[updated:LAST EDITED ON Jun-24-10 AT 07:22 AM (CDT)]We had a leak on the cooling coil made from copper nickel (90/10). I suspect the problem was due to high pH water (almost 10) resulting in dissolution of the material. Is it safe to use titanium or stainless steel cooling coil under these conditions? Are there any other copper family materials, which would be suitable? Thanks a lot

Thanks for commenting on the case - this lack of resistance is exactly what puzzles me. It was put in operation in 1996. There were no defects on this component in the past. The water which was used for cooling until 2008 was basically taken from the river without much of the precleaning; on average its pH was 8,5. After 2008 the special clean water was used for cooling until now; on average its pH is 9,8. I do not have information about the flow rate, but I think this not part of the problem, since there are definitely no signs of any erosion on the inner surface of the cooling coil. We did some failure investigation and it appears that the coil defects are linked to these two periods of operation. There were several through-wall dimples on the inner surface, which had blister-like deposits or their remains above them. Chemical microanalysis found sulphur under the deposits. We believe that these defects are caused by MIC, as we had similar problem on the austenitic steel tubing welds where practically the same water flowed. There are areas in the inner surface with more or less surface corrosion were dezinfication is taking place. Corrosion is reaching up to 1/3 of the wall thickness. I think that this may be caused by high pH water used since 2008. The operator claims it detects the defects immediately. Our conclusion is that there was MIC until 2008, but it did not grow through-wall. After 2008 with changed water chemistry the dissolution of the material took place and caused the final fracture. Would this make any sense to you? Thanks for your time. Mark

Mark Kouba 31 Cu Ni alloys are resistant to high pH water and also from reasonable levels of pitting resistance in river water or now changed to high pH de-min water. From description and failure experienced - since the Cu Ni coil had been in service since 1996 it appears the origin of pitting (usually dimples at some places where due to MIC (organic /inorganic) and some conditions of stagnant water during non-use could have initiated the pitting some years back From two years back when you changed the water chemistry from river water, the pre-existing de-zincified or de-nickel areas could have promoted into fine cracks to give this leak in coil Cu Ni is well proven material for sea water (turbid water with high chlorides / sulphides as well) and in high pH water chemistry. For this water chemistry, it is quite suitable and cost effective also. Cu Ni tubes should easily withstand flow rate of 3-4 m/sec for this high pH water chemistry. You had also not seen any erosion problem for the last 15 years (effectively confirming that the flow rate could have been well within design parameters - 3 -4 metres/sec) Considering the 15 year operational success, you might re-think of any material change at this stage unless you had definitely identified the cause as de-zincification or de-nickelification problem combined with MIC and also pitting initiated earlier not diagnozed properly till 2008. For a compounded problem of 15 years,to me it looks, change of material replacement with Ti could be thought of at a later stage after proper failure investigation going through operational and maintenance histories of the cooling coil in 15 years time. Hope this helps C.V.Srinivasan Nishi Engineers Pvt Ltd India June 29, 2010 E-mail; nishi@vsnl.com >Thanks for commenting on the case - this lack of resistance >is exactly what puzzles me. > >It was put in operation in 1996. There were no defects on >this component in the past. The water which was used for >cooling until 2008 was basically taken from the river >without much of the precleaning; on average its pH was 8,5. >After 2008 the special clean water was used for cooling >until now; on average its pH is 9,8. I do not have >information about the flow rate, but I think this not part >of the problem, since there are definitely no signs of any >erosion on the inner surface of the cooling coil. > >We did some failure investigation and it appears that the >coil defects are linked to these two periods of operation. >There were several through-wall dimples on the inner >surface, which had blister-like deposits or their remains >above them. Chemical microanalysis found sulphur under the >deposits. We believe that these defects are caused by MIC, >as we had similar problem on the austenitic steel tubing >welds where practically the same water flowed. > >There are areas in the inner surface with more or less >surface corrosion were dezinfication is taking place. >Corrosion is reaching up to 1/3 of the wall thickness. I >think that this may be caused by high pH water used since >2008. > >The operator claims it detects the defects immediately. Our >conclusion is that there was MIC until 2008, but it did not >grow through-wall. After 2008 with changed water chemistry >the dissolution of the material took place and caused the >final fracture. > >Would this make any sense to you? Thanks for your time. >Mark