Sintered Density

For many years, inferior corrosion resistance of sintered stainless steel parts has been and continues to be mistakenly attributed to the presence of pores in accordance with the mechanism of crevice corrosion. That this hypothesis is untenable follows from widely available evidence (Ref 7) that sintered stainless steel parts of identical composition and similar pore volumes, pore sizes, and pore shapes, but sintered under varying conditions, may have corrosion resistances (in 5% aqueous NaCl) that can vary by two orders of magnitude. Furthermore, a comparison of wrought and sintered (85% of theoretical density) type 316L for susceptibility to crevice corrosion in 10% FeCl3, in accordance with ASTM G 48, showed that the wrought part was actually more severely attacked than the porous part (Ref 7, 27). More recent studies have shown that most cases of inferior corrosion resistance of sintered stainless steels are due to incorrect sintering, as previously described.

Also, for many years, there existed controversy as to the effect of sintered density upon corrosion resistance. Corrosion testing in an acidic environment, such as dilute H2SO4, HCl, and HNO3, always showed that corrosion resistance improved with increasing density. Testing in a neutral salt solution, however, showed the corrosion resistance to decrease with increasing density (Ref 17, 28, 29, 30, 31), when corrosion testing was done in long-term immersion or salt spray tests, whereas higher density was found to be beneficial in short-term potentiodynamic polarization tests (Ref 32). Maahn and Mathiesen (Ref 8) attributed the failure to observe this important relationship between corrosion resistance and density in short-term, potentiostatic anodic polarization tests to the lack of time for the time-consuming build up of localized attack within the pores, in analogy to the mechanism of crevice corrosion. By using slow stepwise polarization, the expected relationship, that is, a decrease of the stepwise initiation potential with increasing density (equivalent to deteriorating corrosion resistance), was observed (Ref 33).

Recent studies, with austenitic stainless steels (Ref 10, 17, 31) showed that the corrosion resistance in 5% aqueous NaCl (by immersion) can be reduced by up to two orders of magnitude due to the presence of porosity. The negative effect appears at sintered densities of —6.7 g/cirf. reaches a minimum corrosion resistance between —6.9 and 7.2 g/cm3, depending on pore morphology, and thereafter disappears at densities of —7.4 g/cm3 (Fig. 25). To the left of the minimum, corrosion resistance decreases with decreasing pore size due to increasing oxygen depletion and failure to maintain the passive layer. To the right of the minimum, corrosion resistance improves as pores become closed off and inaccessible to the surface. This type of corrosion can be reduced by impregnating the pores with a resin, by metallurgical modification of the pore surfaces, or by the use of higher-alloyed stainless steels, particularly those containing higher concentrations of molybdenum.

Fig. 25 Effect of density on corrosion resistance (B rating) of pressed, sintered, repressed, and annealed 317L parts

Another approach to avoiding the problem of long-term corrosion in a neutral salt solution due to the presence of certain size pores is to make use of the various forms of liquid-phase sintering (transient, persistent, and supersolidus) and to achieve sintered densities >7.4 g/cm3. For austenitic stainless steels, silicon additions of several percent (Ref 34, 35) or smaller amounts of boron (Ref 17), have been used. For ferritic stainless steels phosphorus additions (Ref 35) have been used. The large shrinkage occurring during liquid-phase sintering is often accompanied by increasing loss of dimensional stability. Sizing is usually employed to establish dimensional accuracy. Also, depending on the composition of the liquidphase sintering additive, secondary phases may be present after sintering, which can have a negative effect on corrosion resistance (Ref 17).

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