The heat exchanger body is made of stainless steel, while the head is made of carbon steel; what material should be used for the flange connection bolts?
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Problem description: The material of the heat exchanger body is stainless steel, while the material of the head is carbon steel. It is connected via flanges, and the bolts used are galvanized carbon steel bolts. At the site, these bolts and nuts have rusted, and some inspectors have suggested that stainless steel bolts must be used instead. Dear experts, is it necessary to replace the stainless steel bolts, and what is the basis for that?
The medium depends on the severity of corrosion
Conditions such as pressure and temperature must be considered when selecting fasteners.
After replacing the stainless steel bolts, due to the sticky nature of stainless steel, it becomes difficult to unscrew them every time they need to be removed or installed for maintenance. It is normal for carbon steel bolts and nuts to show signs of wear; lubricant should be applied during regular maintenance
When different materials come into contact, galvanic corrosion mainly occurs. Galvanized bolts, together with stainless steel or carbon steel, form a large cathode and small anode system, which accelerates the corrosion of the bolts. The bolts can be treated with yellow chromium plating. Replacing them with stainless steel bolts is an option as well, but carbon steel flanges will suffer severe corrosion over time. This requires strengthening routine inspections and preparing contingency plans.
First, let’s take a look at what the selection criteria for fasteners are Is there any issue? Do the studs and nuts meet the requirements of the NB/T47020 and 47027 standards? Are material markings present on their surfaces? If not, then there is definitely a problem with the selection. If the above criteria are met, examine the drawings and calculation sheets to see if there are any special requirements specified in them; also confirm the material of the fasteners. Fasteners made of alloy steel should have their surfaces treated with blueing in accordance with standard requirements, rather than being galvanized. There are no explicit requirements specified in the standards for fasteners made of carbon steel materials. For fasteners with a galvanized surface, GB standards set relatively strict requirements, such as specifications for the pitch diameter of the threads before galvanizing and requirements regarding the thickness of the coating. However, in my opinion, there are no special requirements, and manufacturers will not purchase galvanized studs and nuts from external suppliers. It is also possible to use S3048 material for the stud nuts. However, this material is not specified in NB/T47020. Therefore, the standard number field in the drawing cannot indicate the NB/T47027 standard.
For our equipment, there are issues with the user’s operating environment; therefore, all-stainless steel is required, and the same applies to the studs. After conducting the necessary calculations, we decided to go with this material. There are no specific requirements stipulated; what you’re seeing is natural rusting, which is not allowed in our environment.
The basic components of galvanic corrosion include a cathode, an anode, an electrolyte, and a conductor; if any one of these components is removed, galvanic corrosion will cease. The contact area between the two metals involved in galvanic corrosion should be covered or immersed in an electrolyte; as a result, the anode is continuously consumed and dissolves into the electrolyte in ionic form. Therefore, rust reformation on the surface of fasteners is likely to have little to do with galvanic corrosion. Rust is likely the product of a reaction between iron, oxygen, and water; this may be due to rainwater accumulating more easily on the surface of threads, or to damage to the galvanizing layer of the fasteners. Rust formation on the surfaces of fasteners in non-stainless steel equipment is also more severe than on the equipment itself.
Dust, water, and other substances on the surface of bolts are all electrolytes. Coupling corrosion only accelerates the corrosion rate; it is also a type of electrochemical corrosion. The surface of fasteners is galvanized because zinc is more reactive than iron; it acts as an anode in electrochemical reactions, preventing corrosion of the fasteners and providing anodic protection. Rather than the galvanized layer being damaged. Of course, the galvanized layer has a thickness.
Unless there are special requirements, carbon steel bolts are generally used more often; stainless steel bolts and nuts tend to stick together
Could carbon steel bolts + insulating sleeves be considered? Avoid contact between carbon steel and stainless steel to prevent electrode corrosion!