EFFECT OF AGING TIME ON ELECTROCHEMICAL CORROSION BEHAVIOR OF 2101 DUPLEX STAINLESS STEEL

Duplex stainless steels (DSS) are extensively applied in many fields such as petroleum industry, petrochemistry, desalination service and paper-making industry. They are characterized by a two-phase structure., offering an attractive combination of corrosion resistance and mechanical properties. Their best general properties are obtained by keeping approximately equal volume fractions of austenite and ferrite but averting third harmful phases, such as sigma, gamma(2), chi, carbide and nitride, to be precipitated. These harmful phases can be formed from ferrite when DSS are welded or worked at improper high temperatures. Recently, a kind of economical DSS 2101 of typical composition Fe-21.4Cr-1.2Ni-5.7Mn-0.23N-0.31Mo was developed and shows a wide application potential due to its higher yield strength, better pitting corrosion resistance and lower cost than traditional 304 austenite steel. However, limited research has been dedicated to studying the microstructure and mechanical properties of DSS 2101. Especially research on the nature of precipitations and their effects on mechanical properties in DSS 2101 was still lacking in comparison to many researches on the precipitations in DSS 2205 and DSS 2507. In the present work, it aims at investigating how the precipitations can affect the corrosion resistance of DSS 2101. Pitting corrosion resistance of samples aged at 700 degrees C in 1 mol/L NaCl solution was evaluated by potentiodynamic polarization curve test. The electrochemical potentiokinetic reactivation (EPR) test was also used to evaluate the degree of sensitization to intergranular corrosion. Scanning electron microscopy (SEM) and optical microscopy (OM) were used to observe the surface morphology after electrochemical tests. The results demonstrate that both the resistance to pitting (E-b) and intergranular corrosion (R-a) decreases with aging time increasing. Pitting nucleates preferentially in the ferrite phase for the solution-annealed sample, while the initiation of pitting corrosion takes place around Cr2N, in the newly formed secondary austenite for the aged specimen. EPR tests conducted at room temperature are able to detect the sensitization to R-a due to Cr2N precipitation. With increasing aging time, more Cr2N particles are precipitated at phase interfaces and grain boundaries, causing a continuous decrease of resistance to intergranular corrosion.