Mechanism of Accelerated Corrosion in Zircaloy-4 Laser and Electron-Beam Welds

Abstract

The corrosion resistance of a series of Zircaloy-4 laser, electron-beam (EB), and tungsten inert gas (TIG) welds was evaluated in high-temperature water and steam. Corrosion exposures carried out in 633 K water revealed that all welds had excellent corrosion resistance at this temperature. However, corrosion exposures carried out in 673 K steam indicated that the laser and EB welds were susceptible to accelerated corrosion, whereas the TIG welds were not. Auger electron spectrometry (AES) and electron microprobe analysis (EMA) were subsequently performed on most of the welds that were corrosion-tested. AES revealed that interstitial impurities such as carbon, nitrogen, and oxygen were not responsible for the acceleration observed at 673 K. EMA revealed that the welds which underwent accelerated corrosion always exhibited tin, iron, and chromium depletion in the fusion zone. These analyses also revealed that the laser and EB welds which were subjected to multiple fusion passes exhibited more severe alloy depletion levels than did single-pass welds, but that TIG welds sustained no measurable depletion. The strong correlation that EMA established between tin, iron, and chromium depletion and the high-temperature corrosion resistance of the laser and EB welds indicates that alloy depletion effects are the only viable explanation for the accelerated corrosion observed at 673 K. It is believed that alloy depletion and the resultant loss of high-temperature corrosion resistance can be eliminated or minimized by performing welding operations at a power density such that the metal surface is heated above the melting point, but below the boiling point. If the latter is exceeded, depletion of high vapor pressure alloying elements such as iron and chromium will ensue, and the fusion zone will be susceptible to high-temperature accelerated corrosion.