Stabilization steels

Chemically stabilized steels, such as Type 304L have been successfully used in a sulfidic corrodent environment but actual installation tests have not been consistent. 

Intergranular corrosion depends on the length of time the steel is exposed to the sensitizing temperature (500-750°C), even if made from low-carbon or titanium-or niobium-stabilized steel. 

Equipment fabricated from such a steel may undergo corrosion by condensation of even mild corrosives unless it is possible to keep it above the dew-point or to neutralize acidic condensates. This kind of corrosion can be prevented by a preliminary heat-treating at temperatures of 815-915°C. The niobium-stabilized steels respond best to this treatment. 

Similar results were obtained by Clerbois et a1 (45). These authors compared the current-potential curves of two 18 /B stainless steels tempered for two hours at 650°C one of the steels was Ti-stabilized and the other contained no Ti. The Ti-free-steel (A. LS. I. 302) shows a characteristic peak in 20% H2SO4 at a potential of - 50 mV. This peak corresponds to cfee corrosion sensitization of the grain boundaries. The Ti-stabilized steel (A, l. S. L 347) is not sensitized by the same tempering and does not show any peak at - 0.50 mV (Fig, 20). Similar observations have been presented more recently by Serge and Jacquet (4c). 

Special considerations cobalt salts reduce UV and thermal stability steel fiber and acrylic fiber give the best wear retention to brake pads composites containing graphite have much better flame retarding properties than composites containing aramid or glass fiber glass fiber slows down the cure rates of novolac resins ... 

FIGURE 20.27 (a) Time-temperature sensitization curves of stabilized steels as a function of solution... 

The materials used for internals in the ABB Atom BWRs are mainly made of Type 304 or Type 304L materials, but some stabilized steels, such as Type 347, have also been used. 

B) Stabilized steel. Steels containing stabilizers, such as titanium and niobium, which form carbides that are more stable than chromium carbides, are called stabilized stainless steels. 

This phenomenon is restricted to the stabilized grades of steel, such as 321 and 347. The knife-line attack occurs immediately adjacent to the weld and shows as a thin line of intergranular corrosion. It results from intergranular corrosion like the weld decay. It may be noted that weld decay develops at some distance away fi"om the weld. The following is the mechanism suggested for the knife-line attack of stabilized steels. 

Selection of stabilized steels for the piping systems, such as AISI321 and 347. 

Relative to the matrix, the carbides M23C6 and MeC are generally enriched in Cr and Mo (M23C6 and MeC) or in Si and Ni (MeQ, whereas monocarbides MC of TiC/NbC can also be enriched in Mo. Intermetallic phases are generally enriched in Mo, Fe, and Cr and only the Laves phases of Fe2Mo/Fe2Nb type are enriched in Si but depleted in Ni. Moreover, in phosphoms-doped and stabilized steels FeTiP phosphides are observed and in multistabilized materials, Nb can replace the Ti in these compounds [42] and therefore can leave a higher quantity of Ti in solid solution. Such phase instabilities... 

Ultramid A3K BASF High flow, heat stabilized Steel, Cr 6/800/HV surface finish 0.15-0.2 jjim... 

Use stabilized steels, i.e. austenitic steels containing niobium, tantalum, or titanium. 

A practical consequence can be drawn from these results Ti-stabilized steels exhibit better pitting resistance than Ti-free ones, provided that the corrosive medium is not too severe, i.e., the chloride content does not exceed a critical value, depending on the solution pH, which could cause the Ti sulfides destabilization. In the case of crevice corrosion, the situation is more complex. Inside a crevice, the pH decreases and the chloride concentration increases with time. It is an accepted idea that corrosion occurs when the pH... 

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