Oxidation and hydrogenation of the Zr-2.5% 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 joined by flanging with the end joints made up of 403 m type stainless steel (modified). The existence of the galvanic coupling: 403 m stainless steel/Zr-2,5% Nb alloy leads to the oxidation acceleration and especially, to the Zr-2,5% Nb alloy hydrogenation. In order to reduce these 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, continuous, adherent, nontensioned, free of pores and microcraks chromium depositions on 403 m stainless steel. The chromium pelicules were characterized by: metallographic examinations, hardness measurement, X-ray diffraction and electrochemical impedance determinations. The corrosion tests performed by autoclaving at 310 degrees C and 9.8 MPa, in littiated water (pH-10.5), with hydrogen addition, Zr 2.5% Nb samples galvanically coupled with chromated, 403 m stainless steel, evidentiated the reduction in the oxidation and hydrogenation of Zr-2.5% Nb alloy.