The phenomenon in which stainless steel heated to about 500 to 800°C precipitates Cr carbides at grain boundaries, depleting the Cr around them and making the steel prone to intergranular corrosion.
Japanese name: 鋭敏化
Sensitization appears in 2 practice questions across 2 topics, including Corrosion and surface treatment, Materials processing and joining.
Stainless steel resists rusting because a passive film of Cr oxide forms on its surface, which is said to require roughly 11 to 12% Cr or more. When austenitic stainless steel is held at about 500 to 800°C, dissolved carbon combines with Cr and carbides such as Cr₂₃C₆ precipitate at grain boundaries. Carbon diffuses quickly but Cr moves only slowly, so a band forms along the grain boundaries right next to the carbides in which the Cr concentration has fallen too low to maintain passivity. In a corrosive environment this band dissolves preferentially, and corrosion advances along the grain boundaries.
Sensitization occurs through a combination of temperature and holding time. If the temperature is too high, carbon stays in solution and carbides form less readily; if it is too low, diffusion is too slow. It therefore occurs fastest at an intermediate temperature, giving a C-shaped curve on a time–temperature diagram. The higher the carbon content, the more the curve shifts to shorter times, and the more easily sensitization occurs even with short heating such as welding. Given enough time, carbides grow slowly even at lower temperatures, so low-temperature sensitization must also be watched for in welded piping and similar equipment used for long periods at moderate temperatures.
SUS304L and SUS316L, with carbon reduced to about 0.03% or less, form carbides less readily and largely prevent sensitization from welding. Stabilized steels containing Ti or Nb (SUS321, SUS347) tie up carbon first as Ti or Nb carbides. Even in stabilized steels, however, if the region right next to the fusion line reaches very high temperatures, the Ti or Nb carbides dissolve, and later reheating precipitates Cr carbides, so corrosion can advance in a narrow band along the weld line (knife-line attack). Ferritic grades dissolve little carbon and diffusion in them is fast, so they tend to sensitize even while cooling from high temperature.
Whether a steel is sensitized is examined by methods such as electrolytic etching in oxalic acid to see the shape of grooves at grain boundaries, the Strauss test, in which the steel is boiled in a sulfuric acid–copper sulfate solution with copper and checked for cracks on bending, and measuring the current of electrochemical reactivation. Sensitized steel can be restored by solution heat treatment, heating to about 1000 to 1100°C to redissolve the carbides and then quenching. Sensitized grain boundaries are also paths for stress corrosion cracking, and intergranular stress corrosion cracking in the piping of boiling water reactors is known as a case that prompted a review of measures against sensitization.
Answer: B. Pitting corrosion; switch to a Mo-bearing grade such as SUS316
Chloride ions locally break down the passive film at weak points and cause pitting corrosion, in which small holes are dug deeper and deeper. Inside the pit, hydrolysis of the dissolved metal ions acidifies the solution, and chloride ions collect to maintain electrical neutrality, so the corrosion becomes self-accelerating. The large surrounding passive surface acts as the cathode and the bottom of the pit as a small anode, so the pit can advance rapidly to perforation. The countermeasure is to switch to a grade richer in Mo, Cr and N, which strengthen the film (such as SUS316, equivalent to AISI 316, with about 2–3% Mo); the pitting resistance equivalent number calculated from these contents is a guide for selection. Because this is not corrosion that thins the whole surface uniformly, increasing the wall thickness is not an effective countermeasure. More carbon makes Cr carbides form more readily at grain boundaries (sensitization) and actually lowers corrosion resistance.
Answer: C. In zones heated to about 500 to 800°C, Cr carbides precipitate at grain boundaries, leaving Cr-depleted zones. Use SUS304L or Ti- or Nb-added steels
In the heat-affected zone a short distance from the weld line, there is a band exposed for some time to temperatures of about 500 to 800°C. There, Cr carbides such as Cr₂₃C₆ precipitate at grain boundaries, and the Cr concentration right next to the boundaries falls to a level that cannot sustain passivity, so corrosion proceeds along the grain boundaries. This is called sensitization. Countermeasures are to use a low-carbon steel (SUS304L) with less of the carbon that forms the carbides, to use stabilized steels (SUS321 or SUS347) in which carbon is tied up as Ti or Nb carbides, and to give a solution heat treatment after welding to dissolve the carbides again. Austenitic stainless steels do not become martensite on quenching, and Cr evaporation is not the main cause either.
Sensitization is the term for the condition in which Cr carbides precipitate at grain boundaries, forming Cr-depleted zones and making the steel prone to intergranular corrosion. Weld decay is the phenomenon in which welding heat sensitizes the heat-affected zone and intergranular corrosion occurs in that band. The cause is the same; weld decay is the name for the case that arises from welding.
It makes sensitization unlikely during short heating such as welding, but does not prevent it completely. If the steel is held in the sensitization range for a long time, carbides precipitate even with low carbon. For long service at high temperature, choose a stabilized steel or check whether sensitization occurs at the service temperature and time.
Yes. Ferritic grades dissolve little carbon and nitrogen, so Cr carbides and nitrides precipitate at grain boundaries even while cooling from high temperature, and steels with very low carbon and nitrogen or with Ti and Nb added are used. Martensitic grades can also lose corrosion resistance when carbide precipitation at some tempering temperatures depletes Cr.
More carbon makes Cr carbides form more readily at grain boundaries (sensitization) and actually lowers corrosion resistance.
When SUS304 (JIS; AISI 304) is used in chloride-rich seawat…This is called sensitization.
When austenitic stainless steels such as SUS304 are welded,…Passivity Pitting corrosion Stainless steel Heat-affected zone Martensite Carbon equivalent Extrusion Hot isostatic pressing Investment casting Shrinkage cavity