Intergranular Corrosion
When austenitic stainless steels are heated or cooled through the
temperature range 425-900C (800-1650F), chromium tends to combine with
carbon to form chromium carbides. The carbides precipitate preferen-
tially at grain boundaries depleting chromium from the adjacent areas.
This reduces the corrosion resistance of the chromium depleted areas,
sensitizing the alloy to Intergranular Attack (IGA). The extent of
carbide formation is dependent upon time at temperature and the carbon
content of the alloy. Thus, exposure in the temperature range stated
does not automatically mean that sensitization, or IGA will occur.
Sensitization may also result from slow cooling from solution annealing
temperatures, or stress relieving - after welding - in the 425 to 900C
(800 to 1650F) temperature range. In welded fabrications, sensitization
and IGA may occur in corrosive environments in a rather narrow band on
either side of or on the side opposite the weld, known as the heat
affected zone (HAZ).
It is important to note that even if sensitization does occur, it is not
of significant consequence unless the alloy is exposed to a corrosive
environment. Sensitized stainless steel performs in a normal manner and
safe manner in non-corrosive applications.
Order of Resistance
| 904L |
Highest |
This table lists some of the common stainless steels as to general resistance to pitting or crevice corrosion in aqueous environments where corrosive conditions may exist. |
| 317L |
|
| 329 |
|
| 316/316L |
|
| 304/304L |
|
| 430 |
|
| 410 |
|
| 420 |
Lowest |
Methods of Minimizing Intergranular Attack - IGA
1) Solution anneal above 1040C (1900F) followed by a rapid quench.
2) Use type 347, a Cb stabilized grade, or 321, a Ti stabilized grade.
3) Use a low carbon, 0.03% max. carbon grade such as 304L, 316L, 317L or 904L.
With today's technology, carbon is economically reduced to very low residuals. The low carbon grades are the standard for welded fabrication.
ASTM A262 practice A to E are standard tests to determine susceptibility to IGA. Practice E, the Huey test, is widely used.
 |
 |
< < < click here to go back to the previous page :: |
|
