In the nuclear power plants (NPPs), wall thinning of the piping materials is generally caused by a
flow-accelerated corrosion (FAC) and leads to a rupture with no warning unless it is detected and
repaired in a timely manner. To reduce the FAC, it is better to use low-alloy steels, such as 1Cr-½Mo
and 2¼Cr-1Mo, having higher FAC-resistance than carbon steel. Meanwhile, in the secondary water
chemistry at the NPPs in Korea, hydrazine concentration is maintained within the range of 100~150
ppb. For applying these low-alloys to a piping material, we investigated the influence of hydrazine
concentration on their FAC. An experiment was carried out at pH25°C of 9 controlled with ammonia in
a deoxygenated aqueous solution containing 0~250 ppb-hydrazine by using a FAC test loop at 250°C
for 300 hours. Experimental weight loss showed a hydrazine concentration dependency of the FAC in
this concentration range, giving minimum at 150 ppb.
Flow-accelerated corrosion (FAC) is a degradation process that attacks carbon and low-alloy steels. The process has been studied extensively for the last thirty years. Nevertheless piping leaks and ruptures continue to occur. Recent fatal accidents at the Mihama nuclear station in Japan and at the Iatan fossil unit in Missouri demonstrate that plant operators must continue to maintain mitigation programs in order to maintain a high degree of confidence against unanticipated pipe rupture caused by FAC. While FAC is a complex phenomenon, most of its controlling elements are fairly well understood with U.S. nuclear plants using well-developed analytical methods to increase the efficacy of their inspection programs. However, there remains one particular controlling element — described 10 years ago — that is still not widely included in plant inspection programs. This is the entrance effect (or leading-edge effect), which refers to the accelerated attack downstream of a FAC resistant material to FAC susceptible material joint (e.g., a butt weld). This paper provides an update to the previous PVP Conference paper on this subject [1]. Recent plant experience, developments in analysis and the implications to existing plant programs will be discussed.
Abstract
It has been demonstrated that the natural atmospheric corrosion behavior of low alloy steels in marine environments can be accurately described by an equation of the form: ΔW = KtN. With this relationship, reliable predictions of long term weight losses can be made from as few as two sets of determinations obtained in relatively short exposures. The reliability of this relationship has also been established for galvanized and aluminized steels.
Erratum to : On the Corrosion Behavior of Stainless Steels and Low Alloy Steels in Ammonia Synthesis Atmosphere