Stress-corrosion cracking corrodent

It is now well established that the activity of pitting, crevice corrosion, and stress-corrosion cracking is strongly dependent upon the corrosion potential (i.e., the potential difference between the corrod-... 

Figure 9.1 Stress-corrosion cracks result from the synergistic interaction of tensile stress and a specific corrodent.

Slides Corroded automobiles, fences, roofs stress-corrosion cracks, corrosion-fatigue cracks, pitting corrosion. 

The conjoint action of a tensile stress and a specific corrodent on a material results in stress corrosion cracking (SCC) if the conditions are sufficiently severe. The tensile stress can be the residual stress in a fabricated structure, the hoop stress in a pipe containing fluid at pressures above ambient or in a vessel by virtue of the internal hydraulic pressure created by the weight of its contents. Stresses result from thermal expansion effects, the torsional stresses on a pump or agitator shaft and many more causes. 

The corrodent is a liquid metal in this form of stress corrosion cracking. Mercury at ambient temperature and metals including zinc (from galvanized steel-work) and copper (from electric cables) when melted during welding or in a fire cause rapid failure of certain metals. 

Electrochemical noise A variety of related techniques are now available to monitor localized corrosion. No external polarization of the corroding metal is required, but the electrical noise on the corrosion potential of the metal is monitored and analyzed. Signatures characteristic of pit initiation, crevice corrosion and some forms of stress corrosion cracking is obtained. 

Stress-corrosion cracking based on active-path corrosion of amorphous alloys has so far only been found when alloys of very low corrosion resistance are corroded under very high applied stresses . However, when the corrosion resistance is sufficiently high, plastic deformation does not affect the passive current density or the pitting potential , and hence amorphous alloys are immune from stress-corrosion cracking. 

To protect stainless-steel equipment from chloride stress-corrosion cracking by triggering an anodic protection system when the measured potential falls to a value close to that known to correspond to stress-corroding conditions. 

All metals will corrode under certain conditions. Internal corrosion is caused by galvanic corrosion, pitting, corrosion fatigue, stress corrosion cracking, stray currents, etc. 

The electrochemistry of corrosion is a big piece of electrochemistry. It permeates most of the surface aspects of materials science, at least for practical metal systems in contact with moist air. It influences not only the surface but often the bulk owing to its influence an embrittlement and stress corrosion cracking. So, at the beginning, we argued that a corroding metal is rather like a local fuel cell in which the corroding metal has a very large number of pairs of microsized electrodes on its surface, an equal number of them anodic and cathodic, respectively. 

Corrosion—the gradual decay of materials—occurs in many ways, all involving electrochemical surface reactions. The essence of it is the electrochemical dissolution of atoms in the surface into the ion-containing film that is in contact with the corroding metal. However, such dissolution has to be accompanied by a counter-reaction and this is often the electrochemical decomposition of water to form hydrogen on the metal surface. If that occurs, the H in the form of minute protons, H, may enter the metal, diffuse about, and cause a weakening ofmetal-metal bonds and hence stress-corrosion cracking. 

Corrosion studies have been rare. (8), copper, or iron were corroded by carbon tetrachloride when exposed to Co-60 radiation (78). Alkyl halides enhanced the corrosive effect of benzoic acid on iron (79). (1) was found to promote stress-corrosion cracking in zirconium alloys used in nuclear reactors (80). 

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