Forskningsprosjekter, korrosjonsbeskyttelse

Transkript

1 NVF seminar: Korrosjonsbeskyttelse av stålkonstruksjoner Forskningsprosjekter, korrosjonsbeskyttelse Duplex-prosjektet: Belegg med lang levetid Pulverlakk: Varmforsinket stål med pulverlakk Maincoat: Vedlikeholdsbelegg Andre forskningsprosjekter: Offshore vind energi Ole Øystein Knudsen, SINTEF SINTEF Materialer og kjemi 1

2 Duplexprosjektet Metallisering på konstruksjoner i Nordsjøen SINTEF Materialer og kjemi 2

3 TSA on some North Sea installations SINTEF Materialer og kjemi 3

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6 TSZ + 4 coats alkyd / chlorinated rubber 40 years without maintenance in costal climate SINTEF Materialer og kjemi 6

7 Sleipner Riser Platform Coating system applied topside: µm TSA - 50 µm epoxy tiecoat µm epoxymastic - 60 µm polyurethane Put in service in 1992 SINTEF Materialer og kjemi 7

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12 Troll A Coating system µm TSA - 50 µm epoxy tiecoat µm epoxymastic - 80 µm polyurethane Put in service in 1996 SINTEF Materialer og kjemi 12

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15 Experimental investigation What causes the rapid degradation observed? First hypothesis: Galvanic corrosion of TSA in contact with bare steel SINTEF Materialer og kjemi 15

16 Galvanic corrosion experiment Duplex coating PVC Steel A SINTEF Materialer og kjemi 16

17 Current [µa] TSZ TSA Galvanic current (µa) Corrosion rate (mm/yr) TSA TSZ Time [hrs] SINTEF Materialer og kjemi 17

18 TSZ duplex coating 1 cm SINTEF Materialer og kjemi 18

19 TSA duplex coating 1 cm SINTEF Materialer og kjemi 19

20 Degradation mechanism New hypothesis: The cathodic reaction takes place under the organic coating What cathodic reaction? Why only on TSA and not TSZ? SINTEF Materialer og kjemi 20

21 Degradation mechanism Organic coating TSA Steel Al Al e - Cl - O H 2 O + 4 e - 4 OH - e - Formation of aluminium chloride under the organic coating SINTEF Materialer og kjemi 21

22 Behaviour of TSZ and TSA during corrosion TSZ Formation of zinc chloride during corrosion Slightly soluble in water Little effect on ph TSA Formation of aluminium chloride AlCl 3 not stabile in the presence of water Reacts to form hydrochloric acid SINTEF Materialer og kjemi 22

23 Degradation mechanism 2 H e- H 2 Organic coating TSA Al Al e - Cl - O H 2 O + 4 e - 4 OH - Steel e - SINTEF Materialer og kjemi 23

24 5 year field test SINTEF Materialer og kjemi 24

25 TSA TSZ SINTEF Materialer og kjemi 25

26 Blistering around scribe (mm) 5 year field test Field test, 5 years Epoxy + epoxymastic + polysiloxane 4 x alkyd / CR TSA 99.5 Al AlMg5 TSA porous TSZ TSZ ZnAl15 SINTEF Materialer og kjemi 26

27 Blistering around scribe (mm) Different coatings applied on TSA Field test, 5 years B1 B2 C1 C2 D1 D2 E1 E2 R2 SINTEF Materialer og kjemi 27

28 TSA with sealer Why do we not see this degradation? Sealer TSA TSA Steel SINTEF Materialer og kjemi 28

29 SINTEF Materialer og kjemi 29

30 Conclusions Painted TSA on offshore installations have suffered from severe degradation Degradation has often started near details in bare steel connected to the structure A galvanic corrosion mechanism initiate the degradation Hydrochloric acid is formed under the organic coating The organic coating keeps the aggressive electrolyte at the surface TSA with a thin sealer only will not suffer from this type of degradation because the sealer is not able to confine an aggressive electrolyte TSA should not be painted with thick protective coatings At least not for structures to be exposed in corrosive environments SINTEF Materialer og kjemi 30

31 SINTEF Materialer og kjemi 31

32 Pulverlakk To prosjekter: Pulverlakkert og varmforsinket stål for korrosivt miljø ( ) Nålestikk i pulverlakk ( ) SINTEF Materialer og kjemi 32

33 Varmforsinket stål Ren sink Sink-jern legering med gradvis mer jern Stål SINTEF Materialer og kjemi

34 Termisk sprøyta sink 100 µm SINTEF Materialer og kjemi

35 Korrosjon av varmforsinket belegg Kilde: American Galvanizers Association SINTEF Materialer og kjemi

36 ZnMn fosfat sjikt på varmforsinket stål SINTEF Materialer og kjemi

37 Misslykket fosfatering SINTEF Materialer og kjemi

38 Pulverlakkert og varmforsinket stål Pulverlakk Fosfatsjikt (synes ikke) Sinksjikt Stål x SINTEF Materialer og kjemi

39 Average adhesion loss from scribe [mm] Korrosjonskryp på sinken - Ulike forbehandlinger 10 8 NORSOK ASTM 5 years marine 5 years industrial? A (1) ZnMnphosph. B (4) ZnMnphosph. C (8) Znphosph. D (15) ZnMnphosph. E (18) Znphosph. A (3) sweep blasted SINTEF Materialer og kjemi

40 Average adhesion loss from scribe [mm] 10 8 Korrosjonskryp på sinken - Effekt av antall strøk og lakktykkelse NORSOK ASTM 5 years marine 5 years industrial > µm PE/TGIC (8) 150 µm PE/TGIC (9) 75 µm PE/primide (10) 150 µm PE/primide (11) 75 µm primer + 75 µm PE/TGIC (13) 75 µm EP (12) 75 µm PE/TGIC (15) 75 µm EP (16) SINTEF Materialer og kjemi

41 Konklusjoner Varmforsinket stål i marin atmosfære har lang levetid Opplegering med jern under påføring gir metallurgisk binding og ekstremt god heft Overflaterust skyldes oksidering av jernet i belegget og betyr ikke nødvendigvis at belegget har korrodert bort Kvaliteten til fosfateringssjiktet er avgjørende for levetiden til pulverlakken Testing av pulverlakkert og varmforsinket stål viser lite rustkryp og kun korrosjon på sinken (ikke stålet) SINTEF Materialer og kjemi

42 Zinc rich primers Effect of zinc loading Astrid BJØRGUM 1, Ole Øystein KNUDSEN 1, Aws Yonis FAISAL 2, Roy JOHNSEN 2 1 : SINTEF Materials and Chemistry, Norway 2 : Norwegian University of Science and Technology, NTNU, Norway SINTEF Materialer og kjemi 42

43 Background cont. Parameters determining the protective properties of the primers are not fully known Factors important for corrosion protection properties are Zinc loading Zinc particle size distribution Relative pigment concentration compare to critical pigment volume concentration Primer film thickness Conductivity of the epoxy binder O. Ø. Knudsen et al., Progress in Organic Coatings(2005) SINTEF Materialer og kjemi 43

44 Objective Zinc rich primers with zinc loadings of w% are available According to the Norwegian Public Roads Administration primers with % zinc provides better corrosion protection than regular zinc rich primers with wt% zinc Challenges with the higher zinc loadings: Increased zinc loadings are expected to increase the primer porosity Blistering of the top coat due to actively corroding zinc Weaker cohesion strength Main objective has been to investigate the effect of increased zinc loading from 80 to 95 wt% on corrosion properties of zinc rich epoxy primers SINTEF Materialer og kjemi 44

45 Experimental Two sets of zinc rich epoxy primers with different binder systems A wt% zinc loading B wt% zinc loading Primers were applied on steel panels degreased and blast cleaned to Sa 2½ Open circuit potential measurements in artificial seawater were performed to investigate the ability of the primers to protect the steel cathodically Artificial seawater 3 parallel panels of each primer variant SINTEF Materialer og kjemi 45

46 Experimental cont. A three-coat paint system was applied on panels for corrosion testing: Either A or B zinc epoxy primers A commercial epoxy intermediate coat A commercial polyurethane topcoat For primers with zinc loading 90 wt% the intermediate coat was applied on both sealed and unsealed primers using a commercial epoxy tiecoat Ageing resistance corrosion testing in accordance with ISO method A The test was finished after 17 weeks of exposure SINTEF Materialer og kjemi 46

47 Results Open circuit potentials of primed panels in artificial seawater have been measured and compared to performance in cyclic tests Protection potential for steel in seawater is V Ag/AgCl The measurements showed different properties of the two primer sets A and B SINTEF Materialer og kjemi 47

48 Potential [V vs Ag/AgCl] Potential [V vs Ag/AgCl] Potential [V vs Ag/AgCl] Potential [V vs Ag/AgCl] Open circuit potential A primers wt% Zn wt% Zn Time [days] 88wt% Zn Time [days] 92wt% Zn Time [days] Time [days] SINTEF Materialer og kjemi 48

49 Potential [V vs Ag/AgCl] Potential [V vs Ag/AgCl] Potential [V vs Ag/AgCl] Potential [V vs Ag/AgCl] Open circuit potential B primers wt% Zn Zn wt% Zn J1-1 J1-2 J Time [days] 90wt% Zn -0.9 J2-1 J2-2 J Time [days] 95wt% Zn J3-1 J3-2 J J4-1 J4-2 J Time [days] Time [days] SINTEF Materialer og kjemi 49

50 Average corrosion creep [mm] Corrosion creep test 6 A variants B variants Zn content [wt %] Coating systems with A primers: Good corrosion protection No significant effect of zinc loading Coating systems with B primers Improved corrosion protection with increased zinc loading SINTEF Materialer og kjemi 50

51 Average corrosion creep [mm] Effect of tiecoat 6 A variants B variants sealed sealed sealed Zn content [wt %] No effect of tiecoat on corrosion creep (as expected) SINTEF Materialer og kjemi 51

52 Average adhesion strength [MPa] Average adhesion strength [MPa] Results of Pull-off adhesion testing A variants B variants A variants B variants Zn content [wt %] sealed sealed sealed Zn content [wt %] Fracture strength decreased with increasing zinc loading Generally mixed cohesive and adhesive failure Coating system A with 80 and 84 wt% zinc loading in the primer mainly adhesive failure Percentage cohesive failure increased with increase zinc loading No effect of tiecoat on pull-off strength or type of failure SINTEF Materialer og kjemi 52

53 Different performance of primer sets A and B Coating systems with primer A Good corrosion performance Independent of zinc loading Coating systems with primer B Not as good corrosion protection performance as primer A Increased zinc loading improved the corrosion performance Electrochemical properties, A primer Comparable primer properties for zinc loadings 84 wt% Very high capacity for cathodic protection Primer with 80 wt% zinc loading reached the protection potential after months of exposure Electrochemical properties, B primer Improved cathodic protection capacity with increased zinc loading Poor capacity for cathodic protection for zinc loading 85 wt% Primer with higher zinc loading reached the protection potential after 11 months of exposure SINTEF Materialer og kjemi 53

54 Visual examination of B primer with zinc loading 85 wt% Open circuit potential close to the corrosion potential of bare steel after 3 months exposure Corrosion of steel is the dominating anodic reaction No brown corrosion spots observed on the primer surface The rapid potential increase is explained by high resistance between Steel and primer surface Zinc particles Corrosion product is acting as a sealer and thus improving the barrier effect of the primer Steel Epoxy 85wt% Zn : Oxidised zinc particles SINTEF Materialer og kjemi 54

55 Cross sections of unexposed primers A1 A3 80wt% Zn 88wt% Zn 25 µm B1 B3 80wt% Zn 90wt% Zn : Many Zn particles had fallen out SINTEF Materialer og kjemi 55

56 Visual examination of cross-sections Comparing the primers A primers seemed to have higher zinc loadings A primers had larger zinc particle size distribution Both factors indicate better electrochemical properties for A primers Zinc loadings determined from light microscope studies are quite below specified values, particularly for B primers Primer Specified zinc loading [wt%] Determined zinc loading [wt%] A A A A B B B B : Determined values are based on one cross section SINTEF Materialer og kjemi 56

57 SEM studies of cross-sections B primer Zinc particles mainly completely surrounded by the epoxy binder is explaining: rapid potential increase high electrical resistance 90wt% Zn B3 A primer Direct galvanic contact between particles explains a higher cathodic protection capacity Almost identical and low open circuit potentials with lower zinc loadings in spite of fewer particles in direct contact A certain electrical conductivity of the epoxy binder? 88wt% Zn A3 Particles had fallen out SINTEF Materialer og kjemi 57

58 SINTEF Materialer og kjemi

59 Current [ma] Oxidized zinc (g) , ± 20 µm 0,200 0,150 0, , ± 6 µm 0,000 C C Hours SINTEF Materialer og kjemi

60 25 µm SINTEF Materialer og kjemi

61 Polarized -780 mv SCE 25 µm SINTEF Materialer og kjemi

62 Further work Duplex coatings for offshore wind farms Objective: Cost reductions Cheaper coatings But still >20 years lifetime SINTEF Materialer og kjemi 62

63 ISO test i lab Med TSZ Uten TSZ SINTEF Materialer og kjemi 63

64 Uten TSZ Felttest i plaskesone, 2 år Med TSZ SINTEF Materialer og kjemi 64

65 Forenklet beleggsystem TSZ, 100 µm Polysiloxan mistcoat Polysiloxan ca 250 µm Feilet fullstendig i labtest Står bra i felttesten! SINTEF Materialer og kjemi 65

66 Indre spenninger i belegget fører til at det sprekker. På bjelken var belegget litt for tynt, slik at korrosjon startet etter relativt kort tids eksponering. Korrosjonsproduktene førte til belastning på belegget som gjorde at det sprakk i stedet for å tøye seg. Korrosjon spredde seg så i sprekkene og førte til ny sprekking. SINTEF Materialer og kjemi

67 Indre spenning Indre spenning skyldes volumendring i belegget som gjør at det ønsker å trekke seg sammen Når vi belegger et tynt substrat vil det bøye seg SINTEF Materialer og kjemi

68 Slyngerensing med runde shot Blåserensing med skarpkantet gritt SINTEF Materialer og kjemi

69 Shot er runde og gir en avrundet profil Gritt er skarpkantet og gir en tagget profil Shot brukes i slyngerensing fordi det ikke sliter så sterkt på skovlene Slyngerensing er automatisert godset sendes gjennom anlegget der skovler slynger shot mot overflata. Kostnadseffektivt Gritt brukes bare ved manuell blåsing mer arbeidsintensivt og fysisk tungt. Gir vanligvis bedre heft enn slyngerensing SINTEF Materialer og kjemi