Erosion of carbon steel pipes by saline solutions

The material can be upgraded to white steel

A pure medium might be fine, but when other substances are mixed in, corrosion can occur. Alkalis are also highly corrosive media; it takes time for reactions to take place after they are added, and factors such as heat release also have an impact

Is corrosion caused by uneven addition of alkali and inaccurate pH readings?

This post was last edited by Zhi Sheng Wei Hua on 2018-1-24 09:14. Concentration cell corrosion and oxygen dissolution corrosion must be taken into account, as well as intergranular corrosion resulting from differences in the microstructure caused by welding and casting of the materials. All the above corrosions can be regarded as electrochemical corrosion. Concentration cell corrosion: Elbows are the points where the flow direction of the liquid changes; the fluidity of the liquid is not uniform there. The flow velocity is higher in areas with larger curves, while it is slower in areas with smaller curves. This results in old liquid remaining in the areas with smaller curves. It can be regarded as an amplified form of crevice corrosion. Moreover, since the fluid is a neutralizing fluid, variations in the concentration and degree of dispersion of the neutralizing agent within the fluid occur (long-term stirring accelerates uniform dispersion), resulting in concentration differences between old and new fluid. These concentration differences create a potential difference, allowing active ions to cause corrosion of the metal. Oxygen-induced corrosion: It is well known that the presence of oxygen has a positive depolarizing effect. This is evident in the different degrees of corrosion that occur in marine environments, in tidal areas, and in air-filled environments – the most severe corrosion taking place in tidal areas, where there are frequent and significant fluctuations in oxygen concentration. The newly formed oxide layer there is easily depolarized and destroyed, allowing corrosion to progress further. This situation also often occurs at elbows, as the fluid changes direction and strikes the wall of the elbow, which leads to a decrease in the fluid’s solubility for oxygen (air, or other gases dissolved in it, including corrosive gases). The resulting poor oxygen dissolution causes depolarization and changes in potential, thereby accelerating corrosion. The effects of microstructural changes resulting from welding and casting: Elbows are often cast (using sand molds), and there are many factors that affect their microstructure. As is well known, welding can cause microstructural changes in the high-temperature areas (which is why new welding processes such as gas shielded welding were developed); the same is true for casting. Reputable manufacturers strictly control parameters such as the microstructural composition and casting temperature before casting, **thereby reducing the impact of these manufacturing processes on the quality of the product. However, the heterogeneity in the microstructure is an objective reality; such effects often lead to pitting and pit corrosion, which are caused by differences in the uniform potential of the material. Let alone welding; aside from the effects of high temperatures, the material of the welding electrodes used is different from that of the material being welded, which results in a greater potential difference. In summary: When examining corrosion, one should look for the differences among the common elements, and then consider the specifics of those differences, which can help provide some clues. There is no situation in which corrosion cannot occur at all; by appropriately using anti-corrosion materials such as anti-corrosion stainless steel, anti-corrosion linings, and anti-corrosion coatings, the safety of the entire system can be enhanced.

I’m an amateur, but I’d like to share my thoughts as a way to stimulate further discussion. 1. Adding alkali is a good idea; the fact that pitting occurs here rather than full-scale corrosion is good evidence of this. 2. If corrosion occurs on the inside of the elbow while the outside remains intact, it indicates that corrosion is severe under conditions of low flow velocity or low static pressure in the pipeline. Solutions: 1. If the operating conditions permit, try raising the pH value to between 9.0 and 10.0, provided that the temperature does not exceed too much (generally, it’s best not to go above 65 degrees Celsius). 2. If changing the material is an option, low-alloy steel, low-carbon steel, and duplex steel can be tried; sometimes stainless iron can also be very effective (in certain conditions, stainless iron performs better than ordinary steel and is also cheaper). PS: In many cases, carbon steel is more suitable than stainless steel, for example in pipelines used for transporting alkaline solutions at room temperature. I’ve seen things like this: it was said that using carbon steel is fine and that it’s better than 304, but the technicians were still worried, saying that even 304 corrodes severely, so carbon steel definitely won’t work. As a result, they never changed their pipeline; for eight years, they have just been making minor repairs. In another unit, after switching to carbon steel pipelines, it has not been touched for three years.

I read the comments, and one guy said it might not be mixed evenly. I think that makes sense too. Take a look at where your alkali addition point is; maybe you can try using a mixer.

Elbows that change from horizontal to vertical or from vertical to horizontal； It’s hard to imagine that under pH levels of 7–8, elbows submerged in salt solutions that do not corrode carbon steel could develop corrosion of this nature (severe pitting)~~~~~~~ It might be worth trying to replace certain sections of the elbows with non-metallic materials such as FRP+PVC connected via flanges, to see what happens~~~ Whether corrosion occurs or not will be obvious at a glance~~~~~~~~~~~~~

:Lol, what thick pipes those are.