Formation and growth of pits on X5CrNi18-10 austenitic stainless steel in presence of chlorides and sulphates

Abstract

The resistance of X5CrNi18-10 stainless steel to pitting corrosion in a solution containing chlorides and sulphates was tested using the potentiodynamic polarization method. The obtained results show that the stainless steel is significantly resistant to pit formation, but it is susceptible to pit growth and crevice corrosion. Pits formed at the corrosion potential grow continuously. Statistical analysis of the results obtained during pitting corrosion testing was performed. It can be assumed with a probability of 95% that values of indicators of resistance to pit formation (the pitting potential Epit, the metastable pitting potential Empit and the difference Epit-Ekor) will be within the range of several percents. Values of indicators of resistance to pit growth (the amount of charge required for the pit growth q, the protective potential Eprot and the difference Epit-Eprot) will be within the broader range. In addition, the appearance of pits on the surface of the stainless steel, as well as the appearance of the pits bottom, were analyzed. It was shown that the structure of the stainless steel tested was not sensitized to pitting and intergranular corrosion, which means that the stainless steel was not previously thermally treated.

Primenom potenciodinamičke metode ispitana je otpornost nerđajućeg čelika X5CrNi18-10 na piting koroziju, u rastvoru koji sadrži hloride i sulfate. Dobijeni rezultati pokazuju da je nerđajući čelik u značajnoj meri otporan prema formiranju pitova, ali postoji sklonost prema rastu pitova i koroziji u zazorima. Pitovi formirani na korozionom potencijalu nastavljaju stabilno da rastu. Izvršena je statistička analiza rezultata dobijenih pri ispitivanju piting korozije. Pokazano je da će se sa verovatnoćom od 95 % vrednosti pokazatelja otpornosti nerđajućeg čelika prema formiranju pitova (potencijal pitinga Epit, potencijal metastabilnog pitinga Empit i razlika Epit-Ekor) nalaziti u granicama od nekoliko procenata. Vrednosti pokazatelja otpornosti nerđajućeg čelika prema rastu pitova (količina naelektrisanja potrebna za rast pitova q, zaštitni potencijal Eprot i razlika Epit-Eprot) će se nalaziti u širim granicama. Pored navedenog, analiziran je izgled pitova na površini nerđajućeg čelika, kao i izgled dna pitova. Pokazano je da struktura ispitivanog nerđajućeg čelika nije senzibilizovana prema piting i interkristalnoj koroziji, odnosno da nerđajući čelik nije bio prethodno termički tretiran.