Aspects of corrosion protective tape technology

Dr. rer. nat. Michael Quast, Denso GmbH, Leverkusen, Germany

published in 3R international Special Edition 1/2006

Vulkan-Verlag GmbH · Essen · Germany

contact: Nico Hülsdau, Email: n.huelsdau@vulkan-verlag.de

Special 1/2006Pipeline Technology

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Tape composition and structureThe proper choice of a suitable tape coa-ting system starts with a basic require-ment, which is valid for all pipe diametersand operating conditions. This require-ment is related to material compositionand structure. Regarding their composi-tion the available plastics tape coatingscan be assigned to the following mainmaterial classes and combinations the-reof:

Carrier film material

– Polyethylene (PE)

– PVC

Adhesive material

– Butyl rubber

– Bitumen

Among possible tape compositions thecombination of a PE carrier film withbutyl rubber adhesive has proven thebest corrosion protection performance[1], [2], [3]. In contrast to this PVC as car-rier film material, particularly in case ofcontained plasticizers, is susceptible toembrittlement. Bitumen as material basisfor corrosion protection tapes is a lessperforming material also as it provides a

lower ageing resistance than butyl rubberbased adhesives. This lower ageing resi-stance is expressed by a significantlylower electrical insulation resistance,compared to butyl rubber tapes, whichfurther decreases after years of operation[4].

Having made the choice for polyethyleneand butyl rubber as the material basis,the question of the most suitable tapestructure comes up. As a minimum requi-rement the innerwrap tape or so calledcorrosion protection tape should alwaysbe of a three-ply structure with butyl rub-ber adhesive layers on both sides of thecarrier-film [5], [6]. Figures 1 a and bshow typical cross sectional views ofhigh performance three-ply corrosionprotection tapes.

Among these structures the asymmetri-cal one is to be preferred, because itsthick inner adhesive layer ensures higherpeel strength as well as better filling ofsurface irregularities and potential hol-lows, e.g. beside the weld bead. Havinga detailed look on the tape structureshown in Figure 1 a, one can even reco-gnize a four ply structure, which is typicalfür state-of-the-art asymmetrical corro-

sion prevention tapes like DENSOLEN-Tape AS40 Plus. The thin fourth ply bet-ween carrier film and the thick grey butylrubber adhesive is a co-extruded plyfrom a blend of butyl rubber and poly-ethylene. The existance of such an inter-mediate ply can easily be determined bypreparing a microtome cross section asshown in Figure 2. All tape plies indica-ted in the schematic view in Figure 1 acan also be seen in the real photographyof such a cross section. State-of-the-artmechanical protection tapes like DEN-SOLEN-Tape R20HT (Figure 1 c) alsocontain a coextruded intermediate layerbetween carrier film and adhesive.

A co-extruded intermediate ply betweenbacking and main adhesive layer provi-des a homogenous transition from butylrubber to PE. Additionally, the wellknown delamination effect (Figure 3) anda potential long term permeabilitythrough the interface between carrier filmand adhesive is avoided by using tapescomprising co-extruded plies. Same asin DENSOLEN-Tape AS40 Plus, the outerbutyl rubber layer of asymmetrical three-ply tapes should also be manufacturedby a co-extrusion process to also ensureperfect bonding between backing andouter adhesive layer. At this point itshould be clearly distinguished betweentapes of the DENSOLEN-type, where thebacking is produced by a co-extrusion

Aspects of corrosion protectivetape technology

M. Quast

Corrosion protective tape coatings have been used for decades on all types andsizes of buried pipelines. Results thus obtained are as versatile as the available rangeof products. The following paper deals with important aspects that have to be consi-dered to ensure long term performance of corrosion protective tape coatings.

Fig. 1: Cross sectional view of a) asymmetrical 3-ply corrosion protection tape b) symmetrical3-ply corrosion protection tape c) mechanical 2-ply protection tape d) butyl rubber tape

Fig. 2: Microtome cross section of DENSOLEN-Tape AS40 Plus/AS39P

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process and the main adhesive layer islaminated by a calender process, andbetween tapes, where the backing andthe adhesive layers are manufactured ina one-step co-extrusion process. Thelatter type is accompanied by certainlimitations concerning formulation anddimension of the main adhesive layers,while the two-step process offers a widerrange of possibilities concerning adhe-sive composition and tape dimensions.Only this freedom in formulation allows toperfectly adjust the tape properties to itscorrosion protection function.

Why is a three-ply structure of the inner-wrap tape of such importance? As canbe seen from Figure 4 a, an interface andpotential penetration path for water andoxygen remains in the tape overlap, if anonly two-ply tape is used for the inner-wrap. After some time of operation theincompletely sealed tape overlaps inevi-

tably lead to spiral corrosion first follo-wed by complete undermining corrosion.A big percentage of bad experienceswith tape coatings all over the world ori-ginate from and is linked with the effectshown in Figure 4 a. In contrast to this nointerface or penetration paths remains inthe overlap of high-performance three-ply tapes (Figure 4 b). The outstandingfeature of butyl rubber is its ability to self-amalgamate in the overlap areas, resul-ting in a completely sealed, impermeableand sleeve-type coating.

Tape systemsTape systems should always contain atleast two layers of a self-amalgamatingthree-ply tape. The resulting and comple-tely sealed innerwrap ist then normallyoverwrapped with a mechanically pro-tecting outerwrap, which could be eithera three-ply or a two-ply tape.

The even distribution of polyethylene andbutyl rubber plies in a one tape systemas shown in Figure 5 a and b affords amaximum resistance to mechanicalstresses like indentation and impact.Additionally in a one tape system all tapelayers self-amalgamate within their over-laps, forming a sleeve type coatingthroughout the whole tape system. Intwo tape systems using two-ply tapes forthe outerwrap, the overlap between thethird and forth tape layer does not self-amalgamate, which nevertheless is notdetrimental to the corrosion protectionperformance of the whole system. Evencontrary and beside the fact, that theyare economically preferred, there may bealso technical reasons to choose two-plyouterwrap tapes instead of three-plyouterwrap tapes. Particularly in hot cli-mates it could be useful to employ two-ply tapes comprising a white polyethy-lene backing. Such tapes would showonly a minor rise in temperature whenexposed to sun irradiation. The wellknown effect of bubble formation undertape coatings, which are exposed to sun-light, would consequently be avoided.Moreover, there is lower friction betweenthe smooth exterior of polyethylene two-ply tapes and soil, which is of benefit especially for high-temperaturetape systems of the structure shown inFigure 5 d and described below.

Mechanically highly resistant tapesystems according to stress-class C-50(EN 12068) always have a structure asshown in Figure 5 a or b. An alternativetwo tape system makes use of a butylrubber tape instead of a three-ply tapefor the innerwrap. Such butyl rubbertapes, which may contain thin poylethy-lene films (usually 25 to 70 μm) to avoidoverstretching of the tape during appli-cation, are highly conformable and there-fore find use in wrapping of irregularlyshaped objects like weld-on branchlines. In such fields of application therequirement for simple application pre-dominates the importance of maximummechanical resistance. Butyl rubber tapesystems (Figure 5 c) afford minor inden-tation resistance than type 5 a or 5 b tapesystems for the benefit of simple applica-tion.

A special attention has to be directed totape systems for elevated operating tem-peratures, since tape coatings on pipesoperating above +50°C are exposed toparticular stresses. These stresses,which are further indicated below, can becompensated by taking the followingmeasures:

The thermal elongation of the carrier filmleads to wrinkle formation, which in com-bination with reduced peel and lap shearstrength at elevated temperatures wouldpartly lift off tape system from pipe sur-face.

Fig. 3:a) Delaminationduring peel-testb) cohesive peelmode of tape withco-extruded inter-mediate ply

Fig. 4: a) Incompletely sealed tape overlap of two-ply tape wrapping with undermining corro-sion along spiral tape overlap b) Completely sealed tape overlap by self-amalgamation of highperformance three-ply tape

Fig. 5: Corrosion protection tape systems a) one tape system b) two tape system with two-plyouterwrap tapec) two tape system with butyl rubber innerwrap tape d) two tape system forelevated operating temperatures (> 50 °C)

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→Recommendation: Use of a butyl rub-ber tape without carrier film as inner-wrap.

Adhesives get smooth at elevated tem-peratures, which would result in reducedpeel and shear forces.

→ Recommendation: Use of a specialbutyl rubber formulation, which is highlystabilised against thermal degradationand which is self-reinforcing by cross-lin-king at elevated temperatures.

Carrier film of the mechanical protectiontape gets smooth at elevated temperatu-res, which would result in reduced resi-stance to load forces.

→ Recommendation: Use of a high den-sity polyethylene carrier film and additio-

nal use of a load distributing non-wovenrockshield.

Elevated operating temperatures areoften accompanied with alternating tem-peratures, which could result in a longitu-dinal movement of pipe and increase therisk off the coating to be sheared off.

→ Recommendation: Use of a rocks-hield, which decouples moving pipe fromsurrounding soil by providing low frictionbetween rockshield casing and outer-wrap tape.

Standard requirements – stressclassesTechnical properties of a corrosion pro-tective tape coating are described in res-pective national and international stan-dards [4]. It should be noticed that fre-quently employed ASTM standards con-tain well suitable test procedures todetermine single tape properties. Nevert-heless ASTM standards neither defineacceptance criteria nor do they containan adjusted or balanced set of require-ments for tape coatings.

In contrast to this material standards likeEN 12068 ([7], [8]) do not only describetest methods to determine the relevanttape and coating properties, they alsogive a well suitable tool for classificationand comparison of tape coatings by

using a concept of stress classes. Themechanical stress classes A, B and Cessentially differ concerning require-ments for peel strength, impact resi-stance and indentation resistance. Cor-responding ratios for the mentioned pro-perties are shown in Figure 7.

When choosing a suitable tape coating ithas to be considered that the perfor-mance level of a standard factory appliedcoating, e.g. three layer polyethylene (3-LPE) according to DIN 30670, exceedsthe performance level of a field coatingby far. Especially the resistance to loadstresses and peel forces, which are per-manently affecting the pipe coatingduring pipeline operation, is generallylower in case of field applied coatings,which is clearly demonstrated in Figure 8a and b. This different performance levelsoriginates from the general demand foreasy on site applicability, which makes itnecessary to accept lower level ofmechanical strength in case of fieldapplied tape coatings.

Having in mind the different performancelevel of factory coatings and field appliedtape coatings, it seems evident that afield coating with properties as close aspossible to the performance level of theexisting line pipe coating should be cho-sen. The only limiting factor, whichshould influence the decision for a cer-

Fig. 6: Typical field of application for type 5.btwo-tape system: Coating of weld-on branchline

Fig. 7: Differences between stress-class A, Band C requirements for peel strength, impactresistance and indentation resistance accor-ding to EN 12068

Fig. 8: a) Typical residual thickness of a three-layer polyethylene factory coating (3-LPE, DIN 30670) compared to stress-class C-50 and C-30 tape coatings (EN 12068) at 23 °C b) Peel Strength of 3-layer PE factory coatings (DIN 30670) and stress-class C-50 fieldcoatings (EN 12068) at 23 °C

Table 1: Tape systems guideline

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tain tape coating, should therefore be theissue of applicability. Consequently astress-class C-50 tape system (e.g.DENSOLEN-System AS39P-R20HT)should be used for standard wrapping ofwelded joints, bends or full pipe length.Even if the pipeline is operated at tempe-ratures below 50 °C, the higher perfor-mance level of a C-50 system provides ahigher safety margin also at room tempe-rature and narrows the gap to the perfor-mance level of the line pipe coating. Onlyin case of irregularly shaped objects,which require flexible and easy to applytapes, the use of a more conformable butmechanically less resistant two-tapesystem according stress class C-30 or B-30 could represent a reasonable alter-native.

ApplicationThe performance of a corrosion protec-tive tape coating can only be as good asthe quality of its application. Thereforethe corresponding properties and appli-cation equipment play a major role incorrosion protective tape technology. Asa general requirement tape coatingsshould be applied

– without wrinkles and hollows

– with sufficient tape tension

– with constant tape overlap

Observance of the above requirementscan as much as possible be ensured by

– use of appropriate (= pipe diameterdependent) tape width

– limitation to an upper limit of tapewidth even on large diameter pipes

– employment of wrapping machineswhenever possible

Particularly the choice of a suitable tapewidth can influence the quality of a tapecoating. In case of tape application byhand wrapping devices sufficient tapetension can only reliably be applied bylimiting oneself to a maximum tape widthof 150 mm. However, the recommendedmaximum tape width is 100 mm (4"),which allows proper wrapping also incase of more tough mechanical protec-tion tapes. The seeming advantage offaster application when using a widertape is more than compensated by thedisadvantage of a sometimes questiona-ble quality of wrapping due to poor adhe-sion and wrinkle formation.

The range of DENSOMAT hand or motordriven wrapping machines illustrated inFigure 9 a to e offers versatile importantfeatures for perfect site suitability andsimple use.

– built-in break mechanism for constanttape tension

– integrated interleaving take-upmechanism

– low weight

Table 2: Tape width guideline for insulation of girth welds, bends and full pipe lengths

Fig 9: a) Tape wrapping with DENSOMAT 1 hand wrapping machine b) DENSOMAT KGRJunior hand wrapping machine with extension arms c) DENSOMAT 11 motor driven wrappingmachine for field joint coating on a 36" pipeline d) DENSOMAT KGR Junior with closed framee) Easy mounting of DENSOMAT 11 wrapping machine on a 36” pipeline (girth weld insulation)

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– minimum clearance requirement

– possibility to attach extension arms (9b) or circumferentially closed frames(9 c) to wrapping machine for largediameter pipes (DENSOMAT KGR)

– simple mounting and movementacross pipe supports (DENSOMAT 11,9 e).

It is unnecessary to mention that qualityof surface preparation is another impor-tant factor, which affects quality of corro-sion protective coatings. Cold appliedtapes in general and DENSOLEN-Tapesin particular offer a high level of site sui-tability, since they tolerate a surface pre-paration according to ST2 [7] obtainedjust by wire brushing.

ConclusionWhen taking into consideration importantfactors like tape composition and struc-ture, mechanical performance issues andfinally application technique and techno-logy, tape systems offer a well suitable,durable and high performing solution forthe corrosion protective coating of pipesand pipelines in rehabilitation and new

construction, including insulation of girthwelds and irregularly shaped structures.The combined use of DENSOLEN tapesystems and DENSOMAT wrappingmachines provides perfect applicabilityand optimised properties concerningcorrosion protection performance,mechanical strength and durability.

References

[1] Schöneich, H.G.: Umhüllungen - Betrieb-liche Erfahrungen. 3R internat. 40 (2001)No. 1, p. 67-68

[2] Schwenk, W.; Heim, G.; Wedekind, B.;Schäfer, T.: Untersuchungen der Korro-sionsschutzwirkung von Umhüllungen ausStahlleitungsrohren nach langzeitiger Aus-lagerung in Wasser und im Erdboden. 3Rinternat. 35 (1996) No. 12, p. 676-685

[3] Quast, M.: Long term experience withDENSOLEN-Tapes. 3R internat. 43 (2004)No. 13, p. 69-72

[4] Heim, T.: Entwicklung von Korrosions-schutz-Nachumhüllungsmaterialien fürStahlrohre. 3R internat. 44 (2005) Nr. 11,p. 625-633

[5] Wedekind, B.: Plastic material tapes forthe corrosion protection of undergroundpipelines. 3R internat. 13 (1974) No. 5

[6] Bohle, P.: Self amalgamating pipeline tapecoating. CEW, Vol. XXVIII (1993) No. 10, p.61-63

[7] EN 12068 "Cathodic protection - Externalorganic coatings for the corrosion protec-tion of buried or immersed steel pipelinesused in conjunction with cathodic protec-tion - Tapes and shrinkable materials"(1999-03)

[8] Driesen, H. E., 3R internat. 34 (1995) No.10/11, p. 601-606

[9] ISO 8501-1 "Preparation of steel substra-tes before application of paints and rela-ted products - visual assessment of sur-face cleanliness" (1988)

Dr. rer. nat. Michael QuastDenso GmbH, Leverkusen, Germany

Tel. +49(0)214-2602-308Email: quast@denso.de

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