Stress Corrosion Cracking (SCC) testing of weld metal Alloy 182 with tapered specimens in PWR environment

SCK•CEN Mentor

Bosch Rik-Wouter, rbosch@sckcen.be, +32 (0)14 33 30 81

Expert group

Structural Materials

SCK•CEN Co-mentor

Vankeerberghen Marc , mvankeer@sckcen.be , +32 (0)14 33 31 82

Introduction

Due to the ability of nickel-based alloys to offer good mechanical properties, corrosion resistance and compatibility to other materials, Alloy 182 is frequently used as a weld metal for internal components of Pressurized Water Reactors (PWR) and power plants. However this weld metal can be susceptible to PWSCC (Primary Water Stress Corrosion Cracking), when sufficiently stressed and exposed to the primary coolant. Therefore significant work has been carried out to define the conditions under which this Alloy 182 is most susceptible to PWSCC. One of the challenges is to develop an accelerated test methods as lead times of this phenomena can be several decades. 

Recently an accelerated test method to determine stress thresholds for crack initiation of alloy 182 with tapered specimens has been developed. These tapered specimens can be used in slow to very slow SSRT tests under PWR conditions at different temperatures. As the stress along a tapered specimen changes with the width of the cross section, with one test a stress threshold could be obtained. This is done by SEM inspection of the specimen's surface to find out where no cracks are present any more. This is then the stress threshold corresponding with the test conditions i.e. temperature, strain rate and surface conditions. The straining is a way to accelerate the test. However, by using a lower and lower strain rate, a stress threshold can be obtained for "constant load" conditions by extrapolation to zero strain rate.  

This tapered specimen test method has been used successfully to investigate the effect of strain, temperature and surface roughness. For this work, the effect of the surface quality will be investigated. Specimens with industrial, polished and shot peened surfaces will be tested and compared. The aim is to see which surface has the highest resistance to SCC.

Objective

The main objective is to study the influence of the surface quality to the SCC susceptibility of Alloy 182 in PWR environment using the tapered specimen test method. Tapered specimens with different surface treatments are already available for this study.

This can be subdivided in the following tasks:

  • Analyse the tapered specimens with different surfaces conditions before testing (optical microscope, SEM, hardness)
  • Carry out autoclave tests under simulated PWR conditions to determine the SCC susceptibility.
  • Analysis of the tested specimens with optical microscope and SEM to obtain the crack distribution and SCC stress threshold.
  • Try to find a possible mechanism that explains the observed behaviour. 

Experimental work will be carried out in the corrosion lab using the PWR loops and autoclaves.

The minimum diploma level of the candidate needs to be

Master of sciences , Master of sciences in engineering

The candidate needs to have a background in

Chemistry , Electromechanics , Other , Physics , material science