Forms of Metallic Corrosion
33
stainless steel during welding is known as weld decay. The affected zone is
usually a band in the parent plate somewhat removed from the weld bead.
When exposed to corrosive environments, intergranular corrosion takes
place in this zone and the attack gives a granular appearance. During welding, this area remains in the sensitizing temperature range for a sufficient
length of time to permit carbide precipitation, whereas at the weld pool and
its adjacent areas the temperature is high enough and the time insufficient
for the precipitation to take place. Areas further away from this weld decay
zone do not reach the sensitizing temperature and therefore carbide precipitation does not occur.
Due to the longer times at the temperature of annealing or stress relieving, it is possible that the entire piece of material will be sensitized, whereas
shorter times at welding temperature can result in sensitization of a band
usually 1/8 to 1/4 in. wide, adjacent to but slightly removed from the weld.
This region is known as the heat-affected zone (HAZ).
Intergranular corrosion depends on the magnitude of the sensitization
and the aggressiveness of the environment to which the sensitized material
is exposed. Many environments do not cause intergranular corrosion in sensitized stainless steels.
10000 hr.
1000 hr.
100 hr.
10 hr.
1 hr.
10 min.
1 min.
10 sec.
500
600
700
900
0.08%C
0.062%C
0.058%C
0.056%C
0.042%C
°C
800
0.052%C
0.030%C
0.019%C
FigurE 3.1
Effect of carbon content on carbide precipitation. Carbide precipitation forms in the areas to the
right of the various carbon content curves.
33
stainless steel during welding is known as weld decay. The affected zone is
usually a band in the parent plate somewhat removed from the weld bead.
When exposed to corrosive environments, intergranular corrosion takes
place in this zone and the attack gives a granular appearance. During welding, this area remains in the sensitizing temperature range for a sufficient
length of time to permit carbide precipitation, whereas at the weld pool and
its adjacent areas the temperature is high enough and the time insufficient
for the precipitation to take place. Areas further away from this weld decay
zone do not reach the sensitizing temperature and therefore carbide precipitation does not occur.
Due to the longer times at the temperature of annealing or stress relieving, it is possible that the entire piece of material will be sensitized, whereas
shorter times at welding temperature can result in sensitization of a band
usually 1/8 to 1/4 in. wide, adjacent to but slightly removed from the weld.
This region is known as the heat-affected zone (HAZ).
Intergranular corrosion depends on the magnitude of the sensitization
and the aggressiveness of the environment to which the sensitized material
is exposed. Many environments do not cause intergranular corrosion in sensitized stainless steels.
10000 hr.
1000 hr.
100 hr.
10 hr.
1 hr.
10 min.
1 min.
10 sec.
500
600
700
900
0.08%C
0.062%C
0.058%C
0.056%C
0.042%C
°C
800
0.052%C
0.030%C
0.019%C
FigurE 3.1
Effect of carbon content on carbide precipitation. Carbide precipitation forms in the areas to the
right of the various carbon content curves.