The corrosion fatigue crack growth (FCG) behaviour, the effect of applied potential on corrosion FCG rates, and the fracture surfaces were studied for high-strength low-alloy steels, titanium alloys, and magnesium alloys. During investigation of the effect of applied potential on corrosion FCG rates, polarization was switched on for a time period in which it was possible to register the change in the crack growth rate corresponding to the open-circuit potential and to measure the crack growth rate under polarization. Due to the higher resolution of the crack extension measurement technique, the time rarely exceeded 300 s. This approach made possible the observation of a non-single mode effect of cathodic polarization on corrosion FCG rates. Cathodic polarization accelerated crack growth when the maximum stress intensity (K max ) exceeded a certain well-defined critical value characteristic for a given material-solution combination. When K max was lower than the critical value, the same cathodic polarization, with all other conditions (specimen, solution, pH, loading frequency, stress ratio, temperature, etc.) being equal, retarded or had no influence on crack growth. The results and fractographic observations suggested that the acceleration in crack growth under cathodic polarization was due to hydrogen-induced cracking (HIC). Therefore, critical values of K max , as well as the stress intensity range (ΔK) were regarded as corresponding to the onset of corrosion FCG according to the HIC mechanism and designated as K HIC and ΔK HIC . HIC was the main mechanism of corrosion FCG at K max > K HIC (ΔK > ΔK HIC ). For most of the material-solution combinations investigated, stress-assisted dissolution played a dominant role in the corrosion fatigue crack propagation at K max < K HIC (AK < ΔK HIC ).

Mechanisms for corrosion fatigue crack propagation

Corrosion fatigue behaviors of steel wires used in coalmine

The fatigue crack growth rate of Inconel 600 is affected by applied potential, test frequency, and prior thermal treatment. At an applied cathodic potential of -700 mV (SCE), in IN H2SO4, a decrease in test frequency produces an increase in crack growth,da/dN, while a thermal treatment of 700 °C for 0.5 h or more produces intergranular fracture at lowA.K. Intergranular cracking of annealed samples is observed only after precharging with cathodically produced hydrogen. The enhanced FCG rate and tendency for intergranular cracking are mutually exclusive effects in that test frequencies between 1 and 10 Hz do not affect fracture mode, and thermal treatment at 700 °C for 0.5 to 100 h does not affect crack growth rate. It is suggested that since both effects are observed only at cathodic potentials, the data support a hydrogen embrittlement mechanism.

Fatigue crack growth behavior of inconel 600 at cathodic potentials

Life assessment of nuclear LP rotor due to SCC

Corrosion fatigue crack propagation experiments conducted on ASTM A-471 steam turbine disc material in concentrated sodium hydroxide, sodium chloride, and sodium sulfate environments show that crack growth rates are accelerated with increasing cathodic potential. Crack morphology is transgranular at free corrosion potential, but changes to intergranular mode at cathodic potential. At intermediate frequencies and cathodic potential in sodium hydroxide and sodium chloride, cracking was intergranular near both the edges of the compact tension specimen and transgranular towards the center. This is because more hydrogen is available near the free surfaces as compared to the center of the specimen. This coupled with a relatively significant frequency dependence at the cathodic potential indicates that hydrogen diffusion plays a very important role. This should be of concern to nuclear power industry where steam turbine disc cracking has been a major problem. A component of intergranular cracking has been observed in the failed discs which makes one suspect if the turbine disc is somehow seeing a cathodic potential in service. In general, all three environments have been found on failed samples and a similarity of cracking mode in the laboratory at cathodic potentials in all three cases makes a strong argument for hydrogen embrittlement.

Hydrogen embrittlement of a steam turbine disc alloy in aqueous environments

Corrosion fatigue crack growth rates were obtained for an ASTM A 471 Ni-Cr-Mo-V turbine disc steel tested in a room temperature 12M NaOH solution. Tests were conducted under open circuit and applied potentials. The range of applied potentials was −530 to −1280 mVn, which includes the passive, active peak, open circuit, and cathodic regimes. For tests at a frequency of 1 Hz, an R ratio equal to 0.5 and a sinusoidal waveform, the maximum environmental effect was an order of magnitude increase in the fatigue crack growth rate at the lowest cathodic potential. This environmental effect decreased as the applied potentials rose. Applied potentials in the passive range eliminated any environmental acceleration of fatigue crack growth rates. The extent of intergranular fracture increased as potentials dropped in the cathodic range.

J. Begley

Papers

The Effect of Applied Potential on Corrosion Fatigue Crack Growth Rates of a Ni-Cr-Mo-V Turbine Disc Steel in a Room Temperature 12M NaOH Solution