CA2094204A1 - Composition and method for coating metal substrates - Google Patents

Abstract

ABSTRACT
A field-applied, flame sprayed coating for metal substrates is disclosed that comprises a blend of approximately equal parts by weight of a fusion bondedepoxy and an ethylene methacrylic acid (EMAA) copolymer. The coating exhibits excellent resistance to disbondment when applied to cathodically protected substrates. The coating is desirably flame sprayed onto the clean metal substrate at a temperature of about 220°F at a thickness of about 15 mils.
Optionally, the subject coatings can be oversprayed with a flame sprayed-coatingof up to about 10 mils of a powdered EMAA copolymer to achieve enhanced flexibility and resistance to impact or abrasion.

Description

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PFSI-21 ,033 1~ THE UNITED STATiES~PAT~EN'r AND TRADlEM[ARK C9~ilCE

COMPOSITION AND ME:I~OiD ~FOR
. C~ATlN& METAL SUBS'llE~Al[~

BACKGROUND OF THE INVENTION
~; 1. Pield of the Invention This invention relates to flame-sprayed coating systems, and more particularly, to a portable~ field-applied coating and method use~ul for coatingrneeal substrates such as pipelines. Pipeline coatings are required to meet 5 specifications more stringent than required ~or some other applications. Sincernost pipelines utili~e cathodic protection to reduce the rat~ of corrosion, cathodic disbondment testin~g is a cri~ical tese used to evaluate these coating. The coatirlg and method disclosed herein meet or exceed typical cath~ic disbondment specifications forpipeline coating servic~.
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,., 10 ~ ~o Descnption of Related Art FIamè sprayed coating systems ~or thermoplastic materials ~ave previously been disclosed in United States Patent Nos. 4,~3~,309 and 439349s9 and in pending application ~er. No. 7f60~8~6~ filed Septe~ber 16, 1g91, which re inc~porated by re~erence herein.
15~: While the systems, compositions and çoating methods disclos~ in these forego;ng patents and patent application have proved to ~ very use~ or pply~ng proh~ctive :coatings to a wide variety of differeat ar~icles, sucll ~s bxid:ges, ~storage tanks, boa~ hulls, snow pl~wsj collveying f quipment, water :~
treatment:equipment, etc., a specialized coa~ing system, cs:r~positio~L and mef~hod 20~ are ne&d~ for use ~on cathodically protected metaI subst~ s such as girth welds :on plpelines employing cathodic protection. While cathodic protf~tion of ~ - ~
under~round; pipellnes doos substanti~ly eliminate or significantly redllce ~he 2 ~

corrosion due to electrochemical phenomena, this method does not preYent direct chemical attack.
The use of fusion bonded epoxy (FP9E) coatings as pipe coatings is well known. Typically, these coatings are hard and brittle, with very low elongation,S although their hardness provides very good abrasion and impact resistance. A
major drawback to such coatings is that they are not readily applied in the field.
~- They are powder coatings typically applied in factory environments, due to the need for preheating ovens, and in some cases post-curing ovens, for proper applicatiQn. Even with such coatings, it is necessary to leave the weld areas uncoated until the pipe is installed. Qnce the welds are made between adjacent pipe sections, expensive, elaborate equipment is needed to provide a comparable coating for the girth welds. In most cases, an in~erior coaling is appliçd around the girth welds in an attempt to complete the coating withollt su~h equipment.
Shrink sleeves, mastics, tapes, etc., are examples of currently available girth weld coatings. Thus, the relatively low cost of applying high quality, ~usion bonded epoxy coatings to pip;ng in a factory do no~ extend to the field applicasion of such coatings to the pipeline girth welds.
Flame-sprayed ~hermoplastic coatings as previously disclosed generally possess high elongation, good abrasion resistancP, and excellen~ adhesion and '~0 flexibility when applied to clean metal substrates. However, the conven~ional flame-sprayed coatings do no~ proYide the desired degree of resistance ~o cathodic disbondment when applied to cathodically protected subs~ates such as pipeline ;~ girth welds.
Coadngs comprising epoxy resins that are said to be useful ~or proteeting metal substrates from cathodic disbondment have previously been disclosed9 ~or 7 ~ example, in United States Patent Nos. 4,782,124 and 3J578~615~ and in defensive publication T9730150 ~; ~ U.S. 3,578,615 discloses a fluidlzable, heat-curable polyepoxide coating composition possessing improved cathodic disbonding resisfan~e when applied to -' - 30 underground metallic piping. The use of synthetic resins and elaslomers as fillers ~ ~ is ~Iso disclosed. While the use of fillers in amoun~s up to about 35S par~s per 1 .:
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~ne hundred parts by weight of the polyexpoxide (phr) are disclosed, the use of from about 10 to about 60 phr are recommended in order to optimize raw material costs without minimizing coating properties. Application techniques including fluidi7ed beds, spraying as by a compressed air spray gun, and elctrostatic application are also disclosed.
U.S. 4,7~2,124 discloses polycarbonate modified epoxy resins containing carbonate linkages between the epoxy resin and the transesterification induced ~-~ chain scission products of the polyca~bonate or polycarbonate oligomer.
Application to a substrate using known methods such as powder dusting, fluidizedbed processes, electrosta~ic powder spraying, electrosta~ic fluidized bed processes, and others is also disclosed.
U.S. Defensive Publication T973,015 discloses improved resistance to stress cracking or cathodic disbonding of an external polyolefin coating on a cathodically protected pipeline by including 1-18 percent, preferably 4-8 percene, by weight, based on the polyolefin, of calred epoxy resin in at least the region of the polyolefin coating adjacent the pipe. The preferred polyalefin is low-density polyethylene, but may also be an ethylene copolymer, high~ensity polyethylene, polypropylene or a propylene copolymer with up to 20 percent by weigh~ of ethylene. The powdered composition may be coated by electros~atic spraying onto the heatsd pipe, e.g. at 190C for low-density polyethylene, with possible subseguent application by extrusion or otherwise of polyolefin containing no ~' epoxy resin. Strew-coating onto pipe heated ~o a higher temperature, e.g. 320C, is also disclosed.
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SUMMARY OF THE INVENTION
The composition and rnethod disclosed herein provide a non-toxic, high~
build coating of up to 15 to 25 mils thickness without any cure time being required. Application is quick and easy, rçsulting in lower application costs, and the method of the invention does not produce any hazardous waste. The subject methocl and ~usion bonded composi~ion are useful for either new application to pipeline girth welds, or for repairing damage to existing fusion bonded coatings.
According to one embodiment of the invention, a single coating, preferably about 15 mils thick, of the preferred inventive composition (which byr itself meets or exceeds s~ndard pipeline specifications~ is applied to a pipeline girth weld in the field using flame spray equipment as disclosed in U.S. Patent Nos. 4,632,309 or 4~934r~595~ and application Ser. No. 760,866. According to - one preferred embodiment, the composition of the invention comprises a blend of cr~,rogenicrally ground powder containing approximately equal pa~s by weight of t' 15 fusion ~onded epoxy and ethylene methaerylic acid (EMMA) copolymer.
Preferably, the epoxy ralld ~MMA copolymer are each ground to a particle size of ,, .
about 70 mesh prior to blending. Minor amounts of other ingredients sush as UV stabilizer and pigment alei optionally added to retard deig;adation in sunlight, improve appearanse of the coating, and the lil~e.

3~ ~0 According to another preferred embodirnent of the invention9 the coa~ g às recited above is further overcoated with a flame-sprayed layer of EMAA
~!1 copolymer to enhance the overall flexibility9 impact resistance and abrasion resistance of the coating .

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DESCRIPTION OF THE PR~FERRED ~MBODIME~NTS
According to a pre~erred embodiment of the invention, 1/8 inch pellets of natural-colored ethylene methacrylic acid copolymer, most preferably Nucrel~
410 marketed by DuPont Chemical Corporation, are cl~ogenica~ly ground to a 5 particle size of ~out 70 mesh.
Fusion bonded epoxy, preferably Scotchkote~ 206N marketed by 3M
Corporation, is ikewise cryogenically ground to a particle size of about 70 mesh.
Scotchkot~ 206N is a one part, heat curable, thermosetting powdered epoxy coating designed to provide maximum corrosion protection of line pipe.
The cryogenically ground powders thus produced are preferably blended in about equal parts by weight. Blending is preferably done using conventional, commercially available dry blending equipment such as a tumble blender, ribbon blender or vertical cone blender, and blending is continued until substantially homogeneous distribu~ion is achievedO Optionally, about 0.S percent by weight ofthe total blend of an ultraviolet light stabilizer, preferably Tinuvin0 770 marketed by Ciba-13eigy Corp., and abou~ 4 percent by weight of the to~al blend of a commercially available pigment9 preferably blue pigment C102, are blended together with the epoxy and EMAA copolymer.
The substrate to bc coated is pre~erably cl~n and d~y. Desirably, the subskate will be prepared by blast cleaning to a minimum of an SSPC SP 6 (Commercial Blast), with an anchor profile of between two and thrce mils. A~er blasting, the substrate should be cleaned wi~h compressed air (provided ~rom a source that is filtered for oil and water contamillation) to remove any remainiDg powder or o~her particulate matter and dried to remove mo;sture. Drying can usually be done by pass;ng ~he lighted flame sp~y gun over the substrate prior to activating powder llow through ~he gun.
Prior to acti~rating powder flow, the por~ion of the substrate that is to be coated first is desirably preheated to a tempera~ure ranging from about 200 to about 220F. Thereafter, except fos heat-sink areas such as flanges, substIate , . . .

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areas adjacent to the work area being coated at a particular time will normally be preheated by thermal cond~lctivi$y ~rom the work area t~ the adjacent a~ea.
The powder blend as described above is preferably applied to the clean, preheated substrate by flame spraying by the same gene~l methods and with apparatus as disclosed in U.S. Patent Nos. 4,632,309 or 4,934,595, or in application Ser. l~o. 760,866, all of which are incorpoiate~ by reference herein.
During application, Ihe powder blend is pre~erably heated ~y the flame spray gunto a temperature ranging ~rom about 200 to about 220F (compared to a temperature ranging ~om abou~ 170 to about 190F for the EMAA copolymer alone).
A beneficial coating in accordance w;th the present invention is obtained by flame spraying the blended powder onto the substrate at thicknesses of about 15 mils. Furtiher improvements in flexibil;ty, impact resistance and abrasion resis~nce are desirably achieved by subseqllently flame spraying a topcoat;ng ofthe cryogenically ground EMAA copolymer powder over the epoxy/EMAA
copolymer layer at an additional thickness of up to about 10 mils.
Repairability of these coatings is a rnajor advant~ge over pure ~usion bond epoxy coatings previously ~isclosed ;n the prior ar~ pical applicatioll of fusion bond epoxy requires that the substrate bç heated to about 450F, the powder applied, and the residual heat cures the coating.~ These coatings will remelt to some degree due eo the high levels of thermopl~stic material9 which gives a bonding agent Ior the patch rnaterial. No adhesion values have been determined ~o the patch area, but ~he "pocket hlife ~est" indicates excellent adhesion. The overcoat adhesion test results indicate exceillent adhesion at any,~ ~ 25 repair sight.
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TESTING
Samples were prepared using fla~, earbon steel plates, which were cleaned by blasting according to Steel Structures Painting Council (SSPC) SP-6 ~commercial blast) specifications. An anchor profile of about two m;ls was S measured. The samples were preheated to 200-220~F using a flame spraygun as manufactured by Plastic Flamecoat Systems, Inc. Once preheated, a flame sprayed coating of blended epoxy and EMAA copolymer as disclosed ~bove was applied at an average thickness of about 15 mils. The coating rnaterial was preblended with a pigment to give it a dark blue appearance.
A clear, or natural, overcoat of an EMMA copolymer-based compositio (marketed by Plas~ic Flamccoat Systems, Inc. under ~he tradename PF111) was flame sprayed onto half of the samples to evaluate the ef~t on physical properties. In so doing, the previsusly coated sur~ace was preheated to a moltenstate (about 225F) before ~he overcoat was applied. Natural material was used 15 so that any effects of overcoa~ing on the undercoat could be vie~A:ed ~hrough the ~i topcoa~. No discoloration or off-gassing was observed. (In commercial -; applications, the topcoat may be colored to aid UV stabili~ and for gener~
appearance considerations.) Cathodic disbondrnent testing (C DT) was a major consideralion in 20 evaluating the coated p]ates. All CDT's were conducted by Bell Evalu~tion I~bs.
CDT results are reported in terms of zero and reduced adhesion radius, which arecornbined to report ~he total disbondrnent radius. All CDT's were conducted in accordance with ASTM G-95 and ~re reported in millimeters. These ~ests may be conducted at room temperature, or the test may be modified (iaccele~ated) by 25 evaluating peffonnance at elevated tempera~ures.
i ~ Short term (24 hour iand 14 day~ testing was conducted at 150F. rhese tests indicated excellent pe~ormiS~ce~ Testing results are outlined in Tasble I.The 24 hour ~est at 150F resulted in a total disbvndment of one millimeter5, while the 14 day test yielded a ~our to five ~T.illimeter disbondment radius.

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Twenty-four hour testing at 150F was conducted on overcoated plates, yielding aone to two millimeter disbondment radius.
Long term (28 day) testing was conducted at room temperature on samples not having the EMAA copolymer overcoat. The total disbondment S radius on these samples was 13 to 15 millimeters, within ~ypical specification limitsO Some color IGSS was seen in the testing area; however, coating inte~ritywas not affected.
Flexibility was evaluated using the ASTM G10 test method. This ~es~
consists of bending coa~ed plates (with and without overcoating) at room temperature and at freezing. The results are reported in degrees bend per pipe diameter. The temperature difference (75F vs. 32F) did not have a significant effect on the flexibility of the blended epoxy and EMAA ~polymer coating9 indica~ing excellent property retention. Overcoated samples exhibited a slight -~ improvement in bending performance. The test results are presented in Table II.
IS The results ranged between 14.19 and 15.07 bends per pipe diameter.
Other physical proper~ies were also evaluated on Ihe plates coated as Z described above. The test results are reported ;n Tables m, IV and V. In general, physical proper~ies were enhanced on the plates where ~he blended epoxyand EMAA copolymer coating was overcoated with another layer of EMAA
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~ 20 copolymer material. Impact properties were measured using the ASTM ~314 test ;
method. The impact rating of about 30 inch-pounds ~or ~he blended epoxy ~d EMAA copolymer-coated plate was exceeded by about 10 inch-pounds with ~he ~' overcoated sample. Abrasion testing eonducted in accordance with ASTM D1044 indicated a 30 pereent improvement fior the overcoated plate. Adhesion was determined by ASTM 4541, testing the pull of strength of coatings using a portable adhesion tester. Plates ~ame-sprayed only with the blended epoxy and , EMAA copolymer material exhibited cohesive failure at 550 and 600 psi. Plates in which the blended epoxy and EMAA copolymer material was overcoated with the addi~ional layer of EMAA copolymer exhibîted pull-off skengths of 7$0 and ,~
i: ~ .30 900 psi, with adhesive failure see at the interface between the overcoat and the - -~ ~ ~ .
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aluminum dolly. The interface between the steel substrate and the undercoating of flame-sprayed, blended epoxy and EMAA copolymer was unaffected.

TABLE I
CATHODIC DISBOI`IDMENT TESTING (CDT) Electrolyte = 3% NaCl Disbondment Radius Sample Time ~ Volt Zero Reduced Totial A. CSC 115 24 hr150F 3V 1 0 . 10 1 0 ^ 14 day150F 3V 2 2 4 /, 1 4 5 `. 28 day70F 3V 2 11 13 ~.
~ . 1 14 15 .
.i 15 B. CSC 215 24 hr150F 3V 1 1 2 ,~ 1 0 Yi~ .

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TABLE II
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FLEXIBTLITY AS'rM G-lO
Sample TempDFT Deg~/_ipe Dia.
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A. CSC 115 32F 14.5-16.8 14.41 75~1~.4-1608 14. 19 - B. CSC 215 32F 35.4-39.5 14.79 75F36.8-40.0 15.07 .

TABI,E III
., . 10 IMPACT ASTM G-14 , .
Sample Temp DFT Failure .~ A. - CSC 115 72F 26.3-34.1 ~30 in-lbs ,., B. CSC 215 72F 45.0-51.7 :~40 in-lbs .,l _ , .. ____ .. . .
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lS TABI,E IV
ABR~SION ASTM D1044 (:onditions: CS17 Wheel with 1000g Load .~ .. ...
i ~ ~ Samp~le ycles Mg Loss A. CSC llS1000 100 : 1~0 100 . .
B. CSC 2151000 70 ~J
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TABLE V
ADE~ESION ASTM I~4541 Samples Scored Pnor ~o Testing Sample Adhesion Failure S A. l:SC 115 550 CohesiYe 600 Cohesive B. CSC 215 750 Adhesive (Dolly) 900 Adhesive (l~olly) .. . _ .. .
In summary, substrates having a flame-sprayed layer of blen~ed epoxy and EMAA eopolymer demonstrated excellent performan~ when subjected ~
., standard pipeline coating tests. The flame-sprayed, blended epoxy and EMAA
copolymer coating disclosed herein meets or excee~s typic~l per~rrmance specifilcation limits for short and long term cathodic disbondment ~esting. :Impac~
15 -and abrasion properties are nQt significantly high, but the gain in elongation and ,~ flexibility are believed to be a good ~ade-o~f. This coating may be used as an . effectivie, high build, one-step coating that is simple and cost effective ~r use in coating pipeline girth welds. Further improvement in impact resistance, abrasionresistance and adhesion can be realiz~3 by overGoating the base coat with a flame-i 20 sprayed layer of EMAA copolymer.
Other alterations and rnodifications of the invention will likewise become : apparent to those of ordinary skill in the art upon reading ~he present disclosu~, ` and it is intended that the scope of the invention disclosed herein be limited only by the broadest interpretation of the appended claims to which ~he inventors arelegally enti~led.

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Claims (13)

1. A coating for cathodically protected metal substrates, the coating consisting essentially of a field-applied, flame-sprayed blend comprising approximately equal parts by weight of cryogenically ground fusion-bonded epoxy and cryogenically ground ethylene methacrylic acid copolymer.

2. The coating of claim 1 wherein the blend comprises powder having a maximum particle size of about 70 mesh.

3. The coating of claim 1, having a thickness of about 15 mils.

4. The coating of claim 1, further comprising a field-applied, overcoat consisting essentially of flame-sprayed, cryogenically ground ethylene methacrylic acid copolymer.

5. The coating of claim 4 wherein the overcoat has a thickness of about 10 mils.

6. A composition comprising a blend of approximately equal parts by weight of cryogenically ground fusion bonded epoxy and cryogenically ground ethylene methacrylic acid copolymer.

7. The composition of claim 6 wherein the epoxy and copolymer are cryogenically ground to a maximum particle size of about 70 mesh.

8. A method for coating a cathodically protected metal substrate comprising the steps of:
providing a first powder consisting essentially of fusion-bonded epoxy that has been cryogenically ground to a particle size of about 70 mesh;
providing a second powder consisting essentially of ethylene methacrylic acid copolymer that has been cryogenically ground to a particle size of about 70mesh;
blending the first powder and second powder in about equal parts by weight; and flame-spraying the resultant blend unto the substrate.

9. The method of claim 8 wherein the substrate is blast-cleaned prior to flame-spraying.

10. The method of claim 8 wherein the substrate is preheated to a temperature ranging from about 200 to about 220°F prior to flame-spraying.

11. The method of claim 8 wherein the blend is flame-sprayed onto the substrate at a thickness of about 15 mils.

12. The method of claim 8 wherein the flame-sprayed blend is subsequently overcoated with a flame-sprayed layer of ethylene methacrylic acid copolymer.

13. The method of claim 12 wherein the flame-sprayed blend is overcoated by an additional thickness of about 10 mils.