Chloride ions and free chlorine

Saltwater corrosion without free chlorine is generally interfacial corrosion (meniscus corrosion). At the meniscus, there is only a thin layer of saltwater, which is easily saturated with dissolved oxygen due to contact with air; even when oxygen is consumed, it can be replenished promptly, thus creating an oxygen-rich zone. This zone, along with the oxygen-poor zone below the liquid surface, forms a concentration cell for oxygen. The meniscus acts as the cathode where hydroxide ions are generated, while the area below the meniscus functions as the anode where iron ions are produced; rust is formed in the area between these two zones. The specific corrosion rate can be found in relevant materials.

The senior upstairs seems to have explained things in too much depth; I’m not quite understanding it. Can saturated saltwater cause corrosion of stainless steel? In other words, can stainless steel be used in places where carbon steel valves are used?:lol :)

So what you’re asking is: can things that can be bought for one yuan be bought for ten yuan? My answer is: Yes, as long as it’s not ** (saltwater is not acidic)! ;P ;P

Thank you. I originally thought it made sense not to ask, but some people said that chloride ions are present in saltwater, and that carbon steel can handle it while stainless steel cannot; I wasn’t quite sure, so I asked. I’m sorry for embarrassing you, senior :( :'( :'( :'( :'(

Thank you, I learned something new \

1. There are various grades of stainless steel, and different materials should be selected for different media； 2. Ordinary stainless steel, namely what is commonly referred to as 304, is susceptible to corrosion in the presence of chloride ions ； 3. Both brine and concentrated brine are alkaline during the salt production process; carbon steel valves can be used as long as the temperature is not too high and the pH is uniform.

I agree with the opinion from floor 7; 304 steel does not perform particularly well in terms of resistance to saltwater corrosion. As long as the saltwater remains alkaline, ordinary carbon steel is usually the preferred choice.

:) Taking advantage of your valuable time, I would like to ask the experts here: should the pump for frozen saline be made of stainless steel or carbon steel? That is, calcium chloride solution, with concentrations ranging from over 10 percent to over 20 percent. Some friends say carbon steel is sufficient, while others say stainless steel is better.

In media with high chloride ion concentrations, 304 is not only susceptible to corrosion but should not even be considered for use! In the presence of chloride ions, the corrosion of 304 (and 316 as well) is mainly pitting and intergranular corrosion, whereas carbon steel suffers primarily from uniform corrosion. Therefore, stainless steel is prone to perforation or embrittlement at weld areas, and it is also susceptible to fracture under high stress. Its service life is much shorter than that of carbon steel under the same conditions, unless there are no welds. Therefore, when conducting pressure tests on pipes or equipment made of stainless steel, strict requirements are placed on the chloride ion content in the water. 316L can be used in environments with high chloride levels (it can be used in seawater at least, where the chloride concentration is 19 g/L). But the price of 316L is truly exorbitant; all stainless steels were set to see further price increases in 2008! If you don’t believe it, you can ask the manufacturer.

Thank you, brother upstairs. Gained knowledge

I’ve learned it, but where can I find the specific data? Are there any standard regulations? I know that 304 corrodes rapidly in saltwater, but I can’t provide any written evidence

Saturated salt water at 50–60°C contains very little oxygen, and corrosion of carbon steel in such still salt water is extremely slow； When saline water flows, the easy supply of oxygen accelerates the corrosion of carbon steel; therefore, saturated saline storage tanks and pipelines are protected against corrosion or lined. Although carbon steel can be used for the valve, it suffers from severe corrosion after short use. As for stainless steel, chloride ions definitely cause stress corrosion; stainless steel is generally not used in saline environments. Sometimes, skilled workers consider stainless steel valves to be better than carbon steel valves; even salt water pumps use stainless steel instead of carbon steel, mainly because carbon steel is difficult to disassemble once it corrodes, whereas stainless steel can be easily disassembled even if it corrodes.

What materials are generally chosen for non-metallic materials?

CPVC pipes are selected; PVC/FRP

It depends on what type of saline is being used. If it’s saline to which refining agents have been added and that hasn’t been clarified, carbon steel can definitely be used; whereas for refined saline or frozen saline, PVC-lined or stainless steel pipes can be chosen

Why not use PVC valves, or PP ones from CPVC? ! It can be used on saltwater as well! It’s just that the cost is a bit high; once the pipes are lined with rubber, they work fine, right??? We all use either rubber or PU for lining!!