What causes irreversible pitting on the welds of 316 stainless steel heating tubes?

# What is the cause of irreversible pitting on the welds of 316 stainless steel heating tubes? The welding fissures are the most susceptible location for irreversible pitting corrosion in fermentation production, despite the fact that 316 stainless steel heating tubes rely on a chromium-rich passive film for anti-corrosion protection. It is impossible to eliminate pits that have formed on weld areas through simple passivation, and they will progressively expand to create through-wall leakage, resulting in medium pollution and unplanned shutdown losses. The protective layer of welds will be destroyed by a combination of processing, operation, and maintenance factors. The accompanying table categorises the core inducing factors and their corresponding hazard levels. | Inducing Factor | Specific Manifestation on Welds | Degree of Pitting Risk | Effective Control Solution | | ---- | ---- | ---- | ---- | ---- | | Incomplete argon back shielding during welding | Oxidation black oxide layer inside weld seam | Extremely high | Enforce full argon protection and post-weld polishing | | Alkali cleaning temperature exceeding 60℃ | Passive film instant local breakdown | High | Lock PLC interlock to cut power above 60℃ | | Long-term medium chloride over 50ppm | Chloride penetrates film gaps to form micro-pits | High | Install online chloride real-time monitoring | | Unpolished rough weld surface | Dirt and organic matter deposit to form corrosion cells | Medium | Full mechanical polishing for all weld joints | | Missing post-weld local passivation | Weld area lacks dense chromium film | Medium | Supplementary pickling passivation after welding | | Mixed contact with bare carbon steel brackets | Galvanic corrosion accelerates pit expansion | Medium | Lay complete PTFE isolation strips | Weld pitting originates from the destruction of the continuous passive film. After high-temperature melting and cooling, the internal grain structure of welding joints undergoes a significant transformation, resulting in a passive film that is more fragile and thinner than the base pipe body. Oxygen will react with alloy elements within the weld to form loose oxide layers if manufacturers neglect argon back shielding during processing. These layers are unable to form a complete protective barrier, even after the overall tube has been passivated. After weeks of cyclic cleansing, micro-pits develop as a result of the natural infiltration of chloride and alkaline liquid through the small gaps in the oxide layer. Another significant trigger is the use of high-temperature alkaline CIP cleaning. When the temperature of alkali liquid surpasses 60 degrees CelsiusHydroxide ions will swiftly dissolve the chromium oxide film on weld surfaces at a temperature of ℃. Electrochemical corrosion cells are formed instantly when chloride is present in the medium. Within a few production cycles, shallow pits transform into deep, irreversible cavities. In contrast to uniform surface rust, pitting is characterised by the concentration of small points that are concealed beneath grime and difficult to identify during daily visual operations. The only method that can detect concealed thinning weld areas in advance is bi-monthly ultrasonic wall thickness scanning. Fermentation organic residues will be trapped by rough, unpolished weld surfaces, which will decompose to generate acidic substances and further erode the passive film. Weld refining and local post-weld passivation are often omitted by small factories in order to reduce costs, resulting in early weld pitting within the first year of operation. In order to prevent the occurrence of such defects, procurement technical specifications must explicitly mandate full weld refining and secondary local passivation. Isolate all carbon steel supports with PTFE materials and rigorously regulate the temperature and chloride content of alkali cleaning during daily operation. Weld pitting can be effectively delayed, the service life of 316 stainless steel heating tubes can be extended, and sudden leakage failures in production can be prevented by regular wall thickness detection and supplementary passivation.