Stress Corrosion Cracking

Transcription

1 Stress Corrosion Cracking SCC is the brittle cracking of a metal due to the result of combined effects from localized corrosion and a tensile stress.

2 Stress Corrosion Cracking

3 Stress Corrosion Cracking

4 Characteristic Most of areas unaffected Specific to certain environments Brass - SCC in solutions with ammonia. Steel - SCC in caustic (high ph), amine solutions. Stainless steels and aluminium alloys - SCC in solutions containing chlorides. Ti-alloys - SCC in nitric acid or methanol. Can be Intergranular (Al-Cu, Cu base alloys) transgranular (Mg alloy or 18/8SS) or mixed mode of cracking. Residual stresses due to cold working, welding etc., Compressive stress does not cause Threshold stress is required

5 Mechanism

6 Mechanism

7 Mechanism

8 Mechanism

9 Mechanism

10 Mechanism Mechanism Electrochemical theory Fissures at weak points of oxide film favour anodic dissolution & initiation of SCC Stress sorption theory Adsorption of ions on metal weakens metal atom bond Control (Material + Environment+ Tensile Stress) 1. Lower stress level below the threshold value (annealing, thickening the section) 2. Eliminate critical species 3. Apply cathodic protection 4. Add inhibitor (Phosphate in boilers)

11 Hydrogen Embrittlement Brittle mechanical failure caused by penetration and diffusion of atomic hydrogen into the crystal structure of an alloy. (e.g) boiler tubes, plating, crude oil pipe lines pickling Steels in oil and gas industry Formation of cavities in the steel due to hydrogen blistering Failure of wall of a hydrofluoric alkylisation plant due to hydrogen blistering

12 Source of Hydrogen Corrosion process H + + e H H 2 O + e H + + OH H-bearing environment (heat treatment, wielding or other manufacturing process) Decomposition of water vapour or steam on hot surfaces. The corrosion reactions in presence of hydrogen sulphides Anodic reaction: Fe Fe e Dissociation reaction: Cathodic reaction : H 2 S H + + HS - HS - H + + S - 2H + + 2e 2H 2H 2 (gas)

13 Posions - Presence of S 2- and As 3+ delay the recombination of H atoms. P, Sb, Se, Te and Cyanide are other poisons. Fe (Surface) + H2S (gas) FeS (Surface) + 2H (solution) Hydrogen Induced Cracking (HIC) The corrosion damage is in the form of blisters and / or internal cracks in absence of STRESS Sulfide stress Cracking (SSC) Presence of STRESS

14 Mechanism Weakening of the metallic bond strength by the dissolve H. Diffusion of atomic H into metal and forms molecular H 2 in voids/defects forms blisters. This build up high pressure causes rupture. Diffusion of atomic H into metal and reacts with alloying elements to form brittle hydrides Prevention of HE Modification of environments Use of materials resistant to HE (High strength materials with low inclusion/voids)

15 Corrosion Fatigue Brittle failure of an alloy caused by fluctuating stress in a corrosive environment. (Different from SCC) (e.g) Sucker rods & drilling rods in oil wells rail vehicle springs, motor shaft working in corrosive environment Characteristics Large smooth area and a small corroded area Specific to high strength materials Transgranular failure Endurance limit is decreased by corrosive agent Fatigue occurs when a material is subjected to repeated loading and unloading.

16 Mechanism Deep pits are initially formed Cracks initiates & propagates across the metal The frquency of cyclic stress is important. Lower frequency leads to greater crack propagation per cycle. Prevention Lowering the general corrosion rate will delay or prevent CF. Addition of inhibitors Barrier coating (Coating Zn, Cr, Ni, Cu & nitride) Reduce cyclic stresses by shot peening

17 Dealloying Also known as Selective Leaching or Selective dissolution or Parting Dezincification: Removal of one element from an alloy leaving an altered residual structure. Selective removal of Zn from Brass with its yellow colour and its colour changes to red (copper colour)

18 Graphitization Selective dissolution of iron from Grey cast iron (E.g. Water pipes buried in soil) Graphite flakes are cathodic to iron, and corrosion is localized to iron which starts leaching (becomes a porous mass) and leaves a rich residue of graphite flakes.

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20 Characteristics Loss of mechanical strength without change in shape It leaves a porous structure White cast iron does not undergo since C is present as Fe 3 C but leaves behind a network of graphite flakes Mechanism C is present as graphite form Potential difference exists between graphite and iron Local cell promotes corrosion of iron. Prevention Coal tar epoxy coating on metal to prevent graphitization Tackling at desgin stage Cupro-nickel or addition of 1% Sn to brass reduces. Addition of 2% Al to Brass also prevents

21 Fretting Corrosion Loaded metal surfaces under relative motions in the presence of corrosive environment. e.g) Ball bearing, electrical switch gear Discolour & deep pit formation Relative motion small as 10-8 cm Mechanism Wear Oxidation Rupture causes metal removal and oxidation Oxidation wear oxide layer ruptured and oxide debris formed Exposed metal is further oxidized Prevention Lubricate with low viscosity Increase hardness Use gaskets to absorb vibration Roughen the surface

22 Cavitation Damage Corrosion caused by impact of air bubbles in a medium (e.g) Marine propellers, boiler tubes Pitting type of appearance and surface roughening Mechanism Repeated formation of bubbles act as hammer and remove metal. Prevention Metal must be high Corr. Resistant. Coating with neoprene Use dense high strength tensile material.

23 Erosion-Corrosion

24 Erosion-Corrosion Laminar flow Cavitation erosion A = Erosion - corrosion B = Cavitation erosion corrosion C = Turbulent flow corrosion B A C Corrosion Turbulent flow Erosion

25 Types of Flow turbulent flow laminar flow (parabolic profile) y 0 x d notional profile in the absence of friction at the pipe walls Fully developed velocity profiles in a circular section pipe

26 Types of Flow (a) inlet laminar flow (b) inlet entry zone unstable flow turbulence

27 v ( ) turbulent layer intermediate layer y laminar layer laminar sub - layer 0 x x CRIT Hydrodynamic (prandl) boundary layers near the surface of a flat plate formed by a uniform fluid velocity such that turbulent flow can develop

28 boundary layers boundary layers merge and become fully developed entry length Fully developed laminar flow Development of a hydrodynamic boundary layer for solution flowing through a tube

29 Influence of fluid velocity on NAB corrosion performance Corrosion rate mm/year Max. recommended in-service velocity Tidal 1 5 m/s 7.62 m/s m/s 4 Critical velocity is not well defined for NAB. In seawater service conditions a maximum flow rate of 5 m/s is normally recommended

30 Corrosion rate vs Velocity

31 Protective film integrity Protective oxide film on NAB prevents corrosion approx. 800 nm complex layers of CuO, Cu 2 O and Al 2 O 3 Level of protective film damage is dependent on particle impact angle Mechanical removal and/or rupture enables charge transfer at varying rates Recovery aspects then become important (system dependent)

32 Ductile erosion mechanisms For a ductile material, the erosion mechanisms are (1) + (2) microcutting, (3) plastic deformation depending on the angle of impingement.

33 Brittle erosion mechanism Crack systems e.g. oxides at splat boundaries Fracture toughness plays an important role. Can depend on direction (perpendicular/transverse to coating / substrate interface).

34 Synergy T = E + C + S or S = T - ( E + C ) where T is material loss under erosion-corrosion E is material loss by pure mechanical erosion processes C is solids free flow corrosion S is synergy, the difference between erosion-corrosion (T) and the summation of erosion (E) and corrosion (C). Synergistic effects can be: Negative (equivalent to extra safety factor) Positive (additional safety factor required)

35 Microbial Zero Resistance Induced Ammeter Corrosion Immediately after immersion, a metal surface undergoes a sequence of biological and chemical changes that lead to the formation of a biofilm which is causative for corrosion.

36 Microbial Induced Corrosion A biofilm is heterogeneous in nature and the distribution of microorganisms is not uniform: highly complex structures containing voids, microbial clusters or layers. diffusion in biofilms is dependent on flow conditions and structure. Slime, extracellular polymeric substances (EPS) - enzymes, proteins, nutrients, trapped inorganic material possible modification of oxygen reduction mechanisms

37 Sulphate Reducing Bacteria

38 Biofilms and galvanic corrosion Copper alloys are more resistant to biofouling than most metals due to the toxicity of the released copper-ions. When coupled to other metals the release of copper-ions can be greatly reduced. The ennoblement of passive metals, such as titanium and stainless steels is welldocumented: This ennoblement has been related to the formation of aerobic biofilms, although the mechanism governing the processes is still a subject of much debate. Biofilms on titanium have been reported to catalyse the cathodic reduction of oxygen thus increasing the overall cathodic efficiency.

39 References S.N.Banerjee, An Introduction to Corrosion Science and Corrosion Inhibition, Oxonian Press P.Ltd., New Delhi, Zaki Ahmad, Principles of Corrosion Engineering & Corrosion Control, Butterworth Heinemann, M.G.Fontana & N.D. Greene, Corrosion Engineering, McGraw Hill, New York, L.L.Shrier Corrosion, Vol. I & II, Butterworth Heinemann, H.H.Uhlig and R.W.Revie, Corrosion and Corrosion Control, A Wiley Inter Science Publication John Wiley & Sons, New York, 3rd Edition, 1985 and etc.