Recent opinions on the effect of nonmetallic inclusions, especially sulfides, on the nucleation of pits, stress corrosion cracks, and hydrogen-induced cracks are reviewed.

The nonmetallic inclusions present in steels as impurities adversely affect the corrosion resistance of steels. This is true for both the general and local corrosion mechanisms, like pitting corrosion, stress corrosion cracking (SCC), and hydrogen embrittlement (HE).

The effect of nonmetallic inclusions (in particular that of sulfides) on the pitting corrosion mechanism has been the subject of numerous investigations. A fairly aboundant information can be found in vast original literature and in many review articles [1–3]. The role of nonmetallic inclusions in SCC is relatively less covered in the pertinent literature. Since, however, the nucleation of corrosion cracks frequently starts from pits, and pits nucleate at sulfides, the presence of sulfides is likely to affect the SCC process.

Another corrosion mechanism that leads to a local cracking of a metal is the hydrogen embrittlement. The presence of nonmetallic inclusions enhances susceptibility of steel to hydrogen-induced cracking [4].

This review will be concerned with the effect of nonmetallic inclusions on the local corrosion types mentioned.

The effect of sulfide inclusions on the susceptibility of steels to pitting, stress corrosion cracking and hydrogen embrittlement

This paper reports a study of the effects of phosphorus, tin, and molybdenum on the caustic stress corrosion cracking susceptibility of NiCrMoV rotor steels. Constant load tests were performed on these steels in 9M NaOH at 98 ± 1 °C at a controlled potential of either -800 mVHg/Hgo or -400 mVHg/Hgo. Times to failure were measured. The results show that at a potential of -400 mVHg/Hgo the segregation of phosphorus to grain boundaries lowers the resistance of these steels to caustic stress corrosion cracking. When molybdenum is removed from a steel that has phosphorus segregated to the grain boundaries, the steel’s resistance to stress corrosion cracking is improved. High purity alloys, both with and without molybdenum, show very good resistance to caustic cracking at this potential. At-800 mVHg/Hgo segregated phophorus has no effect; only molybdenum additions lower the resistance of the steel to caustic stress corrosion cracking. Segregated tin has little effect at either potential. Metallographic examination shows that one explanation for these results is that molybdenum and phosphorus, probably as anions precipitated from solution, aid in passivating the sides of the crack and thus help keep the crack tip sharp. This sharpness will increase the speed with which the crack will propagate through the sample. Furthermore, removal of molybdenum greatly increases the number of cracks which nucleate. This higher crack density would increase the relative area of the anode to the cathode and thus act to decrease the crack growth rate.

Caustic stress corrosion cracking of NiCrMoV rotor steels—The effects of impurity segregation and variation in alloy composition

This paper summarizes results of studies conducted in the USA over the past five years to identify factors responsible for stress corrosion cracking (SCC) of forged, shrunk-on, low-alloy steel discs in low-pressure (LP) steam turbines used in nuclear power plants. Experience in the UK and subsequent research findings are reviewed, and results of a survey of US experience and of comprehensive experimental programs are reported. Recommendations are presented to minimize SCC of LP turbine discs in existing and future power plants. In general, the work performed revealed: (i) no specific chemical contaminant is required for cracking; crack initiation and growth occur in pure condensed steam; (ii) the yield strength of a disc steel is the dominant factor governing crack growth rates; (iii) other factors being equal, a higher incidence of cracking may be expected in rotor designs having higher applied stress levels. Crack initiation and growth in LP turbines may be minimized by control of the environment, by us...

Low-Pressure Steam Turbine Disc Cracking—An Update

This paper summarizes results of studies conducted in the USA over the past five years to identify factors responsible for stress corrosion cracking (SCC) of forged, shrunk-on, low-alloy steel discs in low-pressure (LP) steam turbines used in nuclear power plants. Experience in the UK and subsequent research findings are reviewed, and results of a survey of US experience and of comprehensive experimental programs are reported. Recommendations are presented to minimize SCC of LP turbine discs in existing and future power plants. In general, the work performed revealed: (i) no specific chemical contaminant is required for cracking; crack initiation and growth occur in pure condensed steam; (ii) the yield strength of a disc steel is the dominant factor governing crack growth rates; (iii) other factors being equal, a higher incidence of cracking may be expected in rotor designs having higher applied stress levels. Crack initiation and growth in LP turbines may be minimized by control of the environment, by use of low-strength alloys, and by designs which eliminate keyways and minimize applied stress levels.

Low-pressure steam turbine dise cracking ― an update

Environmental effects in the stress corrosion cracking of turbine disc steels

Investigations have been carried out to examine the stress corrosion behaviour of some medium strength low alloy steels in high purity wet steam and water. The alloys involved are used for the manufacture of steam turbine discs and rotors.Stress corrosion cracking occurred in both 3% CrMo and 31/2% NiCrMoV steels. Crack growth rates were measured, after exposure in on-site rigs, for periods up to 20,000 h. The effects of applied stress, stress intensity, and strength level were studied in addition to microstructure. The possible role of certain non-metallic incluszons upon the cracking process is highlighted, as is the presence of chromium.

J. G. Parker

Papers

Stress Corrosion Cracking of Turbine Disc and Rotor Alloys