Where Corrosion Comes From: Most Common Corrosion Environments and Failure Patterns of Metal Expansion Joints
Metal expansion joint is responsible for displacement compensation and vibration reduction in pipeline system, but it is often the weakest link in the whole pipeline system. The wall thickness of the bellows is usually only a few tenths of a millimeter to a few millimeters. Once the corrosion penetrates, the media leaks in a flash. Where does the corrosion come from? To put it bluntly, there are no such kinds: medium corrosion, galvanic corrosion, chloride ion stress corrosion, intergranular corrosion, and erosion corrosion.
Take the desulfurization system as an example. The expansion joint behind the flue gas baffle door is exposed to wet flue gas containing sulfide and chloride ions for a long time, and the temperature fluctuates, which is most prone to pitting and stress corrosion cracking. In the chemical industry, if the inner wall of the bellows comes into contact with pickling liquid or cleaning agent residue, even if the concentration is not high, 304 stainless steel can crack within a few months. You don't believe it? One enterprise used a corrugated expansion joint made of ordinary 304 material to transport the process gas containing a trace amount of chloride. As a result, within three months, penetrating cracks appeared at the trough position.
Uniform thinning, pitting pits, cracks caused by intergranular corrosion, dendritic cracking by stress corrosion. Among them, stress corrosion crack is the most terrible, because it has no obvious deformation, looks good, and suddenly leaks. To judge the type of corrosion, you can't just look at the macroscopic surface. If conditions permit, it won't cost much to do a metallographic or scanning electron microscope, but it can help you avoid detours.
Wrong material selection, anti-corrosion in vain: How to choose between substrate and corrosion-resistant alloy
Can you use stainless steel? Stainless steel is also divided into three, six and nine grades. 304 is fine in room temperature clean water, but it stops when it encounters chloride ions. The 316L is a little more resistant to chloride than the 304, but it's only "stronger". To really deal with high-chlorine, high-sulfur, and high-temperature media, nickel-based alloys, such as Inconel 625, C-276, or bidirectional stainless steel 2205, are needed.
Selection of materials is not the more expensive the better, you have to settle accounts. What is the chloride content in the medium? How hot is the temperature? Is there alternating stress? These parameters directly determine the material grade. For corrugated expansion joints used in power stations, 321 or 347 stainless steel is commonly used for main steam pipelines, because it is resistant to high-temperature oxidation and intergranular corrosion; The expansion joint at the entrance of the desulfurization tower often has to be lined with C-276 or an anticorrosion layer, otherwise it can't hold it at all.
The materials of bellows and connecting pipes and flanges are often different. If the potential difference between the two is large, galvanic corrosion will form in the electrolyte solution. At this time, either choose the same potential material, or make insulation isolation at the connection. Don't be cheap, make the bellows into 304, take over the pipe into carbon steel, and paint the surface after welding-the corrosion rate of the weld can make you doubt your life.
Surface Treatment and Coating: What Practices Really Block Corrosive Media
Surface treatment is physical isolation, which, to put it bluntly, separates the metal from the medium. Common ones are pickling passivation, polishing, spraying, lining. Pickling passivation can remove free iron and oxide scale on the surface, forming a dense passivation film, which is helpful for uniform corrosion. However, note that the passivation film is just a layer of window paper in front of chloride ions, so don't expect it to withstand stress corrosion.
For coating, polytetrafluoroethylene (PTFE) is more used. We have PTFE-lined metal hoses in our station, that is, lining the inside and outside of the metal bellows with a layer of PTFE, which is specially used to deal with strong acids and alkali. PTFE has excellent chemical stability and is resistant to almost all media, but the disadvantage is that it is not resistant to wear and high temperatures (generally no more than 200°C). Another practice is to spray Hara (ECTFE) or PFA, which has better adhesion than lining with PTFE, but the price is also higher.
The quality of coating construction is critical. If the surface of the substrate is not treated properly, the coating will blister and fall off, instead forming gaps and accelerating local corrosion. And guess what? For the sake of cost, some manufacturers only make one layer of coating, and the pinhole rate exceeds the standard. Finally, when the customer accepts, they take EDM to detect one dozen accurately. Therefore, the coating is not brushed on, but it has to be done according to the standard: surface sandblasting and rust removal reaches Sa2.5 grade, and the coating thickness and pinhole detection can't be less.
Anti-corrosion Details in Structural Design: Hidden Dangers of Guides, Bellows and Welds
Many people don't take the impact of structural design on anti-corrosion seriously. In fact, a lot of corrosion is "forced" by unreasonable structure. For example, if the guide tube is designed to be too short, the turbulent flow of the medium will directly wash the trough of the bellows, and it is easy to accumulate corrosive medium in that place. Coupled with erosion, the corrosion rate will double. The role of the guide tube is not only to divert the flow, but also to protect the inner wall of the bellows, so the material and thickness should follow the medium. If the medium contains particles, the guide tube must be made of wear-resistant material or made detachable for easy replacement.
The waveform of bellows is also particular. If the wave distance is too small and the wave height is too deep, it is easy to accumulate condensate in the trough, forming a "water nest", which is the starting point of pitting after a long time. For the working conditions with corrosive media, bellows with large wave height and large wave pitch should be preferred, or non-metallic expansion joints (fabric fiber expansion joints) should simply be replaced. People have no problem of metal fatigue and are not afraid of corrosion.
The weld is another pit. The longitudinal weld of bellows is weak, and the welding process is not good, which is easy to produce intergranular corrosion. In the circumferential weld between the joint and the wave shell, the stress is concentrated, and it is often impossible to fully detect flaws. Once the medium penetrates into the root of the weld, the corrosion begins to spread from the inside. There are several solutions: one is to avoid the high stress zone in the weld position, the other is to do solution treatment after welding, and the third is to protect the weld with an inner bushing.
Anti-corrosion traps in installation and maintenance: what to look out for from transportation to operation
Corrosion problems in the installation process are far more common than thought. During the transportation and storage of corrugated pipes, if they are stacked in the open air and rain and moisture enter the inside of the corrugated pipes, they will already start to rust before they are installed. Especially in coastal areas, salt spray is very harmful to stainless steel. Therefore, after the goods arrive at the site, we must check whether the packaging is intact, whether the bellows port is blocked, and the storage environment should be dry and ventilated.
Do not allow weld splashes to land on the bellows surface while installing. Spray of weld slag will destroy the passivation film, forming an activation point, and then corrosion will begin from there. Also, after the installation is completed, it is necessary to press and rinse, and the rinsing water should be drained clean, otherwise it will remain at the bottom of the bellows and will be rusty for you in a few days. I saw a project before, and I forgot to drain it after pressure test. As a result, a set of general-purpose corrugated expansion joints was kept in the warehouse for more than a month, and the inner wall was full of rust spots. Finally, they had to be returned to the factory for treatment.
In the maintenance of operation, the most important thing is to avoid the "small horse-drawn cart"-the medium is over-temperature, over-pressure and super-chloride ion content. If any item is over, the material can't bear it. Regularly check the deflector for wear and tear to see if there are any abnormal noises or vibrations, which are precursors to corrosion and erosion. Enterprises with conditions can set up online corrosion monitoring probes in key parts of the expansion joint, and see the data in real time, which is better than waiting until it leaks and then stopping work.
In the final analysis, the anti-corrosion of metal expansion joints is not as simple as choosing a material and painting a layer of paint. It is a systematic project from design, material selection, manufacturing to installation and maintenance. Which link drops the chain, in the end, it is all leaked to know. Hopefully, this article will save you stepping on a few pits.