Two cases of corrosion in austenitic stainless steel
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Two cases of corrosion in austenitic stainless steel 【Case 1】Intergranular corrosion caused by sensitization in the heat-affected zone of welds I. Overview of the installation (equipment) A nitrogen supply system has a design pressure of 9 MPa; the pipes have a diameter of DN50, and they are made of 0Cr18Ni9 austenitic stainless steel (in compliance with the GB/T 14976-2002 standard for seamless stainless steel tubes for fluid transport). Stainless steel elbows shall conform to the 1.5 times bending radius (long radius) elbow specification in GB/T 12459-2005 \"Steel Butt-Welded Seamless Fittings\", with grade 0Cr18Ni9; the supply condition is solution heat treatment. The elbows and stainless steel pipes were welded manually using TIG welding; the outer surface of the elbow welds was treated with acid washing and passivation, and the post-weld non-destructive testing passed. After the system was installed, a pressure test was conducted on the entire pipeline, and no abnormalities were detected. II. Course of the incident: During an inspection conducted 18 months after the pipeline system was put into use, it was found that there was a continuous drop in pressure within the system. A hissing sound could be heard at one of the pipe bends, leading to the preliminary conclusion that a leak had occurred at that bend. Penetrant non-destructive testing was used to perform a coloring inspection on the problematic pipe elbow, revealing defects and small pores in the heat-affected zone of the elbow weld. Surface penetrant testing was conducted on the elbow sections of the entire pipeline; in some elbows, only pitting was present in the heat-affected zone, with no penetration corrosion ； Some elbows have intact surfaces with no corrosion. III. Cause Analysis: A macroscopic examination of the outer surface of the failed elbow revealed that corrosion was relatively severe in the area near the welds on the outer surface. The corrosion occurred on both sides of the welds, at a distance of about 10 mm from them. The width of the areas with severe corrosion was approximately 10 mm; these areas corresponded to the heat-affected zone of the elbow’s welds, and they had a significantly darker color. There are multiple corrosion spots in the heat-affected zone of the outer surface weld outside the leakage area. Microstructural examination revealed significant intergranular corrosion and intergranular microcracks on the outer surface of the heat-affected zone; the depth of corrosion was approximately 0.2 mm, indicating that the corrosion in the outer surface heat-affected zone was intergranular corrosion. There are significant differences in the microstructure of the heat-affected zone compared to that of the base material and the fusion zone; network-like granular carbides are present at the austenite grain boundaries in the heat-affected zone, indicating sensitization of this region after welding. There is a certain depth of intergranular corrosion in the leakage area; this intergranular corrosion is discontinuous. After layer-by-layer polishing, it was observed that the maximum depth of such intergranular corrosion was approximately 1.5 mm, indicating that the corrosion in the leakage area is of the intergranular type, and that there are obvious leakage pathways present. The base material of the elbow is normal; intergranular corrosion has occurred in the heat-affected zone of the welds on the elbow. In a salt spray environment, S2- and Cl- ions cause localized corrosion, leading to media penetration and leakage. Improper control of welding parameters during the elbow welding process leads to sensitization in the heat-affected zone of the weld, resulting in intergranular corrosion. Pickling and passivation of subsequent welds will exacerbate intergranular corrosion. IV. Rectification measures: During the elbow welding process, the heat input during welding must be strictly controlled; the welding current should be reduced, the welding speed controlled, and the width of the heat-affected zone of the weld minimized to prevent intergranular corrosion. The pipeline is treated to prevent corrosion, in order to resist pitting caused by Cl- in a salt spray environment.
【Case 2】Stress corrosion caused by chloride ions I. Overview of the installation (equipment) The high-pressure fire water pipeline system in the turbine hall of a large coastal coal-fired power plant is designed to operate at a pressure of 1.2 MPa. During operation, the pressure of the fire water ranges from 0.85 to 1.0 MPa. The fluid transported is clean water obtained after treating surface water through coagulation and sedimentation followed by valveless filtration. The material standard for these fire water pipelines is GB/T 14976-2012 \"Stainless steel seamless pipes for fluid transport\", with specifications of φ273×7 mm and made of 304 stainless steel. II. Course of the incident: The high-pressure fire protection water pipeline in the turbine room of Unit 34 began to be pre-fabricated for replacement in March 2021, and leaks started to occur in the pipeline in July. The high-pressure fire protection water pipes in the turbine hall of Unit 56 began to be prefabricated for replacement in May 2021; leaks were detected in November. The leakage sites were located in the main body of the pipes as well as at the welds, and these leaks appeared as penetrating holes – the diameter of these holes was very small, but they were visible to the naked eye ； The corroded areas of the pipe welds showed extensive brown rust; upon further grinding of these corroded zones, through-going cracks were found. III. Cause analysis: In accordance with the standard GB/T 15453-2018 \"Determination of chloride ions in industrial circulating cooling water and boiler water\", the Cl- content in the water used in the fire protection pipelines of units 34 and 56 was tested. The measured Cl- content was 166 mg/L (166 ppm), which exceeds the limit specified in the standard GB 50235-2010 \"Code for construction of industrial metal piping projects\", according to which the chloride ion content in water used for flushing stainless steel, nickel, and nickel alloy pipelines shall not exceed 25 mg/L. The surface of 304 stainless steel is covered with a highly protective passivation film that effectively safeguards the base material from corrosion. However, when there are high concentrations of Cl- in the solution that can destroy this passivation film, it is damaged in a very short time, leading to pitting corrosion. Pitting generally occurs on the surface of stainless steel and then progresses inward, even penetrating the entire cross-section. Pitting corrosion is a type of local corrosion. According to the adsorption film theory, chloride ions form a catalytic corrosion pattern with a small anode and a large cathode in the areas where they are adsorbed. A large concentration of chloride ions accumulates within the pitting area, where they combine with the metal to form unstable chlorides. Upon hydrolysis, these chlorides produce hydrochloric acid and metal cations, resulting in a decrease in pH. To balance the charge concentration in the solution, more anions migrate into that area, thereby accelerating the corrosion reaction in that localized region. There are far more leak points at the welds of these high-pressure fire water pipes than at the base material of the pipes. The welds on the inner wall of the pipeline and the areas adjacent to these welds were not subjected to pickling or passivation; residual internal stresses remain from welding, and the medium tends to accumulate in the uneven surfaces within the welds, thereby further accelerating the local concentration of chloride ions. Under the influence of residual stress, the stable structure within the metal is disrupted, causing grain misalignment, which provides an opportunity for chloride ions in water to adsorb onto and penetrate the passivation layer; as a result, intergranular or transgranular cracking occurs in the stainless steel. The results of the corrosion investigation show that corrosion is primarily concentrated in areas such as the center of the pipe circumferential joints, the weld seams, and the heat-affected zones. These areas have higher residual stresses compared to other parts, which makes the welds more susceptible to corrosion and cracking. At the same time, due to the fact that fire-fighting water is not used frequently, the flow rate of water is essentially zero; as a result, impurities, contaminants, and chlorides settle and accumulate at the bottom of the pipes. Over time, more and more corrosion products containing chloride ions accumulate, leading to an increasing concentration of these substances, which in turn accelerates the corrosion of the walls of 304 stainless steel pipes, until perforation and leakage occur. The results show that in stainless steel pipelines exposed to media with high chloride content, the welds are the weakest points prone to corrosion and leakage. IV. Rectification measures: Stress exists in the welds during pipeline welding, and Cl- tends to accumulate there, making these areas the most vulnerable to corrosion and leakage. High standards are required for the welding process of stainless steel pipelines; stress-relief measures should be taken during construction, along with enhanced quality control ； Flange connections can be used, and the damaged areas of the welds and heat-affected zones can be subjected to secondary pickling and passivation in order to extend their service life. 304 stainless steel boasts excellent resistance to high-temperature oxidation and corrosion, which has led to its widespread use in manufacturing processes. However, the corrosion rate of 304 stainless steel pipes is inversely proportional to the flow velocity of the fluid inside them; as such, it is not suitable for fire protection systems where the flow is intermittent or low.
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