G1RT-CT A. BASIC CONCEPTS F. GUTIÉRREZ-SOLANA S. CICERO J.A. ALVAREZ R. LACALLE W P 6: TRAINING & EDUCATION

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1 A. BASIC CONCEPTS 472

2 PROCESSES LOADING RATE Static or Quasi-Sstatic Dynamic MATERIAL ENVIRONMENT Liquid or Gaseous Environments AGGRESSIVE ENVIRONMENT MECHANIC LOADS Stress corrosion cracking (SCC) Corrosion Fatigue (CF) Presence of damaging simple chemical elements or molecules Hydrogen Hydrogen assisted embrittlement Hydrogen assisted cracking Oxygen Intergranular corrosion cracking 473

3 Crack initiation processes + Propagation processes FRACTURE σ t initiation t propag. t fracture σ SC SC Strength Safe Design: σ σ SC t 474

4 Anodic dissolution Sliding Repasivation Increase in deffect sharpening Catodic reaction H + + e - ½H 2 Hydrogen embrittlement -Adsorption -Absorption -Diffusion -Local damage and failure -Crack advance 475

5 Life estimation (constant environment and stresses) Depends on initiation material surface state (roughness) (surface defects) inclusions... processing... recovering... Depends on propagation Local cracking mechanisms (local fractures after restrained embrittlement) (inherent mechanisms) If previous notches (stress concentration) or cracks exist t life t propagation Design σ σ SCC σ SCC is not only material dependent, it also depends in processing (surface finishing) and design (notches, welds, ) 476

6 Crack propagation rate; da dt is a characteristic of the material (for a given environment and local conditions). Local mechanisms of material transport and embrittlement 477

7 Material behaviour It defines the crack propagation process Stress state + Crack presence Application of Fracture Mechanics Crack propagation rate as a function of the local stress state (K I ), that establishes, together with the environment, the cracking mechanisms da = dt f (K I, environment) 478

8 STRESS CORROSION CRACK GROWTH FRACTURE TIME INCUBATION TIME (t inc ) Crack propagation happens over some characteristic threshold conditions, defining K ISCC (da/dt = 0 for K I <K ISCC,Stage I) - at a quasi-constant rate (da/dt = cte for K I >K ISCC, Stage II) - loading to final fracture at stage III (K I =K IC ) 479

9 DESIGN CONDITIONS AND INTEGRITY MAINTENANCE - Guarantee maximum defect size ( a 0 a Lim ) Determine crack evolution a calc (t) = a Lim + t 0 a Lim da dt dt Material behaviour observable on reception Periodic and cyclic observations to guarantee a real (t) a calc (t) when K I (a calc ) K Ic / F safety Repair, substitute or leave when critical security conditions are reached. 480

10 STRESS CORROSION Example: Intergranular corrosion on stainless steels. Conditions: - Stress state greater than the threshold - Aggressive environment [dissolved oxygen] - Sensitized material 481

11 STRESS CORROSION Example: Intergranular corrosion on stainless steels. Solution: - Reduction of aggresive element concentration ( O 2 ) - Adequate material election - Not susceptible to be sensitized % C to avoid chromium carbides formation at sensitive temperatures and then the IG loss of chromium 482

12 CORROSION - FATIGUE Fatigue conditions + Aggressive environment produce corrosion fatigue 483

13 CORROSION-FATIGUE Similar behaviour than fatigue at inert environment Threshold: ΔK ICF da and crack propagation rate: dn The behaviour depends on: = f ( ΔK I ) Material (microstructure) Stress condition (local) Environment presence + Loading frequency

14 Mechanisms on metallic materials SCC ( metals) Crack advances generally by local fractures or Cleavages or tearing Intergranular (IG) Transgranular (TG) 485

15 BIBLIOGRAPHY / REFERENCES Fontana M.G., Corrosion Engineering, McGraw-Hill, Bradford S.A., Self Study Guide to Corrosion Control, CASTI, Jones DA., Principles and Prevention of Corrosion, Mc Millan, Schweitze PA., Corrosion Engineering Handbook, Dekker, New York, Scully, The Fundamentals on Corrosion, Pergamon Press,