Investigating the Corrosion Behaviour of Zircaloy-4 in LiOH under a Thermal Gradient and Two Phase Flow Regime. Helen Hulme 1 , Alexandra Panteli 1 , Felicity Baxter 1 , Mhairi Gass 1 , Aidan Cole-Baker 1 , Paul Binks 1 , Mark Fenwick 1 , Michael Waters 1 , James Smith 2 woodplc.com
Outline • Part one – influence of two phase flow – Introduction – Testing procedure – Results – Discussion • Part two – relationship between [LiOH] & Temp. – Introduction - Literature data – Testing procedure – Results & Discussion • Summary & Industrial Context
Part one – influence of two phase flow on corrosion of Zircaloy-4 in LiOH 3
Introduction Objective: To understand the influence of two-phase flow (subcooled nucleate boiling) within thick oxide films under a thermal gradient on the corrosion properties of zirconium alloys in LiOH chemistry. Is there potential to cause a localised increase in LiOH concentration – in cracks / pores, as a result of boiling in thick oxide films, leading to accelerated corrosion? What is the critical LiOH concentration required to cause accelerated – corrosion? Could this be detrimental to the use of LiOH without boric acid – additions, eg. For SMR design considerations?
Testing Conditions Heatin ting block ck T esting under a thermal gradient was performed in Wood’s “heat flux rig” Recirculating autoclave loop • Pressurised system • Single coolant temperature • Variable flow across • Specim cimen n No specim cimen Water r flow specimen enclo close sed Δ T across specimen (<90 °C) • 5
Testing Conditions Test Initial Oxide Water Chemistry Thermal Gradient Pre-Stressed Boiling Parameters Thickness (µm) 1 LiOH Yes No No 0 2 LiOH Yes No Yes 3 NH 4 OH Yes No Yes 4 LiOH Yes No No < 20 5 LiOH Yes No Yes 6 LiOH No No No 7 LiOH Yes No No 8 NH 4 OH Yes No No 9 NH 4 OH Yes No Yes > 20 10 NH 4 OH No No No 11 LiOH Yes No Yes 12 LiOH Yes Yes Yes 6 A presentation by Wood.
Testing Conditions Test Initial Oxide Water Chemistry Thermal Gradient Pre-Stressed Boiling Parameters Thickness (µm) 1 LiOH Yes No No 0 2 LiOH Yes No Yes 3 NH 4 OH Yes No Yes 4 LiOH Yes No No Zircaloy-4 sheet < 20 5 LiOH Yes No Yes specimens pre-filmed in 6 LiOH No No No 500 °C air to form 7 LiOH Yes No No relatively thick oxide films 8 NH 4 OH Yes No No 9 NH 4 OH Yes No Yes in a timely manner > 20 10 NH 4 OH No No No 11 LiOH Yes No Yes 12 LiOH Yes Yes Yes 7 A presentation by Wood.
Testing Conditions Test Initial Oxide Water Chemistry Thermal Gradient Pre-Stressed Boiling Parameters Thickness (µm) 1 LiOH Yes No No 0 2 LiOH Yes No Yes Corie ieu et al al, 1962 Corrosion 3 NH 4 OH Yes No Yes 4 LiOH behaviour of Zircaloy-4 in Yes No No < 20 5 LiOH Yes No Yes NH 4 OH similar to water (i.e. no 6 LiOH No No No accelerated corrosion even in 7 LiOH Yes No No extreme concentrations). 8 NH 4 OH Yes No No Using NH 4 OH removes any 9 NH 4 OH Yes No Yes effect of pH > 20 10 NH 4 OH No No No 11 LiOH Yes No Yes 12 LiOH Yes Yes Yes 8 A presentation by Wood.
Testing Conditions Test Initial Oxide Water Chemistry Thermal Gradient Pre-Stressed Boiling Parameters Thickness (µm) 1 LiOH Yes No No 0 2 LiOH Yes No Yes 3 NH 4 OH Yes No Yes 4 LiOH Yes No No < 20 5 LiOH Yes No Yes 6 LiOH No No No 7 LiOH Yes No No 8 NH 4 OH Yes No No 9 NH 4 OH Yes No Yes > 20 10 NH 4 OH No No No 11 LiOH Yes No Yes 12 LiOH Yes Yes Yes 9 A presentation by Wood.
Testing Conditions Test Initial Oxide Water Chemistry Thermal Gradient Pre-Stressed Boiling Parameters Thickness (µm) 1 LiOH Yes No No 0 2 LiOH Yes No Yes 3 NH 4 OH Yes No Yes 4 LiOH Yes No No < 20 5 LiOH Yes No Yes 6 LiOH No No No 7 LiOH Yes No No 8 NH 4 OH Yes No No 9 NH 4 OH Yes No Yes > 20 10 NH 4 OH No No No 11 LiOH Yes No Yes 12 LiOH Yes Yes Yes 10 A presentation by Wood.
Pre-stressed Oxides Standard specimens are pre-filmed unrestrained. • Pre-filmed Oxide T est set-up induces a stress on the Zircaloy specimen upon heating. • Zircaloy Specimen T o reduce this, specimen is pre-filmed attached to the heater block, conditioning it to • Attach to heater the test environment block and Heat-up Pre-filmed Oxide Zircaloy Specimen Steel Heater Block 11 A presentation by Wood.
Results 12
Coolant chem. 2 ppm LiOH Results – influence of oxide thickness Autoclave temp. 250 °C Thick k Oxides es (>20 0 µm) ) Thick = >20 µm re requi uired ed for r accelera rated ted Thin = = <2 <20 µm corrosion osion to occur 15 A presentation by Wood.
Coolant chem. 2 ppm LiOH Results – influence of boiling Autoclave temp. 250 °C Within thick k ox oxid ides es (>20 µm), boiling ing is re requi uired ed for r accelerate rated d corrosion osion to occur ur 16 A presentation by Wood.
Coolant chem. 2 ppm LiOH Results – influence of chemistry Autoclave temp. 250 °C For thick ck ox oxides des (>20 µm) under boiling ing conditions, LiOH is re requir uired ed for r accelera rated ted corrosion sion to occur. Accelerated corrosion is not observed in the presence of NH 4 OH 17 A presentation by Wood.
Coolant chem. 2 ppm LiOH Results – influence of stress Autoclave temp. 250 °C For thick ck oxides des (>20 µm) under boiling ing conditions in LiOH, stress ess in the ox oxide ide is re requi uired ed for accelerate rated d corrosion osion to occur ur. Accelerated corrosion is not observed if the oxide is formed in a pre-stressed condition 18 A presentation by Wood.
Discussion 19
Discussion – key observations For accelerated corrosion to be observed during testing, the following criteria must be met: • Thick >20 µm oxide film – Sub-cooled boiling – LiOH chemistry – Stress – Key observations: Results from pre-filmed specimens under non-boiling conditions do not show • accelerated corrosion, indicating this is not a m memor ory y effect ct. Comparable test conditions using NH 4 OH do not show accelerated corrosion, • demonstrating that LiOH does have e an effect ct. 20 A presentation by Wood.
Discussion – Hypothesised Mechanism Effect of Stress Effect of Boiling Effect of LiOH Stress causes the pores & cracks present in the oxide to open and create a more accessible pathway for the coolant to penetrate nearer the metal / oxide interface 21 A presentation by Wood.
Discussion – Hypothesised Mechanism Effect of Stress Effect of Boiling Stress causes the pores Boiling within the cracks & & cracks present in the pores that have limited oxide to open and accessibility to the coolant create a more causes localised accessible pathway for concentration of LiOH the coolant to solution to levels above that penetrate nearer the of the bulk coolant. SIMS metal / oxide interface analysis agrees with this occuring 22 A presentation by Wood.
Discussion – Hypothesised Mechanism Effect of Stress Effect of Boiling Effect of LiOH Stress causes the pores Boiling within the cracks & & cracks present in the pores that have limited oxide to open and accessibility to the coolant create a more causes localised accessible pathway for concentration of LiOH the coolant to solution to levels above that penetrate nearer the of the bulk coolant. SIMS metal / oxide interface analysis agrees with this observation 23 A presentation by Wood.
Effect of boiling: SIMS results Where accelerated corrosion Accel eler erate ated Accel eler erate ated was observed (as a result of boiling), lithium was leachable from cracks and pores indicating local accumulation of lithium in these regions. Similar observations were seen by Jeong et al . 1999, following accelerated Non-acce accele lerate ated Non-acce accele lerate ated corrosion under a 70 ppm LiOH, 350 ° C isothermal autoclave environment All data from oxides >20 µm 24 A presentation by Wood.
Effect of boiling: SIMS results Leachable lithium content was Accel eler erate ated Accel eler erate ated measured using ICP-OES. This equates to a lithium concentration of 25 ppm within the entire oxide film. Calculations estimate the metal / oxide interface temperature, under conditions for boiling, would be ~315 °C Non-acce accele lerate ated Non-acce accele lerate ated This is significantly lower than that expected for accelerated corrosion ICPOES - Inductively Coupled Plasma Optical 25 A presentation by Wood. Emission Spectroscopy
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