Can 35CrMoA bolts be used for bolts on flanges of equipment in hydrogen chloride-corrosive environments?
Thread Content
There is a heat exchanger unit that needs to be used in an environment subject to hydrogen chloride corrosion. I would like to know whether 35CrMoA can be used as the material for the bolts on the equipment’s flanges; if not, what material should be used for the bolts and nuts? The material of the equipment flange is 316L
It is a mistake to choose stainless steel for media environments containing chloride ions; chloride ions have a very strong affinity for chromium ions, which leads to pitting, also known as corrosion by pits.
Yes, it’s fine to use 316L for the equipment flanges and 35CrMoA for the bolts; this is a standard design. However, care should be taken during regular maintenance – apply some grease or similar to keep the bolts isolated from air as much as possible. It’s nothing. A combination of stainless steel flanges and alloy steel bolts is used, typically in corrosive environments with high temperatures. Water molecules in the atmosphere find it difficult to adhere to the flanges and bolts for long periods; they generally evaporate immediately ； Moreover, the surface of stainless steel flanges usually has a passivation coating, which reduces the likelihood of corrosion. Unless the passivation coating in the area where the flange meets the bolts is damaged, a little butter can solve this problem. I just want to remind maintenance personnel not to apply too much butter – a thin layer is sufficient, haha. . . By the way: guy upstairs, 35CrMoA isn’t stainless steel. . .
Hydrogen chloride? Gaseous? Liquid? Hydrochloric acid? 316L? As long as it’s not an internal bolt, there should be no problem; no corrosion issues
It doesn’t matter whether 35CrMoA is stainless steel or not; as long as there are chromium ions present, chloride ions will definitely cause corrosion to it. A design institute mistakenly selected duplex stainless steel for designing large-scale equipment in an alkali plant; despite the presence of small amounts of chloride ions in the reaction process, corrosion still occurred over time.
This post was last edited by arpcd on 2012-5-3 20:38. The original poster’s question relates to an HCl environment; it is not about some medium but rather hydrochloric acid. Moreover, the bolts do not come into direct contact with the process medium. . The duplex stainless steel equipment you mentioned is meant to come into contact with chloride-containing media, right? Atmospheric environmental corrosion and corrosion due to direct contact with media are two different things. If the design institute chooses duplex stainless steel, it might be a makeshift solution. As a design institute, they must be aware of the corrosive effect of chloride ions on stainless steel, and they also know that corrosion will occur over time. But what is the allowable amount of corrosion? Is it cheaper to replace the equipment during its lifespan, or is it cheaper to use high-quality materials from the start? The choice of duplex stainless steel is definitely made after weighing equipment investment against the need for corrosion resistance; if one wants to avoid any corrosion at all, wouldn’t it be better to choose a nickel-based alloy? Materials such as 254 and 276, which are of high quality, can be used; even niobium-tantalum alloys and zirconium alloys are viable options. However, the cost is a significant factor. In the salt and alkali production industries, there is always a long-term consideration regarding the investment required for equipment made from certain materials as well as the cost of replacing those devices over their lifetime. It ultimately comes down to an economic evaluation issue. For example, if one chooses the 254 nickel-based alloy, the initial cost is 1 million, with a device lifespan of 20 years. If 316L is chosen, the initial cost is 100,000, but the lifespan is only 5 years; thus, replacing the device after 5 years results in a total cost of 200,000 over ten years. Even though it is known that 316L has issues with corrosion resistance, many owners still choose it. . The choice of material is not just a matter of corrosion; cost is also a major issue. . It would be great if all aspects of engineering were determined by technical issues alone. . By the way: 35CrMoA is not stainless steel; it’s important to be careful with such basic facts. . . . It can’t hurt to be serious.