Corrosion in water filled galvanized pipes

Transcription

1 Dr. D. B. Blücher, Prof. Dr. O. Ø. Knudsen (SINTEF) and L. Eliassen, V. Ellefskas (Tyco FPP) Corrosion in water filled galvanized pipes 1

2 Background The reason for this investigation was a pressure increase / 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 dissassembly Galvanized steel has been used in a variety of industries for many years to prevent corrosion of steel in atmospheric conditions. The galvanizing process involves the application of a thin layer (about μm) of metallic zinc to the substrate base metal which is typically mild steel. 2

3 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 3

4 Experimental 2 The surface topography of received samples was measured in an Alicona 3D infinite focus microscope The ph of the system water was analyzed by a ph Meter Radiometer phm 210 4

5 Results 1 In the piping system, the water-fill-level is obvious. 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 5

6 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) 6

7 Results 3 Optical images of the area around the water-fill-level. To the right, a topographical image of the same general area obtained by the confocal Alincona microscope is seen. Water fill level water air 7

8 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 chemical reaction for this is: (1) Zn Zn e - The counter reaction being: (2) O 2 + 2H 2 O + 4e - 4OH - Dissolved oxygen in the water is consumed, or: (3) 2H 2 O + 2e - H 2 (g) + 2OH - Pourbaix diagram for Zn in aqueous solution [Pourbaix 1954] The hydrogen evolution will cause the pressure to increase, as observed. 8

9 Corrosion mechanism 2 Although black steel pipes are used occasionally, new dry and water-filled installations are almost exclusively constructed using hot dip galvanized pipes [Van Der Schijff 2013]. In general, the corrosion rate of zinc is lower in hard water than in soft water or distilled water. This is attributed to the formation of a protective surface layer of zinc carbonates in hard waters. As seen in the table below, the corrosion rate of zinc can vary significantly, from as low as 15μm/yr to 142 μm/yr in different waters [Zhang, X.G 1996]: Roughly corresponds to Norwegian municipal water Harder water in continetal Europe 9

10 mil Corrosion mechanism 3 The relative differences in the corrosion rate of zinc between using 98% nitrogen and compressed air in a variety of different applications was compared to the right. In every environmental condition, 98% nitrogen resulted in a lower metal loss when compared directly to compressed air Average metal loss (1 mil=25,4µm) of corrosion coupons for 98%N 2 (g) vs. compressed air over a 12 month study [Tihen, J., 1974] 10

11 Conclusions When the pipes are filled with water, oxygen in the water and the air inside the pipes will cause corrosion of the zinc. The corrosion process will be slowed down when all the oxygen in the water (originally about 8ppm) has been consumed by reaction (2). If all air pockets are removed by ventilation, this will also decrease corrosion somewhat, but not significantly. 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. 11

12 Solution A possible way to solve the problem with the hydrogen pressure increase is to install evacuation valves at the highest points for the hydrogen gas to escape. Vent the system so that no air pockets can form Use of stainless steel or glass fibre re-inforced piping Dry piping over nitrogen gas The introduction of corrosion inhibitors is not suggested because of their possible negative side effects, e.g. environmental aspects and health issues. 12

13 Backup 13

14 Conclusion Zinc corrodes quite fast in tap water, the result being Zn corrosion products (white) and hydrogen evolution. These corrosion products are gathered in the piping and the hydrogen gas is collected in "pockets" above the water phase in the system. This causes a pressure increase in the whole sprinkler system. 14

15 Results The topography of the corroded pipe inner wall around the water-fill-level. 1mm Water fill level water air 15