Skadeanalyser, viktig bidrag til kontinuerlig forbedring/læring - med fokus på korrosjon

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1 Maintech Konferansen, Trondheim Skadeanalyser, viktig bidrag til kontinuerlig forbedring/læring - med fokus på korrosjon Eksempler Korrosjon i sprinkleranlegg Korrosjon i svømmehaller Skader i korrosjonsbeskyttende belegg offshore Ole Øystein Knudsen (ole.knudsen@sintef.no) 1

2 Dr. D. B. Blücher, Prof. Dr. O. Ø. Knudsen (SINTEF) and L. Eliassen, V. Ellefskas (Tyco FPP) Corrosion in water filled galvanized pipes 2

3 Problem Pressure increase in the pipes Blue flame when welding in a galvanized steel piping filled with municipal water after about 3 years of service White zinc corrosion products were seen after disassembly 3

4 Corrosion mechanism 1 Zinc is a very active material. This is illustrated in the stability diagram of Zn in aqueous solution. The red arrow is the hydrogen overpotential at with Zn corrodes causing the evolution of hydrogen gas. The corrosion of the zinc layer inside the pipes is governed by the selective dissolution of zinc to protect the steel. The anodic reaction for this is: (1) Zn Zn e - The cathodic reaction being: (2) O 2 + 2H 2 O + 4e - 4OH - (3) 2H 2 O + 2e - H 2 (g) + 2OH - The hydrogen evolution will cause the pressure to increase, as observed. Pourbaix diagram for Zn in aqueous solution [Pourbaix 1954] 4

5 Experimental 1 By the formation of these corrosion products, hydrogen gas evolved causing a pressure increase in the piping system (from about 7 bars of municipal water to about 18 bars) A simple test proved the presence off H 2 (g) H 2 (g) from pipe 2H 2 (g)+o 2 (g) 2H 2 O(l) Collecting H 2 (g) from pipe 5

6 Results 1 In the piping system, the water-fill-level is easily seen. Below this level, zinc corrosion products (white) can be seen. Above this level is the gas phase contains air and hydrogen gas Corrosion on mild steel Water fill level Water fill level Zinc corrosion products Corrosion on mild steel 6

7 Results 2 To further investigate the corrosion attack causing the hydrogen evolution and the pressure increase, a section of the galvanized pipe wall facing downwards was removed In this section, the partially removed zinc layer on the inside is evidence of corrosion attack Hole for nozzle facing downwards Pipe outer Zn-layer about 95µm Zn-layer corroded away Pipe inside (water-filled) 7

8 Conclusions Oxygen in the water and the air inside the pipes will cause corrosion of the zinc. The corrosion process will slow down when all the oxygen in the water has been consumed. The corrosion process can further be slowed down by removing all galvanic coupling to bare steel, i.e. at cut edges and auxiliary piping/connections. However, the corrosion will continue by hydrogen evolution from water. As long as there is Zn remaining on the mild steel piping and the pipe is filled with water, the corrosion process will proceed. The corrosion may have two possibly detrimental effects to the system: 1. The hydrogen formed in the corrosion reaction may cause fractures or leaks due to the high pressure. Hydrogen is also a highly flammable gas that is unwanted in the system. 2. The zinc corrosion products may block the sprinklers. 8

9 Korrosjon på deler av rustfritt stål i svømmehall 9

10 Problem Korrosjonsskader på festene for håndløper i svømmehall Håndløper uten skader (AISI 316) Korrosjon på nesten alle festene 10

11 Kjemisk analyse av materialet Prøve merket Feste Feste Løper Løper Standard 316 stål [4] CF8M/ UNS J92900 [5] CF10M/ UNS J92901 [5] Element [Vekt%] C 0,79 0,77 0,02 0,11 Max 0,08 0,08 0,04-0,10 Si 1,56 1,41 0,53 1,34 Max. 0,75 2,0 1,5 Mn 1,94 1,96 1,12 1,72 Max 2,0 1,5 1,5 P 0,037 0,031 0,031 0,039 Max. 0,045 0,04 0,04 S 0,015 0,014 0,005 0,014 Max. 0,03 0,04 0,04 Cr 16,2 16,4 16,6 16,4 16,0-18, Ni 9,8 10,3 9,8 9, Mo 2,2 2,2 2,1 2,2 2,0-3,

12 Mapping av elementer i materialet 12

13 Konklusjon Høyt innhold av karbon i støpelegeringa Utfelling av kromkarbider langs korngrensene i materialet Utarming på krom omkring korngrensene Korrosjon som følge av lavt innhold av krom omkring korngrensene 13

14 Skader i korrosjonsbeskyttende belegg offshore Malt TSA startet å korrodere etter få år Forventet levetid var 50 år 14

15 TSA duplex failures Sleipner Riser 1992 Troll A 1995 Kollsnes gas processing 1996 Ekofisk J 1998 Coating system: TSA 200 µm Epoxy tiecoat 50 µm Epoxy mastic 200 µm Polyurethane 60 µm 15

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18 Galvanic corrosion experiment Duplex coating PVC Steel A

19 0-100 TSZ Current [µa] Time [hrs] TSA Galvanic current (µa) Corrosion rate (mm/yr) TSA TSZ

20 TSZ TSA duplex coating 1 cm

21 Degradation mechanism New hypothesis: The cathodic reaction takes place under the organic coating What cathodic reaction? Why only on TSA and not TSZ?

22 Degradation mechanism 2 H e- H 2 Organic coating TSA Al Al e - Cl - O H 2 O + 4 e - 4 OH - Steel e - 22

23 Traditional crevice corrosion mechanism Aluminium produce acidic corrosion products Al(OH) 2 Cl, AlOHCl 2 - typically ph 3-4 Fontana and Greene Corrosion Engineering Teknologi for et McGraw-Hill, bedre samfunn

24 Tromsøysund bridge ph 3.5 was measured inside the paint blisters 24

25 Pourbaix diagram for Al M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions (London: Pergamon Press, 1966)

26 Analyse av korrosjonsskader oppsummering 1. Informasjon om material og miljø 2. Få en oversikt over korrosjonsskaden. Kan være vanskelig pga motstridende informasjon "vi har gjort dette i mange år uten problemer..." "samme materiale er benyttet mange andre steder uten problem..." 3. Komme opp med en hypotese for skademekanismen 4. Eventuelt gjøre testing, SEM, kjemisk analyse osv for å teste hypotesen 5. Eventuelt ny hypotese og ny testing... osv... til teori og observasjoner stemmer 26