Forms of Metallic Corrosion
61
A feature of HSC is that the occurrence of the fracture is delayed, indicating that hydrogen diffusion in the metal lattice is important for the buildup
of sufficient hydrogen concentrations at the regions of triaxial stresses for
crack nucleation or at the crack tip for its propagation.
The susceptibility to cracking therefore depends on hydrogen gas pressure and temperature, factors that influence the diffusion process. Increasing
the hydrogen pressure reduces the threshold intensity for crack propagation
and increases the crack growth rate for specific stress intensity values. The
threshold stress intensity and crack growth rate are a function of the specific
hydrogen environment.
The susceptibility of steels to embrittlement depends to a large extent
on their microstructure. A highly tempered martensitic structure with
equiaxial ferritic grains and spheroidized carbides evenly distributed
throughout the matrix has maximum resistance to embrittlement compared with normalized steels at equivalent strength levels. The resistance
also increases with decreasing prior austenitic grain size. The presence
of refined austenite is helpful because it either absorbs hydrogen or slows
down crack growth. The effect of individual alloying elements on cracking susceptibility is associated with their effect on the heat treatment,
microstructure, and strength of the steels. In general, carbon, phosphorus,
sulfur, manganese, and chromium increase susceptibility and titanium
decreases the sensitivity to HSC by decreasing the amount of hydrogen
available for cracking.
The behavior of stainless steels in hydrogen environments depends
on their strength levels. Because of the low hardness of ferritic stainless
steels, they are extremely resistant to HSC. However, in the as-welded
or cold-worked condition, they are susceptible. As a result of the higher
strength of the martensitic and precipitation-hardening stainless steels,
they are the most susceptible to HSC. In the annealed or highly coldworked condition, the austenitic stainless steels are highly resistant to
hydrogen cracking.
Although hydrogen stress cracking and stress corrosion cracking (SCC)
are similar, there are certain distinguishing features between the two cracking processes:
1. The “specific ion” effect necessary for SCC is absent in HSC.
2. The application of cathodic potential or current, which retards or
stops SCC, increases the intensity of HSC.
3. Stress corrosion cracks generally originate at the surface, while
hydrogen embrittlement failures originate internally.
4. HSC usually produces sharp, singular cracks in contrast to the
branching of cracks observed in SCC.
61
A feature of HSC is that the occurrence of the fracture is delayed, indicating that hydrogen diffusion in the metal lattice is important for the buildup
of sufficient hydrogen concentrations at the regions of triaxial stresses for
crack nucleation or at the crack tip for its propagation.
The susceptibility to cracking therefore depends on hydrogen gas pressure and temperature, factors that influence the diffusion process. Increasing
the hydrogen pressure reduces the threshold intensity for crack propagation
and increases the crack growth rate for specific stress intensity values. The
threshold stress intensity and crack growth rate are a function of the specific
hydrogen environment.
The susceptibility of steels to embrittlement depends to a large extent
on their microstructure. A highly tempered martensitic structure with
equiaxial ferritic grains and spheroidized carbides evenly distributed
throughout the matrix has maximum resistance to embrittlement compared with normalized steels at equivalent strength levels. The resistance
also increases with decreasing prior austenitic grain size. The presence
of refined austenite is helpful because it either absorbs hydrogen or slows
down crack growth. The effect of individual alloying elements on cracking susceptibility is associated with their effect on the heat treatment,
microstructure, and strength of the steels. In general, carbon, phosphorus,
sulfur, manganese, and chromium increase susceptibility and titanium
decreases the sensitivity to HSC by decreasing the amount of hydrogen
available for cracking.
The behavior of stainless steels in hydrogen environments depends
on their strength levels. Because of the low hardness of ferritic stainless
steels, they are extremely resistant to HSC. However, in the as-welded
or cold-worked condition, they are susceptible. As a result of the higher
strength of the martensitic and precipitation-hardening stainless steels,
they are the most susceptible to HSC. In the annealed or highly coldworked condition, the austenitic stainless steels are highly resistant to
hydrogen cracking.
Although hydrogen stress cracking and stress corrosion cracking (SCC)
are similar, there are certain distinguishing features between the two cracking processes:
1. The “specific ion” effect necessary for SCC is absent in HSC.
2. The application of cathodic potential or current, which retards or
stops SCC, increases the intensity of HSC.
3. Stress corrosion cracks generally originate at the surface, while
hydrogen embrittlement failures originate internally.
4. HSC usually produces sharp, singular cracks in contrast to the
branching of cracks observed in SCC.