Metallic Containers: Corrosion and Materials Issues in Storage and Disposal

1. Metallic Containers:Corrosion and Materials Issues in Storage and Disposal Stuart Lyon, Corrosion and Protection Centre/ Materials Performance Centre

2. Storage of Containers Eventually: Deep underground repository (many countries are ahead of the UK) Currently in the UK: Above ground in vented sheds near the sea.

3. Corrosive Environments Inside container (encapsulant and waste form). Outside container during ventilated storage. Outside container in repository prior to backfill. Outside container after backfill.

4. Forms of Corrosion Corrosion in the active (non-passive) state General thinning (more-or-less uniform recession) Deterministic (predictable) recession rate (generally depends of supply of oxidant) Corrosion in the passive state Slow dissolution and re-formation of passive film (more-or-less uniform recession) Pitting corrosion (localised) Crevice corrosion (localised) Stress corrosion cracking (localised)

5. Passivity �The state of a metal whose corrosion rate is decreased by reaction with its environment through formation of a surface barrier film, usually an oxide�. Corrosion rate dependent on: Macroscopic dissolution rate of the surface barrier film (which is continually re-formed) Local (microscopic and not repaired) disruptions to the barrier film usually caused by changes in local chemistry and leading to localised corrosion.

6. Passivity - Thermodynamics Eh/pH (Pourbaix) diagrams indicate domains of phase stability For example, in the presence of sulphur (e.g. from microbial activity in the soil) iron sulphide appears as a new and less stable phase.

7. General Corrosion - Examples Ductile cast iron water main buried in soil. General corrosion on external pipe wall. Leads to local thinning and perforation. Deepest corrosion front approximately twice average corrosion loss.

8. Localised Corrosion - Pitting Austenitic stainless steel in seawater. Massive, deep pits evident. Accidentally connected to large area cathode � this has driven the pitting corrosion by galvanic effects.

9. Localised Corrosion - SCC Stainless steel pipe carrying hot process fluid contaminated with chloride ions. Residual stresses in pipe from bending during manufacture. Stress corrosion cracking evident perpendicular to principal residual stress field.

10. Localised Corrosion - Pitting Initiation Non-deterministic (fundamentally unpredictable) Can only be modelled statistically (stochastic models) Propagation and Growth Possibly deterministic May be possibly be modelled using electrochemical pit growth models

11. Localised Corrosion - Crevice Requirements Tight mating geometries are required Initiation Requires detrimental change in local chemistry due to diffusion constraint in crevice (may be modelled) Continuation A number of theories (e.g. crevice corrosion is thought to be a form of continuous metastable pitting) but in principle may be modelled.

12. Localised Corrosion - Cracking Initiation Almost invariably from pre-existing pit (sometimes crevice) site. Model as for pit or crevice initiation Transition Pit-to- (short) crack experimentally difficult and, hence, not well characterised or modelled. Propagation In principle �long� crack growth rates may be measured using fracture mechanics methods

13. Materials Selection Strategies Select a material that has been buried from historic times (e.g. lead, copper, iron). Corrosion rate in historic buried environment can be determined directly (perhaps if any is left). Choose a material that undergoes general corrosion, such as iron/steel or copper. Corrosion rate can be determined experimentally in anticipated environment (i.e. predictable) Often dependent on rate of arrival of oxidant from soil activity (e.g. sulphur from soil bacteria) General corrosion rate may be high.

14. Materials Selection Strategies Choose a corrosion resistant alloy (e.g. 316L) General corrosion rate is low � often very low Localised corrosion risk becomes a problem and needs to be quantified (difficult and complex process) Choose a corrosion resistant �super alloy� (e.g. Alloy 22, Ti-0.2%Pd) Localised corrosion risk is reduced to insignificant levels Resource implications for scarce elements (Mo, Ni, Pd, etc.). Are there enough just for Yucca Mountain?

15. Intermediate Level Waste (ILW) Containers

16. ILW Corrosion Issues ILW container material is 316L stainless steel: Highly resistant to general forms of corrosion However, susceptible to localised corrosion: pitting corrosion, crevice corrosion and stress-corrosion cracking Container lid is usually 304L stainless steel: Considerably less resistant than 316L Contains a mesh vent (i.e. a leak path) to release hydrogen from internal corrosion of waste forms (e.g. aluminium) in the encapsulant

17. ILW Internal Environment Cementitious encapsulant for the waste forms Ordinary portland cement (OPC) OPC plus other constitutents such as blast furnace slag (BFS) Alkaline environment Promotes passivity Chlorides in certain waste forms Promote localised corrosion

18. ILW Internal Corrosion Hydroxide (alkalinity) gives good suppression of localised corrosion Cl/Oh ratios for pitting Ditto for scc Ditto for crevice Thiosulphate effects (from BFS)

19. ILW internal galvanic corroison Galvanic corrosion inside with waste forms Graphite, aluminium, etc.

20. ILW Dry Store Environment Controlled atmospheric environment Airborne contamination: Sea salt is very different from sodium chloride due to divalent cations (magnesium and calcium content) Humidity: Controlled at a low level

21. ILW Disposal Environment Two stages of emplacement Prior to backfill: Oxidising and �dry� atmospheric environment, likely to be dust and salt contaminated and at high humidity Post backfill: Reducing and �wet� alkaline clay with groundwater of indeterminate composition probably containing reduced sulphur and chloride species

22. Corrosion: Pre-Backfill Of similar nature to atmospheric environment in above ground dry store. Different in detail, for example: Contamination different Dusts different Humidity probably higher See later in talk!

23. Corrosion: Post-Backfill Of similar nature to internal container environment Different in detail: Wet and reducing Containing reduced sulphur species Containing chloride species Experimentally, much has been done Safe chloride/hydroxide ratios Sulphur assisted corrosion

24. 316L: Crevice Corrosion

25. 316L: Pitting Corrosion

26. Internal Container Environment

27. External Container Environment Condensed surface electrolyte composition depends on: temperature, local airflow, RH, salt and soil deposition, gaseous pollutants, etc. Electrolyte concentration varies from: rather dilute (at higher RH) fully saturated (at lower RH) Focus now on Atmospheric-Induced Stress-Corrosion Cracking (AISCC)

28. Deliquescence of Salts Above a critical RH, all salts will �deliquesce� (absorb moisture from the atmosphere). This: Forms a concentrated, thin electrolyte on the substrate surface Gives a corrosion risk Critical deliquescence points at 25�C for: NaCl (~76% RH), MgCl2 (~33% RH)

29. Atmospheric RH with NaCl NaCl particles contaminating the surface Corrosion starts at RH>75% exactly at deliquescence point

30. Atmospheric Corrosion Atmospheric corrosion affected by the number of wet-dry cycles Most (all) of corrosion occurs during drying episodes

31. Atmospheric Wet/Dry Cycles

32. Atmospheric Corrosion Local crevice corrosion will occur beneath soil/dust deposits on surfaces

33. Atmospheric Particulates

34. Atmospheric Corrosion Corrosion will spread out from liquid droplets arising from deliquescence of salt particles

35. Droplet Corrosion

36. Atmospheric Droplets Micro-droplet formation adjacent to main NaCl droplet (bottom left of picture)

37. Micro-Droplet Expansion

38. Atmospheric Environment Dusts and salts � local crevice attack Electrolyte salts � locally liquid above humidity of deliquescence Chloride concentration in droplets after deliquescence is essentially at saturation Oxygen has free access Local ambient or slightly elevated temperature Rainwater washing etc �

39. Previous Experience

40. Occurrence of SCC

41. 316L: Stress Corrosion Cracking Initiation: Dependent on developing pits, crevices, stress concentrators Propagation: Rule of thumb: �Safe below 50�C in sea water� Actually dependent on time, chloride level, temperature and residual stress state So, ILW containers are going to be OK then?

42. First Evidence of AISCC �Effects of RH and chloride type on stainless steel room temperature atmospheric stress corrosion cracking�, S. Shoji, N. Ohnaka, Corrosion Engineering, 42, p. 877 (1993) Unfortunately only in Japanese

43. Atmospheric SCC of 316L

44. Environmental Hazards Corrosion hazards in the atmosphere Surfaces contaminated with Mg/CaCl2 (e.g. from sea-salt aerosols) will be �wet� at relative humidities > 30-35% Localised crevice corrosion can initiate beneath deposited soots, soils, etc. Cracking may initiate from crevices, pits particularly where there is high stress in austenitic alloys

45. ILW Containers in Storage ILW containers in �dry� storage will be exposed to atmospheres: of varying ambient temperature and humidity, in a ventilated storage shed, consequently, with continuous deposition of atmospheric particulates such as sea-salts and other soils, soots, etc.

46. ILW Container Material 316L localised corrosion susceptibilities: At pH 5 and Cl- > 300 ppm Crevice corrosion at T > 0-5�C Pitting corrosion at T > 15-20�C All these conditions are present: Hence SCC initiation is definitely possible But is SCC crack propagation possible? Pre-2005 would have said NO. Now we have to say YES � serious problem!!

47. Cartoon of AI-SCC

48. ILW Containers Going Forward Factors to be considered in the 500L container design: stress concentration, sharp edges, etc. residual stresses/plastic strain in manufacture inside-out (as well as outside-in) corrosion localised corrosion risk, including SCC �Guaranteed minimum� 500 year life

49. It is possible that ILW storage containers may undergo Atmospheric-Induced Stress-Corrosion Cracking (AISCC) during storage Consequently there is need to quantify the conditions under which AISCC occurs in austenitic stainless steels with regards to: Environmental variablesRH, chloride level and other surface contaminants, temperature Materials issuesStress state, surface finish, sensitisation ILW Containers

50. Design and construct novel electrochemical systems to study crack-tip chemistry and determine the importance of the local environment to AISCC Use of wedge shaped samples/and or C-ring specimens to determine stress range over which AISCC occurs Combine these methodologies to determine the impact of critical factors; i.e. critical chloride level, temperature, materials condition and RH levels Experimental Approach

51. Wedge-Shaped Samples

52. Stress Range for SCC Initiation

53. Atmospheric Domain of SCC Aims: To determine the conditions under which 304L and 316L undergo atmospheric-induced stress corrosion cracking: Temp: RT, 40�C, 60�C and 80�C RH: 30%, 50%, 70%, 80% Chloride contamination levels: 20, 50, 100, 200 and 400 �g/cm2 Using multiple replicate �U� bend samples For as long as it takes �

54. Results so far: SCC observed in 316L at: 80�C, 40%RH at all chloride levels with complete failure after 400 h. 60�C, 60%RH no cracks after 400 h � continuing (however 304L does crack). Once initiated, cracks will propagate Initiation is the difficult process (and stochastic)

55. SCC in 316L

56. Cracks

57. 304L in Magnesium Chloride

58. Droplet

59. Cross-Sectional Analysis

60. Crack Distributions in 304L

61. The SCC Process Stress corrosion cracking generally is a staged process: Initiation, usually by localised corrosion (e.g. pitting, etc.) Pit-to-crack transition Crack propagation All stages may be influenced by: Alloying Mechanical condition (internal stresses, etc.) Environment

62. AISCC Occurrence

63. AISCC � Next Steps

64. Acknowledgements