Questions about wet hydrogen sulfide corrosion in welding
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Regarding wet hydrogen sulfide stress corrosion, Clause 7 of 7.8.2.4 in the standard HG/T20581-2011 stipulates that: “Welded joints between ferritic steel or duplex stainless steel and austenitic steel are not permitted.” Question: 1. Why are welded joints between different types of steel not allowed? What is the principle behind this? Is it that carbon steel and stainless steel materials cannot come into contact, or is it only the welds between them that cannot exist? 2. If a heat exchanger has a wet hydrogen sulfide stress corrosion environment on the shell side, and its heat exchange tubes must be made of stainless steel, in order to avoid joints between different types of steel, a composite plate (carbon steel combined with stainless steel) is used for the tube sheet. If contact between carbon steel and stainless steel is not allowed, then the interface between the base layer and the coating layer of the composite plate will be exposed to the medium; in such a case, cannot a composite plate be used? (The issue of using entirely stainless steel is not discussed here.) If only welding joints between different types of steel are not permitted, then this design is feasible
The second question: is your heat exchanger equipped with a fixed tube sheet or U-tubes? If it has a fixed tube sheet, heat treatment is required after welding with wet hydrogen sulfide; in that case, the explosive cladding will inevitably affect the corrosion resistance of the cladded layer. Would surfacing be a better option in such cases? However, machining is needed after surfacing is completed. Therefore, both surfacing and explosive cladding seem to be unsuitable options when using a fixed tube sheet. U-tubes can also be used
It refers to welded joints, where potential difference corrosion occurs primarily at the main welding areas. Your welding structure of tube sheets and heat exchange tubes poses risks; although stainless steel welds do not require heat treatment, this structure cannot completely eliminate the gaps on the shell side. Double-sided composite panels can be considered. It is recommended to consider all-stainless steel tube sheets.
The weld area is the cladding of the composite plate; it is made of the same steel as the heat exchange tube material, so welding of the same type of steel takes place. Is there a risk of potential difference corrosion?
1) The provision in clause 7 of 7.8.2.4 in HG/T20581-2011 should mean that welding of dissimilar steels is not permitted at the interface in contact with corrosive media. The structure mentioned by the original poster, with no dissimilar steel welding at the interface where the corrosive medium comes into contact, should be feasible. 2) Another condition for stress corrosion is stress. Whether the weld joint is in a high-stress area depends on the overall structure of the heat exchanger and its operating conditions.
If the corrosive medium is on the shell side, should a stainless steel cladding be considered for that side, using an internal weld structure?
Potential difference corrosion means that in a wet hydrogen sulfide environment, \"alloy metal welding joints between ferritic steel or duplex stainless steel and austenitic steel are not permitted.\" In the structure shown in your diagram, the shell side is prone to the accumulation of corrosive agents; therefore, it is best to make modifications.