MC-F-013 Deterioro. Conceptos Básicos

7/30/2019 MC-F-013 Deterioro. Conceptos Bsicos

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472W P 6: TRAINING & EDUCATIONF. GUTIRREZ-SOLANAS. CICEROJ.A. ALVAREZR. LACALLE

G1RT-CT-2001-05071

A. BASIC CONCEPTS

W P 6: TRAINING & EDUCATION


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CORROSION BEHAVIOUR

ENVIRONMENTAL ASSISTED CRACKING PROCESSES

MATERIAL

MECHANIC LOADS

LOADING RATEStatic or Quasi-Sstatic Stress corrosion cracking (SCC)Dynamic Corrosion Fatigue (CF)

ENVIRONMENT

Liquid or Gaseous Environments Presence of damaging simple chemical elements ormolecules Hydrogen Hydrogen assisted embrittlement

Hydrogen assisted cracking

Oxygen Intergranular corrosion cracking

AGGRESSIVE ENVIRONMENT

ENVIRONMENTAL ASSISTED CRACKING


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CORROSION BEHAVIOUR


Crack initiation processes

+

Propagation processes

FRACTURE

t

SC

SC Strength

t initiation

t propag. t fracture

Safe Design: SC


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CORROSION BEHAVIOUR


Anodic dissolutionSliding

RepasivationIncrease in deffect sharpening

Catodic reaction

H+ + e- H2

Hydrogen embrittlement

-Adsorption

-Absorption

-Diffusion

-Local damage and failure

-Crack advance


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CORROSION BEHAVIOUR

Life estimation (constant environment and stresses)

Depends on initiation material surface state(roughness)(surface defects)

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 SCC SCC is not only material dependent, it also depends in

processing (surface finishing) and design (notches, welds,)


inclusions ...processing ...

recovering ...


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CORROSION BEHAVIOUR

Crack propagation rate; is a characteristic of the material (for a given

environment and local conditions).

dtda


Local mechanisms of material transport

and embrittlement


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CORROSION BEHAVIOUR

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 (KI), that establishes, togetherwith the environment, the cracking mechanisms

t)environmen,(K

dt

daIf=


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CORROSION BEHAVIOUR

STRESS CORROSION


FRACTURETIME

INCUBATIONTIME (tinc)

CRACK

GROWTH

Crack propagation happens over some

characteristic threshold conditions, defining KISCC

(da/dt = 0 for KIKISCC, Stage II)- loading to final fracture at stage III (KI =KIC)


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CORROSION BEHAVIOUR

DESIGN CONDITIONS AND INTEGRITY MAINTENANCE

- Guarantee maximum defect size ( a0U a Lim) Determine crack evolution

+=

t

0

Limcalc dta(t)a

Material behaviour

Periodic and cyclic observations to guarantee

areal (t) U acalc (t)when KI (acalc) U KIc / Fsafety

Repair, substitute or leave when critical security conditions arereached.

a Lim observableon reception

dt

da


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CORROSION BEHAVIOUR

Example: Intergranular corrosion on stainless steels.

Conditions: - Stress state greater than the threshold- Aggressive environment [dissolved oxygen]- Sensitized material

STRESS CORROSION


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CORROSION BEHAVIOUR

Example: Intergranular corrosion on stainless steels.

Solution:

- Reduction of aggresive elementconcentration (O2)

- Adequate material election

- Not susceptible to be sensitized

% C to avoid chromium carbidesformation at sensitive temperatures andthen the IG loss of chromium

STRESS CORROSION


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CORROSION BEHAVIOUR

CORROSION - FATIGUE

Fatigue conditions

+Aggressive environment

produce corrosion fatigue



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CORROSION BEHAVIOUR

The behaviour depends on:

Material (microstructure) Stress condition (local) Environment presence

+

Loading frequency...

)K(dNda I= f

CORROSION-FATIGUE

Similar behaviour than fatigue at inert environment

Threshold: KICF

and crack propagation rate:



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CORROSION BEHAVIOUR

Mechanisms on metallic materials

SCC ( metals) Crack advances generally by local fractures

Intergranular (IG) Transgranular (TG)


orCleavagesor tearing


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BIBLIOGRAPHY / REFERENCES

Fontana M.G., Corrosion Engineering, McGraw-Hill, 1986.

Bradford S.A., Self Study Guide to Corrosion Control, CASTI, 2001. Jones DA., Principles and Prevention of Corrosion, Mc Millan, 1992.

Schweitze PA., Corrosion Engineering Handbook, Dekker, New York, 1996.

Scully, The Fundamentals on Corrosion, Pergamon Press, 1990.

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MC-F-013 Deterioro. Conceptos Básicos