Influence of structural phase state on corrosion resistance of tubes made from the E635 alloy

An increase in the duration and tightening operating conditions for cores of VVER-type reactors leads to the need in tightening the requirements for properties and production of zirconium components for fuel assemblies (FA).
As a material of force framework elements (guide thimbles, central tubes, bearing frame angles) in serial production, the E635 zirconium alloy ($\mathrm{Zr}$ — $1\%$ $\mathrm{Nb}$ — $1.2\%$ $\mathrm{Sn}$ — $0.35\%$ $\mathrm{Fe}$) is used. The E635 alloy was also tested in fuel claddings of VVER-1000 reactors. The deterrent factor for expanding its use is enhanced corrosion in comparison with the E110 zirconium alloy ($\mathrm{Zr}$ — $1\%$ $\mathrm{Nb}$), which is the basic material for VVER fuel claddings.
Corrosion resistance of products made from the E635 alloy, among other factors, is determined by the size of second-phase particles (SPPs). The presence of large SPPs in the alloy structure reduces its corrosion resistance. Conducting of the quenching operation immediately before cold-rolling stages is the main way to reduce the SPP size but this is not applicable in industrial conditions due to low workability of the alloy in the quenched state.
In correspondence with the $\mathrm{Zr}$–$\mathrm{Nb}$–$\mathrm{Fe}$ ternary phase diagram, the alloy E635 structure in regular composition is an $\alpha$–$\mathrm{Zr}$-matrix with precipitates of intermetallic particles of the Laves phase type ($\mathrm{Zr(Nb, Fe)}_2$) with an average size of $100$ nm. In the structure of industrial products from the E635 alloy (claddings, sheets, and rods), in addition to the Laves phase, there are observed larger precipitates of $\mathrm{T}$-phase particles ($\mathrm{(Zr,Nb)_2 Fe}$) with a size of up to $1$ $\mu$m, which is connected with the thermal-deformation conditions of their production. The aim of this work was to estimate the influence of size, distribution, and type of second-phase particles on the corrosion resistance of the E635 alloy.
The report presents the results of structural and corrosion researches of model specimens from the E635 alloy produced by different modes and with different structural-phase state. It has been shown that the presence of the $\mathrm{T}$-phase within the structure of the alloy lowers the corrosion resistance of products from the E635 alloy to a larger extent than an increase in the size of the Laves phase.
A manufacturing process for guide thimble tubes from the E635 alloy was developed based on the conducted researches, which allowed one to increase their corrosion resistance by $10\%$.