Estudo de ligas especiais para resistência à corrosão em ambientes agressivos

Abstract

The phenomenon of corrosion deteriorates metallic materials, the corrosive attack is electrochemical and starts on the metallic surface. The relationship between corrosion and metals involves high costs of the order of 5% of the gross domestic product of some countries, these expenses include maintenance or replacement of deteriorated or contaminated products because of corrosion reactions. The corrosive process is not always associated with mechanical efforts and can occur through several mechanisms, the environment to which the metal is exposed directly influences the corrosion variables, such as: speed, temperature, and fluid composition. The increase in the velocity of a fluid accelerates the rate at which corrosion occurs due to erosion effects, in line with most chemical reactions, in corrosion reactions the increase in temperature serves as a catalyst for the reaction and the increase in the concentration of corrosive species produces a higher corrosion rate. In industrial environments, metal alloys are subject to the highest levels of the variables presented, called Aggressive Environments, in aggressive environments metals are exposed to chlorides, bromides, sulfides, hypochlorites, iodides, fluorides and water in motion, making it necessary to use of Special Alloys called superalloys that maintain their mechanical properties even at high temperatures while maintaining surface integrity, are examples of special alloys: duplex stainless steels, austenitic stainless steels, martensitic stainless steels, micro alloyed supermartensitic stainless steels, titanium alloys and nickel superalloys . Special Alloys must present the phenomenon of passivity, losing their chemical reactivity, this phenomenon arises from the ability of the alloy to form an oxide film thatserves as a barrier to corrosion, the formation of the passivation layer in stainless steels is due to the high chromium content which comes into contact with atmospheric oxygen forming the protective chromium oxide (Cr2O3), titanium alloys form titanium oxide (TiO2) and nickel superalloys which, in addition to the formed passivation layer, have high resistance to pitting corrosion due to the presence of molybdenum. The resistance of the alloys was tested by potentiodynamic polarization curves and by calculating the equivalent number of potential resistance to pitting. As a highlight, resistance to pitting and corrosion in general are nickel alloys and titanium alloys.