Coating Performance_3

pdf
Số trang Coating Performance_3 8 Cỡ tệp Coating Performance_3 284 KB Lượt tải Coating Performance_3 0 Lượt đọc Coating Performance_3 0
Đánh giá Coating Performance_3
4.7 ( 9 lượt)
Nhấn vào bên dưới để tải tài liệu
Để tải xuống xem đầy đủ hãy nhấn vào bên trên
Chủ đề liên quan

Nội dung

9.2 Adhesion and Adhesion Strength 469 Table 9.9 Critical adhesion strength values for some coatings (Norsok, 2004); using equipment according to ISO 4624, and carry out test when coatings are fully cured Coating type/Application DFT in μm Pull-off strength in MPa (absolute minimum) Failure mode Thermally sprayed aluminium (or alloys) Thermally sprayed zinc (or alloys of zinc) Potable water tanks Tanks for crude, diesel and condensate Process vessels (<0.3 MPa, <75◦ C) Process vessels (<7 MPa, <80◦ C) Process vessels (<3 MPa, <130◦ C) Vessels for storage of methanol, etc. Fire protection (cement based) Fire protection (epoxy based) 200 100 – – – – – – – 7.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 2.0 5.0 – Cohesive – – – – – – – – results, listed in Tables 9.10 and 9.11, illustrated the complex relationships between preparation methods and applied coating systems. Cross-cut, measured after 36 months, was almost independent on the preparation method for many epoxy coatings; exceptions were coal tar epoxy and pure epoxy tank lining, where wire brushing and needle gunning showed worse results compared to hydroblasting and blast cleaning. Penknife disbondment and impact resistance, both measured after 24 months, showed worst results for the mechanical methods (especially for the wire brushing). Impact resistance was more a function of the coating system than of the preparation method; thus, blast cleaned substrates were, on the whole, only slightly superior to manual preparation under the conditions of the impact testing. Regarding the pull-off strength, measured with a commercial adhesion tester, blast cleaning methods were superior to mechanical methods. Some results are shown in Fig. 9.15. There was a certain trend for the blast cleaning methods that pull-off adhesion increased with time. Under simulated ballast tank conditions, coatings applied to blast cleaned surfaces performed far better than coatings applied to mechanically prepared substrates, and equal to those on hydroblasted surfaces. It was observed that paint failure type was often a mixture of cohesive and adhesive failures, and the appearance of the certain mode was denoted in percent (see Table 9.8). However, as shown in Tables 9.10 and 9.11, substrate failure (denoted “S”) and coat detachment occurred usually from mechanically prepared surfaces, whereas glue failure (denoted “G”) and inter-coat failure (denoted “I”) were the principal failure mode on most of the blast cleaned and hydroblasted surfaces. Björgum et al. (2007) investigated the adhesion of repair coating systems for offshore applications. Pre-rusted steel panels were cleaned with blast cleaning, power tooling and waterjetting. After an accelerated ageing test, the adhesion between coatings and steel substrates was measured with a pull-off device. Although the authors found deviations in the pull-off strength for the different surface preparation methods, these differences were statistically insignificant. Tests on contaminated substrates showed that the level of dissolved salts affected value and type of adhesion of coatings to substrates. With zero contaminants, the mode of failure was cohesive within the primer coat. As the salt level increased, 470 9 Coating Performance Table 9.10 Results of comparative adhesion tests on ballast tank coatings (Allen, 1997) Method Epoxy coating (solvent-less) Wire brushing Needle gunning Hydroblasting (Dw2) Hydroblasting (Dw2 FR) Hydroblasting (Dw3) Hydroblasting (Dw3 FR) Blast cleaning (Sa 21/2) Coal tar epoxy Wire brushing Needle gunning Hydroblasting (Dw2) Hydroblasting (Dw2 FR) Hydroblasting (Dw3) Hydroblasting (Dw3 FR) Blast cleaning (Sa 21/2) Epoxy system Wire brushing Needle gunning Hydroblasting (Dw2) Hydroblasting (Dw2 FR) Hydroblasting (Dw3) Hydroblasting (Dw3 FR) Blast cleaning (Sa 21/2) Glass flake epoxy Wire brushing Needle gunning Hydroblasting (Dw2) Hydroblasting (Dw2 FR) Hydroblasting (Dw3) Hydroblasting (Dw3 FR) Blast cleaning (Sa 21/2) Adhesion parameter Falling ball impacta Pull-off strength in MPab Penknife disbondment in mm 2 1 0 4 0 0 1 2.8/S 2.8/S 6.9/G 3.4/G 3.4/G 4.1/G 5.5/G 6 5 0 0 0 0 0 4 3 0 2 0 2 1 2.1/S 2.4/S 5.2/I 6.9/I 6.9/I 6.9/I 6.6/I 10 7 0 0 0 0 0 2 2 2 0 0 0 0 2.1/S 2.8/S 6.9/G 5.5/G 5.2/G 6.9/G 5.5/G 5 3 0 0 0 0 0 2 1 1 4 0 0 0 2.8/S 4.1/S 6.9/G 5.2/G 3.4/G 5.5/G 6.9/G 5 3 0 0 0 0 0 FR flash rust; Dw surface cleanliness according to STG 2222 a 0 = no cracking, no detachment; 1 = slight cracking, no detachment; 2 = slight cracking and detachment; 3 = moderate cracking, no detachment; 4 = moderate cracking, slight detachment b Failure mode: G = glue, I = intercoat, S = substrate progressively less primer remained adhered to the steel surface. At higher contamination level, there was a change from mixed to totally adhesive failure of the primer (Allan et al., 1995). Baek et al. (2006) reported a notable decrease in pull-off strength if the steel substrate was contaminated with chlorides. The drop in adhesion was very pronounced if a chloride concentration of 7 μg/cm2 was exceeded. Kaiser and Schulz (1987) performed cross-cut adhesion tests on coatings applied to zinc surfaces. If the samples were degreased only, the cross-cut adhesion was very low. The adhesion notably improved if the samples were blast cleaned with coal 9.2 Adhesion and Adhesion Strength 471 Table 9.11 Results of comparative long-term adhesion tests after 12, 24 and 36 months (Morris, 2000) Method Cross-cut in mm Impact resistancea Pull-off strength in MPab Time in months → 36 12 24 36 12 24 36 Solventless epoxy (2 × 125 μm DFT) Wire brushing 0 0 Needle gunning 0 0 Hydroblasting (Dw2) 0 0 Hydroblasting (Dw2 FR) 0 0 Hydroblasting (Dw3) 0 0 Hydroblasting (Dw3 FR) 0 0 0 Blast cleaning (Sa 21/2) 0 12 24 0 0 0 0 0 0 0 2 1 0 2 0 0 1 2 1 0 3 0 1 2 3 2 1 3 1 1 2 2.8/S 2.8/S 6.9/S 3.5/I 3.5/I 4.1/I 5.5/I 3.5/S 5.5/S 7.6/I 11.0/I 11.0/I 8.3/I 12.4/I 2.8/S 5.2/S 8.3/G 8.6/I 10.7/G 11.0/I 10.3/G Glass flake epoxy (2 × 125 μm DFT) Wire brushing 0 0 Needle gunning 0 0 Hydroblasting (Dw2) 0 0 Hydroblasting (Dw2 FR) 0 0 Hydroblasting (Dw3) 0 0 Hydroblasting (Dw3 FR) 0 0 0 Blast cleaning (Sa 21/2) 0 10 2 0 0 0 0 0 1 2 1 1 0 1 0 1 2 1 2 0 1 0 3 3 1 2 1 1 1 4.1/S 2.4/S 6.9/G 3.4/G 7.6/G 6.9/G 6.9/G 4.1/S 5.5/S 11.0/I 15.2/G 10.3/I 16.9/I 13.8/G 2.1/S 8.9/S >17.9/G >17.2/G 9.7/I >17.2/I 13.1/G Low temperature cure glass flake epoxy (2 × 125 μm DFT) Wire brushing 0 0 10 1 1 Needle gunning 0 0 12 1 1 Hydroblasting (Dw2) 0 0 0 2 2 Hydroblasting (Dw2 FR) 0 0 0 2 2 Hydroblasting (Dw3) 0 0 0 0 0 Hydroblasting (Dw3 FR) 0 0 0 0 1 0 0 1 1 Blast cleaning (Sa 21/2) 0 1 2 2 2 1 1 2 2.8/S 4.1/S 6.9/G 5.2/G 3.4/G 5.5/G 6.9/G 4.6/S 3.4/S 17.2/G 14.5/I 15.2/G 16.9/I 13.8/G 7.6/S 12.1/S 16.6/G 11.7/G 10.3/G 13.8/G 12.4/G Modified epoxy (2 × 125 μm DFT) Wire brushing 0 0 Needle gunning 0 0 Hydroblasting (Dw2) 0 0 Hydroblasting (Dw2 FR) 0 0 Hydroblasting (Dw3) 0 0 Hydroblasting (Dw3 FR) 0 0 0 Blast cleaning (Sa 21/2) 0 3 3 0 2 1 0 1 4.8/S 2.1/S 6.9/I 3.8/I 6.9/I 4.1/I 6.9/I 5.5/S 2.8/S 12.8/I 11.0/I 10.8/I 15.2/I 13.1/I 2.8/ S 4.1/ S 10.3/ I 8.6/ I 9.7/ I 7.9/ I 9.7/ G 0 0 0 0 0 0 0 1 2 0 1 0 0 0 1 3 0 2 0 0 0 FR flash rust; Dw surface cleanliness according to STG 2222 a 0 = No cracking; 1 = very slight cracking, no detachment; 2 = slight cracking, no detachment; 3 = moderate cracking, no detachment b Failure mode: S = substrate, I = intercoat, G = glue furnace slag. However, the authors noted an additional effect of the coating to be applied. Chlorinated polyvinyl chloride (PVC), for example, performed especially good if the zinc substrate was blast cleaned. Table 9.12 lists results of changes in adherence of two coatings on aluminium and steel after 500 h in a condensing water environment as a function of the metal pretreatment process. Although the values for the adhesion are higher in the case of the blast cleaned surface, the behaviour after exposure to water was similar for the 472 9 Coating Performance (a) (b) Fig. 9.15 Pull-off strengths after surface preparation-simulated ballast tank conditions. Preparation methods: 1 – hand brush; 2 – needle gunning; 3 – hydroblasting (Dw2); 4 – hydroblasting (Dw3); 5 – dry blast cleaning (Sa 21/2); coating thickness: 2 × 125 μm. (a) Coal tar epoxy after 24 months (Allen, 1997); (b) Glass flake epoxy after 36 months (Morris, 2000). See Table 9.11 for “S”, “I” and “G” 9.2 Adhesion and Adhesion Strength 473 Table 9.12 Adherence of coatings after 500 h condensation (Leidheiser and Funke, 1987) Substrate Coating Adhesion before and after water exposure in MPa Degreased Before After Blast cleaned Before After Aluminium Polyurethane Epoxy-polyamide 11.4 20.3 11.6 22.1 27.6 27.6 28.2 27.6 Steel Polyurethane Epoxy-polyamide 15.4 19.5 3.4 17.4 35.9 25.9 15.2 21.8 degreased and blast cleaned surfaces. There was very little effect of water exposure for both coatings and for both surface preparation methods in the case of aluminium; both coatings exhibited lower adhesion values after exposure to water for both surface preparation methods in the case of steel. The effect of cleanliness on the adhesion of thermally sprayed metal coatings to steel substrates is illustrated in Fig. 9.16. Here, substrate cleanliness is characterised through reflectivity. A value of 100% corresponded to the reflectivity of a light grey tile. The higher reflectivity, the higher is surface cleanliness (this relationship holds for a given abrasive material only). It can be seen that high cleanliness promoted high adhesion strength; the relationship was linear for both abrasive types. Another example for the effects of surface cleanliness is illustrated in Fig. 9.17 in terms of surface preparation grade. The relative adhesion of a metal-sprayed coating dropped down to 50%, if the preparation grade was lowered from Sa 3 to Sa 2. Rider (1987) reported about the bond durability of metals, pretreated with different methods, and adhesives. Wedge style durability tests were conducted, and the durability performance of blast cleaned metallic adherends was compared with standard pretreatments. It was found that blast cleaning at a blasting pressure of p = 0.45 MPa led to a notable reduction in the average length of cracks in the adherendadhesive system. After a root time of 7 h, for example, the crack length was about lC = 107 mm for abrading with distilled water, but it was lC = 60 mm only for blast cleaning. Watts and Dempster (1992), however, who applied wet blast cleaning with aluminium oxide abrasives to adhesively bonded titanium alloys, found that plain blast cleaning did not perform very well; additional preparation steps (anodising and priming) were required to obtain satisfying results. Wedge splitting tests in a corrosive environment were performed by Emrich (2003) for the assessment of adhesion between aluminium substrates and organic adherends. He found that blast cleaning (corundum and glass beads) and subsequent electropolishing reduced the lengths of the cracks in the interface zones between adhesive and substrate compared to samples which were electropolished only. Regarding the two blast cleaning media, the positive effects were stronger for the samples blast cleaned with corundum compared to samples blast cleaned with glass beads. Opposite trends were observed by Emrich (2003) for samples that were blast cleaned and subsequently pickled. In these cases, the pretreatment with corundum and glass beads deteriorated the resistance of the adhesive joint against crack propagation. The shortest crack lengths were measured for the systems where the substrate was pickled only. After an 474 9 Coating Performance Fig. 9.16 Effect of substrate cleanliness (reflectivity) on adhesion strength of arc-sprayed aluminium (Bardel, 1974). Parameters: p = 0.4–0.6 MPa; dN = 8–12 mm; x = 150–300 mm; ϕ = 60 − 90◦ . Upper curve: iron grit (dP = 100–900 μm), Lower curve: silica sand (dP = 600–1,500 μm) exposure time of about 250 h, however, the influence of the surface preparation methods vanished, and the crack length rested on a stable level of about lC = 38 mm. The author could also prove that the crack length depended on the surface roughness of the profile. A coarse profile (as achieved after blast cleaning and subsequent pickling) delivered longer cracks than a finer profile (as achieved after blast cleaning and subsequent electropolishing). Emrich (2003) also noted that the deformation behaviour of the adhesive in the wedge splitting test had an additional influence on the results. A rather rigid, less deformable adhesive promoted a quick crack extension. Bardis and Kedward (2002) performed an investigation into the effects of surface preparation methods on the strength of adhesively bonded composite joints. A double cantilever beam (DCB) test was adapted in order to measure the critical strain energy rates (G Ic ) of the bonded systems. Results are displayed in Fig. 9.18. Blast cleaned adherends had higher failure loads and higher G Ic -values than non-blast cleaned ones, though the failure mode did not change. Load displacement curves for the bonded composites also depended on preparation method. Emrich (2003) 9.2 Adhesion and Adhesion Strength 475 Fig. 9.17 Effects of surface preparation grades on the adhesion strength of metal-sprayed coatings (James, 1984) estimated the change in the shape of a shear–gliding diagram for adhesive layers. The shear–gliding diagram is comparable with a stress–strain diagram, whereby the stress is replaced by the shear stress, and the strain is replaced by the gliding of the adhesive layer. The results showed that a preparation of the substrate due to blast cleaning (corundum, p = 0.6 MPa) and degreasing with acetylene led to a notable change in the shape of the shear–gliding diagram. The use of both methods induced a distinctive drop in shear stress after a number of ten loading cycles in a corrosive medium. However, the shear modulus (ratio between shear stress and gliding) did not change after blast cleaning. Martin (1997) compared the peel resistance characteristics of pipeline coatings as functions of surface preparation procedures. Results of this study are displayed in Fig. 9.19, which shows results of peel resistance measurements after artificial ageing in a salt spray solution. Blast cleaning could notably improve peel strength, but the level of improvement depended on abrasive type and ageing duration. Aluminium oxide and steel grit delivered very good results, whereas glass beads did not contribute to an improvement in the peel strength. The positive effect of blast cleaning seemed to vanish for long ageing duration; after 16 weeks, the adhesion between coating and substrate was completely deteriorated for the degreased and the glass bead blasted samples. Figure 9.20 illustrates the situation after artificial ageing in a hot water immersion chamber. With the exception of the glass bead blasted samples, the peel resistance curves for the different surface preparation methods ran almost 476 9 Coating Performance Fig. 9.18 Effect of blast cleaning on the strain energy rate of bonded systems (Bardis and Kedward, 2002). Preparation methods: 1 – RF–RF, no blast cleaning; 2 – RF–RF, blast cleaning; 3 – VB–VB, no blast cleaning; 4 – VB–VB, blast cleaning (RF–RF = release fabric to release fabric orientation; VB–VB = vacuum bag to vacuum bag orientation) parallel to each other. A gradual reduction in the peel strength with an increase in ageing duration took place. Blast cleaning did not contribute to an improvement in adhesion. However, steel grit showed the best performance among the blast cleaning media in both test situations, and this was contributed to the high roughness at the substrate surface. Substrates with comparative roughness values (glass bead and aluminium oxide) performed quite differently under corrosive environment, and it was concluded that roughness was not the only affecting surface parameter (Martin, 1997). Changes in substrate morphology (contamination) seem to play an important role as well. The worst performance of glass bead can be contributed to the formation of a thin, with Na, Si and Ca, contaminated oxide layer (see Fig. 8.53). Staia et al. (2000) conducted tests on the adhesion of coatings thermally sprayed on steel substrates. The authors blast cleaned the substrate with aluminium oxide (d P = 425–850 μm, p = 0.34–0.62 MPa, ϕ = 75◦ ) and conducted pull-off tests and interface indentation tests. For the indentation test, they found that critical indentation load, necessary to produce a crack at the interface, as well as the critical length of the crack in the interface between substrate and coating increased if the air pressure increased. Pull-off strength also increased as pressure increased. The authors also found a relationship between air pressure and effects of coating thickness on adhesion. For the rather low air pressure ( p = 0.34 MPa), critical indentation
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.