May 24th, 2010 | Author: admin
Alloy Acetate
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Stress corrosion
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Stress corrosion cracking (SCC) is an unexpected sudden failure normally ductile metals or tough thermoplastics subjected to a tensile stress in a corrosive environment, especially at elevated temperatures in the case of metals. SCC is highly specific chemical that certain alloys are likely to undergo SCC when exposed to only a small number of chemical environments. The chemical environment that causes SCC for a given alloy is often one that is only slightly corrosive to metal otherwise. Therefore, the metal parts SCC may appear serious, bright, while full of microscopic cracks. This factor makes it common for SCC to go undetected prior to failure. SCC often progresses quickly and is more common in pure metal alloys. The specific environment is of vital importance, and only very small concentrations of some highly active chemicals are needed to produce catastrophic cracks, often leads to unexpected and devastating failure.
Stress can be the result of charges cracks due to stress concentration, or may be caused by the type of assembly or residual stresses of manufacturing (eg cold working) residual stresses can be relieved by annealing.
Metals attacked
Some austenitic stainless steels and aluminum alloys crack in the presence of chloride mild steel cracks in the presence of an alkali (boiler cracking) and nitrates, copper alloys crack in ammoniacal solutions (season cracking). This limits the usefulness austenitic stainless steel to contain water more than a few ppm content of chlorides at temperatures above 50. Worse, high strength structural steels crack in an unexpectedly weak in a variety of aquatic environments, especially containing chlorides. With the possible exception of the latter, which is a special example of hydrogen cracking, all others show the phenomenon of subcritical crack growth, ie small surface defects in spreading (usually without problems), in conditions where fracture mechanics predicts that failure should occur. That is, in the presence of corrosive media, cracks develop and propagate well below CCI. In fact, the value of the subcritical stress intensity, designated as KIscc, may be less than 1% of the ICC, as the following table:
Alloy
CCI
MN/m3/2
SCC environment
KIscc
MN/m3/2
13Cr steel
60
3% NaCl
12
18Cr-8Ni
200
42% MgCl2
10
Cu-30Zn
200
NH4OH, pH 7
1
Al-3mg-7Zn
25
aqueous halide
5
Ti-6Al-1V
60
0.6 M KCl
20
Polymers attacked
Ozone cracking in natural rubber tube
The polymers can also be attacked by certain reagents, and if is under load, then the cracks will grow only in metals and alloys. Perhaps the earliest known example is the cracking of the rubber ozone layer, where Bond traces of ozone in the atmosphere double attack in the chains of the materials. Elastomers with double bonds in its chains include natural rubber, nitrile rubber and styrene-butadiene rubber. They are highly susceptible to ozone attack, and may cause problems such as car fires (from rubber fuel lines) and flat tire. Today, antiozonants are widely added to these polymers, so that the incidence of cracking is reduced. However, not all products Rubber critical security are protected, and since only begin attack ppb ozone, failures still occur.
chlorine-pipeline attack acetal resin joint
Another highly reactive gas is chlorine, which will attack susceptible polymers such as acetal resin and polybutylene pipes. There have been many examples of such pipes and acetal fittings failing in the properties in the U.S. as a result of chlorine-induced cracking. In essence, the gas attacks sensitive parts of the molecules chain (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, added for its antibacterial action, the attack occur even in parts per million of traces of dissolved gases.
Most step growth polymers can undergo hydrolysis in the presence of water, often a reaction catalyzed by acids or alkalis. Nylon for example, are rapidly degraded and crack when exposed to strong acids. Polycarbonate is susceptible to alkaline hydrolysis, the reaction simply depolymerising the material. Polyesters tend 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 from occurring.
Many polymers are also damaged by UV radiation in the vulnerable points in the structure of its chain. Thus ... (And so on) to get to more information, please visit some products on light truck wheels , argon cylinder. Multi-purpose orthopedics traction bed should show more products here!
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