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A corrosão seletiva de fases de dois tipos de aços inoxidáveis fundidos foi analisada, ao serem submetidos a vários valores constantes de potencial, a partir da região catódica para a anódica da curva de polarização. Os aços estudados foram: austenítico (ACI CF-3M) solubilizado e martensítico (ACI CA-6NM) normalizado e revenido. As curvas de polarização foram traçadas potenciodinamicamente, a uma velocidade de varredura de 0,16 m.V.s-1 em solução aerada contendo H2SO4 1M e NaCl 1M, pH 0,35, à temperatura ambiente. Vários valores de potencial foram selecionados, previamente localizados nas regiões catódica, ativa, passiva e transpassiva da curva de polarização. Potenciostaticamente, foram atingidos os valores de potencial selecionados, sendo eles mantidos fixos por 1800 s. Após o ataque potenciostático, as aparências metalográficas dos aços foram verificadas, desde a região catódica até a anódica. Por microssonda eletrônica, foram quantificados os elementos químicos presentes nas fases dos aços. Para melhor avaliar o início de propagação de pite, foi determinada a porcentagem de íons liberados para o eletrólito, após a imposição de valores de potencial críticos de pite predeterminados pelas curvas de polarização, pelas aparências metalográficas e pelas curvas de densidade de corrente em função do tempo. / The selective corrosion of phases present in two types of cast stainless steels was analyzed, when submitled to several constant values of potential, starting from the cathodic to the anodic regions of the polarization curve. The cast stainless steels were: austenitic (ACI CF-3M) solubilized and martensitic (ACI CA-6NM) normalized and tempered. The polarization curves were obtained at a scanning rate of 0.16 mV.s-1 in 1M H2S04 and 1M NaCI solution, pH 0.35 and at room temperature. Several potential values were selected, previously located in the cathodic, active, passive and transpassive regions of the polarization curve. The values of the selected potential were maintained fixed for 1800 s. After the potentiostatic etching, the metallographic appearances of the steels were verified, from the cathodic to the anodic region. Through electronic microprobe analysis, the present chemical elements were quantified in the phases of the steels. For betler evaluating the beginning of pitling propagation in the studied steels, liberated ion percentage for the solution was determined, after the imposition of critic pitting potential values predetermined from the polarization curves, through the metallographic appearances obtained after potentiostatic etching of the samples and the current density curves as a function of time.

Duplex stainless steel (DSS) that developed in the 1930’s is a type of steel that consists of bothaustenite and ferrite phases in an almost equal proportion. It inherits properties of both ferriticand austenitic stainless steel and so it has good mechanical and corrosion resistance propertiesthat makes it suitable material in different industries with aggressive environment. There aredifferent approaches that used to evaluate the susceptibility of stainless steels to pitting corrosionlike Pitting Resistance Equivalent Number (PREN) and electrochemical methodologies. In thisstudy, focus is on the potentiodynamic (AL 101 5170) and potentiostatic (ASTM G150) testmethods as laboratory evaluations in which standard DSS (2205) is exposed to the differentchloride concentrations 0.1 M and 1 M respectively. In these methods parameters like PittingPotential (Ep) and Critical Pitting Temperature (CPT) can be measured respectively and used topredict the susceptibility of DSS to pitting corrosion. DSS owes its good corrosion resistance tothe oxide film that protects the metal from corrosion. Pitting can be initiated as a result of anyheterogeneities on the metal surface which act as a passive film defect like non-metallicinclusions and intermetallic precipitations such as sigma phase when metal is exposed toaggressive environment and high temperature. The aim of the present work is to make acomparison between two electrochemical methods besides finding how inclusions andmicrostructure can have impact on their results and on pitting corrosion. The results of theexperiment shows that there is a positive linear relation only in longitudinal direction betweenCPT and Ep. Also, among two test methods only ASTM G150 can detect sigma phase existence(<3%). Based on investigations in this experiment, inclusion characteristics such as numberdensity and composition has compatible results in both methods so that samples with big sizeinclusions (>8 μm) have negative effect on corrosion resistance and in terms of composition,samples with nitride inclusions has lower CPT and Ep compared with those with only oxide oroxide/sulfide inclusions. It is also discovered that phase arrangement is not the same in allspecimens that produced by different production methods and samples with finer phases havehigher CPT values in both longitudinal and transvers directions. However, this shape ofmicrostructure only affect the Ep in longitudinal directions but not transvers.

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