The research of the electrochemical behavior of Ni – Cr – Mo alloys and their welded joints by cyclic polarization in a sulfuric acid medium

This paper describes the results of an experimental study of the corrosion characteristics of the base metal and welding seam of alloys based on Ni – Cr – Mo by the method of cyclic polarization in sulphuric acid H2SO4 with the addition of nickel tetrafluoroborate Ni(BF4)2 and iron sulfate FeSO4 as sources of Ni2+ and Fe2+ ions, at temperatures of 20 oC and 60 oC. Five experimental alloys labeled 35, 1, 2000, 22, and 27 were used for conducting electrochemical studies. Two metal plates were studied for each alloy: one from the base material zone and the other from the welded joint zone. From the obtained plates, 10 samples with approximate dimensions of 10×15×5 mm were prepared for research with marking 35/Base and 35/Welded, 1/ Base and 1/Welded, 2000/ Base and 2000/Welded, 22/Base and 22/Welded, 27/Base and 27/Welded. The polarization curves of the studied samples are given, the corrosion potentials (Ecor), pitting (Epit) and repassivation (Erep) are calculated graphically. Calculations of the main (Epit) and additional (Erep) bases of pitting resistance are presented. It was established that the samples made of alloy 1 have the worst corrosion resistance, which probably is explained by the presence of structural defects or distortions in the material. Nickel alloys 22 and 27 demonstrated the best corrosion properties. There is a general tendency for alloys labeled 35, 1, 22, and 27 in which the welded joint either reduces its corrosion resistance or retains its corrosive properties relative to the base metal after the welding process. The exception is the alloy labeled 2000: here the sample of the welding seam of the alloy grade 2000 has a higher corrosion resistance than the sample of the base metal.