326 Steels: Metallurgy and Applicationscarbide precipitation may occur even when the material is water quenched from
the solution treatment temperature. However, intergranular corrosion will only
proceed in sensitized material in aggressive, oxidizing media and, ordinarily,
ferritic stainless grades such as Types 430 and 434 are not exposed to such
environments. Even so, care should be taken to minimize the risk of intergranular
corrosion.
Ferritic stainless steels sensitize more rapidly and at lower temperatures than
their austenitic counterparts, the fastest reaction occurring at a temperature of
about 600"C. However, by holding at a temperature of about 800"C, or by cooling
slowly through the temperature range 700-900"C, the risk of intergranular attack
can be eliminated. Following the initial, damaging precipitation of chromium
carbides, these treatments allow sufficient time for chromium to diffuse into the
depleted zones and thereby eliminate the sensitization effects.
The addition of titanium also reduces the risk of intergranular attack in ferritic
stainless steels. However, whereas the reduction of carbon to below 0.03% is
effective in preventing sensitization in austenitic grades (L grades), significantly
lower levels are required in ferritic stainless steels. Nitrogen is also damaging,
causing chromium depletion via the formation of chromium nitride. Therefore,
the total (carbon + nitrogen) must be reduced to below 0.01% in order to prevent
sensitization in a 17% Cr ferritic steel. The development of low interstitialferritic
steels will be discussed later.Piing corrosion
As its name suggests, pitting is a highly localized form of corrosion which, in
its initial form, results in the formation of shallow holes or pits in the surface of
the component. However, the pits can propagate at a fast rate, resulting in pin-
holing or complete perforation in the wall of the component. Therefore pitting
can be completely destructive in terms of further useful life when only a very
small amount of metal has been attacked by corrosion. In stainless steels, pitting
corrosion generally takes place in the presence of chloride ions and it is widely
held that the initiation stage is associated with attack on non-metallic inclusions.
However, other microstructural features may also play a part. The formation of
a pit in an austenitic stainless steel is shown in Figure 4.17.
Fontana and CJreene 9 have stated that pits usually grow in the direction of
gravity, i.e. downwards from horizontal surfaces, and only rarely do they proceed
in an upward direction. These authors have formulated a model for pitting in
terms of an autocatalytic process for stainless steels in aerated sodium chloride
solution. The sequence of events is as follows:- Anodic dissolution takes place at the bottom of the pit:
M-+ M++e -- A cathodic reaction takes place on adjacent surface:
02 + 2H20 + 4e- --, 4OH-