Forms of Metallic Corrosion
35
temperature prevents the carbides from precipitating. This is what occurs in
the area adjacent to the weld. When the weldment is stress relieved at 932 to
1472°F (500 to 800°C), or is in service in this temperature range, chromium
carbide forms and sensitization takes place; even if niobium is present in the
alloy, it cannot prevent chromium carbide from forming because niobium
carbide requires a higher temperature to form. The same is true for titaniumstabilized alloys.
3.2.2 Ferritic Stainless Steels
Ferritic stainless steels are also subject to sensitization and intergranular
corrosion as a result of chromium depletion. However, there are differences between the sensitization of the austenitic stainless steels and the
ferritic stainless steels. Because the solubility of nitrogen is low in austenitic stainless steels, the percentage of chromium nitride is not a prime
factor for sensitization. However, this is a prime factor in ferritic stainless
steels for sensitization. A second difference lies in the temperature range
of sensitization, which is above 1696°F (925°C), where the solubility of carbon and nitrogen is significant in ferrite. As a result of this difference in
the sensitizing temperature range, the zone of intergranular corrosion differs. The attack in ferritic steels occurs at areas adjacent to the weld or in
the weld itself.
By heating the sensitized steel between 1201 and 1498°F (650 and 815°C),
immunity to intergranular corrosion is restored.
3.2.3 Other alloys
Nickel-based alloys can also be subjected to carbide precipitation of intermetallic phases when exposed to temperatures lower than their annealing temperatures. As with austenitic stainless steels, low-carbon-content
alloys are recommended to delay precipitation of carbides. In some alloys,
such as alloy 625 niobium (columbium), titanium, or tantalum is added
to stabilize the alloy against precipitation of chromium or molybdenum
carbides. These elements combine with carbon instead of chromium or
molybdenum.
Precipitation-hardenable nickel alloys such as Inconel X–750 are susceptible
to intergranular corrosion in hot caustic solutions, in boiling 75% nitric acid,
and in high-temperature water containing low concentrations of other salts.
Inconel 600 is also susceptible in the same media when thermally treated in
the range of 1004 to 1400°F (540 to 760°C).
Copper alloy 260 (70–30 brass) is subject to intergranular corrosion in dilute
aqueous solutions of H 2 SO 4 , Fe 2 SO 4 , BiCl 3 , and other electrolytes.
Red-fuming nitric acid at room temperature will cause intergranular corrosion of titanium. Addition of 1% NaBr will inhibit the attack.
35
temperature prevents the carbides from precipitating. This is what occurs in
the area adjacent to the weld. When the weldment is stress relieved at 932 to
1472°F (500 to 800°C), or is in service in this temperature range, chromium
carbide forms and sensitization takes place; even if niobium is present in the
alloy, it cannot prevent chromium carbide from forming because niobium
carbide requires a higher temperature to form. The same is true for titaniumstabilized alloys.
3.2.2 Ferritic Stainless Steels
Ferritic stainless steels are also subject to sensitization and intergranular
corrosion as a result of chromium depletion. However, there are differences between the sensitization of the austenitic stainless steels and the
ferritic stainless steels. Because the solubility of nitrogen is low in austenitic stainless steels, the percentage of chromium nitride is not a prime
factor for sensitization. However, this is a prime factor in ferritic stainless
steels for sensitization. A second difference lies in the temperature range
of sensitization, which is above 1696°F (925°C), where the solubility of carbon and nitrogen is significant in ferrite. As a result of this difference in
the sensitizing temperature range, the zone of intergranular corrosion differs. The attack in ferritic steels occurs at areas adjacent to the weld or in
the weld itself.
By heating the sensitized steel between 1201 and 1498°F (650 and 815°C),
immunity to intergranular corrosion is restored.
3.2.3 Other alloys
Nickel-based alloys can also be subjected to carbide precipitation of intermetallic phases when exposed to temperatures lower than their annealing temperatures. As with austenitic stainless steels, low-carbon-content
alloys are recommended to delay precipitation of carbides. In some alloys,
such as alloy 625 niobium (columbium), titanium, or tantalum is added
to stabilize the alloy against precipitation of chromium or molybdenum
carbides. These elements combine with carbon instead of chromium or
molybdenum.
Precipitation-hardenable nickel alloys such as Inconel X–750 are susceptible
to intergranular corrosion in hot caustic solutions, in boiling 75% nitric acid,
and in high-temperature water containing low concentrations of other salts.
Inconel 600 is also susceptible in the same media when thermally treated in
the range of 1004 to 1400°F (540 to 760°C).
Copper alloy 260 (70–30 brass) is subject to intergranular corrosion in dilute
aqueous solutions of H 2 SO 4 , Fe 2 SO 4 , BiCl 3 , and other electrolytes.
Red-fuming nitric acid at room temperature will cause intergranular corrosion of titanium. Addition of 1% NaBr will inhibit the attack.