Customized test for detecting the atmospheric corrosion resistance of uncoated AISI 316 type stainless steel in marine environment

Transcription

1 EuroCorr Paper Customized test for detecting the atmospheric corrosion resistance of uncoated AISI 316 type stainless steel in marine environment Troels Mathiesen, Henrik Bang, Torben Jensen FORCE Technology Denmark

2 AISI 316L in marine atmosphere Superficial stains caused by marine exposure Rigging on sailboat AISI 316. Polished finish. Natural washing by rain

3 Atmospheric rates of stainless steel Micropitting of stainless steel after 20 years exposure at UK sites Maximum Pit Depth, µm Finish Site 17% Cr 18%Cr 10%Ni 17%Cr 12%Ni -2.5%Mo Exposed and washed Sheltered and unwashed Exposed and washed Sheltered and unwashed Exposed and washed Sheltered and unwashed Cold rolled, softened and descaled Marine Semi-industrial Rural Dull polished Marine Semi-industrial Rural Heavy-industrial AISI 316 max. pit depth mm in marine atmosphere Source: Avesta Sheffield Handbook for Stainless Steel, 1984

4 Look out for hot surfaces Collaps of tank Wet, chloride containing insulation may cause stress cracking (SCC) or corrosion under insulation (CUI) Condensate pipe Guidelines for hot stainless steel surfaces, type 316 Requirements for insulation material: mineral wool: max. 20 ppm chloride+50 ppm silicate Apply external coating when operating in temperature region C - Epoxy or silicate paint - Thermal spray aluminium (TSA) - Aluminium foil wrap or upgrade to resistant grade

5 SCC temperature limits in ISO Nickel-base and titanium alloys are practically immune to SCC

6 AISI 316L in marine atmosphere - basic mistakes! Iron contamination or sparks from cutting Residues of ink-marking Both promote pitting at temperatures below safe limit

7 AISI 316L in marine atmosphere when shielding and deluge tests are unavoidable Least Affected Most Affected More Affected Less Affected Instrument tubing on offshore platform. Tropical climate Sheltered location Frequent fire drills with seawater Result: through-wall pitting Source: Schiroky et al. 2013

8 Slimline double block and bleed valve (SDBB) Schematic of SDBB Several leakages in 316 type SDBB operating at C on offshore platform Root cause analysis revealed - corrosion on sealing face - no material defects - poor design

9 Qualify new valve by customised testing 6-hour cycle, test duration 32 days Neutral salt spray at 35 C for 1 hrs according to ISO 9227 Dry period. Internal heating to 50 C with hot water for 2 hrs Soak period at 100 % RH and 35 C for 3 hrs Salt spray chamber

10 Qualify new valve by customised testing Valves were inspected weekly Initial rust stains apparent after first week Corrosion from cavity

11 Dismantling and detailed inspection after 32 days Failure mode seen in service was reproduced in test, i.e. corrosion along sealing face Probable loss of tightness evaluated

12 Summary for 8 valve types presence of corrosion Type Material Primary valve Secondary valve Vent valve SS conical sealing face 01 zn* 316 SS none (zinc paste) SS graphite gasket sealing face and gap on outer part of valve sealing ring and gap on outer part of valve lock pin at o-ring sealing face sealing face and gap on outer part of valve None lock pin at o-ring sealing face SS none (teflon gasket) none (teflon gasket) thread inside valve SS sealing face sealing face and thread on outer valve part SS sealing face sealing surface sealing face sealing surface pin and thread on outer valve part SMO none sealing ring (316 SS) sealing ring (316 SS) 07 25Cr DSS none sealing ring (316 SS) sealing ring (316 SS) critical borderline good No valves pass without remarks after 32 days testing Upgrading material rather than modifying sealing face seems most feasible

13 Conclusions AISI 316 in marine atmosphere works well in many applications However, small margin to corrosion promoted by contamination, shielding and elevated temperature Customised cyclic salt spray testing was able to reproduce corrosion and leakages observed on SDBB valves of AISI 316 Test was used for evaluating 8 valve designs All AISI 316-type valves show corrosion in at least one critical area that might affect tightness Application of zinc paste may delay corrosion in cavities 6Mo and Superduplex valves generally perform well in the test, but inferior AISI 316 parts compromised the result Upgrading material rather than modifying sealing faces seems most feasible

14 THANK YOU FOR LISTENING! Hope to see you in Copenhagen for 17 th Nordic Corrosion Congress May 2018