Stress Corrosion Cracking & Hydrogen Embrittlement (Simon Hogg) Flashcards
What is Stress Corrosion Cracking?
Simultaneous corrosion action while under an applied tensile stress
Fine cracks propagate at a normal to applied stress
Failure is delayed as cracks grow very slowly
Damage is mostly hidden and can result in very fast failure
Why does corrosion happen at the crack tip?
Anodic reactions lead to metal dissolution
Hydrolysis reactions happen forming H+ ions, lowering pH at crack tip
What is hydrogen embrittlement?
H+ ions produced in hydrolysis are adsorbed and diffuse into metal, just ahead of crack tip which causes embrittlement in certain materials.
What are the 3 requirements for SCC?
Susceptible material
Specific environment that causes corrosion
Sufficient stress
What are the 4 SCC mechanisms?
Anodic dissolution - regions with precipitates or additives segregating on grain boundaries can produce a local anodic path which leads to intergranular SCC as grain boundaries dissolve
Film rupture - passive film ruptures and exposes bare metal at crack tip, small anodic surface area will dissolute and cracking can propagate, stops by crack tip blunting which lowers stress intensity and metal can be repassivated and process continues
Stress-sorption - damaging ions are adsorbed at crack tip which reduces cohesive strength of atomic bonds and reduces the fracture stress
Hydrogen embrittlement - H+ ions produced in hydrolysis can rapidly diffuse into metal as an interstitial impurity either forming brittle hydrides, diffusing ahead of crack tip to make deformation and fracture easier, or producing hydrogen gas in voids which causes internal pressure
How can you protect against SCC?
Lower stress intensity below threshold for SCC - residual stresses can be heat treated but not possible for large structures and can have side effects to metal
Ensure alloy/environment wont cause SCC - better thought of at design stage
Apply a coating to act as a barrier between metal and environment
What could be the source of extra hydrogen that could end up causing embrittlement?
Corrosion (hydrolysis reaction)
Storage of hydrogen gas
Electroplating
Picking of metals in acids to remove surface scale
How does temperature affect hydrogen adsorption?
Gaseous hydrogen isn’t normally adsorbed.
At higher temps, molecular hydrogen can dissociate into atomic hydrogen which can be adsorbed.
Solubility of hydrogen is higher in liquid metals than solid.
What is de-embrittlement and how do we do it?
Heat treatment performed no more than a couple hours after processing (usually after electroplating).
Heat for 2-4 hours at 200C.
Allows some hydrogen to diffuse out of the metal but distributes the remaining hydrogen to reduce local concentrations.
How can you control hydrogen embrittlement?
Choose materials with good resistance
Remove hydrogen from environment
Apply coatings to act as barrier to hydrogen
Use ‘hydrogen trapping’ where you use dislocations and vacancies to trap hydrogen and reduce its mobility in the metal
