SUS316 and SUS316L's Characteristic and Stress Corrosion Resistance

The characteristics of SUS316 and SUS316L
Japan SUS series SUS316L stainless steel, carbon lower than the SUS316 stainless steel, better resistance to intergranular corrosion. SUS316L stainless steel resistant to carbide precipitation and better performance than the stainless steel SUS316, SUS316L stainless steel, does not require annealing after welding. If you can not do after welding to do low-temperature annealing or stress relieving treatment, can avoid the use of SUS316L intergranular corrosion. Welded and exposed in the 800 ~ 1500 ° F (427 ~ 826 ° C) temperature range, SUS316L with corrosion resistance. Need to do stress for the container, in this temperature range for short, and do not affect the corrosion resistance of normal metals.

L levels of a large steel after annealing, the high temperature without the need to accelerate the cooling treatment. Low-carbon of the L level, in terms of resistance to stress corrosion cracking has no special advantages, but when operating under stress relieving, L level is still the first choice, because such an environment can cause intergranular corrosion.

SUS316L alloy corresponding carbon content compared with the same corrosion resistance and mechanical properties, prone to intergranular corrosion in applications, SUS316L is a competitive advantage. Encountered in welding and stress relieving short-term heat, although not sufficient to cause intergranular corrosion, but it is worth noting that continuous or prolonged exposure to 800 to 1500 ° F (427 到 826 ° C) temperature range of SUS316L to that are harmful. 1100 到 1500 ° F (593 到 816 ° C) temperature range to do stress relieving treatment, may cause a slight rattle of SUS316L.

The chemical composition of SUS316 and SUS316L (%)
  C Si Mn P S Ni Cr Mo
SUS316 0.07 0.041 1.42 0.045 0.030 11.00 17.10 2.61
SUS316L 0.014 0.040 1.33 0.029 0.001 12.20 16.65 2.15

SUS316 and SUS316L resistance to stress corrosion
Austenitic stainless steel because of its excellent corrosion resistance and mechanical properties, including the refining system is widely used in many contacts, including chemical plants rot in turbid media. Refining the system almost every step of varying degrees may cause stress corrosion cracking of austenitic (SCC) of the corrosion medium H2S and Cl-. Although the content of Cl-is not necessarily very high (usually only a few to dozens of ppm), but due to micro cracking caused by Cl-is a lot of accidents. Cracking usually occurs in high temperature parts, in particular, heat transfer speed and alternating wet and dry sites. L0ppmCl-containing water and the concentration of 1ppm Cl-in water to generate steam that have occurred in austenitic stainless steel stress corrosion cracking incidents. Austenitic stainless steel production equipment in use in the process, sometimes inevitably occur the phenomenon of sensitization of stainless steel, and may also sensitized austenitic stainless steel in H2S-Cl - H2O in the behavior of SCC. Therefore, this special solution and sensitization of SUS316 and SUS316L stainless steel in H2S saturated solution of the SCC behavior of Cl-, CO2-micro and medium pH.

Using U-bend specimens of solution and sensitized austenitic stainless steel SUS316 and SUS316L in four different Cl-containing solution in saturated H2S stress corrosion cracking (SCC) behavior. Experimental results show that in this environment, SUS316L and SUS316L sensitized metallic surfaces were tested, no SCC. The sensitization of SUS316 and SUS316 were formed on the surface of the black surface film to film and found to have varying degrees of pitting.

Sensitized SUS316 and SUS316 specimens cracks are mainly distributed in U-sample at the top of the arc and the top of the key transition (at the maximum tensile stress), the radial cracks were fired along the arc-shaped distribution, this type of crack with a hydrogen embrittlement SCC features. Cracks along the grain and transgranular cracking hybrid, serious corrosion of the grain and a deep gully there. Sensitized SUS316 of crack initiation is intergranular began the process of crack propagation in both intergranular cracking have transgranular cracking, transgranular based. There is a small branch of the crack tip, beginning at the crack due to stress large, fast crack, crack no branch. In the ensuing process, the stress is released, slowing the pace of expansion, the crack began to branch. Wearing crystal grains show corrosion of many parallel grooves. These are austenitic stainless steel in Cl-media containing SCC occurs when the brittle cracking along the traces of crystal faces.

Experiments show that, Na2CO3 on the sample in H2S-Cl - H2O System SCC susceptibility plays a role in mitigation. Soaked in this solution, shiny surface of the sample, there was no SCC. As harsh corrosive environment, and at the crack tip may lead to further acidification, crack propagation speed, no branch. Sensitized SUS316 and SUS316, compared to a much narrower crack, and crack length than SUS316. This treatment makes the SUS3l6 that subtlety is more sensitive to the SCC.

H2S in the presence of the aqueous solution only, SUS316 and SUS316L are not cracking. This is because, although the reaction of H2S and iron atoms can enter the steel in H, but due to the higher pH values, so that the original passive film is not easy to destroy, H be a certain degree of entry barriers. H in austenitic stainless steel and the diffusion coefficient is small, the amount of hydrogen into the steel enough to cause embrittlement and cracking.

Cl-in the sample surface pitting, corrosion pits in the medium and further acidification and Cl-concentration, the interaction of stress and corrosion crack initiation under the lead. Effect of the occluded cell crack tip, pH, greatly reduced, Cl-concentration was significantly increased, the resulting H makes the crack tip martensitic transformation occurs, causing brittle materials, crack expanding. Adding hydrochloric acid in the accelerated tests, most of the samples have undergone cracking. This is because the hydrochloric acid on the surface of the specimen are dissolution of iron sulfide film.

The addition of Na2CO3 SCC susceptibility of stainless steel makes the greatly reduced. Na2CO3 role is to make the PH value of the Bu Zhiyu too low, H2S and Fe in the reaction is slowed down, the surface coverage of sulfur less. Had been sensitized SUS316 generated in the grain boundary Cr23C6, generally easy to crack. However, in this study, sensitized SUS316 cracking along the grain pattern is not all, but rather mixed intergranular and transgranular cracking and transgranular based. The SCC is not anodic dissolution of the SCC, but the hydrogen embrittlement type of SCC. These results say that subtlety is not largely increase of SUS316 in the H2S-Cl - H2O solution cracking sensitivity.

The results show that the addition of Cl-and lower pH values ​​will increase the experimental sensitivity to stress corrosion of steel, while the small amount of Na2CO3 can reduce the SCC susceptibility. In short, H2S saturated solution at room temperature and saturated H2S +5% NaCl +0.05% CH3COOH solution, SUS316L showed high resistance to sulfide SCC performance, and SUS316 is more sensitive; in the solution after adding small amount of Na2CO3 significantly SUS316 stainless steel in this environment to improve resistance to SCC under capacity; plus HCl lower pH, making SUS316 and SUS316L SCC in the solution increased sensitivity; after treatment SUS3l6 sensitization to intergranular corrosion resistance is poor, and slightly in solution increases the sensitivity of the SCC. And SUS316L showed a strong immune sensitization.

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