Stainless Steel Corrosion Resistance
316Ti stainless steel has traditionally been specified by German engineers and users as a specification type, with the original UK grade being 320S31, which is essentially a standard carbon 316 stainless steel with titanium stability. The addition of titanium effectively reduces the risk of intergranular corrosion in stainless steel materials in the 425-815°C heating temperature range.
When austenitic stainless steels are subjected to prolonged heating in the 425-815°C temperature range, the carbon in the steel diffuses into the grain boundaries and precipitates chromium carbide. This removes chromium from the solid solution and leaves a lower amount of chromium combined near the grain boundaries. This is called sensitization. Subsequent exposure to a corrosive environment makes the grain boundaries susceptible to preferential damage. This type of corrosion is called intergranular corrosion.
The addition of titanium reduces the risk of intergranular corrosion because titanium carbamates are formed before chromium carbides and have the effect of maintaining the correct distribution of chromium throughout the steel structure. So the result is that the areas adjacent to grain boundaries formed by the carbomers do not deplete the chromium to levels where localized corrosion may occur within the grain boundary regions. Another way to reduce the risk of intergranular corrosion attack is to reduce the carbon content level to less than 0.03%. By doing this, the 316 grade is virtually the same IC as 316Ti. This is the basis of the 316L type.
Under most conditions, 316Ti stainless steel and 316L stainless steel applications are interchangeable, i.e., 316L stainless steel is suitable for applications where the use of 316Ti stainless steel is specified. In aqueous corrosive environments or ambient temperature environments, either 316L stainless steel or 316Ti stainless steel can be used. In some cases, 316L stainless steel may be a better choice. However, the presence of titanium in 316Ti stainless steel does provide some improvement in mechanical strength, particularly at temperatures higher than about 600°C, so care must be taken in choosing 1.4404 as a replacement under these conditions. However, 316Ti stainless steel at ambient temperatures may have poorer impact properties compared to 316L type stainless steel.
Machinability of 316Ti may also be an issue, as titanium carbon ammonia particles can lead to higher tool wear and may not be as susceptible to cold forming or cold heading as 316L stainless steels.The titanium carbon ammonia in 316Ti may also lead to problems that require a high standard of polished surface finish. During the polishing process, TCA particles can produce "comet tail" streaks on the polished surface.
There is also some evidence that 316Ti stainless steel may have poorer pitting and stress corrosion openness than 316L stainless steel. The alloy-stabilized 316Ti grade may also be susceptible to knife-line attack in heat-affected areas of the weld, very close to the molten zone where the carbon-nitride is redissolved in the solid stainless steel matrix. In addition the weldability of 316Ti and 316L is similar. Niobium stabilized fillers should be used to weld 316Ti, especially where high temperature weld strength may be important. In other cases, 316L filler should be able to provide water-soluble corrosion resistance of the weld metal that matches that of the parent 316Ti and 316Ti materials.