Everything about Stress Corrosion Cracking totally explained
Stress corrosion cracking (
SCC) is the unexpected sudden failure of normally
ductile metals or tough
thermoplastics subjected to a constant
tensile stress in a
corrosive environment, especially at elevated temperature (in the case of metals). This type of
corrosion often progresses rapidly. The corrosive environment is of crucial importance, and only very small concentrations of certain highly active chemicals are needed to produce catastrophic cracking, often leading to devastating failure.
The stresses can be the result of the crevice loads due to
stress concentration, or can be caused by the type of assembly or
residual stresses from fabrication (eg. cold working); the residual stresses can be relieved by
annealing.
Metals attacked
Certain austenitic
stainless steels and
aluminium alloys crack in the presence of
chlorides, mild
steel cracks in the present of alkali (
boiler cracking) and
nitrates,
copper alloys crack in
ammoniacal solutions (
season cracking), and many metals
crack when exposed to liquid metals or when coated by metals nearby their melting point (
mercury,
gallium, and at high temperatures even for example
cadmium). This limits the usefulness of stainless steel for containing water with higher than few ppm content of chlorides at temperatures above 50 °C. Worse still, high-tensile structural steels crack in an unexpectedly brittle manner in a whole variety of aqueous environments, especially containing chlorides. With the possible exception of the latter, which is a special example of
hydrogen cracking, all the others display the phenomenon of subcritical
crack growth, for example small surface flaws propagate (usually smoothly) under conditions where
fracture mechanics predicts that failure shouldn't occur. That is, in the presence of a corrodent, cracks develop and propagate well below
KIc. In fact, the subcritical value of the stress intensity, designated as
KIscc, may be less than 1% of
KIc, as the following table shows:
| Alloy |
KIcMN/m3/2
|
SCC environment |
KIsccMN/m3/2
|
| 13Cr steel |
60 |
3% NaCl |
12 |
| 18Cr-8Ni |
200 |
42% MgCl2 |
10 |
| Cu-30Zn |
200 |
NH4OH, pH7 |
1 |
| Al-3Mg-7Zn |
25 |
Aqueous halides |
5 |
| Ti-6Al-1V |
60 |
0.6M KCl |
20 |
Polymers attacked
Polymers can also be attacked by certain reagents, and if under load, then cracks will grow just as in metals and alloys. Perhaps the oldest known example is the
ozone cracking of
rubbers, where traces of ozone in the atmosphere attack
double bonds in the chains of the materials. Elastomers with double bonds in their chains include
natural rubber,
nitrile rubber and
styrene-butadiene rubber. They are all highly susceptible to ozone attack, and can cause problems like car fires (from rubber fuel lines) and tyre blow-outs. Nowadays, anti-ozonants are widely added to these polymers, so the incidence of cracking has dropped. However, not all safety-critical rubber products are protected, and since only
ppb of ozone will start attack, failures are still occurring.
Another highly reactive gas is
chlorine, which will attack susceptible polymers such as
acetal resin and
polybutylene pipework. There have been many examples of such pipes and acetal fittings failing in properties in the USA as a result of chlorine-induced cracking. Essentially the gas attacks sensitive parts of the chain molecules (especially secondary, tertiary or allylic carbon atoms), oxidising the chains and ultimately causing chain cleavage. The root cause is traces of chlorine in the water supply, added for its anti-bacterial action, attack occurring even at
parts per million traces of the dissolved gas.
Most step-growth polymers can suffer
hydrolysis in the presence of water, often a reaction catalysed by
acid or
alkali.
Nylon for example, will degrade and crack rapidly if exposed to strong acids, a phenomenon well known to savvy ladies who accidentally spill acid onto their tights.
Polycarbonate is susceptible to alkali hydrolysis, the reaction simply depolymerising the material.
Polyesters are prone to degrade when treated with strong acids, and in all these cases, care must be taken to dry the raw materials for processing at high temperatures to prevent the problem occurring.
Many polymers are also attacked by
UV radiation at vulnerable points in their chain structures. Thus
polypropylene suffers severe cracking in
sunlight unless
anti-oxidants are added. The point of attack occurs at the tertiary carbon atom present in every repeat unit, causing oxidation and finally chain breakage.
Crack growth
The subcritical nature of propagation may be attributed to the
chemical energy released as the crack propagates. That is,
» elastic energy released + chemical energy = surface energy + deformation energy
The crack initiates at
KIscc and thereafter propagates at a rate governed by the slowest process, which most of the time is the rate at which corrosive ions can diffuse to the crack tip. As the crack advances so
K rises (because crack length appears in the calculation of stress intensity). Finally it reaches
KIc, whereupon fast fracture ensues and the component fails. One of the practical difficulties with SCC is its unexpected nature.
Stainless steels, for example, are employed because under most conditions they're 'passive', for example effectively inert. Very often one finds a single crack has propagated while the rest of the metal surface stays apparently unaffected.
Examples
SCC caused the catastrophic collapse of the
Silver Bridge in December 1967, when an eyebar suspension bridge across the Ohio river at Point Pleasant, WV, suddenly failed. The main chan joint failed and the whole structure fell in less than a minute into the river, killing 46 people in vehicles on the bridge at the time. Rust in the eyebar joint had caused a stress corrosion crack, which went critical as a result of high bridge loading and the low temperatures. The failure was exacerbated by a high level of
residual stress in the eyebar. The disaster led to a nationwide reappraisal of the state of the nation's bridges.
A
nylon 6,6 connector in a
diesel fuel line fractured when a small drop of
sulfuric acid leaked from the
lead-acid battery overhead. It formed a small crack which grew until fuel started leaking. As the critical crack grew, leakage increased until the line parted and fuel fell unrestricted into the road, and caused several crashes to other motorists. The driver of the vehicle should have spotted the leak before it became critical.
An
acetal resin junction in a water supply system suddenly fractured over a weekend, causing substantial physical damage to computers stored below in the building. The junction failed at
injection moulding defects by
chlorine attack of the
polymer. The water supply contained only 5 ppm of chlorine, but it was enough to trigger stress corrosion cracking of the defective moulding.
Further Information
Get more info on 'Stress Corrosion Cracking'.
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