OXIDATION AND HYDROGENATION OF THE ZR-2.5 -PERCENT NB ALLOY, GALVANICALLY COUPLED WITH 403 M STAINLESS-STEEL, ELECTROLYTICALLY CHROMATED

In the PHWR-CANDU nuclear reactor, the Zr-2,5% Nb pressure tubes are j oined by flanging with the end joints made up of 403 m type stainless steel (modified). The existence of the galvanic coupling: 403 m stainl ess steel/Zr-2,5% Nb alloy leads to the oxidation acceleration and esp ecially, to the Zr-2,5% Nb alloy hydrogenation. In order to reduce the se phenomena, we used a coupling with chromated 403 m stainless steel. The electrochemical experiments performed were meant at establishing the optimum chemical composition and parameters for the uniform, conti nuous, adherent, nontensioned, free of pores and microcraks chromium d epositions on 403 m stainless steel. The chromium pelicules were chara cterized by: metallographic examinations, hardness measurement, X-ray diffraction and electrochemical impedance determinations. The corrosio n tests performed by autoclaving at 310 degrees C and 9.8 MPa, in litt iated water (pH-10.5), with hydrogen addition, Zr 2.5% Nb samples galv anically coupled with chromated, 403 m stainless steel, evidentiated t he reduction in the oxidation and hydrogenation of Zr-2.5% Nb alloy.