Question

Can someone tell me "what Electrochemical Factors affecting corrosion behaviour of austenitic stainless steel" ? and a general explaination for each of the factor?

Answer 1

I am giving a brief explanation on this please read it properly:

Corrosion is the electrochemical degradation of a material especially metallic alloy due to interaction with its environment. This phenomenon is characterized by the removal of atoms from the metal to form deleterious compounds in the presence of water and gases. The effects and consequences of corrosion has been known to man since the earliest metallurgical times and has been a constant drain on his productive activities, as such, purposeful attention have been focused on the problem both by scientists and engineers. The corrosion initiation process and propagation mechanism on stainless steels is of great importance due to its complication and insidious nature. The corrosion resistance of stainless steels is through the formation of thin, passive protective films on their surfaces which coats the base alloy. The film formation occurs instantaneously within an oxidizing environment. Despite these attributes they are vulnerable to specific corrosion types in astringent atmospheres such as pitting and uniform corrosion. Acidic solutions are specifically aggressive to this film causing severe pit formation and growth in addition to general corrosion  Pitting is a localized form of corrosion in which metallic dissolutions occurs at vulnerable sites and/or regions on the metal surface resulting in the formation of cavities in passivated metals or alloys that are exposed to aqueous, nearly neutral solutions containing aggressive anions. It generally occurs in chloride, halide or bromide solutions. If a fault in the passive layer or a surface defect results in the local destruction of the former, dissolution of the steel underneath leads to a build-up of positively charged metallic ions, which in turn causes negatively charges (e.g. chloride ions) to migrate near the defect. The chloride ions undoubtedly caused more deleterious degradation on the stainless steel, the consequence of which was that of severe active corrosion reactions of anodic dissolution of the tested alloy. In the presence of the reacting species (Cl- ), at high concentrations, the ability of the stainless steel to repair its protective film was drastically reduced and the protection was lost. The relative concentration of metal cations in within the pit solution increases with increase in chloride concentration which results in increase in the dissolution rate of the steel. Through an autocatalytic process pit forms and becomes continuously loaded with positive metal ions from anodic dissociation. The Cl− ions concentrates in the pits for charge neutrality and encourage the reaction of positive metal ions with water to form a hydroxide corrosion product and H+ ions which combine to release hydrogen gas. At this region the passive film is electrochemically unstable, undergoing potential-dependent transpassive dissolution. Progression from the passive state to pitting corrosion phase can also be explained on the basis of competitive adsorption interaction between the Cland the elemental atoms responsible for passivity whereby Clions migrate to the metal/liquid interface under high electric field intensity resulting from an impressed current sequentially reaching the critical potential (Epit), which corresponds to the Clconcentration necessary to displace adsorbed oxygen species and facilitate the oxidation of iron atoms. The presence of adsorbed Clincreases the potential difference across the passive film thereby enhancing the rate of Fe2+ diffusion from the metal/film interface to film/solution interface.

Pit creation on metals is incalculable though there is preference for sites associated with flaws or cracks. In harsh conditions, general corrosion of stainless steel is inconsequential. The inception of pitting corrosion on occurs in two phases (nucleation and collapse of passivity) leading to oxidation and anodic dissolution. Inhomogeneities on the surface of the alloy are initiation sites for pit formation. The necessary condition for an active pit is the presence of an aggressive environment inside the pit. Under this condition, the pit remains active, behaving as a small anode, while the surrounding surface acts as a large cathode. The rapid production of metal ions within the pit induces the diffusion and migration of anions such as Clchiefly responsible for breakdown of the passive film. Figure below depicts a propagating pit in an iron based alloy containing chromium in a chloride containing environment

In the presence of Cl- , the hydrolysis of Fe2+ is accelerated, as shown in the reactions below;

The electrons given up by the anode flow to the cathode (passivated surface) where they are discharged in the cathodic reaction:

Electrochemical diffusion of chloride ions within the pit activates the redox reactions. This increases the entropy of the ions and electrolytic reactions in the pit, including hydrogen evolution reactions (eqn. 11 & 12) thereby accelerating the confined corrosion process on the steel. The corrosion process is due to the induced ionization of metallic atoms in the pit which as cations thus attracting the negatively charged Clions. The differential size between the anodic and cathodic sites catalyzes the rate of corrosion and the aftermaths of the process i.e. the formation of FeCl2, Fe(OH) 3etc within and at the outer regions of the pit. Breakdown of the passive film can be seen partially dependent of the circumstances associated with pitting corrosion. At a particular potential the passive film perforates due to anion adsorption. Once a pit nucleates, metal dissolution self-propagates causing the film to lose its capacity to repassivate.

Chloride is a strong electronegative ion, relatively small and easily diffuses through perforations or pores. Metallic cations ionizes and reacts strongly in chloride solutions.Increment in NaCl concentration influences the redox portion of the polarization curve resulting in higher values of icorr and less noble values of Ecorr . The Cl− ions are aggressive enough to attack and initiate pitting corrosion on the steel samples. Cl− ions are capable of penetrating through the passive layer under induced corrosion. This propagates the diffusion of Cl− ions into the pits to perpetuate electrical neutrality and breakdown of the corrosion products within it thus obstructing repassivation. The mechanism is under activation control due to increased acidity which accelerates the corrosion rates.

The passive film formed on the stainless steels is composed of chemical combination of iron and chromium oxides in addition to hydroxides and oxyhydroxides. At the onset of passivation there is a chemical reaction between metal cations and water to form hydrated species resulting in the formation of the oxide film by deprotonation of the hydroxyl ion .Under anodic polarization the stainless steels samples acquired a passive state. Pitting corrosion is induced by an attack of the Clions diffuse into the passive film through electrochemical migration under electrostatic influence. The electrolytic transport of chloride ions through the film is due to the selective nature of the layer on the passive film in the solution. The chlorides accumulate in the region leading to the formation of pits.

Corrosion pit nucleates on ferrous alloys in the absence of oxygen. Oxygen depletion prevents the production of hydroxyl ions. This electrochemical mechanism causes the accumulation of ionized metal atoms on the surface creating strong electrostatic affinity from the chloride anions resulting in accelerated metal dissolution within the pit. The experimental condition was well aerated to aid the passivation of the steel samples. The stainless steels were unable to repassivate in the presence of atmospheric oxygen in addition to reciprocal alterations which depletes the protective film. The oxygen in solution oxygen reacts with metal cations from the anodic dissolution process to form protective oxides on the metal interface. This reduces anodic oxidation responsible for corrosion; however decreased supply of oxygen caused the propagation and progression of micro pits. Under induced corrosion, the impressed current evokes the diffusion of chloride species to the pit interior, destroying the passive film in the process, thereby accelerating the mechanism of corrosion reaction.

Although explanation is huge but i hope usefull.