Performance characteristics of protective coatings ...

Cold Regions Science and Technology 127 (2016) 76–82

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Cold Regions Science and Technology journal homepage: www.elsevier.com/locate/coldregions

Performance characteristics of protective coatings under low-temperature offshore conditions. Part 1: Experimental set-up and corrosion protection performance A.W. Momber a, M. Irmer b, N. Glück b a b

Muehlhan AG, Schlinckstraße 3, 21103 Hamburg, Germany Fraunhofer Application Center for Large Structures in Production Engineering, Albert-Einstein-Straße 30, 18059 Rostock, Germany

a r t i c l e

i n f o

Article history: Received 19 June 2015 Received in revised form 19 October 2015 Accepted 31 March 2016 Available online xxxx Keywords: Coatings Corrosion protection Icing Mechanical loads

a b s t r a c t Six organic coating systems are investigated according to their performance under Arctic offshore conditions. Four performance groups are considered: corrosion protection performance, performance under mechanical loads, surface status and icing performance. The investigations involve the following tests: accelerated corrosion protection/ageing tests, tests for coating adhesion, hoarfrost accretion measurements, impact resistance tests, abrasion tests, and wettability tests. The test conditions are adapted to Arctic offshore conditions, which mainly cover low temperatures down to −60 °C. A testing facility for hoarfrost performance tests is developed. The coating systems are organic coating systems which differ in generic coating material, hardener, number of layers, dry film thickness and application method. Part 1 describes the testing programme and discusses the results of the corrosion protection performance tests. Part 2 discusses the results of the surface topography measurements, wettability assessment, hoarfrost formation and mechanical testing. A procedure for the ranking of the coating performance is developed. The best performing system in the scope of evaluation is a three-layer system with high thickness (1400 μm), consisting of two glass-flake reinforced epoxy coats and a polyurethane topcoat. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Oil and gas exploration in Arctic regions is a future scenario for energy delivery. In due of this development, protective coating systems will face a number of challenges, mainly caused by the harsh environment. The environment is characterized by violent wind, high waves, very low air temperatures, infrared radiation, floating and pushing ice, rime and snow. The latter issues, in particular, will have effects on the performance of surface protection systems. It is known that steel corrosion will not be accelerated in low-temperature sea water (Melchers, 2002) or low-temperature atmosphere (Mikhailov et al., 2008), although the water may show increased oxygen contents. It is not an increase in corrosivity, but rather the question how surface protective coatings will respond to the harsh environment, that will determine the performance of organic coatings. This would specifically include the following items: - corrosion protection capacity; - icing and de-icing behaviour; - response to mechanical loads.

E-mail address: [email protected] (A.W. Momber).

http://dx.doi.org/10.1016/j.coldregions.2016.03.013 0165-232X/© 2016 Elsevier B.V. All rights reserved.

In terms of corrosion protection capacity, the low air temperatures may be a special challenge to the coatings. Temperatures as low as − 60 °C can be expected in Arctic regions. Standard testing scenarios for offshore coatings (ISO 20340, 2009; Norsok M-501, 2012) request air temperatures up to − 20 °C only, and it is not known how organic coatings may perform at lower temperatures. The response to mechanical loads, namely to impact, will also be affected at low temperatures. It is known that the response of organic materials changes from plastic response to elastic, or elastic–plastic, response (Hainsworth and Kilgallon, 2008), and to higher rigidity modulus at low temperatures (Murase and Nanishi, 1985). This would, among others, induce a susceptibility to cracking (Bjoergum et al., 2011). Mechanical near-surface parameters, such as scratch hardness, are also affected at low temperatures (Hainsworth and Kilgallon, 2008). These modifications are also considerable for the entire system “substrate-coating”, which may react in a modified way if exposed to very low temperatures. A parameter describing this response is the adhesion strength between substrate and coating. Icing and de-icing are crucial processes in terms of efficiency and safety of offshore structures (ISO 12949, 2001; ISO 19906, 2010; Ryerson, 2011). ISO 12949 (2001) distinguishes between two types of atmospheric ice: glaze and rime, whereby the formation of either ice type depends mainly on air temperature and wind speed. Glaze, which forms in the splash water zone, is caused by freezing water; it

A.W. Momber et al. / Cold Regions Science and Technology 127 (2016) 76–82

features a high density and rather high adhesion/cohesion strength numbers. Rime typically forms in the atmospheric zone due to incloud icing; it has a moderate density, and it is usually vane-shaped. Another phenomenon is hoarfrost, which forms due to direct phase transition from water vapour into ice; it is common at low temperatures. Hoarfrost is of low density and strength (Fikke, 2006). On offshore platforms, either type of ice may be found. Although active icing prevention strategies, such as heating, are powerful, they cannot be used elsewhere on a platform. Passive icing prevention in terms of icephobic coating surfaces is a very attractive alternative. The same is true for anti-icing. Active methods, such as vibrations, heating or mechanical scraping, could be replaced or supported through coating surfaces that promote a weak adhesion to adhering ice (Antonini et al., 2011). There are no studies known dealing with the above discussed problems in a systematic way, particularly not for offshore conditions. In due of a long-term (13 years) exposure study, Hattori et al. (1991) investigated the response of 12 coating systems on an Alaska site at temperatures between −25 °C and +16 °C. The coating systems included inorganic zinc-rich paint, thermally sprayed zinc and aluminium, rubber, epoxies and polyurethanes. The authors measured adhesion strength, top coat chalking, coating thickness and steel corrosion. Coating deterioration due to freezing and thawing could not be detected, neither due to UV radiation. In terms of corrosion protection, the authors found that vinyl chloride and chlorinated rubber coatings did not perform well at low temperatures. Zinc-rich paints and zincsprayed coatings along with epoxy/polyurethane systems were found to perform suitably. The performance of high-performance composite coatings (HPCC), suitable for protecting pipelines, under a wide range of temperatures (− 50 °C to + 95 °C) was investigated by Singh et al. (2005). The authors found that the impact resistance decreased with a decrease in temperature. Shore hardness, in contrast, increased moderately if temperature dropped. Coating flexibility was not affected in the temperature range between − 30 °C and − 40 °C. More recently, Bjoergum et al. (2011) investigated the behaviour of five coating systems under temperatures between − 10 °C and − 60 °C. They found that polysiloxane topcoats did not perform well under corrosive and mechanical load at low temperatures. Reinforced polyester coatings and vulcanized rubber performed better, whereas a conventional 3layer system featured very high corrosion creep numbers. Regarding the adhesion strength of the coatings, the authors could not establish a proper trend, because some coating systems showed an increase in pull-off strength at lower temperatures, while others showed a decrease. Impact tests delivered good results, except for the polysiloxane coating. Indentation tests delivered an increase in hardness at lower

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temperatures. Ice adhesion tests revealed definite relationships to coating system and temperature: rubber and reinforced polyester showed the highest values, and the values increased with a decrease in water temperature. An effect of the water composition (distilled water versus sea water) was also noted. None of these investigations considered offshore conditions. It is the objective of this paper to systematically investigate, and to evaluate, the performance of organic coating systems suitable for standard offshore applications under simulated low-temperature offshore conditions. 2. Testing programme 2.1. Coating systems Six organic coating systems with proven records for the protection of steel structures under offshore splash zone and offshore atmospheric zone conditions were selected for the investigations. Their basic compositions and properties are listed in Table 1. The systems included 1-pack and 2-pack systems. The 2-pack systems consisted of a resin (generic type) and hardener to be mixed together for curing. The 1-pack systems, in contrast, did not contain hardener, but cured due to moisture contained in the surrounding environment. The systems also included coatings with different hardeners and generic types; materials with and without fillers; different filler materials; low- and high-solid materials; and systems with different layer compositions. The total dry film thickness numbers for the systems ranged from 375 μm to 1500 μm. The coating systems were supplied by diverse coating material manufacturers. All coating systems were applied according to the specifications of the suppliers. Steel substrates were prepared by blast-cleaning to a surface preparation grade Sa 2½ according to ISO 8501-1 (except system 1, which required Sa 2 only). The roughness of the substrates was between Rz = 50 μm and 75 μm. 2.2. Testing procedures The testing procedures employed for this study are summarized in Table 2. They consisted of four testing groups, namely (i) corrosion protection, (ii) mechanical properties, (iii) surface status, (iv) icing. 2.2.1. Accelerated ageing test The corrosion protection performance was investigated by means of an accelerated ageing procedure prescribed in ISO 20340 (2009) for offshore coatings. This procedure includes a combination of UV/condensation, salt spray, and low-temperature exposure cycles. The exposure

Table 1 Investigated coating systems. System

Layer

Generic type (resin)

Hardener

Filler

Solids in vol.-%

Density in kg/l

DFT in μm

1

1 2 3 1 2 3 1 2 3 4 1 2 3 1 2 3 1 2 3

Epoxy

Polyamine

Glass flakes

88

1.34

Polyurethane Epoxy

Aliphatic Polyamine

– –

67 79

1.29 1.60

Polyurethane Polyurethane

(moisture-hardened) (moisture-hardened)

Zinc dust –

65 72

2.80 1.40

Epoxy Epoxy Polyurethane Epoxy

Phenalkamine Amine Isocyanate Phenalkamine

– – – Aluminium

95 47 57 60

1.50 1.53 1.21 1.23

Epoxy Epoxy

Amine Phenalkamine

– –

51 95

1.34 1.50

400 400 400 175 175 175 50 150 150 150 500 40 75 150 150 75 500 500 500

2

3

4

5

6

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Table 2 Testing methods. Group

Property

Test

Standard

Corrosion protection

Surface status

Offshore protection Coating adhesion Impact strength Abrasion resistance Wettability

Icing

Surface topology Hoarfrost accretion

Accelerated ageing Pull-off strength Impact resistance Taber testa Contact angle Surface energy Roughness Icing test

ISO 20340 ISO 4628 ISO 6272-1 ISO 7784-2 DIN 55660-2 DIN 55660-1 ISO 8503-4 No standard

Mechanical properties

a

Slightly modified because of smaller test panels.

cycle lasts one week (168 h), and it includes the following stresses: 72 h (3 days) of exposure to UV (UV [A] lamps) and water; 72 h (3 days) of exposure to salt spray; and 24 h (1 day) of exposure to low temperature (− 60 °C ± 2 °C). The complete testing period included 25 cycles (25 weeks). The testing temperature for the low temperature exposure (− 60 °C) was notably decreased for the testing, because the original procedure specifies −20 °C only, which was assumed insufficient for a realistic scenario. Three samples were tested under each condition. Specimens are shown in Fig. 1. They were injured with an artificial scribe, which went through the entire coating system down to the bare steel. This scribe characterizes a mechanical damage to the coating. Assessment criteria for the corrosion protection performance were the following: degree of rusting, degree of blistering, and scribe delamination. These parameters were summarized in an “Anticorrosive Effect” (AE) as suggested by Kalenda et al. (2004):

AE ¼

AðBlisteringÞ þ BðDelaminationÞ þ 2 CðRustingÞ : 4

ð1Þ

Here, AE is the anticorrosive effect. A is a term related to the blister degree (according to ASTM designation); B is a term related to scribe delamination; and C is a term related to rust degree (according to SSPC designation). A value of AE = 100 characterizes the performance of a newly applied, undeteriorated coating system. The parameters A to C must be estimated according to a matrix procedure (Kalenda et al., 2004; Momber et al. 2015). This integral parameter was successfully utilized for the assessment of coating systems for offshore wind towers (Momber et al., 2015).

Fig. 1. Specimens under accelerated ageing.

2.2.2. Coating adhesion The coating adhesion tests were performed at different temperatures according to ISO 4624 (2014) with a hydraulically driven digital pull-off tester “Positest AT-M” (DeFelsko, Ogdensburg, USA). The setup is shown in Fig. 2. The dolly size was 20 mm, and the stress rate was 0.25 MPa/s. The adhesive was a cyanacrylat-based material (“Ruderer 5102”). Adhesive and coating were cut to the substrate around the circumference of the dolly. The tests were performed for each coating system under four different sample temperatures: +20 °C, 0 °C, − 20 °C and − 60 °C. Pull-off strength and fracture type were documented for all individual tests. Fracture types were visually estimated at the loaded coating section and at the reverse side of the dolly with a standardized accuracy of 10% (see Fig. 2b). Three measurements were done for each combination. 2.2.3. Wettability and surface topography The wettability was assessed based on static contact angle measurements according to DIN 55660-2 (2011) and on specific surface energy measurements according to DIN 55660-1 (2011). In contrast to previous investigations, wettability was assumed to be a strictly local parameter that varies even over the rather small sample size. For this reason, wettability-topographies were established on the sample surfaces. Each sample surface was subdivided into nine (smaller samples) or 15 (larger samples) fields of equal size (see Fig. 3a). On each field, five contact angle measurements were performed. The measuring machine was a video-based system “OCA 20” (DataPhysics, Filderstadt, Germany) with an accuracy of ΔθC = ±1°. Fig. 3b provides a screenshot of a settled drop. Wetting liquids were deionized water, diiodmethane and foramide. The drop volume was 5 μl, and the dosing rate was 2 μl/s. The specific surface energy was estimated with the Owen–Wendt method (DIN 55660-1, 2011):

ð1 þ cosθC Þ σ L P 1=2 1=2 ¼ σ S 2 σ DL

σ PL σ DL

!1=2

1=2 þ σ DS :

ð2Þ

The parameter σ is the specific surface energy, the subscripts S and L stand for solid and liquid, and the superscripts D and P stand for dispersive and polar. The surface profile parameters were estimated with a mechanical stylus instrument “Hommel-ETANIC T1000” (Jenoptic, Jena, Germany). They were measured on the same local fields where wettability measurements were performed. Three measurements in horizontal direction and three measurements in vertical direction were made in each field. A stylus instrument according to ISO 3274 (1998) was utilized. The cut-off length was 4.8 mm. The assessment parameters are arithmetic mean roughness (Ra) and average maximum roughness (Rz) according to ISO 4287 (1997). 2.2.4. Hoarfrost accretion A special testing device was developed for hoarfrost formation assessment; it is illustrated in Fig. 4a. It consisted of a heated water quench, an icing chamber, and a cooled sample holder. The heated water quench was designed to keep the relative humidity higher than 80%. The icing chamber (size 350 l) featured the following parameters: temperature range −70 °C to +180 °C; relative humidity 10 to 98% (as from + 10 °C); temperature ramp 3.5 K/min. The test duration was 90 min. Hoarfrost formed due to resublimation of wet air on the cooled coating surfaces. The sample surface temperature was kept on a stable level of − 2 °C during the tests; it was permanently controlled by means of a thermocouple. The ambient air temperature was kept on +1 °C. Assessment criterion was the thickness of the ice layer formed. The thickness was measured with a specially designed gauge, which can be recognized in Fig. 4b. It works in a similar way as a wet film gage known from coating application work.


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Fig. 2. Set-up for coating adhesion (pull-off strength) measurement, a — Pull-off tester (1) with sample (2), dolly (3) and fracture area (4) b — Fractured areas (1 to 3) and reverse side of testing dolly (D) after testing; arrow marks separation cut between tested circular area and surrounding coating.

2.2.5. Mechanical properties Mechnical testing included impact tests and abrasion tests. The impact resistance test was performed according to ISO 6272-1 (2011) by means of a falling 2 kg-weight with a 20 mm tip diameter. The testing arrangement is provided in Fig. 5. The weight was dropped from different heights which corresponded to different impact energies. The tests

were performed at four temperature levels: +20 °C, 0 °C, −20 °C and − 60 °C. The impact height (H) was successively increased in 5-cmsteps. Results are reported based on visual evaluation of the impact area using a stereo microscope at ×10 magnification. The assessment criterion was the impact energy (in J) that caused cracking in the coating system. Impact energy is simpy: E = m·g·H, with g = 9.81 m/s2.

Fig. 3. Contact angle measurement arrangement, a — Subdivision of specimen surfaces into measurement fields; right column lists contact angle (water) ranges in ° b — Screenshot of a settled drop and contact angle definition.

80


Fig. 4. Hoarfrost testing device, a — General arrangement b — Hoarfrost thickness measurement.

The abrasion resistance was estimated with a rotating abrasive wheel (Calibrase CS-10, load: 1 kg ± 10 g) according to ISO 7784-2 (2006). The testing configuration is illustrated in Fig. 6a. The configuration was slightly modified in order to consider smaller sample sizes. Abraded material debris was collected for further investigations,

particularly for SEM inspections. The tests were performed at two temperature levels: 20 °C and 0 °C. A total of 2000 revolutions were performed for each test. The coating thickness was measured after each 250th cycle on eight defined measurement points. The assessment criterion was the reduction in coating thickness (Δh).

Fig. 5. Impact testing apparatus and cold chamber with temperature control for sample storage.

Fig. 6. Set-up for abrasion resistance tests, 1 — weight; 2 — abrasive wheel, 3 — coated specimen, 4 — wear track, 5 — debris suction nozzle.


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3. Results of corrosion protection performance tests 3.1. Accelerated ageing test Table 3 contains images of coated samples for three situations: before testing (new), after the accelerated ageing procedure (25 weeks) at very low temperature (−60 °C) in the testing chamber, and after inspection. Numerous coating degradation features can be distinguished on the samples, namely chalking, blistering, underrusting, and scribe delamination. It can also be seen that the coating systems performed differently. Some of the systems showed severe scribe delamination. The AE numbers, based on Eq. (1), are listed in the lower row in the table. They range from AE = 25 to 71; thus they vary by 284%. The low numbers witness a notable deterioration of the coating systems during the ageing process. The numbers are in tendency notably lower than AE numbers published by Momber et al. (2009) for organic coatings subjected to the standard ageing test (ISO 20340), where values between AE = 65 and 82 were reported. Obviously, the temperature reduction from −20 °C to −60 °C contributed to an accelerated deterioration of the coating systems. For the zinc-dust based coating system (3), which performed best, the AE-value 71 corresponds well with numbers reported by Momber et al. (2009) for this type of coating. Systems 2 and 5 show the worst performance. System 2 was not particularly specified for offshore conditions, but is known to perform excellent under C5-M and Im2 corrosivity conditions. However, this system seems to be very sensitive to steep temperature changes. Fig. 7 summarizes the results for the two different temperature scenarios. The AE-numbers are considerably larger at − 20 °C for all samples (between 76 and 94) compared to the results for −60 °C. These results clearly indicate a deterioration of the corrosion protection performance of the coating systems at the very low temperature. There were, however, differences in the sensitivity of the coatings to temperature variations. Systems 1 and 6 were rather insensitive (moderate change in AE-number), whereas systems 2 and 5 were very sensitive to temperature modifications. Two examples are provided in Table 4. The standard deviations for AE showed of higher numbers at the lower temperature. Differences in local coating structure, or localized corrosivity in the chamber, seemed to be more influential on the corrosion protection capacity at the very low temperature.

Fig. 7. Effects of coating system and testing temperature on AE numbers.

recognized. Firstly, the values for the pull-off strength vary in a wide range from low numbers for the system 3 to very high numbers for the systems 1, 2 and 6. Secondly, pull-off strength increases with a decrease in temperature in tendency. This trend confirms results reported by Bjoergum et al. (2011) for epoxy and rubber coatings. The low values for the coating system 3 can be related to the fracture in the zinc-dust primer layer. Zinc-dust is known to be susceptible to mechanical load because of the low-strength interfaces between zinc particles. Thirdly, two groups of coating systems can be distinguished. The first group is rather unsusceptible to changes in testing temperature; this group includes systems 2, 3, and 6. The second group shows an explicit relationship between testing temperature and pull-off strength; this group covers systems 1, 4, and 5. Interestingly, systems 4 and 5 also showed a high temperature-sensitivity with respect to the accelerated ageing test (see Table 6). No general trends could be noted with respect to the fracture type. However, for some coating systems, for example 2 and 5, the frequency of cohesive fracture (in the coating layer) increased at lower temperatures. System 4 responded mainly with cohesive fractures at all temperature levels. System number 1 showed mixed fractures in all cases. System 6 showed adhesive fractures between 3rd coating layer and glue.

3.2. Coating adhesion test A good adhesion of the coating to the substrate is an essential preposition for a good corrosion protection capacity. The results of the adhesion pull-off tests are summarized in Fig. 8. Three trends can be

Table 3 Results of the accelerated ageing tests for −60 °C. Coating system Situation

1

2

3

4

5

6

88 ± 2.5

25 ± 2.5

71 ± 6.6

47 ± 7.6

32 ± 14.4

78 ± 0

New

After ageing

After assessment

AE number

82


Table 4 Sensitivity of coating systems to very low temperatures. Situation

Coating system 1 (low sensitivity)

2 (high sensitivity)

−20 °C

−60 °C

−20 °C

−60 °C

94

88

76

25

After ageing

After assessment

AE-number

Fig. 8. Effects of coating system and temperature on pull-off strength.

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