Atmospheric corrosion mechanism

A high-nickel alloy is used for increased strength at elevated temperature, and a chromium content in excess of 20% is desired for corrosion resistance. An optimum composition to satisfy the interaction of stress, temperature, and corrosion has not been developed. The rate of corrosion is directly related to alloy composition, stress level, and environment. The corrosive atmosphere contains chloride salts, vanadium, sulfides, and particulate matter. Other combustion products, such as NO, CO, CO2, also contribute to the corrosion mechanism. The atmosphere changes with the type of fuel used. Fuels, such as natural gas, diesel 2, naphtha, butane, propane, methane, and fossil fuels, will produce different combustion products that affect the corrosion mechanism in different ways. 

Tantalum is severely attacked at ambient temperatures and up to about 100°C in aqueous atmospheric environments in the presence of fluorine and hydrofluoric acids. Flourine, hydrofluoric acid and fluoride salt solutions represent typical aggressive environments in which tantalum corrodes at ambient temperatures. Under exposure to these environments the protective TajOj oxide film is attacked and the metal is transformed from a passive to an active state. The corrosion mechanism of tantalum in these environments is mainly based on dissolution reactions to give fluoro complexes. The composition depends markedly on the conditions. The existence of oxidizing agents such as sulphur trioxide or peroxides in aqueous fluoride environments enhance the corrosion rate of tantalum owing to rapid formation of oxofluoro complexes. 

Atmospheric corrosion can be prevented by using volatile inhibitors which need not be applied directly to the surfaces to be protected. Most such inhibitors are amine nitrites, benzoates, chromates, etc. They are mainly used with ferrous metals. There is still some disagreement as to the mechanism of action. Clearly, any moisture that condenses must be converted to an inhibitive solution. There is no doubt that the widely used volatile inhibitors are effective in aqueous solutions containing moderate... 

In situ Raman spectroscopy is being used to investigate corrosion products from zinc in a humid atmosphere and sodium chloride70 and from Type 304L stainless steel in aerated water at elevated temperatures and pressures.71 The changes in detected species over time helped identify possible corrosion mechanisms and the effect of different variables on corrosion rates and mechanisms. 

Like all other non-oxide ceramics Si3N4 is metastable in air or combustion gases, both at room and at elevated temperatures. Detailed understanding of oxidation and corrosion mechanisms and the influence of the surrounding atmosphere on the lifetime are necessary before Si3N4 ceramics can be applied under oxidising or corrosive conditions [431-437]. 

Graedel, T.E. (1992) Corrosion mechanisms for silver exposed to the atmosphere. 

The role of alloying elements in weathering steels consists of the effect of formation of the protective layer of corrosion products increase in mechanical strength and toughness and improved weldability. The protective qualities of the corrosion products on the steel depend on the continuous growth of the adherent, compact, inner layer and on low porosity within the layer. The kinetics of atmospheric corrosion were found to obey the equation,... 

Alnico alloys are brittle and hard, they can only be machined by grinding, spark erosion, and electrochemical milling, and they resist atmospheric corrosion well up to 500 °C (Fiepke, 1990). The mechanical behavior, in particular creep, of Alnico-type, Fe-Ni- Al alloys has been studied in detail, and both Fe-rich alloys with precip-... 

High mechanical and electrical resistance, wide operation temperature range (from 260 toi - -260°C), low friction coefficient. Surpasses all known materials in chemical resistance. Resistant to atmospheric, corrosion and radiation effects. 

For nongalvanized steel, cosmetic corrosion generally involves a cathodic delamination mechanism the surface under the paint becomes cathodic and the surface exposed in the hole becomes anodic. To slow down or prevent atmospheric corrosion, it is therefore important that the siuface treatment be a good cathodic inhibitor in the finished product. The phosphate layer increases corrosion resistance by limiting the available free surface for the cathodic reaction. In general, the activity of the free surface is further reduced by passivating posttreatments or by the deposition of amorphous phosphate films between the crystals. 

Corrosion mechanisms in molten sulfates consist of a sequence of chemical reactions and transport processes including oxide dissolution, transport of dissolved species through the salt film, and subsequent precipitation of oxide within the salt film in contact with the gas atmosphere. 

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