Metals Flashcards

Question

What makes stress corrosion cracking complex?

Answer 1

As the processes occur at a highly localised level (typically at the crack tip. Different microstructural variables and heterogeneities, stress concentrations, temperature and modified chemical environments all contribute to the interaction of the material, environment and the stress. e.g. hydrogen induced environmental cracking has lead to brittle intergranular failure.

Answer 2

1.Anodic Reaction at Crack Tip Process: Localized metal dissolution Result: High metal ion concentration → Hydrolysis → H⁺ formation Effect: Low pH at crack tip (pH ~3–4), even if bulk solution is neutral (pH ~7) 2. Hydrogen Generation H⁺ Ions: Reduced to atomic hydrogen (not H₂ gas) Result: Hydrogen adsorbs and diffuses into metal ahead of crack tip 3. Embrittlement Effect: Local hydrogen accumulation → Hydrogen embrittlement Materials Affected: Susceptible alloys like Aluminum and steels

Answer 3

1. A susceptible material Generally alloys with a good general corrosion resistance (Al, Ti, stainless steels etc), will undergo SCC. Note some steels are susceptible to both general corrosion and SCC. 2. A specific environment that causes SCC in the material (specific metals with different environments like hydroxides or nitrates or methanol or sea water etc ref pg10] 3. Sufficient stress Externally applied and/or residual stress (say from machining or welding) or self generating (wedging from corrosion products). In some alloy/environment combinations the threshold stress for SCC can be as low as 5% of the yield strength.

Answer 4

Dissolution Effects 1.Anodic dissolution (sometimes called active path dissolution). 2.Film rupture Absorption/mechanical effects 3.Stress-sorption cracking 4.Hydrogen embrittlement

Answer 5

heterogeneous regions in the microstructure (precipitation or segregation to grain boundaries) produce a local anodic path. This will lead to intergranular SCC as the grain boundary regions are dissolved. SIDE NOTE: in sensitised(from the precipitation of Cr containing carbides at the grain boundaries so the grain boundaries deplete of Cr so these regions are anodic compared to undepleted grains) stainless steel the cracking can be transgranular or intergranular.

Answer 6

* Here a passive film ruptures due to the stress to reveal bare metal at the crack tip. * This leads to anodic dissolution in the presence of the stress. The crack then propagates a small distance (say 1µm) and is then arrested due to crack-tip blunting (which lowers the stress intensity). * The metal can then be repassivated and the process continues again. * The fracture surface shows evidence of discontinuous fracture and this mechanisms is seen in SCC of brass in ammonia, where dealloying can result in a brittle film being produced.

Answer 7

If damaging ions are adsorbed at the crack tip they can reduce the cohesive strength of the atomic bonds. The adsorption process leads to a reduction in the surface energy of the solid. From fracture mechanics, this reduces the fracture stress

Answer 8

Atomic hydrogen can be produced by reduction of H+ ions and due to its small atomic size, it can rapidly diffuse into the metal lattice as an interstitial impurity. This can lead to metal embrittlement either by: 1: The interstitial hydrogen causes an internal pressure if the hydrogen atoms combine to produce hydrogen gas in voids or flaws in the material. 2. Brittle metal-hydrides can form (particularly in Mg, Zr and Ti alloys, which are easily fractured. 3. Hydrogen is adsorbed at the crack tip to reduce the surface energy (as for stress-sorption). 4. Hydrogen diffuses ahead of the crack tip and makes plastic deformation and cleavage fracture easier

Answer 9

SCC is not just a process corrosion activity causes a stress concentration which then leads to failure. It is a combination of the material, environment and the stress leading to crack propagation. The stress concentration due to corrosion is unlikely to lead directly to failure in an inactive environment. the stress intensity required for cracks to propagate under stress corrosion cracking condition is less than that required for brittle fracture.

Answer 10

Some form of defect: * A pre-existing crack-like defect (such as porosity in a weld), groves from machining operations that could lead to cracking in (say) brittle martensitic microstructures, cracked inclusions etc. * A corrosion induced defect e.g. a corrosion pit, regions where a passive film is reduced such as at grain boundaries, intergranular corrosion (localised corrosion at grain boundaries due to galvanic effects), crevice corrosion, dealloying etc. * Localised cracking of a passive oxide film due to the mechanical stress or chemical attack (e.g. by Cl- ions), which then leads to pitting.

Answer 11

1. No crack propagation is observed below K1SCC, with this value dependent on the environment, the alloy and the alloy microstructure. 2.At low stress intensity values (short crack lengths) the crack growth rate increases rapidly as K increases (region 1). It will then approach a constant crack growth velocity (region 2) when some rate limiting process is established – say transport of chemical species to the crack tip. 3.Region 3 corresponds to the transition to the point where the stress intensity exceeds that for fracture in an inert environment and final brittle fracture will occur. This region seen only in very susceptible alloys.

Answer 12

Transgranular (i.e. the crack propagates through the grains) or Intergranular (i.e. the crack path is along grain boundaries). The nature of the crack path depends on the alloy and/or the environment.

Answer 13

1. By lowering the stress intensity below the threshold for SCC, cracks will not be able to propagate. Be aware of residual stresses that can arise during fabrication or additional stress concentrations from notches, sharp changes in cross-section etc. Residual stresses can be relieved by an appropriate heat-treatment, but this may not be possible for very large structures and the heat-treatment process can cause other deleterious effects (such as sensitisation in stainless steels). 2. Ensure that the alloy/ environment combination will not give rise to SCC. This can be difficult and requires that at the design stage there is appropriate materials selection. Additionally, different heat-treatments for the same alloy will give different behaviour. SCC-resistant materials can be very expensive. 3. Coatings can be applied to remove the environment from the material, but careful consideration must be made of what happens if the coating is damaged.

Answer 14

* Storage/containment of hydrogen gas * Corrosion (see previous slides) * Electroplating processes * Dissolved hydrogen due to liquid-state processing of metals, e.g. welding or casting * Picking of metals in acids (used to remove surface scale)

Answer 15

Gaseous hydrogen is not normally absorbed into metals. However as the temperature is increased, molecular hydrogen can dissociate and into atomic hydrogen and high-temperature heat treatments etc. can result in hydrogen absorption. The solubility of hydrogen is higher in liquid metals than in the solid state. The dissolved hydrogen gets largely trapped in interstitial sites. For example hydrogen can be dissolved during welding, leading to subsequent cracking. (Poorly controlled casting conditions - say excess moisture present - can also result in hydrogen-related problems, e.g. gas porosity.)

Answer 16

Depending on how efficient the plating process is, electrodeposition processes (e.g. electroplating) can result in hydrogen by cathode reactions. But atomic hydrogen, not molecular hydrogen, is absorbed into the material (although recombination can cause molecular hydrogen to form in the material). e.g either hydrogen can get absorbed into the metal surface due to its high mobility(freely move in interstitial locations) and bond with another H to form H2 within the metal. ORR a metal hydrogen bond is formed which can weaken the bond (stress sorption) so then more reactions can happen to form H2 or evolution of gas via desportion if lots of H2 is absorbed

Answer 17

the fracture surfaces of hydrogen embrittled failures vary from 1.microvoids (as for ductile fracture) to 2. ‘quasi-cleavage’ (like cleavage for brittle fracture, but with the fracture plane not a typical cleavage-plane) 3. to intergranular.

Answer 18

De-embrittlement is a heat-treatment performed as soon as possible (within 1-3hour) after processing (normally after electroplating) – for ~2-4 hours at ~200°C. This allow some of the diffusible hydrogen to leave the metal, but also redistribute the remaining hydrogen, to reduce areas of high local concentrations.

Answer 19

* Choosing materials with good resistance (or not using alloys particularly susceptible). * Removing hydrogen from the environment – either in service or during processing (e.g. use of mechanical removal of oxide scale rather than acid pickling). * Applying coatings to act as barriers to hydrogen ingress, but the practical use of coatings depends on the environment and the coating process may induce hydrogen. * Ensuring plating processes have a high cathode current efficiency (so that the current applied works to apply the coating and not produce hydrogen). * Use ‘hydrogen trapping’ which involves inducing defects (things like dislocations, vacancies, certain atom-types etc.) in the metal to trap hydrogen and reduce its mobility

Answer 20

 There would be fine cracks propagate though the material in a direction perpendicular to the applied stress. It may also be possible to determine a crack initiation site, such as a pit, grain boundary etc.  Cracking can be either transgranular (i.e. the crack propagates through the grains) or intergranular (i.e. the crack path is along grain boundaries).  The fracture surface could show different fractures such as microvoids (as for ductile fracture) to ‘quasi-cleavage’ (like cleavage for brittle fracture, but with the fracture plane not a typical cleavage-plane) to intergranular fracture.  The SCC investigation would require a range of microscopy techniques such as optical microscopy to determine the crack path and scanning electron microscopy to analyse features on the fracture surface.

Answer 21

Small cracks in the material lead to a large stress concentration at the tip of the crack. The crack can be very sharp (of the order of the atomic spacing in the material and = stress levels at the crack tip that exceed the cohesion strength of material. So although the applied stress may be lower than the theoretical fracture strength, the stress concentrating nature of the crack means the local stress at the crack tip is sufficient to break the bonds = fracture.

Answer 22

a brittle material is more sensitive to cracks in the material. For a brittle material, the yield strength is high so there is little or no plastic deformation as there's no way to relieve stress at the crack tip. Ductile materials have void formation and growth absorbs energy so the localised plastic deformation at the crack tip acts to blunt the crack and so the stress concentration effect is not as severe as in brittle.

Metals Flashcards

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