7. Discussion
7.1. Oxide film and analysis methods
For a long term service pipe, the oxide film on the crack surface has been
built up for many years. The service crack in the cross-section, as shown
in Fig. 3, was open at the beginning, and became narrower gradually to the
crack tip. The analysis methods used in this work were AES and SEM. The
AES analysis showed that the thickness of the oxide film decreased from the
crack beginningto the crack tip. But with the limitation of AES resolution
(sputter spot size about 0.5 mm), it is impossible to analyse the
thickness of oxide film right at the crack tip. SEM can give us the second
electron (SE) image and backscatter elec- tron (BSE) image. BSE image can
give the composition information because the heavy elements scatter back
more electrons to generate a much brighter image than the light elements.
Due to its oxygen content, the oxide film is darker than the steel matrix.
But a thin layer of oxide film may produce an identical con- trast compared
to the steel matrix. This circumstancemay happen at the fresh crack tip.
Fig. 14. Detailed observation of fresh crack surface.
7.2. Crack tip
Since the oxide film at the crack tip can not be characterised by AES and
SEM, the structure of the crack tip became the important factor. As shown
in Fig. 15, there are three types of crack:
Type I: Actively growing crack with the crack sides covered by an oxide
film
Type II: Old dormant crack with a newly initiated crack tip
Type III: Dormant crack with the crack sides and crack tip both covered by
an oxide film
The first two kinds of cracks have an active crack tip which is not large
in size, so the crack tip should be a narrow area. Because the crack tips
are freshly produced without too much corrosion they should reveal IGSCC
features. There may be a thin layer of oxide film on the crack tip
surface, but this film can not be identified using SEM. For the third kind
of crack, the oxide film should cover all the crack surface including the
crack tip, and there should be no region between oxide film covered crack
and the tearing structure produced by the overload. For a type I crack,
the crack propagates gradually and the oxide film is built up on the crack
surface gradually, so the thickness of the oxide film on the crack surface
should gradually decrease from the crack beginning to the crack tip front.
For a type II crack, the oxide film should cover the whole old dormant
crack surface except for the newly initiated crack tip. The newly
initiated crack tips for types I and II can be identified from the
structure observed by SEM. For a type III crack, the oxide film should
cover all the crack surface including the crack tip.