Sensitization (noun): the loss of corrosion resistance in austenitic stainless steel that occurs when chromium carbides form along grain boundaries during time at about 800 to 1,600 °F, leaving chromium-depleted zones that corrode first. In welds, you prevent it by using L-grade or stabilized stainless, keeping heat input low, and controlling interpass temperature.
Sensitization is also called carbide precipitation, and the corrosion it causes in welds is often called weld decay. You cannot see it with the naked eye. A sensitized weld can pass visual inspection, radiography and a hydro test, and then fail months later in acid, chloride or hot water service. This guide explains how it forms, where it shows up, and how QC teams test for it.
What happens inside the metal?
Stainless steel resists corrosion because chromium forms a thin, self-healing oxide film on the surface. The steel needs at least about 10.5% chromium in solution to keep that film.
When 300-series stainless sits in the sensitizing range, carbon moves to the grain boundaries and bonds with chromium. It forms chromium carbides, mostly of the type Cr23C6 (often written M23C6). Carbon moves much faster than chromium. So the chromium near each grain boundary gets used up before more chromium can move in to replace it.
The result is a thin, chromium-poor band along each grain boundary. That band can drop below the level needed for a passive film. In a corrosive fluid, the attack follows those bands. This is intergranular corrosion. Grains can loosen and fall out, and the metal can crack under stress.
Time and temperature both matter. A standard-carbon grade such as 304 can sensitize in a short time near the middle of the range. An L grade takes much longer at the same temperature, which is why L grades survive normal welding.
Where does sensitization show up in a weld?
It usually does not show up in the weld bead. It shows up in the heat-affected zone (HAZ), in a band a short distance from the fusion line.
- The weld metal cools fast, usually contains some delta ferrite, and is made from low-carbon filler. It rarely sensitizes.
- The metal right at the fusion line gets so hot that carbides stay dissolved, then cools quickly through the range.
- The band just beyond peaks inside the 800 to 1,600 °F range and stays there long enough for carbides to form. This is where weld decay appears, often as two parallel lines of attack on either side of the weld.
Stabilized grades have their own pattern. In 321 and 347, a narrow band right next to the fusion line can be attacked after the part is reheated, for example during stress relief or hot service. This is called knife-line attack.
| Heat source | Sensitization risk | Why |
|---|---|---|
| Single-pass TIG on thin sheet or tube | Low | Short time in the range; fast cooling |
| Heavy multipass welds on thick plate | Higher | Each pass reheats the HAZ of earlier passes |
| Weld repairs in the same spot | Higher | The same HAZ goes through the range again and again |
| Stress relief of attached carbon steel | High for standard grades | Typical carbon steel stress relief holds the part inside the sensitizing range for an hour or more |
| Service inside the range | High for standard grades | Long time at temperature, even without welding |
How do you prevent sensitization when welding?
Specify L grades or stabilized grades
Lower carbon means fewer carbides can form. L grades cap carbon at 0.030%, compared with 0.08% for standard grades. Stabilized grades add titanium or niobium, which bond with carbon before chromium can.
| Grade | Max carbon (wt %) | Stabilizer | Weld sensitization risk |
|---|---|---|---|
| 304 | 0.08 | None | Moderate to high |
| 304L | 0.030 | None | Low |
| 316 | 0.08 | None | Moderate to high |
| 316L | 0.030 | None | Low |
| 321 | 0.08 | Titanium | Low; watch for knife-line attack |
| 347 | 0.08 | Niobium (columbium) | Low; watch for knife-line attack |
Carbon limits shown are for plate and sheet. Many mills sell dual-certified 304/304L and 316/316L, which meets both carbon limit and strength needs. Match the filler: use ER308L on 304L and ER316L on 316L. See ER308L vs. ER316L filler rods.
Keep heat input low
Less heat means less time in the range. Heat input in J/in equals volts times amps times 60, divided by travel speed in in/min. To keep it down
- Travel faster and run stringer beads instead of wide weaves.
- Use the lowest amperage that gives full fusion.
- Use pulse TIG on thin parts to cut average heat.
- Clamp copper or aluminum chill bars near the joint to pull heat out.
Control interpass temperature and repairs
Let the part cool between passes. Check it with a contact or infrared thermometer before each pass. Limit repeat repairs in the same area, because each one sends the HAZ back through the range.
Keep carbon off the joint
Oil, grease, marking crayon and shop dirt add carbon to the surface. That carbon can raise local carbon content in the weld and HAZ. Clean the joint with acetone and use stainless-only brushes and grinding discs.
Plan heat treatment with care
Avoid stress relief of standard-grade stainless inside the sensitizing range. If a 304 part is welded to carbon steel that needs stress relief, use an L grade or change the build sequence. Our guide to TIG welding 304 stainless to carbon steel covers that case.
Repair a sensitized part by solution annealing
Solution annealing dissolves the carbides and restores corrosion resistance. The part is heated to about 1,900 °F or higher and cooled quickly, usually by water quench. This can distort finished parts, so prevention is cheaper.
How do QC inspectors test for sensitization?
Because sensitization is invisible, testing is done on samples or test coupons. The common standard is ASTM A262, which has several practices
- Practice A, oxalic acid etch. A fast screening test. The etched surface is checked under a microscope. A “step” structure is acceptable. A “ditch” structure means the sample needs one of the other tests. Practice A can accept material but cannot reject it.
- Practice B, ferric sulfate and sulfuric acid. Measures weight loss after exposure.
- Practice C, boiling nitric acid. Also measures weight loss, over several exposure periods.
- Practice E, copper and copper sulfate in sulfuric acid. The sample is bent after exposure and checked for intergranular cracks.
- Practice F, copper and copper sulfate in 50% sulfuric acid. Measures weight loss.
Acceptance limits for weight-loss practices are not set in A262 itself. The buyer sets them on the purchase order or in the governing spec. Some buyers also use the electrochemical EPR test.
For incoming material, check the mill test report. Confirm carbon is at or below 0.030% for L grades, and confirm the material was supplied solution annealed.
What should go on the drawing or purchase order?
Write the requirements down so the shop and the inspector work to the same rules
- The exact grade, including “L” or dual certification, and the filler
- Maximum heat input or interpass temperature, if the service is aggressive
- No stress relief of stainless inside the sensitizing range without engineering approval
- The ASTM A262 practice and acceptance limits, if testing is required
- Back purge and heat-tint limits on the root side
Sensitization matters most where the fluid is corrosive: refinery and chemical service in oil, gas and petrochemical plants, and acidic cleaning cycles on sanitary tube welding for food and dairy lines. TIGTechs welds L-grade stainless through our precision stainless TIG service.
Have a corrosive-service part that cannot afford weld decay? Request a quote with the grade, service fluid and test requirements. We will build the weld plan around them.
What is polythionic acid stress corrosion cracking?
Polythionic acid stress corrosion cracking (PTA SCC) is the refinery version of sensitization failure, and it is one of the main reasons refiners specify low-carbon or stabilized stainless steels.
While the unit runs, sulfur in the process stream forms a sulfide scale on stainless piping, heater tubes and vessels. That scale is harmless while the equipment stays hot and dry. During a shutdown, air and moisture reach the scale and turn it into polythionic acids. Those acids attack the chromium-depleted grain boundaries of sensitized metal. Combined with residual stress from welding or forming, the result can be cracks through the wall in a matter of days.
Prevention works on two fronts
- Material and fabrication: use L-grade stainless, or stabilized grades such as 321 and 347, sometimes with a stabilizing heat treatment. Keep weld heat input low so the heat-affected zone does not sensitize.
- Shutdown practice: NACE SP0170 (now published by AMPP) describes how to protect equipment during downtime. Common methods are a dry nitrogen purge to keep oxygen out, or an alkaline soda-ash wash to neutralize acids that form.
If you buy stainless fabrications for sulfur-bearing service, state the grade, any heat treatment and the intergranular corrosion test requirement on the purchase order.
How is sensitization different from other weld corrosion problems?
Several corrosion problems show up near stainless welds, and each has a different cause and fix. Telling them apart saves time in a failure review.
- Weld decay (sensitization): intergranular attack in a band a short distance from the weld, where the heat-affected zone spent time in the sensitizing range. The fix is low-carbon grades, stabilized grades or solution annealing.
- Knife-line attack: a very narrow band right at the fusion line in stabilized grades such as 321 and 347, usually after reheating in service. The fix is a stabilizing heat treatment or a different grade.
- Heat tint corrosion: the oxide colors on the surface sit on a thin chromium-depleted layer. It starts at the surface rather than at grain boundaries, and pickling or electropolishing removes it.
- Chloride pitting: local pits caused by too little molybdenum or chromium for the service, often from using ER308L filler on 316L. The fix is matching or over-alloyed filler.
- Chloride stress corrosion cracking: branching, mostly transgranular cracks in warm chloride service under tensile stress. It can happen in fully annealed metal, so low carbon alone does not prevent it.
Frequently asked questions
Austenitic stainless steel sensitizes between about 800 and 1,600 °F. The longer the metal stays in that range, the more chromium carbides form. Standard-carbon grades such as 304 sensitize much faster than L grades.
It greatly reduces the risk during normal welding, because 304L has 0.030% maximum carbon. Long exposure inside the range, such as stress relief or hot service, can still sensitize an L grade. For long service in the range, engineers often choose a stabilized grade such as 321 or 347.
No. A sensitized weld looks normal and can pass visual, radiographic and pressure tests. It is found by lab tests such as ASTM A262, or by corrosion after the part is in service.
Heat tint is a surface oxide layer caused by poor gas shielding, and you can see it as straw, blue or gray color. Sensitization is a change at the grain boundaries caused by time at temperature, and it cannot be seen. Both lower corrosion resistance, and each has its own fix.
Solution annealing dissolves the chromium carbides and restores corrosion resistance. The part is heated to about 1,900 °F or higher and cooled quickly. Because this can warp finished parts, preventing sensitization during welding is the cheaper path.
A stabilized stainless steel contains an element that ties up carbon before chromium can. Type 321 uses titanium and Type 347 uses niobium (columbium). Because carbon forms titanium or niobium carbides instead of chromium carbides, the grain boundaries keep their chromium and resist sensitization.
ASTM A262 is the standard set of practices for detecting susceptibility to intergranular attack in austenitic stainless steels. Practice A is a quick oxalic acid etch screen. Practices B, C and E (ferric sulfate, nitric acid and copper-copper sulfate tests) give pass or fail results when the screen is not conclusive.
