Preventing
peeling
P. Collet, BS Coatings, France,
discusses ways of tackling
coatings disbondments.
S
everal pipeline coa...
Disbonding of an HSS field joint coating, leading to
corrosion caused by the “cathodic protection shielding
effect”, obser...
protected and the (mostly) physical/mechanical anchoring of
the coating to be applied.
However, the force of such linkages...
Benefits for the coating performance
The performance of this new Silpipe®
SCT solution has
been tested through the evaluat...
epoxy-metal substrate interface. This process quickly leads
to a loss of adhesion of the epoxy vis-à-vis the steel. This
p...
of 5

PreventingPeeling

Published on: Mar 4, 2016
Source: www.slideshare.net


Transcripts - PreventingPeeling

  • 1. Preventing peeling P. Collet, BS Coatings, France, discusses ways of tackling coatings disbondments. S everal pipeline coating failures have been reported by corrosion experts over the last few years. The major causes of the various corrosion problems reported may be summarised in the following two groups: The operating environment of the pipelines, where extreme conditions may be encountered due to high temperature or humidity or a combination of both. The discrepancy between the real service conditions and the conditions the coatings were actually designed to meet under daily operating conditions, which consequently may have adversely impacted the choice of the coating system and the associated surface preparation of the steel pipes. ñ ñ Facing disbondment issues Recent case studies have reported large-scale coating disbondments, which could involve corrosion problems beneath the coating and lead to potential damage. Examples have recently been reported for ‘old’ and ‘modern’ coating systems: Loss of adhesion of coal tar coatings, reported in Kuwait1 , where the general poor adhesion was the main source of disbondment. Blistering problems of FBE coatings appearing above 90 ˚C, observed in France and Angola.2 Problems of massive loss of adhesion between epoxy and steel reported in several countries, such as India3 and South America4 with a 3LPE system. ñ ñ ñ
  • 2. Disbonding of an HSS field joint coating, leading to corrosion caused by the “cathodic protection shielding effect”, observed in Angola.2 Investigation and required coating properties In-depth technical evaluations and R&D efforts have been sponsored in order to enhance our understanding of the phenomenon and its causes. As far as the investigation and the possible explanation for disbonding of 3LPE is concerned, various assumptions are listed hereafter: water and oxygen diffusion through PE, water saturation of the FBE layer, superficial corrosion of the steel surface forming magnetite, all these steps being accelerated by temperature.2 Therefore, one consequence has been to investigate the adhesion strength of FBE primers on steel, taking into account the water diffusion through the topcoat within the lifetime of the pipeline and the water permeating through the polyolefin layer in less than 300 days at 60 ˚C.5 A methodology based on the ageing of bi-layer FBE/adhesive coatings was then proposed to evaluate the FBE adhesion ñ to steel during wet ageing. Other technical experts have also recommended a change to longer term (120 days) hot water immersion tests at operating temperature so as to provide a better prediction of loss of adhesion after ageing.6 A property known as “wet Tg” has also been newly considered to assess the mechanical and electrical properties of the FBE.2, 7 These efforts have led to new criteria for selecting materials, so that mono-layer and multi-layer coatings achieve optimum bonding to steel and chemical bonding between the layers. At this point, it is also important to mention that the disbondment of three-layer coatings is possibly caused by internal stresses.8 Although this has to be considered, this influence is not covered in this article. Materials selection Extensive works have been led by the major FBE producers to develop new grades to match the forthcoming requests from operators to solve the above-mentioned technical issues. These works have focused on the improvement of the barrier effect against corrosive components such as water and salts by increasing the Tg and the hydrophobic behaviour of the epoxy primer. These developments have led to major improvements in terms of adhesion performance at high temperature, less sensitivity to hot water absorption, higher wet Tg, tensile strength improvement even after immersion in water and better cathodic disbondment resistance at high temperatures.7 Necessary but not good enough The optimisation of the anti-corrosion protection power of the coating is therefore linked. Firstly, to its intrinsic characteristics that enable the material to withstand the service constraints and, secondly, by maintaining its adhesion vis-à-vis the substrate. The interest of strengthening the adhesion of FBEs used as mono-layer coatings or as primers in multi-layer systems is linked to two reasons: The abrupt variations in temperature that occur during the coating application process lead to thermal shocks within each constituent (including the steel), which cause mechanical stresses at each interface, possibly reducing inter-layer adhesion.8 The penetration of species such as water, oxygen and salts into the coating causes a reduction in the affinity between the metal substrate and its protective coating at the interface, resulting in blistering and loss of adhesion. The purpose of the surface preparation consists of two objectives: Obtaining a clean substrate. Optimising the surface roughness. This is to create a strong link between the coating and the substrate by improving the ‘wettability’ of the steel to be ñ ñ ñ ñ Figure 2. Disbonding HSS. (Photo courtesy of TOTAL, France). Figure 1. Disbonding 3LPE. (Photo courtesy of TOTAL, France). Reprinted from April www.worldpipelines.com
  • 3. protected and the (mostly) physical/mechanical anchoring of the coating to be applied. However, the force of such linkages is less than that of a covalent chemical bond. A technical answer but not an environmentally-friendly solution Chemical treatments have been used for many years to prepare steel prior to it being coated in order to obtain maximum adhesion of the (FBE) coating on the (steel) substrate, thanks to this chemical bond. The main chemical treatments (chromation, acid washing) efficient in assuring chemical bonding between the coating and the steel substrate have been restricted or banned due to health, safety and environment issues (toxic substances, waste disposal). As far as the required technical performance for the external pipe coatings are concerned, the current situation is hence ‘bottlenecked’ by the HSE limitations of the current solutions for chemical surface treatment, as illustrated by Figure 3. A sustainable solution for sustainable performance Silpipe® SCT is a silane-based solution specifically designed by additivation for the chemical surface treatment of steel pipes as a technical environmentally friendly alternative to the chromation technique.9 The associated patented SILPIPE® process is based on this Silane-based Chemical Treatment (SCT) of a grit blasted surface, before heating the steel pipe up to the correct temperature for epoxy powder coating application, as shown in Figure 4. This process takes place in several steps, as described in Figure 4. As far as the application of the Silpipe® SCT solution is concerned, the amount applied to the metal substrate is around 50 g/m2 . No surface rinsing is necessary before heating up the steel pipe to the required temperature for applying the FBE. SILPIPE® SCT has to be specially designed for its compatibility with the epoxy groups of the FBE and modified to improve the anticorrosion properties. It is an aqueous solution in the form R-Si—(OH)n, presenting the following advantages: No Si-OR alkoxy functions, avoiding the presence of co-solvent (through hydrolysis). Very high stability of the aqueous solution. Fast direct reaction of the Si—(OH) function with the metal. Furthermore, this solution is a solvent-free and VOC-free alternative. ñ ñ ñ Figure 5. Peeling vs. immersion time of the three-layer coating, with SILPIPE® SCT chemical metal treatment. Figure 6. Peeling vs. immersion time of the three-layer coating, without SILPIPE® SCT chemical metal treatment. Figure 3. Process for selecting and applying coating at the required quality level. Figure 4. Diagram of the SILPIPE® process. Reprinted from April www.worldpipelines.com
  • 4. Benefits for the coating performance The performance of this new Silpipe® SCT solution has been tested through the evaluation of both FBE coatings and three-layer coatings and the discussed results are focused on the adhesion performance after hot water immersion. Mono-layer coating The characteristics of the selected FBE are shown in Table 1. It was applied by means of an electrostatic spray, on grit blasted (roughness between 70 and 90 µm/Sa 2.5 surface cleanliness) steel plates and the coating applied between 350 µm and 450 µm was post cured for 10 minutes at 200 ˚C. Tests on the FBE monolayer were run by monitoring the adhesion following hot water immersion, as described hereafter: The metal plates were immersed in tap water at 80 ˚C, for 40, 80 and 120 days. Pull-off adhesion tests, according to the ISO 4624 Standard, were carried out after 120 days immersion time. Peeling tests, according to the EN 10289 Standard, were carried out after 40, 80 and 120 days immersion times. The compared adhesion performance between chromate and Silpipe® SCT chemical treatments, summarised in Table 2, shows that the treatment efficiency of the SILPIPE® SCT solutions used matches the efficiency of the chromate solutions in coating adhesion after hot water immersion for 120 days. Three-layer epoxy-polyolefin coating The following results are based on a study conducted on a three-layer coating applied on a pilot scale application line and further at an industrial scale. Notes relative to the coatings materials used: The coating comprised an epoxy powder primer, an adhesive and a top coat. The epoxy powder was identical to that previously described, while the adhesive (ca. 250 µm thickness) was a polyolefin grafted with maleic anhydride-based groups, the softening point of which, determined by DSC, was 135 ˚C and the top coat (ca. 3 mm thickness) was made of HDPE. Pilot trial The coating was applied to the external surface of a 7 mm thick steel pipe, with an external diameter of 116 mm. Two series of test pieces were considered: Series 1: coating applied on clean grit blasted surface. Series 2: coating applied on clean grit blasted surface with Silpipe® SCT surface treatment. Before immersion in water at 60 °C, on each test piece (10 cm long sections), the three-layer coating was cut through its whole thickness. Two incisions, 2.5 cm apart, were made over the whole circumference of each test piece to facilitate the ingress of water at the level of the ñ ñ ñ ñ ñ ñ Table 2. Adhesion assessment of FBE coatings versus immersion time in tap water at 80 ˚C Surface treatment Loss of adhesion (mm) EN10289 Pull off test (N/mm2 ) ISO4624 After 40 days After 80 days After 120 days After 120 days 5% chromate solution 1 mm 2 mm 2 mm 20 +/-2 10% chromate solution 1 mm 2 mm 2 mm 20 +/-2 5% Silpipe® SCT solution 1 mm 2 mm 2 mm 20 +/-2 10% Silpipe® SCT solution 1 mm 2 mm 2 mm 20 +/-2 Table 3. Adhesive characteristics Melt index (190 ˚C/2.16 kg) ISO 1133 5 g/10 mm Melting point DSC 134 ˚C Vicat softening point ISO 306 121 ˚C Table 4. Top coat characteristics Melt index (190 ˚C/2.16 kg) ISO 1133 5 g/10 mm Melting point DSC 128 ˚C Vicat softening point ISO 306 120 ˚C Table 1. Main characteristics of the FBE Methods Results Particle size Laser spectroanalyser Average: around 50 µm < 10% above 96 µm Specific gravity ISO 2811 Around 1.4 g/ml Moisture content Weight loss at 105 ˚C < 5% Gel time at 180 ˚C ISO 8130-06 Around 70 sec Tg of cured film NFA 49-706 Around 105 ˚C Minimum time of cure DSC analysis 180 ˚C 160 sec 220 ˚C 70 sec 240 ˚C 50 sec Performance of 400 µm cured film applied on steel substrate Methods Results Impact resistance ISO 6272 > 20 J Flexibility CAN CSA Z245.20.02 0 ˚C Pass -30 ˚C Pass Water absorption after immersion at 100 ˚C for 100 days ISO 62 Less than 7% Cathodic disbondment at 23 ˚C for 28 days CAN CSA Z245.20.02 Less than 6 mm Reprinted from April www.worldpipelines.com
  • 5. epoxy-metal substrate interface. This process quickly leads to a loss of adhesion of the epoxy vis-à-vis the steel. This performance can be visually observed in Figures 5 and 6. It demonstrates the improvement in adhesion after hot water (60 ˚C) immersion for more than 120 days as suggested test6 to predict loss of adhesion after ageing. Industrial scale experiments The purpose of this study was to check if these results could be duplicated at an industrial scale on an existing installation without any special investment. To carry out this assessment, a 3LPE coating was tested, because it appears to be more sensitive to delamination in hot and humid environments than a single layer FBE coating. Adhesive (Table 3) and Top Coat (Table 4) were applied with the FBE corresponding to the one previously used and described. The application process is described according to Figure 4 with indicative dwelling times for applying a multi- layer coating on steel pipes with the following dimensions: Length: 12 m. External diameter: 114 mm. Thickness of steel: 3.6 mm. The 5% Silpipe® SCT solution was applied at a rate of around 50 g/m2 on the steel surface after grit blasting to achieve a Sa. 2.5 (ISO 8501-1) surface cleanliness and 55 - 65 µm Rz roughness. Two series of test pieces were used for the adhesion evaluation: the first series being chemically treated with a Silpipe® SCT solution, the second series without any chemical treatment. ñ ñ ñ Figure 7. Peeling determination after immersion in hot water for 40 days. Figure 8. New perspective for selecting and applying coating with the required quality level. The immersions were carried out in tap water at 65 ˚C and 80 ˚C for 40 days. The adhesion evaluations were carried out by an independent laboratory according to the prEN ISO 21809-1 Standard. Figure 7 shows the drastic improvement compared to conventional surface preparation based on measurements of the peeling force after immersion in hot water for 40 days at two different temperatures. New perspectives Firstly, the new approach initiated through this patented Silpipe® process opens new perspectives regarding the problems linked to the disbondment of various external coatings, thus meeting the concerns of operators when designing new coating systems. The benefit of the chemical surface treatment has been demonstrated to maximise the adhesion performance, specifically after hot water immersion for a long period (120 days) as suggested by experts for ageing simulation to predict disbondment. Secondly, this innovative process gives the opportunity to coating applicators to offer this technical solution without any HSE restrictions. It allows the process (Figure 8) to be ‘debottlenecked’ when choosing and applying the right coating system with a sustainable approach to guarantee durable lifetimes of pipelines. References ‘Integrity management of non-piggable pipelines – a KOC Direct Assessment Case Study’, Allied Engineers & KOC, 9th Oil and Gas Pipelines in the Middle East Conference, The Energy Exchange, Abu Dhabi, 4 - 6 May 2009. ROCHE, M., MELOT, D., PAUGAM, G., TOTAL SA, ‘Recent experience with pipeline coating failure’, 16th International Conference on Pipeline Protection, BHR Group, Cyprus, 2 - 4 November 2005. TANDON, KK., SWAMY, G.V., SAHA, G., ‘Performance of three-layer polyethylene coating on a cross country pipeline – a case study’, 14th International Conference on Pipeline Protection, BHR Group, Barcelona, 29 - 31 October 2001. PORTESAN, G., TAVES, J., GUIDETTI, G., ‘Cases of massive disbondment with three-layer PE pipeline coatings’, Cathodic protection and associated coatings, Cefracor, EFC Event nr 254, Aix-en-Provence, France, 6 - 7 June 2002. SAUVANT-MOYNOT, V., DUVAL, S., KITTEL, S., LEFÈBVRE, X., ‘Contribution to a better FBE selection for 3 layer polyolefin coatings’, IFP, 16th International Conference on Pipeline Protection, BHR Group, Cyprus, 2 - 4 November 2005. JOHN, R., ALAERTS, E., Tyco Adhesives, ‘The importance of the hot water immersion test for evaluating the long-term performance of buried pipeline coatings’, 16th International Conference on Pipeline Protection, BHR Group, Cyprus, 2 - 4 November 2005. GAILLARD, G., BOULIEZ J. L., BS Coatings, ‘The interest of new hydrophobic epoxy primers for three-layer coatings’, 16th International Conference on Pipeline Protection, BHR Group, Cyprus, 2 - 4 November 2005. CHANG, B. T. A., JIANG, H., SUE, H-J., GUO, S., STJEAN, G., PHAM H., WONG D., KEHR, A., MALOZZI, M., LO ,K. H., ‘Disbondment mechanism of 3LPE pipeline coatings’, 17th International Conference on Pipeline Protection, BHR Group, Edinburgh, 17 - 19 October 2007. GAILLARD, G., BOULIEZ, J. L., ‘A new application process that assures good adhesion of fusion bonded epoxy coatings exposed to very severe conditions’, 18th International Conference on Pipeline Protection, BHR Group, Antwerp, 4 - 6 November, 2009. 1. 2. 3. 4. 5. 6. 7. 8. 9. Reprinted from April www.worldpipelines.com

Related Documents