Can metal corrugated expansion joints still be used if they are rusted? Don't be in a hurry to change, look at these points first
Two days ago, a customer from the cement industry called and scolded as soon as he opened his mouth: The quality of your expansion joint is not good. It has only been half a year since it was installed, and the surface is full of rust spots. I asked him to take a picture and take a look-yes, how can 304 stainless steel hold up to the chloride ion environment? This is not a quality problem, but a material selection rollover. So don't blame the product as soon as you come up, first look at how the rust came from.
Several Common Causes of Rust-Don't Blame Product Quality
Many people think that stainless steel will not rust, which is a misunderstanding. The "stainless" of stainless steel is relative, mainly protected by a dense chromium oxide film on the surface. Once this membrane is destroyed, corrosion comes to your door.
Improper material selectionIs the number one killer. 304 stainless steel is commonly used for general-purpose corrugated expansion joints, but 304 is like paper paste in the face of cement desulfurization and chemical acidic media. When the concentration of chloride ions in the medium is high, pitting and stress corrosion will crack in minutes. The right thing to do is raise the material-316L, duplex stainless steel, or even nickel-based alloy. Don't end up paying for downtime losses just to save hundreds of dollars.
Welding residual stressAlso often overlooked. If the corrugated pipe is not treated with solution or passivated by pickling after welding, the corrosion resistance of the weld area will be significantly reduced. In addition, when installing, the guide tube is installed backwards (the direction of the arrow is not according to the instructions), and the airflow rushes directly to the root of the corrugation, accelerating local corrosion. There is also water accumulated under the insulation layer-the insulation material absorbs water and sticks closely to the surface of the bellows, and the corrosion rate can be multiplied several times in the alternating dry and wet environment. During transportation, the low-level problem of packaging damage and water entrance can occasionally be encountered. When you open the package, you can pour water out of the bellows.
How serious are the consequences of rust?
Many people think that rust is just not good-looking, just wipe it. Wrong. Corrosion can directly weaken the wall thickness at the crest. Especially the high-temperature axial expansion joint used in the power station industry has a design temperature of 500 to 600 degrees and frequent thermal cycle. Once it is partially thinned, fatigue cracks will appear soon. The crack spread to the end, the bellows opened, and the medium leaked.
More insidious are pitting perforations. It looks like a small pit, the size of a fingernail, but the inside has formed through holes. This kind of corrosion is particularly common on the corrugated expansion joint matched with the desulfurization flue gas baffle door-the medium is sulfur-containing and moist, with low pH value and extremely fast pitting. Once the compensator fails, the thermal displacement of the pipeline can't be absorbed, and the thrust is unbalanced. In the slightest case, the insulation layer is damaged, and in the worst case, the pipeline support is displaced and the flange surface is pulled off. Don't wait until the braces are all crooked to regret it.
How do you judge whether you want to change the rust?
Let's make a simple on-site judgment first. Pick the rust layer with your nails-the floating rust that can be picked off is mostly the oxide layer of surface pollutants. Wipe it clean and apply some anti-rust oil to last for a while. If the rust layer is brittle, peeling, or the underlying potholes are sanded with sandpaper, that is a big problem.
The regular practice is to measure the wall thickness with an ultrasonic thickness gauge. If the wall thickness is reduced by more than 10% of the design value, or a pit with a depth of more than 0.3mm occurs at any location, it is recommended to replace the part directly. Don't gamble on luck on key occasions such as power stations and chemical industry. In addition, the corrugated joint matched by the electric plug-in insulation door or the smoke baffle door, and the internal corrugated valley bottom are the hardest hit areas by corrosion. It is recommended to open holes for inspection every six months. Don't bother, one leak costs you enough to replace several sets of equipment.
Preventing rust is not so mysterious-the source is stuck in the material selection stage
Prevention is much simpler than treatment. The corrugated expansion joint matched with the desulfurization flue gas baffle door, the medium contains sulfur and is moist, and it is directly applied to 316L or duplex stainless steel. During installation, the orientation of the guide tube must be in the direction of the arrow, so as to avoid the air flow directly washing the corrugated root. The external surface of the directly buried expansion joint is coated with epoxy coal pitch, which is ten times more reliable than simply painting. When civil backfilling, care should be taken to protect the coating from scratches by rubble.
Daily inspection depends on whether the drainage hole is blocked. Pooled water that doesn't drain is a hotbed of rust, especially those that are installed in the open or expansion joints in gutters. If you find that the drainage hole is blocked, just stab it with a wire. The damaged insulation layer should be repaired in time, and the rain should not seep in.
What to do with it if it has rusted?
Look at the corroded area. Small areas-such as rust spots one or two centimeters in diameter-are hand-polished to reveal a metallic luster, passivated by acid pickling, and then sprayed with high-temperature resistant paint. You can refer to the anti-corrosion treatment method of the flange surface of the rubber compensator, and apply anti-corrosion coating after cleaning. Don't feel distressed if you have a large area of rust, just change it. Once, in order to save money, a factory wrapped a layer of rubber PTFE compensator around the rusty expansion joint. As a result, the inside continued to rot. Finally, the bellows burst and shut down for three days, resulting in hundreds of thousands of losses. Coating if the surface is not thoroughly treated is equivalent to wearing a raincoat on bacteria-it looks waterproof, and the inside rusts more happily.
The rust of metal corrugated expansion joints is not terrible, but the terrible thing is that I don't know how much rust it is and whether it should be replaced. Check it again according to the steps mentioned above, whether it is repaired or changed, and you will have the bottom in your heart.