of calcium carbonate, it was found that 42 weight percent of calcium carbonate could be added to the fully plasticized grade at 450 F., while 54 weight percent calcium carbonate could be added to the unplasticized grade at 428 F., without noticeably impeding the fluidity of the mixture.

Calcium carbonate can be added to the primer in an amount equal to or greater than those shown for the coal tar enamel, for in the case of the primer, solvent is added to produce the proper fluidity.

In the matter of a primer, materials other than chlorinated rubber have been tried with indifferent success.

A series of test panels for the comparative study of the behavior of inhibited and uninhibited coatings under simulated cathodic protection were prepared from 4-inch by 4-inch by fli-inch hot rolled carbon steel plates, sandblasted and cleaned in trichlorethylene vapor. Primer (chlorinated rubber) was applied by brush, after mixing in additions of inhibitor for those test specimens requiring it. All primed and unprimed test panels were coated by dipping them into a container of molten coating bitumen (plasticized coal tar enamel) for a length of time (usually about 3 seconds) sufficient to result in a coating thickness of 0.09:0.03 inch when removed and allowed to drain in air. Mixtures of coating bitumen and inhibitor were prepared by heating the bitumen to the application temperature of from 450 to 490 F. and adding the 'desired amount of inhibitor (25% by weight calcium carbonate for both primer and coating).

Preliminary viscosity tests were performed todetermine the maximum amounts of the various types of inhibitors which could be added to the coating at the recommended application temperature. Amounts less than the maximum quantities thus determined were employed in subsequent disbonding tests.

After coating the panels, the procedure for the disbonding tests comprised the following steps:

( l) A A-inch diameter hole (holiday) was drilled in the center of a panel through the coating to the bare steel to provide an intentional holiday.

(2) A 3-inch diameter by 4-inch long glass tube, open at both ends, was cemented to the panel with epoxy cement in a manner to form a container about the holiday.

(3) A magnesium anode (1 inch by 1 inch by 4 inches) was electrically connected through a l-ohm resistor to a steel panel, and suspended in an electrolyte, about 1 inch above the holiday. The anode was immersed in the electrolyte for a distance of about 2 inches to provide cathodic protection by means of the sacrificial magnesium anode. I p

(4') The electrolyte comprised 1% sodium chloride (NaCl), 1% sodium sulfate(Na- S0 and 1% sodium carbonate (Na CO in 300 ml.'of distilled water.

(5) At one week intervals, in a continuous -day test period, the liquid level was maintained by additions of distilled water, and the current measured by determining the voltage across a one-ohm resistor with an electrometer. I I,

(6) Following the 120-day exposure, the cell was dismantled, the bulk of the primer and outer coating was pried away from the holiday region with a spatula, and the disbonding measured. i

Linear disbonding measurements in centimeters were made on each test specimen from which the coating had been removed after test; These measurements represent total disbonding, and were made from the periph'ery of the holiday to the periphery of the opening made bythe disbonded coating. Primer-outer coating and primer-steel disbonding represent, respectively, the portion of the specimen where the outer coating has disbondedfrom the primer, and primer has disbonded from the basemetal, and the sum of the two types of disbonding is referred to as total disbonding in Table I below.

Table I lists results for two eries of tests, the first series having no inhibitor and the second series containing inhibitor in both the primer and outercoating. Three specimens were tested for each series. Current is shown as an average of the weekly determinations.

nated rubber primer containing 25% CaCOz powder.

1 No inhibitor. 3 Inhibitor.

The average reduction in current required for the inhibited specimens was 52%, while the average reduction results for a number of the different variables considered for this second set of tests. As in Table I tests, specimens were tested in triplicate and the current results averaged. While the differences in current and disbonding between inhibited and uninhibited specimens is not as great in Table II as in Table I, it will be appreciated that the 30- day tests are relatively short term as compared with those of four months.

Applicants invention is designed to give improved protection to pipelines and underground tanks for a matter of years, and it is over such long-range use that the greatest bene-fits from the invention are realized. The tests used to supply results for both tables are accelerated tests with.the results of Table I showing the increasingly greater benefits obtained with longer use of the inhibited coatings.

The purpose of the test results in Table II, as previously mentioned, is to provide a means of comparing variables, there being sufiicient showing of improvement in corrosion resistance shown by the inhibited specimens, even in the relatively short -day tests, to point up certain preferred practices in performing my invention.

TABLE II Average Total dis- Primer Coating total onding current, centi- Test series No. Type Inhibitor Type Inhibitor milliamps meters,

I specimen:

1 1, 610 0. 83 2 Chlorinated rubber- None Plasticized coal tar. None 1, 980 0. 60 3 1,790 0. 80

1, 540 0. 47 do 25% CaCOareagent ..do 25% CaCO; reagent.-. 1,260 0.47 3 1, 370 0. 50 III specimen:

1 1,360 0. 50 do 20% CeCOa reagent ..do 20% CaCO: reagent 1,500 0.45 1,290 0.45

1,340 0.52 .do 25% limestone passed 400 mesh ..do 25% limestone passed 400 mesh. 1, 470 0. 52 3 1,480 0.55 V specimen:

1 1,530 0. 62 do 25% dolomite passed 400 mesh do 25% dolomite passed 400 mesh.. 1,330 0. 1, 350 0.55

1, 560 0. 67 do None d0 25% 021005 reagent 1,010 0.67 1, 270 0. 70

1,100 0.42 do 40% limestone passed 400 mesl1 d0 35% limestone passed 400 mesh. 1, 190 0. 48 1,260 0. 48

A 1,980 0. 42 do 25% Ca'COs reagent do None 1,750 0.57 2, 020 0. 20

l Koppers 708 external tar enamel.

in total linear disbonding for the same specimens was 67%.

The results in the above table show outstanding differences, in both current required and amount of disbonding, between the specimens inhibited by the method of this invention, Test Series II, and uninhibited specimens, Test Series I.

Another set of tests was run for a period of 30 days. In these tests, specimens were prepared and tested in exactly the same manner as in Table I tests, except for the shorter test period. By virtue of this shorter testing period, the inventor was enabled to test hundreds of specimens in a reasonably short time.

From the table, it will be noted that when both primer and outer coating were inhibited with at least 20% of a calcium carbonate or dolomitic material, the total disbonding was measurably decreased over the resultant total disbonding when no inhibitor was used. Inhibitor in amounts as low as 5% produce beneficial results, a lthough to a lesser degree than for amounts of 20% inhibitor or greater. When no inhibitor was used in the primer, but was included in the outer coating as in Test Series No. VI, total disbonding was not appreciably reduced. However, the average total current required for protection in the case of Test Series VI was an improvement over that required for Test Series I in which no Table II, given below, is a representative listing of inhibitor was used in the primer or in the outer coating.

7 Similar improvement in total current required was noted for the Test Series from 11 to V and VII. In Series No. VIII, with inhibitor in the primer only, there was improvement in total disbonding, but no reduction in current required over Series I with no inhibitor.

In all specimens for which results are shown in Tables I and II, the primer thickness was within the range of from 0.0005 to 0.0015 inch, and the coating thickness was within the range of from 0.065 to 0.120 inch.

All percentages given above and in the appended claims represent weight percent.

I claim:

1. A process for protection of the outer surface of a metallic article exposed to a corrosive underground environment which comprises applying to said surface an elastomer primer coating and an outer coating of a bitumen wherein each of the coatings contains more than by weight of an alkaline earth metal carbonate, and subjecting the thus-coated article as a cathode to an electric current sufficient to retard corrosion of the surface of said metallic article.

2. A process according to claim 1 wherein the metallic article is a ferrous article, the elastomer is chlorinated rubber and the bitumen is coal tar enamel.

3. A process according to claim 2 wherein the ferrous article is a pipe or tank.

4. A process according to claim 2 wherein the primer coating and outer coating each contains not less than 20% by weight of calcium carbonate.

5. A cathodically protected metal pipeline having the outer surface of the pipeline protected by a coating comprising: I

(a) a first layer of chlorinated rubber bonded to the outer metal surface of's aid pipeline,

(-b) a second layer of bitumen bonded to said first layer wherein each of said layers contains a dispersion of more than 5% by weight of calcium carbonate, and

(c) an anode in electrical circuit with said pipeline as the cathode.

I 6. An article according to claim' 5 wherein said first and second layers each contain at least 20% by weight of calcium carbonate.

7. An article according to claim 6 wherein the first layer has a coating thickness between 0.0005 and 0.0015 inch and the second layer has a coating thickness be-' tween 0.065 and 0.120 inch.

References Cited UNITED STATES PATENTS 2,054,769 9/1936 Holtz 138-145 2,365,427 12/ 1944 Moore 138145 3,001,919 9/1961 Petrocokino 204-148 3,354,063 11/1967 Shutt 204-197 3,409,525 11/1968 Taylor et al. 204-147 3,484,349 12/ 1969 Vrable 204-496 3,553,094 1/1971 Scott et a1 204197 TA-HSUNG TUNG, Primary Examiner US. Cl. X.R.

-117'79, 92, 128.7;"133, 138-446, 178, Dig. 6;