Experimental
2.1. Test apparatus
The SCC test apparatus with an in situ observation system used in experiments is briefly
described as follows: The autoclave, whose inner volume was 1.3 L, was made from Type
316 stainless steel, and allowed a maximum pressure and temperature of 100 kg/cm2 and
300 C, respectively. The autoclave had a Pt counter electrode and an Ag/AgCl pressure balanced
external reference electrode to measure and control the electrochemical condition.
The surface of the specimen set in the autoclave could be observed during the
SCC test through a transparent Al2O3 single crystal rod inserted into the observation port
of the autoclave. Size of the rod was 20 mm in diameter and 160 mm long. A gap between
the end of the rod and the specimen surface was 10 mm to avoid fluctuation of the
observed image due to the spatial distribution of solution density induced by thermal convection.
A high-performance charge-coupled device (CCD) camera system was employed
to observe and record the cracking images. The observation area of the CCD camera system
was about 4 mm · 5 mm when a 50· objective lens was used. In order to observe all
the cracks on the whole gauge area 4 mm wide and 10 mm long, the head part of the CCD
camera was mounted on an X–Y–Z stage which could move over the volume
20 mm · 20 mm · 20 mm. The stage and the recording system were operated synchronically
using a computer. It was confirmed that reasonably clear images of the crack initiation
and growth behavior could be obtained in the high temperature water up to 300 C.
Details of this apparatus are described elsewhere